AX84 BBS

Amp Parts and Kits

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

Single ended neverending story?(by mmarino)

Thu, 11 Mar 2010 17:50:09 -0500

Fellas amp builders, I guess most of you spent a lot of time experimenting with single ended designs, like I did, and these are few of my conclusions... These amps have that something in the sound thats worth all the trouble and trials and errors on the way, and thats because it is very difficult to make one that is really good sounding. Or if you want "push pull" good sounding, but with mid range complexity and dirtiness of a single ended. After this philosphic rubbish, to the matter. I've experiment with EL34 SE (before also 6V6 and KT77), and preamps from P1EX, to JCM800 2204, to SEL. Well, my 2204 EL34 SE has at most one sweet spot in settings where it sounds really good but not "to die for" and I've tried many things that have influence. different Tubes, preamp designs, defferent preamp voltages, grid resistors, different speakers but with the same guitar it's still not quite there. I can't get rid of slight harshness thats more or less annoying dependant on my mood :), and believe me I've tried everything. Almost. To the point that I'm convinced SE is not for me. Could somebody compare from experience the difference between EL34 sound at usual plate voltages (360V-380V) and lets say 300V-330V, more into pure class A? This is something left to try for me, but it's difficult to test without a new PT. I'll definitely stick to two-tube preamp design and afterall I'll try something else: a marshall superlead preamp with 3 triodes and fourth will be a phase inverter half to drive an EL34. For some reason, only few designs I've seen with PI stage incorporated. Don't get me wrong, I have a decent push pull amp, but SE design is something I won't give up on, as it has some magic hidden inside somewhere...

EL34 pp bias question(by tgray)

Thu, 11 Mar 2010 16:59:10 -0500

I have a question about bias voltage and cathode current for a push pull EL34 power amp. I have 461 volts on the anode, 460 volts on the screen for 2 EL34 tubes. I measured bias voltage of -52 volts and cathode current for both EL34's of 80mA. Actually my current meter has only 2 significant digits so the cathode current is between 70 mA and 80 mA. The arrangement for measuring cathod current is that I tied bothe cathodes together and made a soldered connection to a banana jack. I then have another banana jack connected to ground. This makes it easy to put my current meter in series with the cathode current to ground. When I'm done with my current measurment, I just put a jumper between the two banana jacks. I trust a direct measurment of current as opposed to a voltage measurement accross a 1 ohm resistor. Anyway, my question is... do these measurements seem reasonable? For some reason I was expecting a lower (absolute value) voltage than -52 V. Does -52 volts sound in the ball park for an EL34 tube with 460 V on the anode or should I shop for another meter? tgray

VU Meters - how/where to install(by Michael Mo)

Thu, 11 Mar 2010 15:28:51 -0500

I have managed to get my hands on a few 0 - 500ma VU meters with lights etc.. but I dont fully understand how they work or where to install them. If I plug it into my variable power supply I cant feed it much more than 1v before it maxes out... so how would this be installed in an amp in a way simular to ashdowns? Any help would be appreciated Mike

GREED GREED GREED I need advise(by LuisG)

Thu, 11 Mar 2010 18:07:11 -0500

Hello guys, I have been looking for a cheap cab for the past few months, on craigslist and guitar shops. I have yet to find one, a Marshall 1x12 is 250 dlls. With 250 dlls I can buy all the equipment, build a cab and then sell it. I find this quite frustrating because I live on a condo and I can't have fancy equipment. ANYWAY to the point. I play at home 99% of the time so I play at low volume. Currently I have a VOX VT50 amp I am probably going to sell. Reasons are it hums on the high gain models, and its a PITA to carry around as it weights a ton and the times I do go out to try and Jam with a friend its alwas a PITA. Yesterday I placed an order on Amazon for this guy a Crate Blackheart BH 110 Killer Cab Guitar Amp Cabinet, 1 x 10 [link] but today I feel greedy because the 1x12 version of it is like 15 dlls more. But I really don't think I need a 1x12 just yet. 150 dlls might be ok to pay and not have to go through the trouble of building my own cab. Mostly because I will probably endup wanting it with round edges + Tolex and all the whole enchilada. Another thing is that I have seen 4x12 marshalls loaded with V30 for 175 dlls on Craigslist so I feel paying 150 for this is a ripoff. any 1x10 lovers/haters advise please :) any 1x12 lovers/haters advise as well please :) Thanks Luis

Doing some amp mods(by Nick Glover)

