AX84 BBS

Amp Parts and Kits

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

Hey to Merlin(by bhorste)

Sat, 31 Jul 2010 13:35:36 -0400

Just thought I should let you know that I ordered a copy of your book, so there's another edition sold. Can't wait for it to come in, and hope that the rest of it reads like the first chapter.

Thoughts on building the P1(by efudd)

Sat, 31 Jul 2010 12:17:05 -0400

It's bright, at least with sc pups and the Celestion G10-20 in there (which really isn't bright.) If you are going to build this I would just add a switch from the start for the .001 cap (C13). I'm going to use a push/pull on the gain pot. You can use a guitar 500KA DPDT pot from Stew-Mac but AES now has Alpha equivalent R-VPP-500KA (knurled unfortunately)for $4. The bright switch seems essential to me. The designers here might just want to design this straight into the schematic. Also if using the Hammond PT you want the 115V tap for the higher B+ and .4 ohm on the filaments is just the ugly-ass kluge you're looking for - hit right at 6.3V. For anyone interested in building this you are not going to play jazz or country even at bedroom levels with this, but it does rawck-out. Just ran it through a 2x12 w/ Peavey 1230s (also bright) and it's JTM50 living room fun. If you are building a combo I'd opt for a 12" over a 10, maybe something like a Weber 15W Silver Bell.

Suggest a decent cheap soldering iron?(by Steve K)

Sat, 31 Jul 2010 12:40:25 -0400

I need to buy a soldering iron for the job. I have a decent station at the house, but for the job I just need something for occasional soldering. Something in the $25 range. Any suggestions? This will be used for pots, terminal strip wiring, possibly some components like power transistors etc.

Valveart EL34's(by Charlie Cyr)

Sat, 31 Jul 2010 11:04:01 -0400

My original P1Extreme kit came with a valveart EL34. The tube went microphonic a while ago but it was always my favorite tube in that amp. It seemed to be a cross between an EL34 and 6L6 in tone. I just put the tube in mo old modified champ and found it has the same tone in that amp so I am going to look for another. Physicly it doesn't really look like any of my other EL34's so I was wondering is this really a different tube sold as an EL34 or are there EL34's just that different.

UL wiring(by stringbend)

Sat, 31 Jul 2010 10:27:35 -0400

What would happen if the UL taps were cris-crossed relative to the primaries?

LTP plate resistor bypass caps(by AleksiS)

Fri, 30 Jul 2010 10:48:28 -0400

Hi everyone, just something I started wondering about with my last build: Suppose you have a regular LTP as your PI. Now adding a small cap between the PI plates kills the treble (as the out-of-phase signals cancel out each other). But what if you have that cap in place (let say 100pF) but still add a bypass capacitor to only one of the PI's plate resistors (lets say 500pF)? Now what happens? The 500pF kills some treble and creates an imbalance on the PI. So does it actually lessen the effect of the 100pF cap between the plates as the signals are unequal in amplitude? Just something I can't seem to visualize in my head. Thanks in advance :) BR, Aleksi

Tube mismatch by half?(by JeffW)

Sat, 31 Jul 2010 10:54:51 -0400

So I have a PP PA with KT77 tubes in it that has one valve drawing 20ma and the other 40ma. At first I thought it was my bias meter so I soldered in two 1ohm resistors and still the same. Switched tubes and it is the actual tube causing the mismatch. Is this a bad valve or just some super poor QC from JJ. I realize that there are differences but 20ma? Anyway, if you have any advice or insight please pass it along. Thanks

Tolex ?(by mac dilard)

Fri, 30 Jul 2010 20:06:37 -0400

How do you do the corners on a flat board. Say a 1/2"x 4" X 18"? I have tried every way I can think of and still can't get it right.

Fixed Bias Problems(by Bonfire)

Sat, 31 Jul 2010 03:02:40 -0400

Hey guys, I've switched to fixed bias in a build of mine and I can't get a negative voltage from this circuit...in fact, the DMM pretty much shows no potential at all (similar to when the probes are not touching anything, i.e. some millivolts which is unchanged when removing the probes from the test points)... [link] Any ideas what might be wrong? I've switched both caps and the diode to rule out busted parts, the resistors all test fine, all connections seem solid and show zero resistance.

power supply for all 12ax/au7 amp(by mynyka)

Fri, 30 Jul 2010 08:16:40 -0400

So I recently built an amp based loosely on a 2203 but using a pair of 12au7s for the power amp (3x 12ax7 + 2x 12au7). It works well but if I want to run all the plates at 250v or so, should I take 1 tap off the last filter to feed them all or filter and feed each one separately? The reason I ask is, I have it wired up the first way at 220v but if I raise the voltage to 250 I get bad hum when I turn up the gain. Reasonably quiet at 220v though...Seems like I need more filtering..... but 5 caps....????

