AX84 BBS

Amp Parts and Kits

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

New schematic - Bogen Conversion(by kenbonner89)

Wed, 19 Jun 2013 04:55:30 -0400

Completely gutted an old Bogen CHA-33 PA. Going for a cleaner, loud guitar amp. Any thoughts? Places I should tweak? [link] Some things I don't have yet is NFB going to PI and figuring out if the parallel switch will actually work (going for emulating jumpering a JTM45). I might have to change out a jack from being switched?

mute switch for output transformer with headphones wiring(by lukasz)

Wed, 19 Jun 2013 03:18:32 -0400

Hello, I got output transformer with headphones wiring for my first P1 (its from some old TV). I'd like to put mute switch to play only through headphones, so it would have to be between output wiring and speaker jack. Can I just put a switch that disconnects speaker jack (so amp would work like there was no speaker connected)?

Unique situation: Lowrey organ tube amp help needed.(by GreenCanoe)

Tue, 18 Jun 2013 10:45:04 -0400

An electrical engineer I work with has a side gig as a Yamaha keyboard authorized service tech. He has a church a fair drive from here that uses a Yamaha keyboard that is plugged into a vintage tube Lowrey Organ amp to power the speakers that serve the church. Until now the Lowrey organ amp has been serviced by an old time service guy that would keep it running for them. Of course, that guy has now retired and they were hoping my friend could take on the power amp service as well. My friend knowing I was into tube amps asked if I had any idea where he could get a schematic for this Lowrey power amp before he drove out to this church. I checked the usual sources and struck out. Here's what I'm looking for: Lowrey LC1B or LC-1B Really, they just need to buy a used Crown poweramp and never worry about it again. But that's not my call...

Creative Inductor Ideas(by musicisphysics)

Mon, 17 Jun 2013 00:10:53 -0400

I need some creative ideas on how to use inductors in a tube amp. I recently acquired about 3000 Hamlin coils which were intended to be used in 50vdc reed relays. [link] They measure about 1.9k each, so they should be able to handle around 25mA. My inductance meter isn't able to read their value for some reason, but as the following datasheet will attest, coils of this nature can easily exceed 1 henry. [link] So what should do with them? High frequency plate/grid choke for reverb recovery? Some sort of power supply filtering? Tremolo circuit? Resonance circuit??? I'm already using coils in a tone stack. Since I have so many, it would be no problem to use a lot of them them in parallel or in series. I don't want to make relays with them; please, give me some ideas!!!

Hum issue(by brownie772000)

Mon, 17 Jun 2013 03:12:37 -0400

Hi I have a firefly PCB build that started out fine but has developed an intermitant slight hum. The odd thing is if I touch the transformer (power) case with my .finger the hum goes away. Anybody have any ideas? Thanks, TB

running an amp at lower voltage(by bustapr)

Sun, 16 Jun 2013 02:03:47 -0400

hello. Ive been looking into building an amp which requires 500V on the power tube plates(per its schematic). this setting makes a 50w amp. Im not all too keen on making a 50W amp and would like it lower. so my initial thought was to run the entire amp at a lower voltage like 400-450V on the power tube plates. this would also lower the B+ side exponentially also. I also have a problem with my power outlet where it puts out around 125-128V. this would put a 720V transformer(as in the schematic) to run the power tube plates at around 530V. im not comfortable running 6L6GC tubes at this voltage since their datasheet says 500V-max. Id also like to experiment with different power tubes. so thats another reason Id like to run the amp at a lower voltage. theres also a really nice Edcor PT at a great price Id like to use. My question is if I can do this without changing any values on the amp itself. would the headroom difference be too dramatic? How much wattage(a guess?) would 100V less make a notmally 50W amp? would a 6L6GC lifespan be greatly affected when running at 530V? is it easier to get sag at lower voltages? forgive if these questions are too rookie. I have few years experience building electronics stuff at my college, but this is my first audio project(first guitar amp).

