| Author: | nickt (guest: search) |
| Date: | Tue, Mar 13th, 2007 @ 21:18 ( . ) |
I followed a long tortured thread on 18watt.com and discovered the simplest power scaling idea that that actually works. Unfortunately it does not work on SE amps or those with unusual PIs (Paraphase, Cathodyne...). However... All that's required is a reverse log 50k pot and two bits of wire. The pot goes between the shared cathode of the PI tube and the PI cathode resistor (usually 470R or 1k2). That's it! In case you're tut-tut'ing at this point. The mod (its not worthy of the term circuit) is what's used by Mojave on their amps! YMMV. |
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| Author: | jimmy lee (guest: search) |
| Date: | Tue, Mar 13th, 2007 @ 21:46 ( . ) |
what does it sound like? |
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| Author: | nickt (guest: search) |
| Date: | Wed, Mar 14th, 2007 @ 01:53 ( . ) |
On 03/13/2007 @ 20:46, jimmy lee wrote :
what does it sound like? : -- Apparently "normal" - in other words same as the amp at hi volume. Like I said all I'm doing is quoting the 18watt.com folks. The reason I'm interested is that I hate Master Volumes - they're just UN-natural (IMHO anyway). The Mojave amps are kinda 50w marshalls with double size OTs (quoting again). So the output stage is pretty clean and has feedback around it. Not sure what this mod will sound like with a "no NFB" amp. Haven't had the time to check the mod out personally - just wanted to share it (recalling a Mojave related "what the...?" discussion here a while ago). |
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| Author: | Merlin (registered user: 4129 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 07:28 ( . ) |
So it's basically a variable cathode resistor on the PI? Presumably you bias the PI colder and colder then, until you force it to run practically in class B?! I'm not convinced... |
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| Author: | stevem (guest: search) |
| Date: | Wed, Mar 14th, 2007 @ 07:50 ( . ) |
If you are not making the B+ on the output tubes vari downward,and the bias also in proportion to that B+ voltage change then you are not powerscaling in a way that will truly make the amp sound flat out only at a lower volume. |
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| Author: | AletheianAlex (registered user: 3532 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 16:24 ( . ) |
On 03/14/2007 @ 06:50, stevem wrote :
If you are not making the B+ on the output tubes vari downward,and the bias also in proportion to that B+ voltage change then you are not powerscaling in a way that will truly make the amp sound flat out only at a lower volume. -- True. And even "real" power scaling isn't really scaling anything... it is just shifting the operating point away from the optimal transformer impedance load.... so yeah, the wattage is dropping, but it is kinda like jacking up the reflected transformer load, unless i am looking at it wrong. |
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| Author: | Pete Allen (registered user: 889 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 08:01 ( . ) |
Unless it's actually meant to go in the tail. That would decrease the headroom of the PI, so it would crunch earlier. However I can't see how this would sound much different from a post pi MV Pete |
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| Author: | Mark L (registered user: 1446 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 08:23 ( . ) |
I think it would sound different than a post PI master volume because the PI will be going into cold cut-off distortion rather than saturation type distortion of the normal PI bias. So I'd guess the PPIMV would probably sound better, but we'll never know unless we try it. |
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| Author: | Zoe N Iain (registered user: 7776 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 08:26 ( . ) |
[Iain] I wonder if using a dual gang pot and implementing this and PPIMV at the same time might work better? |
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| Author: | Pete Allen (registered user: 889 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 09:59 ( . ) |
On 03/14/2007 @ 07:23, Mark L wrote :
I think it would sound different than a post PI master volume because the PI will be going into cold cut-off distortion rather than saturation type distortion of the normal PI bias. So I'd guess the PPIMV would probably sound better, but we'll never know unless we try it. -- If you put the resister in the tail (not what the original post specified) then the bias of the PI would stay the same. The headroom would decrease. Pete |
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| Author: | Balijukka (guest: search) |
| Date: | Wed, Mar 14th, 2007 @ 12:37 ( . ) |
you don't get the penode dirt, simple as that. :( jukka |
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| Author: | Mark L (registered user: 1446 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 16:05 ( . ) |
On 03/14/2007 @ 11:37, Balijukka wrote :
you don't get the penode dirt, simple as that. :( : : jukka -- That's right, and isn't that what the 18W is mainly about. |
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| Author: | tkdrew (registered user: 593 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 16:11 ( . ) |
I have a stong feeling that it would make a nice 18 watter sound more and more like a Spitfire as it is turned up. I recently converted my Spitfire clone to an 18 watt because so much of the dirt was coming from the PI. Very fizzy to say the least. |
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| Author: | Merlin (registered user: 4129 posts ) |
| Date: | Wed, Mar 14th, 2007 @ 18:24 ( . ) |
Not to mention the DC on the pot- scratch-mania! |
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| Author: | Zaphod (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 06:53 ( . ) |