Convertible Phase Inverter(by Zoe N Iain)

Sat, 31 Jul 2010 12:06:01 -0400

Just in case anyone is interested in how you do this... [link] With the switch in the current position, it's a standard LTPI. Flip the switch and it's a floating paraphase with about double the gain. Both are fairly well balanced when running clean, but the paraphase will create more even harmonics when overdriven as more imbalance creeps in. There are ways round that, but it kinda defeats the object of using a paraphase in the first place. Hope someone finds it of use.

"Unecessary Quotes"(by Alex G)

Fri, 30 Jul 2010 07:21:39 -0400

Thought you "guys" would "like" this. There's definitely "a lot" of this that goes on in "amp message boards!" (i.e. "my posts"). [link]

This dead HRD(by nonzero)

Thu, 29 Jul 2010 15:45:49 -0400

I've got iron from a dead (dropped and shattered) HRD, so I would naturally like to use it to build something. The PT delivers 440VDC and about 65mA rectified. The OT is already set up for 6L6 PP with both 4R and 8R outs. I would like to make a 6L6 PP power section. I've got a Philips 5879 I'd like to use for the input (seems like it'd have just enough gain). I'd probably use a 12AX7 for my LTP PI. I'm not too interested in a FMV tone stack, but I've got some current to spare (just more than enough for another 12AX7) so I'd like to do something interesting. Any ideas?

50w PP - first attempt at PTP(by GuiteLectrique)

Sat, 31 Jul 2010 10:49:25 -0400

Hi everybody! Sorry it's been a while, I have been playing instead of building :) Now I'm back at it in my first attempt at point to point. I would be grateful if some of you guys could check my layout and tell me if I did something wrong. The amp is a lead preamp - 50w pp with a single knob tone control. I know the speaker output section looks odd but I've got my reasons for that. By the way, the control knobs will be on TOP of the chassis (a little bit over the circuit). Thanks!

In Brussels...(by foobar)

Thu, 29 Jul 2010 09:48:18 -0400

I'll be at Brussel the first week of August. Any good music shows going there ? I won't be able to attend the Summer Festival 2010 which is a few weeks later. Also, any recommended music store. Already missing my guitar !!!

Bit of detective work(by Wattie)

Fri, 30 Jul 2010 10:15:44 -0400

Just received a donated amplifier from an acquaintance who is moving country, and this is excess baggage. His only statement was "I think there might be something wrong with it". This is a well known P1 type kit build, that I helped him put together. I will hesitate to mention which one because there is absolutely no blame on the kit or how it was built. What is wrong with it is that it hums, it smells, and the two resistors directly in series with the EL84 screen look like toast. My other half made me put it in the garden - it smells so bad. I summise that a bad tube went in, but there won't be any more information from my "pal". Could be he left it on for a few weeks with no speaker, I don't know. We are talking about R3 and R7 if it were a standard P1 - these are obvious replacements. My questions are, if the forum can help: What components should I be double checking and replacing before going through the start up procedure? Apart from the toasted resistors. What might have caused this unfortunate event? A crappy tube?

Power Transformer choice(by stsski14)

Wed, 28 Jul 2010 12:22:42 -0400

I generally have a good background in electrical concepts but am unfamiliar with how power transformers are rated. Say I have a 250 Volt CT 100ma transformer. What is a good way to estimate estimate subsequent B+ voltage under load? Any help would be appreciated or if you could refer my to a site. Thanks!