Thu, 11 Mar 2010 14:16:09 -0500

Hi everyone, this is my first post here... I've been doing plenty of DIY effects and learning all I can about electronics, but I'm still not at the point where I feel comfortable doing mods without some confirmation from more knowledgeable people. Alright, I'm trading my Blues Jr. for a Vox Night Train, which I like much better, but it still doesn't have more than one channel or an FX loop. Neither of those seem particularly difficult to add, since all I really want is to have the same channel twice--I dig the distortion I get from cranking the gain, but it would be impractical to try to switch from bright to thick and crank the gain in the middle of a song. So it seems like basically all I'd have to do is put together a copy of the EQ/gain section and preferably also the master volume knob section so that I can have two different sets of settings, then link those to a switch that could also be footswitchable. I'm not really sure how to do the footswitch, but the rest I'm pretty solid on, except for exactly where to insert it on the PCB. Here's a link to the schematic: [link] It looks to me like I would cut the trace before C29 and run that to the switch, which would then run back to that spot on the PCB and also to my copy. The output would then go to where #2 is on V3B on the schematic. The thing I'm not sure about that is that lug 6 of the bright/thick switch goes to a separate section of the circuit, so how would I do that with channel 2? Do they both just connect there, or am I going to need diodes or to also wire that to the switch? Then the Volume section would also need to be wired to the switch so the gain channel isn't louder than the clean channel, but that knob seems to be connected two places on the schematic. Do I just need to splice in at lug 1 of the volume knob, or am I going to need to splice in farther back? Beyond that, I also need to know how to make the channels work with a footswitch. I really have no idea how to wire that to work in addition to the amp switch. Mesas have a middle position on the switch for the footswitch, which could be nice, but if it's going to make it more complicated, I'll pass. Also, when I switch channels, am I going to have to worry about "pop" when I switch? Finally, I really don't know where to stick the FX loop. It seems that, when adding an FX loop to an existing amp, you also want to add some level knobs. Where do I splice that in, and how would I wire it up?

Fake caps in SEL(by Bizzie)

Thu, 11 Mar 2010 14:32:53 -0500

I have recently made a SEL using parts I sourced myself and have been really happy with the sound and the build went well with few problems but after reading about the fake caps going round i have decided to check mine and found they are fake. My question is would it be OK to use some 100u 450v caps in place of the fakes as I have cut one up and found it contained a non branded 47u 400v cap inside and this worries me as the voltage rating is a bit low.

When mods go bad part II(by joeyvoltage)

Thu, 11 Mar 2010 18:12:12 -0500

Check this gem out!LOL they say a picture is worth a thousand words, allthough I bet this could just be summed up in a few [link]

Matchless Cheiftain Circuit(by Ben Sykora)

Thu, 11 Mar 2010 12:35:43 -0500

Couple questions about the cheiftain... [link] How beneficial are parallel triodes really? For example, the cheiftain reverb could be build with 3 preamp bottles if not for the parallel business. One of the arguments is lower noise, but there are other methods to address that. Is there disadvantage to running the reverb path into the "pull" of the PI?

Turret board drilling(by LuisG)

Thu, 11 Mar 2010 13:19:01 -0500

For those of you who use a hand drill to drill the board, what do you use to make the drilling perfect square? Thanks Luis

Random TV tubes(by gemini97526)

Wed, 10 Mar 2010 22:15:28 -0500

I won an auction on Greedbay for a 28 piece tube lot, now I have these random tubes that I will never use. (2) 6DA4 (2) 6DW4 1 each of 6Q7 6W4GT 12AX4GTB 6AU4GTA 1G3GT All rectifier tubes, with the exception of the 6Q7, which is a triode/dual diode. Almost all of them used in TV aplications, but I know the 6DW4 can be used as a damper in tube rectifier supplies to ramp the B+ up slower. I don't know, 2 bucks each? Shipping is likely only a couple dollars as well. Email me at my screen name @ hotmail.

Just finished my first build(by Lou Glorfeld)

Thu, 11 Mar 2010 01:05:45 -0500

I just completed my final wiring before going through an abbreviated start up procedure. Please take a look and see if you spot something obviously wrong. [link] I spent almost three weeks wiring it and was *very* careful to get everything correct, but stupid mistakes can still happen. I have looked at all connections with a magnifying glass, and see no problems. I've traced the schematic through twice, and although I haven't quite finished the second one (except for a few details it is almost done) they both check out. One question. When I power up the PT should I measure from the secondary CT to each side of the secondary or just across the whole secondary?