If the pot is in the PI tail, the increasing PI distortion should sound "browner" and more pleasant than the hard-clipping of an increasing cathode resistor. On 03/14/2007 @ 15:11, tkdrew wrote : I have a stong feeling that it would make a nice 18 watter sound more and more like a Spitfire as it is turned up. I recently converted my Spitfire clone to an 18 watt because so much of the dirt was coming from the PI. Very fizzy to say the least. |
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| Author: | tkdrew (registered user: 593 posts ) |
| Date: | Thu, Mar 15th, 2007 @ 09:55 ( . ) |
Either way I'll still take my converted amp now run through an Airbrake attenuator anyday of the week compared to how it sounded before. I couldn't get the smoothness that it has now, it always had a fizzy quality to it. If you look at the Spitfire schematic you can't keep from clipping the PI pretty severly no matter how its biased, it runs off a lower B+ than the preamp so the headroom is limited. I always felt that the PI was breaking up way before the outputs started to, it was hard to get clean with any volume and the dirt just wasn't that great. Just my experience,YMMV. |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 06:00 ( . ) |
IMO the main problem with the Spitfire's PI is the 1.2k cathode resistor. That's roughly equivalent to a 2.4k cathode resistor on a single triode. It's just destined to hard clip in a fizzy sounding way. I have often commented on this. On 03/15/2007 @ 08:55, tkdrew wrote : :.... I couldn't get the smoothness that it has now, it always had a fizzy quality to it. If you look at the Spitfire schematic you can't keep from clipping the PI pretty severly no matter how its biased, it runs off a lower B+ than the preamp so the headroom is limited. I always felt that the PI was breaking up way before the outputs started to, it was hard to get clean with any volume and the dirt just wasn't that great. : |
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| Author: | Balijukka (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 07:12 ( . ) |
On 03/16/2007 @ 05:00, Zaphod wrote :
IMO the main problem with the Spitfire's PI is the 1.2k cathode resistor. That's roughly equivalent to a 2.4k cathode resistor on a single triode. It's just destined to hard clip in a fizzy sounding way. I have often commented on this. That must be a monday morning design. :) jukka |
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| Author: | Zaphod (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 06:52 ( . ) |
Not if the tail resistor value is being varied, rather than the cathode reistor. On 03/14/2007 @ 07:23, Mark L wrote : I think it would sound different than a post PI master volume because the PI will be going into cold cut-off distortion.. |
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| Author: | Mark L (registered user: 1446 posts ) |
| Date: | Thu, Mar 15th, 2007 @ 07:16 ( . ) |
Yep, that would be better than the variable bias resistor. Maybe a 100k or 500k pot in series with the tail resistor. |
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| Author: | Zaphod (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 06:49 ( . ) |
Yes, I'm certain it actually goes in the tail resistor. It would sound different than a post-PI MV, because at the same time the drive to the power tubes decreases (and hence power tube distortion), you would get increasing "brown" distortion from the PI to compensate. On 03/14/2007 @ 07:01, Pete Allen wrote : : Unless it's actually meant to go in the tail. That would decrease the headroom of the PI, so it would crunch earlier. However I can't see how this would sound much different from a post pi MV : |
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| Author: | beer (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 02:47 ( . ) |
Thank you nickt, I have been wondering what that control was for awhile now. I asked about it here a long time ago and I could see that it is just one pot and 2 wires. At the time the thought was that it was something like you see on some of the Matchless amps with a resistor across the 2 outputs of the PI. The Mojave's sound great, I have played the Coyote and the Scorpion. The power dampening control works very well, sounds great, very musical. |
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| Author: | nickt (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 04:24 ( . ) |
On 03/15/2007 @ 01:47, beer wrote :
Thank you nickt, I have been wondering what that control was for awhile now. I asked about it here a long time ago and I could see that it is just one pot and 2 wires. At the time the thought was that it was something like you see on some of the Matchless amps with a resistor across the 2 outputs of the PI. : : The Mojave's sound great, I have played the Coyote and the Scorpion. The power dampening control works very well, sounds great, very musical. -- Thanks beer, I remembered discussion on the Mojave which is why I posted the info. Glad to be of help to someone! ;-) Funny how it seems to be really simple things like this circuit that sound good. I'm totally convinced tubes and guitars are more magic than science. Nick |
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| Author: | Zaphod (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 06:45 ( . ) |
That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. On 03/13/2007 @ 20:18, nickt wrote : : : All that's required is a reverse log 50k pot and two bits of wire. The pot goes between the shared cathode of the PI tube and the PI cathode resistor (usually 470R or 1k2). That's it! : : In case you're tut-tut'ing at this point. The mod (its not worthy of the term circuit) is what's used by Mojave on their amps! : |
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| Author: | Doug H (registered user: 4650 posts ) |
| Date: | Thu, Mar 15th, 2007 @ 09:23 ( . ) |
On 03/15/2007 @ 05:45, Zaphod wrote :
That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. That's an interesting idea, given that PI distortion plays a big role in "the sound" for a lot of these p/p amps. So by reducing the PI headroom you don't need to turn the amp up as loud to get the distortion, hmmm.... The clips of the mojave amps at their site sound good, BTW. |
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| Author: | Zoe N Iain (registered user: 7776 posts ) |
