AX84 BBS

Amp Parts and Kits

Doberman Music Products, LLC offers a variety of parts and kits

Diode Coring for Noise Reduction(by Chris Hurley)

Wed, 10 Mar 2010 19:12:23 -0500

I had to whack the previous thread due to it containing a commercial amp schematic. The question was related to series diodes in the signal path and their purpose. If I'm not mistaken, Mr. Aiken indicated that this is a noise reduction technique. He pointed out that this process "cores" out the low level signal, but that one wouldn't really notice in practice. He produced this simulation as an example: [link] There was also a mention of smoothing caps, but I'm not sure where they would go. Anybody care to draw up a cleanroom example of this technique?

L2L quirkiness(by IvanC)

Wed, 10 Mar 2010 18:47:19 -0500

Hey fellas. As some of you know, I recently completed the L2L Club. You also know I had a brain fart in the beginning when I mistakenly put the 2 bias supply caps in backwards. Luckily, that was spotted and now we are all well again. Well, almost. :) I was gonna do some recordings this weekend but unfortunately, I had an unhappy misfortune. Suffice it to say, I am a clutz and now, a new set of 6V6's later I am functioning again but....I have discovered a strange anomaly. Basically, when I dime the treble pot I get this awful sound (reminiscent of the sound of a blown or oversaturated transistor) and then, the note dies out after a couple of seconds. I have replaced the treble pot and, I soldered in a 50K resistor across the mid pot (changing it to a 25K pot) in the hopes that that would reduce the voltage going into the MV and PI, but, neither had any effect on this anomaly. The MV (which is pre-PI) has no effect on this anomaly either thereby, ruling out the PI. As a result, Brian determined it was definitely an issue with the tone stack or 4th stage. So..., I decided to shuffle the tubes around. Eventually, I found a combination of placements that finally stopped the note decay almost completely. I say almost completely because, I can still hear this weird anomaly but, it's now more or less in the background and, is still only happening when I dime the treble pot. No other combination of settings, with any of the other pots, produces this anomaly. I never realized (until now) how substantially different the gain of one tube can be to another. Even of the same make and model (I have 3 Russian Groove Tube 12AX7's in there). One of the tubes has so much gain that, when I put it in that position (V4), the amp started squealing like crazy. So...., i'm gonna guess that 2 things are contributing to this phenomenon. 1) the particular gain of the tube that I place in that position and 2) the inherent gain of the 4th stage. Obviously, the tube that's in there now has less gain than the one I originally had in there which, ultimately, means the 4th stage is running too hot, no? When I look at the voltage readings that I took when I had that first set of 6V6's in there (R.I.P), I notice that the voltage across the cathode (of the 4th stage) is the lowest reading in there. Would I be correct in assuming that if I changed R31 (820r) to a higher value that I would in essence, lower the gain of that stage? Basically, i'm making the assumtion that, by using a larger resistor I would be creating a larger voltage drop across the cathode resistor. This in turn would drop the voltage across the plate resistor as there's only so much voltage available which, is set by the B+ (i'm thinking kirchoff's votage divider law). And, if i'm not mistaken, the plate resistor is what sets the gain of a stage, correct? I'm figuring that I don't want to mess with the plate resistor as that is already a lowly 47K hence, my idea to change R31. As an alternative, maybe I can voice the amp a little brighter and then, I will never need to dime that treble pot (even though I probably won't have a need to anyway). But, that'd just be putting a bandaid on it and not really addressing the issue at hand so..... As always, i'm all ears. At least this quirk is no longer as troublesome as it was but, it is nonetheless still there. Any ideas, anyone? Ivan

Got a Bogen, now what?(by Soundmasterg)