Is this worth the effort?(by 3rdof3)

Sat, 31 Jul 2010 12:00:49 -0400

I've got a design that I'm looking to get a bit more gain/tonal change out of and was wondering if this circuit would be worth the effort. The bootstrapping would be switchable. Thanks! [link]

a few questions on the theory.(by monongahela monster)

Thu, 29 Jul 2010 15:35:22 -0400

I am researching to gear up for my first build here and i have a couple elementary questions. okay so since the cathode is connected to ground i assume that that is where the current is coming from. and i am assuming it is ac.( or is it?) i was also under the assumption that current in a tube only flows out of the plate. so why have a cap to block ac signal in the cathode circuit? it seems like either ac is coming out of the tube from the grid via the cathode ( which doesn't sound right.) or it is blocking ac coming in, ( which leads me to believe it must be dc but i see no rectifier anywhere near here.) what the hell's going on? also, the theory page for the p1ex has two preamp tubes. I was under the impression that it had one preamp tube ( and that having two would make it a sel)also some of the resistors are labeled differently on some schematics rather than others, this doesn't worry me any but i would like someone to let me know if it should....and lastly , is there some other way to look previously asked questions up rather than endlessly scrolling through potential answers in marginally related threads? whew. I know that was a lot but any help would be appreciated. thanks.

this power transformer...(by nonzero)

Wed, 28 Jul 2010 14:08:18 -0400

...reads 3.6V across the two green wires. 345V across the two red wires. The yellow ct is for the B+ (172V from red to yellow). Green to yellow reads 66V. Its plugged into 120V wall voltage. I don't have data on this pt other than that. Is it supposed to be for 240V? Is there any reliable way to get heater voltage from this?

HOT PT - Overfiltering?(by Alex G)

Fri, 30 Jul 2010 12:27:11 -0400

I've used Magnetic Components 100-watt Marshall style power transformer in both 100-watt and 50-watt builds. I noticed in both cases that the PT becomes really hot. After playing full bore for about a half hour or so, I can't put my hand on the transformer for more than 5 seconds (which I've read is a very general rule for determining whether a PT is running 'too' hot). The OT stays cool, however. Being paranoid, I thought it wouldn't hurt to write the company, especially because I'd read other reports of these particular transformers running hot, however not everyone had this experience. This was the response I received: "Yes, these are built to withstand high continuous running temperatures without any problems. All of the magnetic wire and layer insulation in this unit is rated for these high temperatures. It is normal for these particular units to run hot. Please rest assured that these should perform well for you." I thought this was an adequate response, especially since I'd read on a HIFI forum that certain transformers run hot with no problems because of the materials used. Still a little paranoid, my question is about using too much filtering for the first filter cap. I used 100uf in both the 100 and 50 watt amps I built. That particular transformer is intended to be used with only 50uf of filtering, I presume, like the original 100-watt design, so I'm wondering if the PT can tolerate double the filtering? Here's a link to the datasheet: [link] And FWIW, I'm not using the Center-tap, and I'm not sure if that affects anything concerning current draw. Thanks!

Help in understanding Transformers more so(by Michael Mo)

Wed, 28 Jul 2010 05:17:31 -0400

Just when I thought I was begging to understand this stuff. I have been looking at the Hammond guitar amp replacement transformer specs and noticed that the Marhsall JCM800 50watt PT secondary is only rated at 690v/150mA And the 50w JCM900 is rated at 330v/100mA, Surely these are way under spec�d for two EL34 or am I missing something? It is done deliberately to allow the voltage to sag allot. According to these specs the fender deluxe reverb could almost be used in the JCM800

freq dependant triode strapping(by AleksiS)

Tue, 27 Jul 2010 08:15:18 -0400

Hello everyone, quick question... I've been thinking about triode strapping the EF86 for the high frequencies in my last build. It's basically set up as a "regular" pentode, but I'm simply adding a smallish cap between the plate and screen. Is the cutoff frequency simply calculated with the screen impedance and the capacitance used between the plate and screen? BR, Aleksi

50W_PP purpose of R11(by stsski14)

Wed, 28 Jul 2010 01:21:48 -0400

I was just taking a look through the projects and was curious about the 220ohm resistor after the output transformer. Can anyone explain what necessitates this resistor? I haven't seen anything similar in my travels.

Reverb Madness(by Jason C)

Wed, 28 Jul 2010 06:16:34 -0400

Well, I just finished my second effects pedal - Reverb Madness [link] I'm also having fun with iWeb... I haven't labeled the controls or jacks yet. With the artwork I chose it isn't going to work to put the labels on top, I might put the on the front. From left to right it's tone (fully CCW is transparent - I probably could have left that control off actually), Dwell, and level which is really the wet/dry mix. It's based off the belton digi-log reverb module which is a really nice sounding reverb. I might even consider putting this module into an amp rather than a spring can.