First time P1 build, few questions(by Squidtubes)

Thu, 11 Mar 2010 09:04:03 -0500

Hello Friends! I have been reading from this site for a few years and finally decided to build a P1. I bought the components and made the board a few years ago, when the 060223 revision was current, so all of my build is based on that schem/layout. Here is a link to pictures of what i have so far : [link] On to the questions! 1.) Can I use a LED as a pilot light (resisted to 4.5 volts at 30 miliamps)? I don't really understand the 6.3v leads from the PT (269EX, the new one with 115 and 125 volt windings) , neither are marked any different, so is there really a positive or a negative? Current direction is of course a problem if not.. 2.) Could someone link me a picture of the input jack? I did lots of searches and couldn't find a picture of the jack like mine. It has three tabs, one for the ring, one for the tip, and one for the little grounding terminal that grounds the tip when no jack is plugged in. 3.) After reading the Paul Ruby startup guide, I am worried about a bleed resistor and was hoping someone had a good picture of this added to a P1 build. Anyway, I'd like to start with those questions, please feel free to look at my photobucket album there and tell me what you think :) I want to do this right, so I need to ax a few questions! Thanks guys

Oscilloscope question(by Dan Moos)

Thu, 11 Mar 2010 00:54:44 -0500

I have a vintage heathkit IO-12 oscilloscope that seems to be working well. I don't have probes, and have so far just used wires with alligator clips. How do I know what the thing can handle? I have ordered a reprinted manual for it, but in perusing what is availiable to read online, I can't find this information in other heathkit 'scope manuals, and am not expecting this one to be any different. for instance, can I safely assume that I can test the full signal path of an amp? I know the currents are pretty low in a tube amp. This scope 'is all tube, and I would presume can safely handle the voltages in a tube amp since the scope itself is running on similar voltages. Any thoughts?

Tone Stack Values(by RWood)

Thu, 11 Mar 2010 01:47:24 -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

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?

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

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?

Low power on a build, questions(by Soundmasterg)

Thu, 11 Mar 2010 06:40:34 -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

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

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

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

Fuzzy decay sound to notes(by Mark M)

Wed, 10 Mar 2010 02:12:46 -0500

Hi Guys. I have been fooling with my 4-4-0 build for awhile. I built it awhile ago but had to set it aside. One thing that I can't seem to rid in the amp is a is a fuzzy overtone to the sustained notes as they decay. I built this amp first to the original schematics, then have incorporated all changes as they have evolved. I have tried a significant number of tube swaps from my stash of tubes in all the various spots with no relief on this artifact. I have also built a nice TW Liverpool prior to this so I am aware of how lead dress and layout can affect the amp design. Its been vexing to me and I can't enjoy the amp with this niggling issue. Has anyone encountered this? I am using a JJ can capacitor (40/20/20/20)in this amp and a 80 uf sprague atom up front at the first power supply node. I don't think its capacitor related but any tips would help.

Hi Octane / Renegade / Two channel amp layout for review(by NTBlade)

Tue, 09 Mar 2010 21:16:38 -0500

Hi all, OK, I've started a layout for a 2 channel switcher since there isn't one published here and I want to build one. It's basically the HO + Renegade clean channel added on (HO with clean I suppose) which will be mated to the HO SE EL84 Power amp so I've kinda dubbed it as an "AX205" (Cool eh?). Please have a look at this and give some feedback. When reviewing bear in mind: This is a first (ever) draft!! Ignore component values since at this point I'm only looking for any glaring layout gotchas, no-nos etc. I went for multicaps to free up some space for the clean channel. The layout is by no means complete but I've got to the stage where I should really seek advice in case I've broken any cardinal rules and if you're familiar with the schems then the layout should hopefully make sense (especially since if robbed most of it from the HO). I wasn't sure whether to continue the ground bus and tack on the clean channel or not so I went for a sort of hybrid star/bus approach. Please comment Volume pots are upside down to make room for channel indicators - (Clean, rhythm, lead using the HO gain switch mod.) Shielded cable is indicated by coloured and black dashed lines, (transparency indicates under board connections) the screen grounding points should be visible and are copied from the HO layout except where they connect to PS caps feeding the stage where I'm using multi-caps Channels switching: Relays for the small signals and an SSR for the "Boost" function which isn't shown. switching by means of logical "ANDed" signaling (Front panel in series with footswitch) driving the relays / SSR derived from a dedicated small transformer Relays / SSRs on small vero "daughterboards" The point of all his is to try and get myself and others a layout for a switcher without re-inventing the wheel. It probably isn't ready for review but hey some feed back should speed the process up! All the best, NTB PS - If this is in the wrong section then (mod) please move it. PPS any mistakes are of course, intentional and are meant as a test for you! ;�) [link]

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.