Wed, 10 Mar 2010 18:25:02 -0500

Hey guys, I picked up a Bogen CHB100 the other day.(schematic at schematic heaven if you want to see) I got it for a good price, and feel like modding it once I get done with my current project. I'm looking for ideas for what to do with it. It uses a half wave doubler in the pwoer supply, has about 440v on the plates of four 7868's according to the schematic, so I figure it should be about 80 watts or so depending on the quality of the OT. They mounted the OT right next to the PT...seems a bit crazy to me, but it doesn't seem to hum too badly. The amp uses a 6C4 as a splitter, and has a 6EU7 and a 12AX7 in there also. I don't mind wholesale changes and was planning on changing to a full wave doubler with 4 diodes among other things. Anyone have any cool ideas for what I could make this thing into? Greg

Low power on a build, questions(by Soundmasterg)

Wed, 10 Mar 2010 18:21:56 -0500

Hello, I've been working on a clone of a Vox AC100 for awhile. I added a second high gain channel to it, and I mix both channels at the first triode of the phase inverter. The inverter is a seesaw paraphase, using a 12AU7, and for the first triode I have another half of a 12AU7 in parallel, with half the value of plate resistor and cathode resistor of a normal single triode. The output section has four KT77's with about 470v fixed biased and 440v or so cathode biased. It seems low on power as I'm only getting about 50 watts out or so. I'm thinking the phase inverter isn't driving the power amp enough, though I'm not sure why. The circuit I used is exactly the same as what Vox used, with the exception of the parallel triode setup. The OT is a Vox AC100 clone that I got from Heyboer. I have no idea why the power is so low....I was thinking it should be about 80 watts cathode biased and 100 watts fixed biased. Does anyone have any ideas about what I can try to look for? Greg

HOLY CRAP! This OT is Gigantic!(by Doug H)

Wed, 10 Mar 2010 18:14:37 -0500

I really need to post a photo of this... I got my Hammond 100W Marshall replacement OT in yesterday to put in my Windsor and my God! This thing is HUGE!!! It is almost twice the size of the stock Peavey OT (which I thought was pretty big), and it dwarfs the power transformer as well. Can't wait to install it tonight and see how it sounds. In similar news- I got the choke in too and wired it in last night just to see how it sounds- *Major* improvement in the sound/feel/touch/response of this amp. Much more clarity and the amp really sings now. Well worth the $30! Now, on to Gigantor, the Output Transformer that ate Cleveland...

Calling the experts(by S1vaD)

Wed, 10 Mar 2010 18:10:59 -0500

I tried to run this question in on the bogus caps thread, but that had gotten so long an cumbersome, and I think everybody had beaten that horse as much as it would take that I didn't get a response. SO......I will try to pick the brains of the experts here and gather some knowledge. First, the bogus caps seemed to ultimately terminate in a smallish radial cap that was marked approximately the same value as the outermost case. In trying to look at the specs of these caps, I saw that their ripple ratings were much lower than the caps they were pretending to be. What effect would that have on the tone of a P1 amp? Second, stop me before I screw up if I am about to. I ordered 4 100uF, 350 VDC axial caps to replace my impostors. They would not have a negative effect on my amp, would they?

Where Can I Get These Components?(by Hankenstein)

Wed, 10 Mar 2010 17:50:37 -0500

Part of the characteristic sound of Dumble amplifiers comes from Dumble's choice of parts: the type of capacitors used in the tonestack and in the overdrive stages the type of plate resistors the output transformer the lead dress, and the use of shielded wire. the width and breadth of the crystal lattice the tiny magnets used to collect and reconstitute all of the fragile harmonics. ....especially the crystal lattices and the tiny magnets

Tone Stack Values(by RWood)