Jensen P12T(by bankeny)

Mon, 17 Jun 2013 03:27:31 -0400

I found an old Jensen alnico speaker the other day that needs a recone. I can't find much info about it other than it has a fairly low wattage rating (5W or so) Is the P12T a desirable speaker? Or would it not be worth the investment to have it repaired? Thanks!

Parafeed Volume Pot & Mute Switch(by musicisphysics)

Sat, 15 Jun 2013 19:44:00 -0400

I designed a CF mixer, and I'm wondering if there might be a problem with preceding the OT of the parafeed with a mute switch, and following it with a volume pot(as in the attached pic). I did try it and it seemed fine to me, but there may be some issues which I'm unaware of. [link]

Can I use these transformers?(by swinginguitar)

Mon, 17 Jun 2013 20:45:38 -0400

Question #1 I had purchased some parts back when to do a Princeton type build, but am now looking at the Building Blocks ideas. So, I have the Hammond OT/PT sold for Princetons - 1760E and 291AX. What if any changes would I need to use this with the Push Pull design for that OT with 6v6? I see that it is advertised as a 20 watt design, but I'm assuming that is tube type and bias dependent...? The OT is rated for ~15 watts so should I bias it down a little bit? Question #2 I'm undecided between the Simple preamp (which looks very Fendery to me), and the crunch (which looks Marshally to me) - would the crunch offer some reasonable cleans as well? How would the Crunch preamp sound into a 6v6 PP arrangment? What are the other preamps modeled after?

Dual-gang pots(by Caprotesta)

Fri, 14 Jun 2013 16:56:22 -0400

Hi everybody, Does anybody know where I could find two 'regular sized' 500KA pots? I have two mini pots that I could use, but I'm concerned that they might not be able to handle the wattage for the amp I'm building. Thanks in advance for your help!

Selecting correct OT Primary Impedance(by thehoj)

Sat, 15 Jun 2013 08:41:35 -0400

I've got a homebrew 2x6v6 amp, and I'm having trouble selecting the correct primary impedance for my OT. I'm effectively running at 271V (284 (B+) - 13 (Cathode voltage)) on the 6V6's, biased at 43mA (11.7W plate dissipation) per tube. Should I go with 8000ohm? Or go 10,800? I don't know how to tell from any of the datasheets I've looked at. I see on tdsl for class AB1 PP it shows: Va Vg2 Vg1 Ia Ig2 Ra S Zout Pout AB1 P/P 285 285 -19.0 70.0 4.0 70,000 3.6 8,000 14.0 AB1 P/P 250 250 -15.0 70.0 5.0 60,000 3.75 10,000 10.0 I'm between the two Va numbers, and my bias is -13.. I don't really fit into either of these specs..

Modified Masco PA(by phatass)

Fri, 14 Jun 2013 11:18:02 -0400

Well, a friend of mine picked up an old Masco PA. I had gathered it was an MA-35 amp and was doing a little bit of research about them to see what could be done to make a neat little guitar amp. Come to find out, the thing has a bunch of 12ax7s installed, which tells me somebody's already modified it for something, as the originals had 7c7s in them. So, it looks like we may have something that's already part of the way there for guitar purposes. Anybody here do anything like this? Any caveats? Any ideas? We've got a chassis that's already wired with 4 AX7s and 2 6l6s. I haven't dug into the things quite yet, but I'm really interested in this deal.

Vox type amp(by Drhorn496)

Wed, 12 Jun 2013 15:01:27 -0400

Looking for an amp project with 3- 12ax7 preamp tubes and 2 el84 power amp tubes resembling a vox ac15 or a Vox AC30, does anyone have a schematic like this or a suggestion?