| Date: | Thu, Mar 15th, 2007 @ 09:32 ( . ) |
(Iain) I wonder what it would sound like if you made a LTPI from little power pentodes, like the pentodes from an ECL80, or something? Drive a couple of KT88s or something clean. Then vary the tail resistor in the same way to drop the drive, while keeping the distortion. |
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| Author: | Arthur B (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 14:51 ( . ) |
On 03/15/2007 @ 08:32, Zoe N Iain wrote :
(Iain) I wonder what it would sound like if you made a LTPI from little power pentodes, like the pentodes from an ECL80, or something? Drive a couple of KT88s or something clean. Then vary the tail resistor in the same way to drop the drive, while keeping the distortion. -- I think using an interstage transformer would be a better idea. Output control could be preformed by varying the screen grid voltage. |
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| Author: | AletheianAlex (registered user: 3532 posts ) |
| Date: | Thu, Mar 15th, 2007 @ 16:24 ( . ) |
On 03/15/2007 @ 13:51, Arthur B wrote :
On 03/15/2007 @ 08:32, Zoe N Iain wrote :
: (Iain) I wonder what it would sound like if you made a LTPI from little power pentodes, like the pentodes from an ECL80, or something? Drive a couple of KT88s or something clean. Then vary the tail resistor in the same way to drop the drive, while keeping the distortion. : -- : : : : I think using an interstage transformer would be a better idea. Output control could be preformed by varying the screen grid voltage. -- basic idea behind a lot of old compressors and AVC circuits. As far as the pentode thing goes... might be interesting, you have to be careful with pentodes in a LTPI though because they naturally want to regulate their own current, so it is hard to get a stable operating point since they will be fighting against the tail resistor for control. you 'll also have a harder 3rd harmonic and even less second than usual. It might be better to use a phase splitter like a concertina, and then feed each phase to it's own pentode stage and go from there. |
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| Author: | beer (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 12:07 ( . ) |
On 03/15/2007 @ 05:45, Zaphod wrote :
That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. So it is in series with the tail resistor? |
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| Author: | Balijukka (guest: search) |
| Date: | Thu, Mar 15th, 2007 @ 14:44 ( . ) |
If you want to get a good overdrive sound from the PI do a Melissa. :D jukka |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 06:02 ( . ) |
It has to be or else it will really sound c**p at reduced settings. On 03/15/2007 @ 11:07, beer wrote : : : So it is in series with the tail resistor? : |
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| Author: | beer (guest: search) |
| Date: | Sat, Mar 17th, 2007 @ 14:04 ( . ) |
On 03/15/2007 @ 11:07, beer wrote :
On 03/15/2007 @ 05:45, Zaphod wrote :
: That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. : : : : : So it is in series with the tail resistor? -- After reading thru a lot of forum posts ie Metroamps I would say the pot is placed as nickt originally posted in series with the cathode resistor. I would think that the pi would cease to work with too much resistance in there but apparently it does the job. In the Metroamps thread there is a link to pics of a Mojave Scorpion built for Billy Gibbons [link] and the pics give it up. Unless some wiring was color changed under the board to fool the amp porn lurkers, the power dampening is a real simple solution. Apparently it has been used on some of Buddhas stuff and was also used on Sundown amps. Which I know nothing about just relaying info. Pretty damn interesting. Makes me want to build a Plexi. |
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| Author: | nickt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 05:33 ( . ) |
On 03/15/2007 @ 05:45, Zaphod wrote :
That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. Not on the schematic I saw. It was also extensively discussed as going between the cathode and cathode resistor - nothing to do with the tail resistor. (I'd draw it in ASCII art but I'm too tired) |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 06:05 ( . ) |
Yes, I saw that schematic that someone drew, and also read through the relevant threads. I belive there was an error made in the schematic (ie it's wrong), and the guys in the thread were a little confused about what was really going on. On 03/16/2007 @ 04:33, nickt wrote : : : Not on the schematic I saw. It was also extensively discussed as going between the cathode and cathode resistor - nothing to do with the tail resistor. (I'd draw it in ASCII art but I'm too tired) : |
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| Author: | nickt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 05:40 ( . ) |
On 03/15/2007 @ 05:45, Zaphod wrote :
That's actually incorrect. It goes *below* the cathode resistor (ie where the cathode and grid resistors join) and forms part of the PI tail resistor. As the pot increases in value , it gradually reduces the PI's headroom and ability to drive the power valves. [link] For those interested in the actual thread and the actual circuit. |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 06:06 ( . ) |
Tell you what - you build it both ways (it's easy to change) and tell us which way you prefer. I know where I'm placing my money.... :o) |
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| Author: | nickt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 06:29 ( . ) |
On 03/16/2007 @ 05:06, Zaphod wrote :
Tell you what - you build it both ways (it's easy to change) and tell us which way you prefer. I know where I'm placing my money.... :o) -- I don't get it - folks who have actually repaired Mojave amps say it's one way you say its another based on theory. Why? All I was doing was sharing some info - I'll really think twice before bothering again. All I know is Mojave amps sound pretty good and folks were interested in the Power pot. I'm not a Mojave expert. Geeze... |