Wed, 10 Mar 2010 17:07:39 -0500

Hi, In my 6G6 Bassman build (bass channel, for bass not guitar), the 25k bass pot has very little effect, except that it is sucking a lot of gain unless it's lifted from ground. Original schem is here, and I am using the Bass channel only, with no 470k mixing resistor to the PI: [link] I tried to sim it using Duncan's Tone Stack Calculator, but I am not sure what to use for Zsrc, so I used the 100k Rk from the previous triode's cathode follower. I believe I need to lower this, to account for the tube's own resistance in parallel with the Rk Since the bass control in this circuit is isolated from the treble, I had to substitute a "full on " value in this sim for the "treble" resistor feeding the bass pot, and another for the mid-pot value, as there is no mid in this design. I was also not sure what to use for the Load (R5), so I went with the 250K Vol pot that follows the bass control Could you take a look at my first pass at the sim, representing my starting point, to see if I've got it right? [link] And then could you point me in the direction of which components I should begin tinkering with, to get more range out of the bass control. Maybe something other than 25K pot would be a start? I thought that was weird, especially since the Normal Channel on this same amp uses a 250K bass pot. Thanks, RWood

LED instead of pilot lamp(by JHoff)

Wed, 10 Mar 2010 17:07:23 -0500

Hi all. I would like to use an LEDin place of a standard 6.3 Volt pilot bulb. Can anyone suggest a basic schematic, or a reason why this is a bad idea? Thanks

Behringer Ultrabass- how do you open it?(by Miles ONeal)

Wed, 10 Mar 2010 17:03:50 -0500

Some friends asked me to look at why their Behringer Ultrabass 30W "bass workstation" buzzes so badly. It's a small, closed back box with a front load speaker hard wired in with a short cord. Can't pull the amp with the speaker in place. The speaker cord doesn't seem long enough to pull the speaker out the front to disconnect or desolder or whatever, and the cord disappears into the amp sans plug. Anyone ever tried to open one of these up? I don't see a way to do it. Yeah, I know it's cheap junk, but they don't have the money to get anything else right now.

Tube Alternatives(by Peliass)

Wed, 10 Mar 2010 17:03:17 -0500

Hello guys, I wanted to know if anyone knew of any good alternative tubes that can be used in SE configuration in the 5- 7 watt range apart from the usual 6v6 and el84 that had their own distinct sound. Does anyone have any experience with the 6973 or 7591/7868? Thank you.

Squeal comes and goes(by Spartan)

Wed, 10 Mar 2010 16:46:04 -0500

OK - so I decided to build a P1 extreme. Made my own chassis from stainless steel, bought my own parts and ended up with an amp. Decided to make a 112, with a 12" Eminence Legend. Everything is done (sort of) - plug in the guitar and it sounds great for about 10 minutes, a little humm but thats OK, and then the squealing pig starts. The noise comes and goes. I have reflowed the solder joints, rewired the input cable even done the chop stick test. No luck so far. I used a grounded input jack - and also grounded underneath the stadoff near the jack. I then thought - ground loop so I lifted the ground from under the standoff - no improvement. Any thoughts would be appreciated...

L2L Clips(by Brian S)

Wed, 10 Mar 2010 16:15:09 -0500

Here are a couple of L2L clips. the clips were recorded with a Zoom H2, gains on 10, tone controls on 5, MV on 5. My son is playing. He has only been at it for a year. They demonstrate the range of tones using just the guitar's volume. The L2L Club [link] The L2L SE [link] Brian

Squish Control(by Charlie Cyr)

Wed, 10 Mar 2010 16:13:47 -0500

How does a pentode squish control work. I had thought they varied the screen voltage but the cap between the pot and ground should block DC so now now I'm thinking it wouldn't vary the screen voltage. What am I missing?

The Burriss Royal Bluesman(by FredB)

Wed, 10 Mar 2010 16:02:00 -0500

I checked out one of these amps [link] out at a local store today. I was well impressed. As far as I was concerned it tone was just right for blues with plenty of gravel. The breakup is easily controlled with the guitars volume and playing intensity. Only a couple of minor issues that bothered me. I would like the vibrato depth to be able to go more shallow and the speed to go more slowly. Other than that it was a pleasure to play through.

