AX84 BBS Recent Posts as of Fri, Mar 27th, 2015 @ 00:15:38 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com RE: Lowering B+ and Zener diodes(by Alva Goldbook) Just a thought....have you considered just using the 550 Volts, and maybe instead using a tougher power tube than a standard 6L6, like say a KT88? Thu, 26 Mar 2015 20:00:22 -0400 http://ax84.com/bbs/dm.php?id=489887 RE: Bridge rectifier with zener B+ lowering and bias(by Darren Porter) If you are thinking 400V is too high for the EL84, keep in mind that there are amps out there that run 400V on them. Peavey Classic 50 comes to mind. If it's just a design choice, well that's another story.Thu, 26 Mar 2015 19:06:35 -0400 http://ax84.com/bbs/dm.php?id=489886 RE: Tonestacks. (by Alva Goldbook) I haven't tested this circuit yet, but I drew this out the other day. It combines features of the Fender Pro 6G5 tone stack with features of the Jubilee tone stack. Some of the components values may need to be tweaked. <a href="http://i.imgur.com/r6ixUQp.png" target="newwin1427429742">[link]</a> Thu, 26 Mar 2015 17:56:40 -0400 http://ax84.com/bbs/dm.php?id=489885 Bridge rectifier with zener B+ lowering and bias(by AneirinE) I'm building an amp with an EL84 poweramp. The PT doesn't have a CT on the high voltage secondary so I am using a bridge rectifier. The PT doesn't have a bias tap so I thought I could try the AC coupling technique in a Marshall Artist 3203: <a href="http://www.drtube.com/schematics/marshall/3203pwr.gif " target="newwin1427429742">[link]</a> After rectification I'm getting too high a B+ (about 400v), so I thought I could lower this with some zeners by about 45v (3 x 15v/5W zeners in series). So - I have some questions: 1) To bring the bias within range for a pair of EL84s, do I just reduce R1? If so, how much to? I was going to experiment with 15k or so but that is a total guess. I was also going to replace R2 with a 15k resistor in series with a pot to ground so it is adjustable. 2) How can I do this with zeners to reduce B+ as well (can I even do it?) and what kind of effect does this have on the bias voltage etc...? Thanks! Thu, 26 Mar 2015 19:06:35 -0400 http://ax84.com/bbs/dm.php?id=489884 RE: Tonestacks. (by Alva Goldbook) Adam's Amps has a very good breakdown of various tone stacks. <a href="http://amps.zugster.net/articles/tone-stacks " target="newwin1427429742">[link]</a> What's interesting is how the 2 control tone stack is on the Fender Pro 6G5 & 6G7. It's similar to the standard Fender tone stack, but it looks like the bass controls are set up more like a James type circuit, perhaps proving less interaction with the treble controls. Thu, 26 Mar 2015 12:28:07 -0400 http://ax84.com/bbs/dm.php?id=489883 RE: Tonestacks. (by Alva Goldbook) Perhaps. But I looked at a demo Chris George did of the reissue, and he was touting how well the mid controls worked on that amp. So I'm thinking the EQ was done this way as an improvement upon the standard Fender/Marshall circuit. There's also a triode between the diode clipping and the EQ circuit, so I'd think there'd be a decent amount of isolation between the two parts of the circuit. Thu, 26 Mar 2015 12:23:36 -0400 http://ax84.com/bbs/dm.php?id=489882 RE: Settling on this one I believe(by Roosville) <div class="quoteblock" >On 03/26/2015 @ 08:37, PaulP wrote : <a href="http://www.geofex.com/article_folders/vintvolt/vintvolt.htm " target="newwin1427429742">[link]</a> -- </div> <div class="quoteblock" >On 03/26/2015 @ 08:37, PaulP wrote : <a href="http://www.geofex.com/article_folders/vintvolt/vintvolt.htm " target="newwin1427429742">[link]</a> -- </div> Thanks, good find, not quite what I described but along the same lines non-the-less. The design in the link drops the whole mains, so it will also drop the heater voltage and bias if you have one. The point I was raising is that you need to bridge over the HT transformer and just oppose the HT winding only. I found when implementing a Zener in one of my own amps it just seemed such a bad idea in every way. Oddly enough though, it seems to be a very popular idea along with the MOSFET. I also think it is worth discussing the regulation issue when impementing a fixed drop in a variable supply, which doesn't seem to be part of the conversation on this matter. Cheers.Thu, 26 Mar 2015 09:28:54 -0400 http://ax84.com/bbs/dm.php?id=489881 RE: Settling on this one I believe(by PaulP) <a href="http://www.geofex.com/article_folders/vintvolt/vintvolt.htm" target="newwin1427429742">[link]</a> Thu, 26 Mar 2015 08:37:03 -0400 http://ax84.com/bbs/dm.php?id=489880 RE: Tonestacks. (by steve25) I think Marshall may have gone with that tone stack due to the diode clipping used in the amps pre section, as the clipped signal wave form does not change, just its signal level so to speak?Thu, 26 Mar 2015 06:44:44 -0400 http://ax84.com/bbs/dm.php?id=489879 RE: Tonestacks. (by sambo69) you could also take a look at the tone stack of the Commanchero amp here in the Associate projects section. I built one of these amps ( thanks Cliff !) and did a little tweaking around on the tone stack. Wed, 25 Mar 2015 17:57:37 -0400 http://ax84.com/bbs/dm.php?id=489878 RE: Tonestacks. (by Alva Goldbook) The Pultec PEQ was a tube driven passive equalizer, and in those days they used massive multi-tap inductors, whose values you could switch between with a rotory switch. The inductor values varied between 100mH and 625mH. The switching is rather elaborate, but circuit is pretty straight forward. <a href="http://www.gyraf.dk/gy_pd/pultec/pultech.gif " target="newwin1427429742">[link]</a> I was quite surprised when I found these on Mouser, but it appears to me that these days you can find shielded axial inductors, not much bigger than a resistor, in values that would be appropriate for audio use. <a href="http://tinyurl.com/q75e8k6 " target="newwin1427429742">[link]</a> As I understand it, microphones work off of much smaller source signals than guitar pickups. This is why microphone cables require 3 wires, because extra shielding is necessary due to the worse noise to signal ratio. Of course, even tube driven microphone preamps didn't have to use the massive power transformers and output transformers found on guitar amps. Guitar amps tend to be overly noisy as it is, and perhaps even a shielded inductor would bring too much noise to a tonestack circuit. There is certainly something to be said of breaking up a tone stack circuit. This page has several interesting takes on a James-like tone circuit, along with response curves, and you can see that you always seem to see a decrease in performance with passive tone stacks, especially when you add more controls. <a href="http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html " target="newwin1427429742">[link]</a> Many of those circuits are active, and intended for hi-fi use. I believe Ampeg made a guitar amp with active EQ a few years back, and the down side with active EQ in guitars is that it interferes greatly with how much overdrive/distortion you're using. Treble controls become interactive with your Gain control. The Marshall Silver Jubilee has an interesting tone stack. I won't post the schematic since Marshall is making them again, but you can find the schematic online. It looks like a standard Fender Marshall tone stack, but the bass controls are taken out of their normal configuration, and a dual gang pot is instead used with an RC filter. I haven't figured out how to do this yet, but I'd like to modify this circuit to be driven with tubes instead of op amps. At the bottom of the page there's a Bootstrapped Twin T with variable Q circuit. Ideally, I'd like to make a middle control that would allow for peaks and notches, with variable Q, as well being able to select between a range of different frequencies. I suspect it would require at least three triodes to make it work properly. <a href="http://www.geofex.com/article_folders/eqs/paramet.htm " target="newwin1427429742">[link]</a> Wed, 25 Mar 2015 14:54:57 -0400 http://ax84.com/bbs/dm.php?id=489877 RE: Tonestacks. (by PaulP) The original Matamp GT100 has an inductor in the NFB voltage divider to form a variable mid-boost (NFB LC notch). Pretty clever, until you realize the NFB has to wrap around to the gain stage before the cathodyne phase inverter; it must have been a real delight to keep stable. The Orange Graphic schematic from '72 shows the LC filter moved to between the grid and cathode for the boost circuit, and by the '74 schematic it's gone entirely and the "boost" is now just variable cathode bypassing. It is a shame that so many amps use the old FMV, but since it was used in a bunch of amps that are now "classics" and featured on classic recordings, and a decent portion of amp customers just want to emulate those sounds that their heroes made, I think it's here to stay.Wed, 25 Mar 2015 12:22:56 -0400 http://ax84.com/bbs/dm.php?id=489876 RE: Tonestacks. (by Merlin) <div class="quoteblock" >On 03/25/2015 @ 07:27, Alva Goldbook wrote : I have never once seen a single inductor used in a tone stack. Is there any reason for this? </div> Yes: inductors are expensive, bulky, non-linear, not available in many values / styles, you usually need large values for audio, and they're sensitive to picking up hum. <div class="quoteblock" > these days you can get a shielded axial inductor that's about the same size as a resistor and cheaper than what most people pay for capacitors in tube guitar amps. </div> Tiny inductors like that rarely have enough inductance to be useful for audio EQ. These days gyrators are used instead of inductors -they're far more flexible. However, for