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| Author: | Balijukka (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 07:24 ( . ) |
Please Nick, don't take this personally, and do not stop bringing in new ideas. This is what this site is all about, someone brings in a new interesting topic and then we all taste it and digest it, some like it, some dislike it, but everyone has their say about it, or about something relevant to it, or more often about something that has nothing to do with it, or even something that is Texan. If you had not started this thread, at least I wouldn't have known about it. jukka |
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| Author: | nickt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 07:50 ( . ) |
On 03/16/2007 @ 06:24, Balijukka wrote :
Please Nick, don't take this personally, and do not stop bringing in new ideas. This is what this site is all about, someone brings in a new interesting topic and then we all taste it and digest it, some like it, some dislike it, but everyone has their say about it, or about something relevant to it, or more often about something that has nothing to do with it, or even something that is Texan. : : If you had not started this thread, at least I wouldn't have known about it. : : jukka -- Yeah - you're right - thanks. Sorry folks - bad day at the plant. |
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| Author: | Merlin (registered user: 4129 posts ) |
| Date: | Fri, Mar 16th, 2007 @ 09:11 ( . ) |
How about replacing the cathode resistor with the pot, then bringing bothe the grid leaks to the wiper. Allowing you to vary between full head room but cooler bias, and less headroom hot bias... Just for laughs you understand! |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 09:55 ( . ) |
Yes, please don't take it personally. It's not against you - or well, really anyone! :o) However, as I said, I read through the relevant threads, and to me it appears that someone either mis-drew or mis-described what he saw inside a Mojave amp and then the rest just followed along. It's a pretty easy mistake to make... |
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| Author: | Paul Ruby (registered user: 6263 posts ) |
| Date: | Fri, Mar 16th, 2007 @ 12:04 ( . ) |
I haven't read this whole thread, nor the thread at 18W. But, this is just another form of MV where the signal getting to the power tubes is reduced (and distorted with colder PI bias). It may very well sound good, so I'm not disputing that it is a cool thing to do. It is not power scaling and we would call it. It does raise the basic question about power amp tone: Do you rerally need to drive the power tubes to cut-off for great power amp distortion? Or, does a hard clipped grid signal sent to them sound the same (or close enough)? Again, without haveing read any of the postings, if the consensus is that it sounds "right" then it is right... - Paul |
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| Author: | stevem (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 13:48 ( . ) |
To some extent I think the way the duty cycle gets changed as the output stage clips gives a type of distortion that most of us here like. |
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| Author: | Balijukka (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 15:48 ( . ) |
I like the way power tubes distort just before clipping. That smooth rich moment there. Think Jimi plexi cleans. :) jukka |
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| Author: | AletheianAlex (registered user: 3532 posts ) |
| Date: | Fri, Mar 16th, 2007 @ 16:20 ( . ) |
On 03/16/2007 @ 11:04, Paul Ruby wrote :
I haven't read this whole thread, nor the thread at 18W. But, this is just another form of MV where the signal getting to the power tubes is reduced (and distorted with colder PI bias). : : It may very well sound good, so I'm not disputing that it is a cool thing to do. It is not power scaling and we would call it. : : It does raise the basic question about power amp tone: Do you rerally need to drive the power tubes to cut-off for great power amp distortion? Or, does a hard clipped grid signal sent to them sound the same (or close enough)? Again, without haveing read any of the postings, if the consensus is that it sounds "right" then it is right... : : - Paul : : -- i agree. it is just another form of PPIV MV. Triode LTPI clipping is nothing like PP power tube clipping. Sure it is probably more similar than regular preamp clipping because it is a bootstrapped differential stage, but that only gets it half way there. You are still missing the sag and it's conductanc-limiting effect on the grids, transformer saturation... and let's not forget about the negative feedback loop causing a pinched negative waveform shape if there is one, and the effect of the AC coupling time constant on varying grid conductance There are a million different schemes out there ranging from simple pots to drop signal, regulators that drop B+ and change the bias, even circuits that drop the filament voltage to change the emission. They all do similar things in different ways |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 17:50 ( . ) |
That's what I first thought. But there's actually more to it than that. In a regular PPI MV, the level of PI distortion stays the same, and just the drive to the power tubes reduces. In this particular scheme, the proportion of PI distortion will also increase at the same time as the amount of drive to the power tubes. I guess this is intended to help compensate for the decrease in power tube distortion (assuming the amp in question has much of that). On 03/16/2007 @ 15:20, AletheianAlex wrote : : : i agree. it is just another form of PPIV MV. : |
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| Author: | AletheianAlex (registered user: 3532 posts ) |
| Date: | Fri, Mar 16th, 2007 @ 18:27 ( . ) |
On 03/16/2007 @ 16:50, Zaphod wrote :