New Marshall SL-5C(by Mark McL)

Fri, 14 Jun 2013 13:25:48 -0400

HI All, I wanted to see how this new Slash amp (SL-5C)compares to some of the SEL projects here-but they must be hiding the schematics- Are the newer amp schematics not available to the general public? Thanks all,

HO Project Re-boot(by charisjapan)

Fri, 14 Jun 2013 07:47:34 -0400

Hi, it's me again ... has it been 5 months?! ;) 2 daughters getting married put amps on the back burner. So, after all that fuss, I went back to an almost standard 10.10.04 H.O. schematic, but with a switchable paralleled Stage 3 and a switchable 5U4GB tube rectifier. The Stage 3 purpose is to get a little more "fat" tone, using half a tube that is kind of idling. The tube rectifier is mostly for show, honestly. However, it may indeed give a pleasant alternative tone ... Pre-emptive Call for Help: 1) What do you think of that Parallel Stage 3+ ? ... [link] Switch up and it sends a parallel signal to V3A grid, switch down and the grid is sent to ground. Also, was wondering about evening out the V3A&B cathode resistors to both 1.5K as was suggested ... yea, nay, or maybe? 2) What is that connection [box]A between V2 pins 3&4, filament and cathode? 3) I've used a "split ground bus scheme" successfully, 1st filter cap (in this case C1+C2) to the P.T. bolt, all the rest on a bus securely bolted to the far end of the chassis ... comments? I appreciate all your patience and help! Will post layout and progress pics as soon as I make a few final decisions. Btw, this will be housed in a small chassis, tubes up, and use point-to-point with a few terminal strips ... hope to keep lead dress as short and simple as possible. Here's the link to my present schematic ... [link] And a link to the previous thread, "Small head HO "Exposed" ... [link] Thanks, Randy aka charisjapan

Simplest way to add clean channel to this amp(by Dan Moos)

Thu, 13 Jun 2013 23:12:43 -0400

I designed and built this amp about 2 years ago. Its my first design, and I am more than happy with it. I have been gigging pretty regularly with it and really dig the tone. My church wants me to build an amp for them. They also like how my amp sounds and I don't want to change the root tone much or at all. A clean channel would be nice though, for a myriad of reasons I won't get into here. There are basically three ways I can think of to do it. A)create a true second channel with its own preamp and controls. This is my least favorite idea. B)have the second channel just use the parts of the dirt channel it needs. Sounded simple to me at first, but when I started trying to make it work on paper, it wasn't as simple as I'd hoped. C)just switch resistors in and out of the gain and volume pot circuits. In other words, change the "setting" of those knobs by adding or subtracting series resistors to the pots as required. This sounds like the simplest to me. Its the way I hope can be made to work. The amp definitely can be set to sound like I want the clean setting to sound like just by changing gain and volume settings, so I know it can work. D)a variation of the third idea, where instead I actually switch to a different pair of pots. This could be cool because then it wouldn't be just a clean setting, but whatever you set it as. This is the method I prefer if there are no hidden "gotchas" to doing it this way. Some notes about the schematic. The new amp will almost certainly have the second gain control hardwired at some ideal setting so as to simplify the front panel for the various players. This complicates my idea because I would have that pot set differently for the clean setting. Maybe a resistor would have to be switched here too. The FX send level will also be hardwired. I leave it in the same place all the time, and for pedal safety don't want it too adjustable on a community amp. I probably chose strangely with the 12au7 in the FX loop, but it sounds good and I don't want to mess with it. Future amps will probably have a better tube for this position though. I welcome all ideas! here's the schematic: [link]

EV SRO/15's ? (by Mors08)

Thu, 13 Jun 2013 19:03:54 -0400

A while back I scored some old speaker cabs a neighbor threw in the alley. Ended up being all that was any good (not water logged/rotted) was an EV SRO/15 speaker. It�s got the big coffee can (alnico I believe?) back and it�s heavy as hell. I built a cab for it and it works fine at 8 ohms. However, it only sounds good CLEAN - even then it's a bit shrill. This might be fine if I were playing country licks through a Twin but I like a little dirt. (tweeds) I think the problem may be the big aluminum dust cap? It just does not budge whatsoever when pushed. So any amp distortion ends up sounding extremely nasally and generally painful. On top of that, I swear it resonates sympathetically at certain tones (usually C and Eb) both when being played through and when it�s just next to another amp. Question is � is this thing worth re-coning (paper) and taking advantage of it�s incredibly high efficiency and sheer ability for volume? (150 watts and 100+ sens.?) Or � is this speaker worth much more as it is to someone else and would it be smarter to use it to fund a more appropriate speaker for my needs? It was free in either case so I�m ahead no matter what � I�d just rather have something that works for me than have a valuable boat anchor taking up space. If selling, I see $100 or so seems to be a fair price??