Loss of Volume on 50 Watt pp(by tgray)

Wed, 10 Mar 2010 15:54:12 -0500

I am closing in on the completion of an amp that really challenges my amp building skills. I married a core 50 watt pp power amp with the core crunch preamp. In addition, I put in a relay switching circuit and power scaling to boot. I have built a similar amp without relay channel switching and power scaling and I loved it. Now if I can get the power scaling and relay switching to work I'll have my "dream amp" So far the dream has been kind of a night mare. The problem I'm having on the amp is a loss of volume when a chord or note is played. The note starts out good and strong but looses volume as if someone turned the volume pot down while the note is played. I looked in a troubleshooting guide from this site and found a list of things to check out including: 1. Failing power filter caps 2. Faulty phase inverter 3. Faulty Power or Preamp tubes The tubes are easy to figure out, just get known good tubes and swap them out. The filter caps, I suppose I'll have to remove them one by one and check them. Does anyone have a good test technique for caps? I have a cap meter but I'm not sure if a cap can fail and still produce the correct capacitance. If the cap leaks I guess it may cause the voltage to drop in the power supply. If I just put a DVM voltmeter on B+, B+2,...B+n and watched the voltage when I play a note to see any sags. Would that be adequate? Checking the functionality of the phase inverter poses some problems too. If I hook a 2 channel O scope on both halves of the inverter and got a trace for each half of the signal I should be able to see if the signal is distorted. Any words of advice?

Firefly output transformer hookup(by Nukehit)

Wed, 10 Mar 2010 13:04:12 -0500

I am Looking at John Calhouns firefly guide and the 125A transformer hookups are 2 and 4. Doug Hammonds Rev 3 schematic has this as 2 and 5. Which is correct?

October Studio Build(by RickB)

Wed, 10 Mar 2010 13:04:03 -0500

Well, I was going to build the P1-eX, however the tone of the October sucked me in. I know this is not a recommended first amp, but... I just couldn't help it. The october just has the tones that I really like. I realize there are more parts in the october and more chances for screw ups, I realize that I may get this built and have to debug and futz with it. But, thats ok. I'm up for the challenge. I will simply take my time. Double check each joint prior to solder, check after each solder.

P1-eXtreme Alive and Kicking!!!!!!!!!(by Playingwfire)

Wed, 10 Mar 2010 11:27:20 -0500

IT'S ALIVE! My P1-eX struck it's first notes tonight after seveal days of running through the powerup doc. It's very quiet at idle at half gain half volume. It starts to make a little noise at full gain and three quarters volume but it seems to be right in line with my factory amps. I had a little pause for concern when I was getting high voltlages at the pramp tube but I had noticed it was loose in the socket and when I put the retainer on it everything came down to where it belonged. The voltages are all well within expected ranges. The only thing I've noticed that's odd (or maybe it isn't) is the caps drain very quickly with the tubes in. It's down to less than 10v in about 15 seconds. Also I put it in standby with a chord ringing and it took about 5 or 10 seconds for it to go silent. Is that normal operation? I neglected to adjust the bias before I plugged a guitar in and it was biased hot, 31v for it's debute and the bottom end was "flabbly" I guess you would call it. I had to put the guitar away to work on that so I haven't played it since I adjusted it to 25.4v. I haven't really cranked it dimed out yet either, the gilfriend was sleeping. I'm very curious to see how it sounds on a good speaker. I played through the speaker of a 1980s 35 watt Peavy Back Stage 1 x 10 I solderd a jack to. I never did like that amp much but it got me started playing because it has a headphone jack to keep the parents happy at the time. Speaking of that old Peavy, it has reverb. Would it be possible to use the reverb tank on an HO build into that cab? I was thinking about striping it out and using it for that project anyway. Well now that I have it going I think I'll finish it up as kind of a bare bones head. I was going to go to Angela.com and pick up a cage top and a bottom plate unless someone might know a better place for that kind of stuff. I'm also thinking about tearing it down and painting the chasis, burgundy and gold, Hail to the REDSKINS. I guess I might pick up a couple of caps to replace those ELONs while I'm at it. I wouldn't want one to go poof and take somehting with it. Thank you to all that responded to my posts and special thanks to Paul F. and company who very patiently helped me through the layout for the VB section. Also thank you to Chris H. and everyone at the AX-84 Project without who none of this would be possible. Keep selling kits Chris! Rock on everybody! Sean