That's what I first thought. But there's actually more to it than that. In a regular PPI MV, the level of PI distortion stays the same, and just the drive to the power tubes reduces. In this particular scheme, the proportion of PI distortion will also increase at the same time as the amount of drive to the power tubes. : : I guess this is intended to help compensate for the decrease in power tube distortion (assuming the amp in question has much of that). : : On 03/16/2007 @ 15:20, AletheianAlex wrote : : : : : i agree. it is just another form of PPIV MV. : : : -- Yeah, just another implimentation. you could get a similar effect by wiring up a pre and post phase inverter master volume like a lot of Matchless volume/master and Lee Jackson master1/master2 designs, and using a dual pot to controll them in inverse. Then you are not decreasing headroom but instead increaseing the drive to the PI while bleeding off the extra voltage swing that is produced before it hits output tubes. |
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| Author: | kurt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 19:52 ( . ) |
there's a big discussion about this on the hoffman forum, a couple people tried it out. [link] |
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| Author: | Zaphod (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 20:18 ( . ) |
A big discussion and a pretty confused one IMO... |
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| Author: | beer (guest: search) |
| Date: | Sat, Mar 17th, 2007 @ 00:40 ( . ) |
In this thread the pot is shown in series with the cathode resistor. Right now all I have is SE amps. Next one will be a PP and I can try this damn thing. I know for a fact it works really good on the Mojave amps. |
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| Author: | Beerzgood (registered user: 718 posts ) |
| Date: | Thu, Aug 21st, 2008 @ 15:13 ( . ) |
Well I finally got around to trying out this scheme. I converted my old Carvin X100B to a Mojave Scorpion based plexi circuit with the 'power dampening' control. Aka RA50k pot in series with the cathode resistor of the PI. It works great imo, the distortion you get out of this is not what you get from cascaded gain preamps, it isn't higain in any way, but for a classic blues based rock guy it is great. Without the dampening, this amp would be about useless and would require a distortion pedal at practice volumes. As it is, I am happy with the tones straight into the amp and with an overdrive like a Boss SD1 or OCD it really smokes. Billy Gibbons has a Mojave Scorpion in his road rig and has another custom built Scorpion combo. My amp isn't a Scorpion cause I don't have that big MM OT but all in all I am pleased. If anyone has a too loud PP amp this mod is well worth doing, and when it is dialed all the way up it is out of the circuit leaving a stock amp. It isn't scaling at all but it is a great little toy that gives nice distortion at lower volumes. Steve K On 03/17/2007 @ 00:40, beer wrote :
In this thread the pot is shown in series with the cathode resistor. Right now all I have is SE amps. Next one will be a PP and I can try this damn thing. I know for a fact it works really good on the Mojave amps. -- |
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| Author: | Doug H (registered user: 4650 posts ) |
| Date: | Fri, Aug 22nd, 2008 @ 12:27 ( . ) |
I did this to my 10W "Vox AC-15" and it works real well. [link] |
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| Author: | Beerzgood (registered user: 718 posts ) |
| Date: | Fri, Aug 22nd, 2008 @ 18:34 ( . ) |
Looks like a cool little build. I used the pot in series with the cathode resistor like the Mojave. I didn't know it would work on the tail. Goes to show, if it sounds good then it is good. |
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| Author: | Doug H (registered user: 4650 posts ) |
| Date: | Sat, Aug 23rd, 2008 @ 11:22 ( . ) |
On 08/22/2008 @ 18:34, Beerzgood wrote :
Looks like a cool little build. I used the pot in series with the cathode resistor like the Mojave. I didn't know it would work on the tail. Goes to show, if it sounds good then it is good. -- It is in series with the cathode resistor. |
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| Author: | Beerzgood (registered user: 718 posts ) |
| Date: | Sat, Aug 23rd, 2008 @ 14:06 ( . ) |
Ahh. The way it was drawn cornfused me :) |
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| Author: | Doug H (registered user: 4650 posts ) |
| Date: | Sat, Aug 23rd, 2008 @ 20:38 ( . ) |
Yeah, I was running out of room.;-) |
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| Author: | Beerzgood (registered user: 718 posts ) |
| Date: | Sun, Aug 24th, 2008 @ 01:58 ( . ) |
Hey man I just listened to your clips. That's a sweet sounding amplifier. Nice playing too. |
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| Author: | nickt (guest: search) |
| Date: | Fri, Mar 16th, 2007 @ 20:55 ( . ) |
On 03/16/2007 @ 11:04, Paul Ruby wrote :
I haven't read this whole thread, nor the thread at 18W. But, this is just another form of MV where the signal getting to the power tubes is reduced (and distorted with colder PI bias). : : It may very well sound good, so I'm not disputing that it is a cool thing to do. It is not power scaling and we would call it. : : It does raise the basic question about power amp tone: Do you rerally need to drive the power tubes to cut-off for great power amp distortion? Or, does a hard clipped grid signal sent to them sound the same (or close enough)? Again, without haveing read any of the postings, if the consensus is that it sounds "right" then it is right... : : - Paul : : -- Totally agree - If it sounds good it *is* good. As a card carrying MV hater one that sounds good would be fantastic. I hate hating things ;-) Given all the comments in this thread it may be that this approach might not be generally applicable - ie it just works in Mojave type amps (ie Marshall plexi variants). |
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| Author: | beer (guest: search) |
| Date: | Fri, Mar 23rd, 2007 @ 16:42 ( . ) |
Giving this thread a bump... Has anyone tried this out yet? |
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| Author: | nickt (guest: search) |
| Date: | Sat, Mar 24th, 2007 @ 02:53 ( . ) |
On 03/23/2007 @ 15:42, beer wrote :