guitar use this is overkill. In any case, active EQ is often not suited to guitar use where things get overdriven a lot. You can do quite a lot with passive EQ if you are willing to split the controls up and distribute them throughout the amp instead of trying to do it 'all in one go' with one big tone stack. A Bandmaster treble/bass stack and a separate bridged-T mid control can be really quite powerful...Wed, 25 Mar 2015 11:53:54 -0400 http://ax84.com/bbs/dm.php?id=489875 RE: "Raffler" DC drive (mod) schematic(by Stephen Keller) Google does pretty good for this one with: raffler site:ax84.com As it happens I had it in my bookmarks.Wed, 25 Mar 2015 10:55:23 -0400 http://ax84.com/bbs/dm.php?id=489874 RE: "Raffler" DC drive (mod) schematic(by proton45) Thanks Stephen... Its "funny" how search engines work, search for "Raffle" and I would have found this, search for "Raffler" and, Bupkiss! You will never find it! Thanks again...cWed, 25 Mar 2015 10:44:21 -0400 http://ax84.com/bbs/dm.php?id=489873 RE: Udating Ampeg Gemini II... how far can I go to keep vintage valu(by stringbend) I would think the IEC connector would devalue the amp much more than a mere 2 to 3 prong conversion. YMMV. Alterations that don't deface the instrument, especially those that can be reversed will have much less impact on market value, IMO. Wed, 25 Mar 2015 10:18:05 -0400 http://ax84.com/bbs/dm.php?id=489872 RE: Udating Ampeg Gemini II... how far can I go to keep vintage valu(by Stephen Keller) <div class="quoteblock" >On 03/25/2015 @ 07:42, steve25 wrote : One other way to do it is to use a crimp on wire ring and bolt it down to one of the Transfomer mounting bolts. </div> If you want to meet UL standards, then the earthing connection may not be used for any other purpose, including such purposes as holding down a transformer. I would not solder the connection; rather I'd use a crimp connector on un-tinned wires and bolt it to the chassis with toothed lock washers designed to bite into the metal of the chassis. UL also specifies that if you use a captive mains input cable, that the earthing lead must be longer than the hot and neutral to ensure that if the cable is somehow pulled out, the earthing remains the last one connected. Merlin floated a document around a while back about UL and European standards. Maybe in this thread: <a href="http://www.ax84.com/bbs/dm.php?thread=488908 " target="newwin1427429742">[link]</a> StphWed, 25 Mar 2015 09:52:27 -0400 http://ax84.com/bbs/dm.php?id=489871 RE: Math error correction, sigh...(by Stephen Keller) <div class="quoteblock" >On 03/24/2015 @ 18:12, Stephen Keller wrote : For example, they show an L4 d.c. resistance of 210 with 450 mA and a 20 volt drop. Ohm's law says you'd need 950 mA to drop 20 volts across 210 ohms. </div> Sigh... I slipped a decimal point. You'd need 95 mA to drop 20 volts across 210 ohms. Still not the drop I'd expect from that amplifier. Other weird stuff with the voltages. They show a five volt drop across the 420 ohms of the OT primary. Assuming they mean that as the d.c. resistance and not the a.c. impedance, it takes about 12 mA to drop that. Not really what I'd expect for a typically biased 6BQ5 output stage. The 7.5 volt drop across the 150 ohm cathode puts the combined screen and anode currents at 50 mA. Hard to believe the screen is drawing 38 mA. My original contention that the voltages or component values are fishy on that schematic stands; it's just my math that should be treated with suspicion. StphWed, 25 Mar 2015 09:41:18 -0400 http://ax84.com/bbs/dm.php?id=489870 RE: Udating Ampeg Gemini II... how far can I go to keep vintage valu(by steve25) If the amp is in pretty good condition overall than the IEC will cut a bunch of value off of it, if the cabinet is beat and the amps chrome face has a lot of rust that can be seen from even 3 ft away than its a toss up on the amps value! You do not need special solder of the chassis, but you will need to scrape it up some a have a 120 watt big ass iron to solder to it, or pre heat the area with a heat shrink torch! One other way to do it is to use a crimp on wire ring and bolt it down to one of the Transfomer mounting bolts.Wed, 25 Mar 2015 07:42:20 -0400 http://ax84.com/bbs/dm.php?id=489869 RE: Tonestacks. (by steve25) take a look at the schematic for a Soundcity 120 as it has a active tone section!Wed, 25 Mar 2015 07:33:42 -0400 http://ax84.com/bbs/dm.php?id=489868 RE: Got it working!