Increasing 5E3 distortion(by Hoiekvam)

Tue, 11 Jun 2013 16:19:16 -0400

A friend wants more distortion in his 5E3. Would adding a SPDT switch be an effective way to increase preamp distortion in the Bright channel of a 5E3 circuit? My thought is to put a SPDT switch between pin 6 and the extant 100k resistor, and between the same pin 6 and a new 220k resistor. The switch simply connects one resistor, simultaneously disconnecting the other. Result: the Normal channel is maintained. The stock tone of the Bright Channel can be maintained, or switched over to a "Magnatone"-like overdrive if desired. Any thoughs on better ways to accomplish this goal?

Guitar tone control has effect only at min or max(by Henry Ramsey)

Wed, 12 Jun 2013 13:47:57 -0400

Is this normal? I have two different guitars, both are SHB, one is an Epiphone w/ humbuckers the other is a Washburn/Oscar Schmidt OE30 I refitted with P90s that fit in humbucker slots. In both cases the volume and tone controls are stock, but tone has little noticable effect except at minimum or max rotation. The amp I'm playing through has the blues preamp in it's stock form without the tone control mode that Zoe&Iain made later. I am using a 6AW8A though not a 7199. Is the lack of effect due to some interaction between the guitars and the amp? How can I make it more effective if so?

Treble Peak Circuit Placement(by 3rdof3)

Sat, 15 Jun 2013 08:21:57 -0400

Do any of you have experience or opinions on the placement of a standard 470k/470p treble peak circuit either before or after the Gain pot? I see it both ways on some old M@rsh@ll schem's and am curious about the tonal difference. Thanks!

Changes in the 12ax7(by steve25)

Tue, 11 Jun 2013 07:54:24 -0400

How has the beloved 12AX7 that originated in 1947 came to become what it is today? How do we expect any new small plate version of this tube to last a normal life that a 1950s version would when these tubes have some 40% less plate and cathode surface area? Alot of us hate the thin sound of some small plate Russain tubes, is it any wonder why this would be and why they die so fast? Sorry I need to rant, I just spent 1 hour testing 10 of the new TUNG SOL braned (read high priced)12ax7 tubes, and only 2 out of 10 could match what the average Electo Harmonics 12ax7 could pull off gm wise and at a cheaper price.

Lead pre & 50 W P-P start up...(by ormen)

Fri, 14 Jun 2013 10:01:18 -0400

Hi, I just soldered together an amp and followed the design of these stages rigorously. I have followed Paul Ruby's start up guide and made the voltage measurements stated there and I would like to know if they are are acceptable since this is my first amp build from scratch. Here they are. B+1 = 417 VDC B+2 = 414 VDC B+3 = 385 VDC B+4 = 354 VDC B+5 = 350 VDC Plates = 417 VDC Screen grids = 415/410 VDC V4a Pin 1 = 254 VDC Pin 2 = 40.5 VDC Pin 3 = 68.4 VDC V4b Pin 6 = 256 VDC Pin 7 = 41 VDC Pin 8 = 68.5 VDC V6a Pin 1 = 353 VDC Pin 2 = 203.8 VDC Pin 3 = 205.2 VDC V6b Pin 6 = 203.7 VDC Pin 7 = N/A Pin 8 = 1.2 VDC V7a Pin 1 = 234 VDC Pin 2 = N/A Pin 3 = 1.7 VDC V7b Pin 1 = 267 VDC Pin 2 = N/A Pin 3 = 2.17 VDC As far as my knowledge goes (it doesn't go far) it seems OK to me, voltage levels is just a little bit high. So now I need to ask the pros here if it is okay? I measured all of these to ground, I don't even know if this is the correct way to do it but it seems like that. I'm a newbie so any help would be greatly appreciated. If I get the amp up and running and functional without too much hum I will post some sound samples as soon as I get the opportunity.