Giving this thread a bump... : : Has anyone tried this out yet? -- I'm going to but haven't had a chance to scratch myself recently. Will post when done.. along with the Flushman clips... and Lightning clone clips... and all the other things I haven't got round to... |
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| Author: | beer (guest: search) |
| Date: | Sat, Mar 24th, 2007 @ 12:30 ( . ) |
On 03/24/2007 @ 01:53, nickt wrote :
: I'm going to but haven't had a chance to scratch myself recently. Will post when done.. along with the Flushman clips... and Lightning clone clips... and all the other things I haven't got round to... -- I want to try this out myself but I need a PP amp to do it. Thinking of making my old Carvin 100W into a 50 watt Plexi with the power dampening control. That way I already have iron and chassis. |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sat, Mar 24th, 2007 @ 16:30 ( . ) |
First I did it like Zaphod Phil suggested. I trust his judgement. I used a 47k pot . I'm guessing linear since there is nothing other than the "47k". I put it between the tail resistor and ground on my Trinity SIII 18 watt clone. I noticed very little change thru-out the sweep. I then put it as per the diagrams posted on the above links. It worked to cut the volume as well as most of the ka-hoo-neez. Then it occured to me that the beloved 18 watters get their sound from the power amp being overdriven by the PI. This mod reduces the drive of the PI, thereby cutting the ka-hoo-neez. Since I was using a linear pot there was a small spot near the end of the sweep that did all of the change in about 5 degrees of rotation. It sounded the best at the very end. When I removed the pot I measured that end at zero ohms. LeeMo |
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| Author: | Randy F (guest: search) |
| Date: | Sat, Mar 24th, 2007 @ 17:54 ( . ) |
Didn't you just add a cold bias stage? Did you happen to maintain a fairly constant AC voltage to the grids to the output tubes? Keep in the mind the gain is going to drop to perhaps 1/25th of the original so yes the effect of losing cojones could be slightly unreliable. Level differences are very important in subjective listening |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sat, Mar 24th, 2007 @ 19:46 ( . ) |
With the amp crankin hard and sounding marvelous, I turned the pot and the effect was very similar to turning the volume control on the guitar. The volume dropped and the crunch and distortion went bye-bye. It could just be my amp, but the 18 watt amp is known for hammering the power tubes with the PI. That's why it sounds so good on 13. Oher amps that have pre-amp distortion could benifit from this mod. Mine did not. LeeMo |
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| Author: | Zaphod (guest: search) |
| Date: | Sat, Mar 24th, 2007 @ 20:46 ( . ) |
I would never trust anything anyone called Zaphod says. Just look at his name. I mean that's gotta be a fake, right? Surely the outlawed president of the galaxy, who stole a spaceship isn't going to be hiding out on the mostly harmless planet with his double heads wrapped in a towel, building tube amps.... I suspect that with a 47k tail resistor, a 47k pot isn't going to make that much difference in proportion. In a Plexi, a 50k pot would be about 4 times the size of the tail resistor. With an 18W you would therefore need something like a 200k pot. As the pot is increased and the PI drive decreases, the increased PI distortion should help somewhat to compensate for the decreasing power amp distortion. But it still won't sound quite the same. On 03/24/2007 @ 15:30, LeeMo wrote : First I did it like Zaphod Phil suggested. I trust his judgement. I used a 47k pot . I'm guessing linear since there is nothing other than the "47k". I put it between the tail resistor and ground on my Trinity SIII 18 watt clone. I noticed very little change thru-out the sweep. I then put it as per the diagrams posted on the above links. It worked to cut the volume as well as most of the ka-hoo-neez. Then it occured to me that the beloved 18 watters get their sound from the power amp being overdriven by the PI. This mod reduces the drive of the PI, thereby cutting the ka-hoo-neez. Since I was using a linear pot there was a small spot near the end of the sweep that did all of the change in about 5 degrees of rotation. It sounded the best at the very end. When I removed the pot I measured that end at zero ohms. : |
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| Author: | beer (guest: search) |
| Date: | Sat, Mar 24th, 2007 @ 22:59 ( . ) |
On 03/24/2007 @ 19:46, Zaphod wrote :
: I suspect that with a 47k tail resistor, a 47k pot isn't going to make that much difference in proportion. In a Plexi, a 50k pot would be about 4 times the size of the tail resistor. With an 18W you would therefore need something like a 200k pot. The pot is in series with the cathode resistor check this out [link] You can see the wire coming off the cathodes and joining with the wire coming off the cathode resistor. Then you can see the 2 wires headed up to that pot. Apparently BG likes the dampening a lot, see the setting called Mo Funk. |
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| Author: | Paul Ruby (registered user: 6263 posts ) |
| Date: | Mon, Mar 26th, 2007 @ 12:41 ( . ) |
Beer is right. The pot is clearly in series with the BIAS resistor, not the tail. The pot is reducing the bias current to make the PI colder and colder... It is not in series with the tail. In this case, a 47K pot would make much more sense than 1M. |
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| Author: | LeeMo unlogged (guest: search) |
| Date: | Mon, Mar 26th, 2007 @ 13:27 ( . ) |
I played with it again this morning and put a 250kA in there. Much more usable swing. |
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| Author: | Mark L (registered user: 1446 posts ) |
| Date: | Sun, Mar 25th, 2007 @ 08:33 ( . ) |
On 03/24/2007 @ 19:46, Zaphod wrote :
I : I suspect that with a 47k tail resistor, a 47k pot isn't going to make that much difference in proportion. In a Plexi, a 50k pot would be about 4 times the size of the tail resistor. With an 18W you would therefore need something like a 200k pot. : : I'd try a 500k log wired in reverse, i.e. zero ohms when turned fully clockwise. |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sun, Mar 25th, 2007 @ 15:14 ( . ) |