(by steve25) Cool deal!Wed, 25 Mar 2015 07:31:39 -0400 http://ax84.com/bbs/dm.php?id=489867 Tonestacks. (by Alva Goldbook) I've been pondering tone stacks lately, and it rather bugs me that so many amps have used the Fender/Marshall tone stack. Because, in all honesty, it's a rather lousy circuit. The performance of the mids is just awful. In recording studio gear, no one would tolerate an EQ circuit that performed this poorly. I'm not sure of the origin of this circuit, but I once read something about how Leo Fender found it in an old manual from the 1920's or 1930's that was originally designed for telephone use. I think for new amp designs, this circuit ought to be put to bed. In a lot of tube driven studio gear, passive EQs tend to be far more sophisticated. Which would be a bit too cumbersome and expensive to place in a guitar amp. But with the exception of Ampeg SVTs, I have never once seen a single inductor used in a tone stack. Is there any reason for this? Perhaps inductors were expensive years ago, but these days you can get a shielded axial inductor that's about the same size as a resistor and cheaper than what most people pay for capacitors in tube guitar amps. So I was thinking that we should have a thread here for tonestack circuits other than the standard Fender/Marshall tone stack. I was reading through this the other day. <a href="http://home.online.no/~jaeioluf/sound/eqpassiv.htm " target="newwin1427429742">[link]</a> And so I pieced this tone stack together based upon it. It's sort of similar to a James type circuit, and I'm sure it could be improved upon. <a href="http://i.imgur.com/BOYmeLd.png" target="newwin1427429742">[link]</a> Thu, 26 Mar 2015 17:56:40 -0400 http://ax84.com/bbs/dm.php?id=489866 RE: Got it working!(by GeoMan) I re-did a late 50's Supro amp that belonged to a buddy's grandfather a couple of months ago. It's awesome to see a little piece of history run again and how happy it makes the player.Wed, 25 Mar 2015 07:22:33 -0400 http://ax84.com/bbs/dm.php?id=489865 RE: Custom chassis(by Gruszie) <div class="quoteblock" >On 03/25/2015 @ 06:38, Gruszie wrote : What is your expected price range? I'm looking for 24"x8"x3" and couldn't find anything reasonable in price for that size. So If you already have plans on doing chassis that would work out better than having to buy a new box brake. I'm just up the road in winchester, VA so good to head you are local. Give me anot email on, what you expected prices will be and we can get a few chassis ordered as son as the new brake arrives. : thanks : careygruszie@yahoo.com -- </div> typing on this phone sucks sorry. Wed, 25 Mar 2015 07:20:45 -0400 http://ax84.com/bbs/dm.php?id=489864 RE: Settling on this one I believe(by Roosville) Sorry to resurrect, but I found the idea of using a Zener, MOSFET or resistor all to have failings. The issue with a resistor is that it is just putting the PSU design all at sea. The idea of introducing a fixed drop means that as the supply into the amp changes with different wall voltages, then you can end up with a wider than normal change in the B+. For example; the wall supply can vary from place to place so you need to design with the highest voltage in mind. At max input voltage say you have 500V and want no more than 430V at 220mA for the B+. You need to lose 70V so you put in a Zener string of 70V which will also need to include thermal de-rating because we are generating ~15W of heat here. You would possibly need to design for 25W min. Now you plug into a different venue, and the supply here is low, so the 500V becomes 450V (drop of 10% , the UK allows 16% variation), we still take out the 70V so B+ is now 450-70 which is 380V. You can see whilst the wall voltage has dropped to 10% the fixed Zener has dropped the B+ by 12%. With the Zener design (and fixed MOSFET) you end up with the larger B+ variation and lumbered with the (lack of) reliability of semiconductors producing heat. It also looks out of place in a valve amp. However I have an alternative simple, cheap solution which scales with the input voltage, so at a lower input voltage a lower B+ drop is applied. It also produces less heat and is inherently a lot more reliable. It is called an opposing voltage. If you connect a small transformer to the same input mains supply as the power transformer, and then put the secondary winding in series with the B+ but OUT OF PHASE, you will drop the B+ by the output of the small transformer. As the above only needs 220mA then the VA rating transformer is relatively small and hence cheap. Split bobbin would ensure breakdown voltage safety. Being connected to the same mains, as the input voltage drops the opposing voltage drops and conversely as it rises the opposing voltage rises, hence improved regulation. You can discover the correct phase by looking at the output against the B+ with an AC multimeter before connecting in series. Transformers are very efficient and reliable devices. If you want to go a bit further get a transformer with a multi-taped output. See //valvesphere.blogspot.co.uk/2015/02/a-better-ht-b-reduction-scheme-or-my.html for diagrams. Wed, 25 Mar 2015 06:55:43 -0400 http://ax84.com/bbs/dm.php?id=489863