Merlin - RE: Glass Blower(by headspaz)

Wed, 10 Mar 2010 11:15:17 -0500

Merlin, I want to try building your Glass Blower. Your design in quite intriguing. The output buffer surfing between bootstrapped rails is pure genius !!! Would you be so kind as to share your PCB layout??? I'm especially interested in the smaller layout... the single dual-opamp version... as shown in the gutshot. Just a pic of the copper side would be a great help. Thanks

Completed my Hi Octane(by sjexperience)

Wed, 10 Mar 2010 11:10:21 -0500

Hi Guys, This is Steve here, working and living in Bangkok, Thailand. I'v been working after hours and weekends on this project and I have completed the Hi Octane. I wanted more power and a bit of the Marshal Crunch , so I decided to go with an EL34 for the Power Tube. So I am not entirely sure if she qualifies as a Hi Octane or an SEL?? I got the trannies made on order and they cost me a fraction of what i had seen online. Also i needed primary for 220 V and output 180-0-180 and 275-0-275 as i was not sure what i would use. I made a mistake in ordering the Output tranny and got one with 2.5 K primary and 4-8-16-0 secondary. But on further research I came up with the solution of connecting an 8 ohm speaker to the 16 ohm out of the Transformer. Next i needed something to mount the parts on . Tried to get a chassis built but living in the city with no workshop/equipment, i decided to try it on a 4$ aluminium tray used in small restaurants. Drilled small holes and used a round file to enlarge the holes to fit the pots, input and output jacks and the tube sockets. So far so good, but upon inspection, i found out the vendor gave me 2 octal sockets and 1 noval socket. So I had to wait one week for Saturday to get it changed. Changed some Caps also to Higher voltage ratings. I also scourged around for a descent speaker and came across a shop selling 8 inch speakers for 3 $. These speakers were reconditioned and all i wanted was a bigger cone in the magnet. So i got one of these. The sticker on the Magnet says CORAL SPEAKERS. I also stripped a burnt out SMPS from an old computer and reused the power jack connector for the amp. I have kept the 12 V fan in case I want to fit it to cool the amp later. Reused the wires from the SMPS also. Talk about recycling stuff!! With all these I set out soldering , a little bit everyday. Had to be very careful in the wiring and soldering because i was not working continuously and mistakes are easy to overlook. Finished the wiring after a couple of days and time to test the circuit. The wiring I have to admit is messy and I was skeptical when turning on the amp for the first time. Said a prayer and switched it on, power indicator lit up , then switched on the standby, poof!! blew the fuse.I checked the circuit, replaced the fuse, this time it blew before the power indicator could light up!!! Checked for short or leakage but found none. Turns out i used a very low amp fuse, 0.125 amp!!! so replaced it and tried again, this time nothing blew, no smoke. was time to fit the tubes and give it a go. The first 12ax7 went in fine, the second one wouldn't go in in one of the holes. Turns out i over heated and applied too much solder and the molten solder crept into the socket pin holes!! Had to clean it out and fitted the tube. Octal tube went in just fine. After all this, it was time to try the amp for the first time. connected the speaker, turned on the main switch, tubes started to glow, all three, which i didn't expect. turned on the standby switch, slowly turned up the volume, no sound........... got my guitar, plugged it in, and VIOLA!!!! sound!! I had used a triode/pentode switch also in my initial design and after some playing, i flicked this switch, the overall sound became mellow. but after sometime, the sound kind of clipped and the volume was 5% of the original. I switched off the amp and turned it back on after a few minutes, worked fine for a few seconds, and went down again. Looked like something was overheating and clipping/shorting out. So after some testing and experimenting, my conclusion was that the triode/pentode switch was causing an overload due to the output tranny beinf 2.5 k instead of 4 K primary. I didn't like the sound of the triode/pentode switch it either so i took it out and just went with the circuit as per the schematic. This time the amp worked fine and i tested it for several hours and all looked good. The amp is silent when no input is plugged in, gives clean sound on low gain settings and pretty good overdrive with both gains at max and a humbucker. I replace the bass cap with 0.01 uF and got a better bass response. the amp squealed at 80%+ volumes but I am guessing it's just feedback through the pickups. But trust me guys, at 70-80% volume, the amp is loud!! Last week i went out again and got a cabinet for the speaker and another speaker cabinet for the amp. Cut out the parts i didn't need and am about to finish the cabinet with the amp. I need some Tolex or fake leather as we call them here to cover the amp and we'll be all set. Building this amp was fun and i throughly enjoyed it. I've been working in the kitchen, the small dining table in out apartment and have generally made a mess of the place, but it was worth it. I have a few pictures at the moment but i will upload more and some clips too. Thanks a lot guys at AX84. Rock on!!!!