My name is LeeMo and I am a cheapskate. I've now tried it on my Trinity, again. I put a 1meg linear pot in series between the PI's cathodes and the 820 cathode resistor. I wired it as a rheostat, one leg to center wiper and one leg to an outside terminal on the pot. Very simple and very effective. This could warrent drilling another hole in my (amp's ) chassis. I first tried it with a 47k pot to no effect. I almost gave up , but due to this thread and a suggestion by Zaphod Phil to raise the value, I stumbled onward. Zaphod , I tried it in series with the tail resistor, also. Minimal change if anything. If someone has a "Decade" resistor box, try it and let us know what values get the best results. Until then , your fathful servant. LeeMo |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sun, Mar 25th, 2007 @ 15:21 ( . ) |
I forgot to thank you guys. MarkL, beer, Randy F, Zaphod Phil nickt and others this thing shows potential. Thanks!!! LeeMo Cheapskate-aholic off the wagon. |
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| Author: | Mark L (registered user: 1446 posts ) |
| Date: | Sun, Mar 25th, 2007 @ 17:29 ( . ) |
After your little experiment I decided to run some LT spice sims on this. The Spice sim agrees that the rheostat in series with the cathode resistor is more effective. With 18k the output is reduced by 12dB, 50k = 18dB and 100k = 22dB. |
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| Author: | beer (guest: search) |
| Date: | Sun, Mar 25th, 2007 @ 19:25 ( . ) |
On 03/25/2007 @ 16:29, Mark L wrote :
After your little experiment I decided to run some LT spice sims on this. The Spice sim agrees that the rheostat in series with the cathode resistor is more effective. : With 18k the output is reduced by 12dB, 50k = 18dB and 100k = 22dB. -- Did you look at the link I posted? It clearly shows that this pot in is the cathode circuit, the original poster posted that it was in the cathode circuit and then was more or less told he was mistaken. Which he wasn't. Which was why I posted the link to the pic that started all the discussion to begin with. |
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| Author: | Mick S (registered user: 1245 posts ) |
| Date: | Sun, Mar 25th, 2007 @ 16:14 ( . ) |
On 03/25/2007 @ 14:14, LeeMo wrote :
My name is LeeMo and I am a cheapskate. : If someone has a "Decade" resistor box, try it and let us know what values get the best results. : Until then , your fathful servant. : : LeeMo : -- LeeMo, You have the ultimate Decade resistor box right there in your pot. How about you tell US what gives the best results. :-) Mick |
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| Author: | The Radium King (guest: search) |
| Date: | Sun, Mar 25th, 2007 @ 22:12 ( . ) |
3 parakeets. |
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| Author: | Merlin (registered user: 4129 posts ) |
| Date: | Thu, Apr 05th, 2007 @ 08:42 ( . ) |
By chance I was looking at the Carlsbro 60TC and I see it has a "limiter" control, which is very similar to this "power scaling" idea! The difference is the Carlsbro allows you to bias the LTP anywhere from normal voltage at the grids, to zero volts. Schem: [link] |
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| Author: | Pete Allen (registered user: 889 posts ) |
| Date: | Thu, Apr 05th, 2007 @ 09:39 ( . ) |
Interesting circuit! I'll have to look at that properly sometime. And thanks for the link to Carlsbro schematics. I've never found them before. I have a TC60 twin at home (different circuit), so it saves having to dig out that hard copy of the schematic each time I want to play. Pete |
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| Author: | Joe V (guest: search) |
| Date: | Thu, Apr 05th, 2007 @ 11:37 ( . ) |
On 03/24/2007 @ 15:30, LeeMo wrote :
First I did it like Zaphod Phil suggested. I trust his judgement. I used a 47k pot . I'm guessing linear since there is nothing other than the "47k". I put it between the tail resistor and ground on my Trinity SIII 18 watt clone. I noticed very little change thru-out the sweep. I then put it as per the diagrams posted on the above links. It worked to cut the volume as well as most of the ka-hoo-neez. Then it occured to me that the beloved 18 watters get their sound from the power amp being overdriven by the PI. This mod reduces the drive of the PI, thereby cutting the ka-hoo-neez. Since I was using a linear pot there was a small spot near the end of the sweep that did all of the change in about 5 degrees of rotation. It sounded the best at the very end. When I removed the pot I measured that end at zero ohms. : : LeeMo -- Thanks for the detailed play-by-play. Very interesting. For the record, it's cojones. :-) |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Thu, Apr 05th, 2007 @ 22:16 ( . ) |
I lost my dictionary! ; ) LeeMo |
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| Author: | JKCavitt (guest: search) |
| Date: | Thu, Apr 05th, 2007 @ 19:13 ( . ) |
Yes, I just tried it. I'll be dipped, it really does work. Played it for around 20 minutes, my son & I. I modified my Bassman clone to have a 1987 preamp last weekend. What a riot! And now adding this 25K pot makes the setup much more versatile. I wouldn't characterize this as power scaling, but I reckon it is a very effective MV alternative, certainly has a far more effective range than the Pre-PI MV on my son's JCM 800 Clone. The high do have a tendency to roll off a bit, but it is within adjustment range with my tone stack (tweed). I usually have treb & mids around 5 or 6, bass around 0 or 1. Cranking up the preamp and dropping the ... MoJo knob (PI Bias Adjust) results in a mild decrease in the highs that can be brought up with the treble and mid controls. Distortion doesn't get buzzy, "farty", or really change character. Yeah, this is certainly worthy of an additional hole in the faceplate. |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Thu, Apr 05th, 2007 @ 22:23 ( . ) |