Preferred Speaker Impedance (by Mark McL)

Fri, 07 Jun 2013 13:04:35 -0400

Hi All, I have a question about matching valve amps and speaker impedance. Lets assume that your valve amp is designed/built correctly and has the proper valves and multi-tap OT. You will only use this amp with only a single speaker cabinet. Does it matter what impedance (4,8, or 16 ohm) your speaker is? Any advantages to one over the other? Can anyone hear the difference? Thanks

ECC99 push-pull power tube design exercise, 1st attempt(by cfwahl)

Sun, 09 Jun 2013 12:45:09 -0400

I've been trying to graft an interesting preamp onto a low-power output section, that uses a single ECC99 tube driven by a cathodyne PI. I read Merlin Blencowe's single-ended and push pull articles on his website, and tried to confirm questions I had by reading other sources. I have on hand a Hammond 269EX PT (190-0-190V) and an MPS OT5PP OT (22.5K primary impedance, 30H inductance) so that's what I've based the following graphic example on: [link] which I drew in CAD over an image of the datasheet's anode characteristics. I welcome any comments about where I may have erred. The conclusions I come up with are these: 22.5Kohms primary impedance basically sets the slope of both Class A and Class B operation load lines. If I'm not mistaken, the Class B line pretty much has its foot anchored on the x-axis to the chosen B+ voltage (potential delivered to the center tap of PP OT, which Merlin B. calls HT), but the Class A operation load line may be shifted upward by choosing a bias point at some desired quiescent anode current above the B+ voltage. With the loads of the preamp>PI>output circuit that I have in mind, and analyzing by DuncanAmos' PSUD2, I reckon that the max. V I can expect to get out of the PT is about 261 -- that's without a dropping resistor between rectifier and ECC99 load, only filter capacitors. I reduced that to about 238V for this exercise, based on a couple other circuits I've seen, which puts the bias point closer to the middle of the Vg curves/range on the chart. With the 22.5K impedance of the OT, it looks like there really isn't any transition to Class B operation at all, within the normal operating range of the grid. In order to force some Class B operation, I'd have to slide the Class B load line over by raising the B+ voltage (get another power transformer). Or, get another OT with a lower primary impedance, to make the A and B load lines steeper. Or both! Am I doing OK so far? In addition to the red curve representing max. plate (anode) dissipation, I drew another gray one at 85%. When the Class A line is slid upward to be tangent to the 85% curve, I get a quiescent bias point of about 17.7mA over 238V, at Vg = -8V. That translates, if I understand this correctly, to a cathode resistor of Abs(-8.0/0.0177) = 450 ohms, or half that if one resistor is biasing both triodes. Is that the correct procedure? Is it advisable, if I stick with the PT and OT that I have on hand, and recognizing that operation is strictly Class A, to bias the thing hotter or cooler (higher or lower current) than 85% max. dissipation? Should I change the B+ to about 220V, to get the bias point closer to the tangent of Class A line and dissipation curve? With a push-pull circuit, should I change the OT to a lower-impedance type, to bring AB operation into play? Naturally, I welcome any advice concerning this, from you who are all much more experienced at this than I. Thanks, -- CFWahl

OT's primary Max. Current?(by Jussi)

Thu, 06 Jun 2013 03:22:27 -0400

Hey, Very basic question... I'm choosing output transformer for my project with two 6V6 in push-pull. In the specs some transformers I see for example: "Primary (input) Max.Current 150mA". If my power transformer has 200ma does it mean I cant use that OT?