fizzy distortion + low output, SE 6V6 12AU7(by danhei)

Wed, 10 Mar 2010 11:02:24 -0500

Working on a SE 6V6 amp with 2x12AU7 preamp. Schematic: [link] I'm getting a fizzy sounding distortion when driving the preamp hard (with the master volume low). My reading of Merlin's cathode follower page suggests that may be part of the sound of a CF. Perhaps related I'm only getting 1.3W out before clipping. I need to scope this but the CF may be clipping before the output. All preamp plate and cathode resistors were chosen with potentiometers, using a ~100mV 1KHz sine input and scope to get max output. Anything obviously wrong with the circuit? I was thinking of switching the CF and tone stack to follow the second gain stage so the third gain stage could drive the 6V6 harder. But I'm not sure if that would solve the fizzy issue. Could also convert the CF to another gain. Looking to get a reasonable preamp distortion sound out of the amp.

tone controls....(by RichT)

Wed, 10 Mar 2010 10:53:00 -0500

anyone have any new or interesting designed tone controls....contour, presence, shape, body, sweep other than the popular ones found at... [link] ?????

Relay Channel Switching -- Noisy?(by Craig W)

Wed, 10 Mar 2010 06:58:30 -0500

Hey guys, I'm planning on using relays for switching in a build and I'm wondering if relay switching is noisy? And if it is, what are some tricks to reduce/eliminate the noise?

The end of the Firefly PCB....(by John Calhoun)

Wed, 10 Mar 2010 06:30:17 -0500

I think I'm going to wrap up selling the Firefly PCB. When the last of the kits and PCB's are gone I doubt I'll have more PCB's made up. Things are getting busier for me and I don't have as much spare time as I used to. I have perhaps another 50 PCB's ��I'll put together kits as the orders for them come in. When the last PCB is gone though I'll close up shop so to speak. Historically, I would expect the 50 PCB's to last me until perhaps the summer. It's possible they will go quicker of course since I am posting this.... It's a bit funny though since this whole venture began because I wanted a PCB for myself but didn't want to bear the $100+ burden of a professionally manufactured one-off PCB. My big idea was to have 25 made and then sell the 24 I didn't need. Somehow that kept snowballing.... The oldest email I have for the PCB is from July 2007. Who knew this would continue for almost the next three years.... Strange. I've had fun though. All the more when people have sent me photos of their completed amps, posted to YouTube, etc. I truly enjoyed helping a lot of people start that insipid addiction to tube electronics. I hope I can find some free time (perhaps this Winter) to start up another project like this one. Thank you, everyone. And particularly thank you Doug for the Firefly design and Chris for this wonderful site. John Calhoun