It's funny. The first time that I posted about this, I used a 47K linear pot. I didn't find it very useful after messing with it for all of five minutes. Then I tried it with a 500K audio taper pot and it worked well. Then , I figured I'd try a 1M and maybe it would be more usable. After thinking about it , I went back to the 500k, then a 250K , and now finally I'm back to the 47K which works better now for some reason. My guess is "operator error" . LeeMo |
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| Author: | JKCavitt (guest: search) |
| Date: | Thu, Apr 05th, 2007 @ 22:49 ( . ) |
Well, I also just tried a 50K pot I had laying around. I was thinking it might give a little more range of control. Not much difference, but either one has more range of control than the pre-PI MV on my son's JCM 800 clone. I used linear tapered pots here - not log or reverse log tapered pots, but the effect was easily controllable, did not jump. I've got several others laying around including a dual ganged 1MA pot for a post PI MV, but have yet to try it. However, I think your approach is a pretty good one. I have no idea if this will affect the life of the PI tube, a 12AX7. I have also not tried this with humbuckers yet - so far just P90s. One thing I did notice is that with the preamp volume knobs dimed (jumpered inputs), and the "Mojo" control maxed (50K ohms) the sound did not really clean up when I rolled back the guitar volume knob. Thanks for the R&D and pointers there LeeMo! Bang for the buck, this is right up there. Good luck with your 18watt. |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sat, Apr 07th, 2007 @ 20:15 ( . ) |
I just went to the "shack" and got a rotary switch and a "bag-o' zistors". One half watt carbon comp 5%. I replaced the pot with the switch. Starting at 56k, 22k, 15k, 10k, 4.7k and a jumper for zero ohms. It works better than the pot for the simple reason that it is predictable and solid. Part #'s 271-306 and 275-1386. Ten whole smackeroos. I'll skip lunch a couple of days next week. My name is LeeMo... and I'm a mod-aholic. |
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| Author: | dai h (guest: search) |
| Date: | Sat, Apr 07th, 2007 @ 21:54 ( . ) |
I think this mod is described pretty much as detailed in the metroamps thread in TUT1 (5 to 50 times the 470 ohm) except that Kevin O'Connor stated it wasn't usable due to the noise from the DC shift IIRC (and KOC made no mention of using a rev.log pot). |
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| Author: | LeeMo (registered user: 188 posts ) |
| Date: | Sun, Apr 08th, 2007 @ 00:40 ( . ) |
That's pretty amazing, 5 to 50 times the resistor. In my 18 watt amp the catode resistor in the PI is 820 ohm. Five times that is close to the 4.7k at which I arrived and fifty times that is close to the 47k pot that I was using. 56k was the closest resistor that was in the pack so that's what I used. Contrary to Mr. O'conner, I believe that it is usable . Especially in conjunction with a slight amount of attenuation to the output. LeeMo |
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| Author: | beer (guest: search) |
| Date: | Sun, Apr 08th, 2007 @ 02:11 ( . ) |
On 04/07/2007 @ 20:15, LeeMo wrote :
I just went to the "shack" and got a rotary switch and a "bag-o' zistors". One half watt carbon comp 5%. I replaced the pot with the switch. Starting at 56k, 22k, 15k, 10k, 4.7k and a jumper for zero ohms. : It works better than the pot for the simple reason that it is predictable and solid. Part #'s 271-306 and 275-1386. Ten whole smackeroos. I'll skip lunch a couple of days next week. : : My name is : LeeMo... : and I'm a mod-aholic. -- I read somewhere that Mojave replaced the pot with a rotary switch too. |
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| Author: | Balijukka (guest: search) |
| Date: | Sun, Apr 08th, 2007 @ 05:19 ( . ) |
If you use a rotary switch and resistors, you can as well drop the B+ at the power tubes to get more realistic cranked power stage sound at low volumes. [link] If the switch is dual, you can also tweak the bias at every level according to your liking, just as in KOC's power scaling. and NO SOLID STATE COMPONENTS required. :D jukka |
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| Author: | JKCavitt (guest: search) |
| Date: | Sun, Apr 08th, 2007 @ 12:30 ( . ) |
Jukka, Your poweramp is cathode biased right? So your approach is to dump excessive B+ prior to the power tubes with selectable load resistors, right? Wouldn't that get more involved with a fixed bias amp? I didn't notice any evidence of DC (scratchy sound) with the pot in series with the cathode resistor. Wonder if the KOC approach mentioned is the same one we're talking about here? I tried following one of those threads and the reverse log explanations made my head hurt. LeeMo, We should be buying you lunch instead of you skipping it! |
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| Author: | David L (guest: search) |
| Date: | Sat, Sep 29th, 2007 @ 10:10 ( . ) |
Hi all. I am a complete noob to electronics, but wanted to find out whether this mod would work on my Carvin V16 112 combo. It is a Pentode/Triode 16w/5w valve amp with a Power Soak which apparently Allan Holdsworth had a hand in designing. However it sounds best to my ears in Pentode mode, and is far too loud for my needs, and even with the Power Soak turned all the way up and the volume low, there is hardly any discernible overdrive. But if all the tone controls are wound up, it will start to feedback, but then it is considerably loud. I am not looking for an O.T.T. heavy saturation, what I want is the singing sustain quality of a Dumble, think Larry Carlton, Robben Ford, Scott Henderson and all those guys. I use an Xotic BB Preamp which is great, but I don't want too much dirt, and want to be able to cut back to clean just using my guitar volume control. Now the problem is that this is a hybrid amp, so I don't know whether this power scaling mod would work (I would have a tech do it anyway incidentally, although here in Spain I wouldn't know who to consult). I can provide the schematics if it is any help... Thanks for your advice.... David |
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