high octane push pull

[Beezerboy]

A guy gave me this circuit board. he thought it was a Trainwreck (its not), then thought maybe it was an AX84. the preamp looks similar but its got 2 power tubes (6BQ5/EL84). I got the board and a diagram of the lay out. the board is already built and looks pretty well done. I've checked the components and they match the diagram except for a couple numbers that I'm not sure of because the print is not too good. I've been to AX site but there is nothing about this one. other searches I've done are dead ends, the old sites are gone or infected, etc. anyway... what I want is the schematic. or at least the transformer specs. and the pots, especially the ganged GAIN pot. the others I can prolly guess. I posted this in other places but no responses yet



 

[Auke Jolman]

I build een AX84 SEL amp years ago. The preamp looks somewhat similar.

[Willabe]

A guy gave me this circuit board. he thought it was a Trainwreck (its not), then thought maybe it was an AX84. the preamp looks similar but its got 2 power tubes (6BQ5/EL84). I got the board and a diagram of the lay out.

You should draw out a schematic from the layout drawing.

Then you can easily see what it is. 

[Beezerboy]

all I have is the board... no chassis, transformers, pots, or tube sockets, etc. I've built half a dozen amps but they were based on common Fender and Vox designs. this one has some odd stuff that I have never seen. there are ohm numbers that I can't be sure of, plus there are no voltages given. I think if I had the power transformer spec I'd be OK

and the AX84 is similar in some respects buy definitely not the same preamp. and PP instead of SE

[Willabe]

Is the layout drawing in the OP the amp?

If so, then you can make a schematic from that. Do as much as you can and fill in the rest as you go forward.

[JPK]

All the pots should be same as attached HO schematic PDF below. Your 470ohm voltage divider resistor values after the dual gain on your layout matches the HO schematic so I'd use 1Meg. Just pick them on the Hoffman store linked below. Be careful to note the suffix "a" for Audio (log) and "l" for linear. The OT is a little harder to pick but just match the OT tube impedance, needed B+, and required heater voltage, etc. Let me know if I can help any further. I built a SE HO but never did the PP mod. If the layout in the OP is indeed for your board, as asked above, making your own schematic would be a really good idea and a good exercise. I think my attached HO schematic's pre-amp circuit would be almost identical but I didn't study to that detail. So that part might be the same. Use the output stage from Auke Jolman's SEL PP schematic, etc etc.

https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=PotsKnobs&ORDER_ID=713782388

[bmccowan]

photometers are light meters Note - I see JPK changed it to "pot" which is the abbreviation for Potentiometer - good. Sorry to be the word police
And I'll join the chorus suggesting that you draw a schematic

[Beezerboy]

heres what I got...<a href='https://postimg.cc/z3fKBh3z' target='_blank'><img src='https://i.postimg.cc/sfYcT9rh/HO-PP-circuit.png' border='0' alt='HO-PP-circuit'/>[/url]

see link below. slucky already gave me some good stuff but keep it coming

[Willabe]

No, that's a layout drawing. You should draw up a schematic.

Any time your dealing with an amp, you want a schematic, a layout drawing is also helpful but, that's just for finding where the parts are in the chassis for trouble shooting/repair.

Most amp companies only had a schematic drawing, they did not release a layout drawing.

It's much harder to see what's going on in the circuit with a layout drawing.

The schematic is much more important than a layout drawing.

[GlideOn]

I don't believe there is such thing as a Hi-Octane 'push-pull' amp from AX84.

The P1, P1 Extreme, Hi-Octane and SEL were all single ended. Meaning no V3 phase invertor. Meaning completely different preamp typography.

There was a "November" push-pull amp but that was a 6V6 design which require different output impedance transformer primary than EL84 and obviously different tube sockets.

The Dual Gang pot seems to just be the gain for V1A and V1B. It's a cascaded preamp, so why it is on a variable resistance vs being a fixed RC "peaker" value on a typical JMP is kind of an odd design choice. It may be attempting to be bit more linear in brightness throughout the gain range as an alternative to a fixed "bright cap."

The preamp isn't so much a concern as the power amp. You need to find out for sure what is intended to go in those power tube slots so as to select the appropriate Output Transformer primary load.

[labb]

Just before ax84 shut down they went to what they called the building blocks where they had different preamps and different power ends. I think the closest to a high octane pp would have been the Lead Preamp with the 20 watt pp power end. That lead preamp has Marshall stamped all over it. Lots of gain.

[GlideOn]

Just before ax84 shut down they went to what they called the building blocks where they had different preamps and different power ends. I think the closest to a high octane pp would have been the Lead Preamp with the 20 watt pp power end. That lead preamp has Marshall stamped all over it. Lots of gain.

I spent some time on that site for about 6 months back in the early days before building my own single ended SEL. There were all kinds of one-offs as it was both filled with beginners and those who completed their amps, eager to tweak them. Some were sold as kits like Doberman Amps (now defunct) and they came with weak transformers which were somewhat lacking in the bass and fidelity department. Lots inc myself tried to make them more powerful as a result. Even recently I've been working on this amp.

It's was a melting pot of creativity there. Sad times when potential hobbyists have faded from interest in doing such things.

[Merlin]

Just before ax84 shut down they went to what they called the building blocks where they had different preamps and different power ends. I think the closest to a high octane pp would have been the Lead Preamp with the 20 watt pp power end. That lead preamp has Marshall stamped all over it. Lots of gain.

+1. It's a HO preamp bolted onto the building block EL84 output stage (which is the same as a Marshall 18W). HO schematic attached.
It seems to have a CT-interrupting standby switch and a ton of reservoir capacitance? You could leave out the switch and one capacitor though.
Power transformer: something between 430V CT and 480V CT, 100mA DC.
Output transformer: 8k, 15-20W.

[labb]

I just finished a version of this. I used the preamp from the old SEL and a modified version of the Train Wreck Express power amp. Used the Pacific  Train Wreck express transformer set. It can run either EL 34 or 6V6 power tubes. Have only tried the EL 34 to date. Looks like a winner if you like Marshall. Found it to be too bright at first so had to do a little work on that.

[Beezerboy]

"The preamp isn't so much a concern as the power amp. You need to find out for sure what is intended to go in those power tube slots so as to select the appropriate Output Transformer primary load."

thats why I posted the layout. I had no idea what the transformers were supposed to be. and no voltages or values for the pots. I was hoping someone would recognize it because it appears to be an odd one. well.... it is an odd one, I've searched many hours and still have not found THAT one. some that were kina close, but not really.  over at the "garage"  Slucky posted his version of it and its pretty close. good enough for me to make it work. he also suggested getting rid of some of the cap in the power filter section. I'll probably do that. as for lifting the CT for standby... I might give that a try, as well as the dual pot on the gain.

I have trannys on the shelf for a Brit style 18 Watt PP so I'm going to use those. the power tubes will be EL84s

I'm dealing with the seasonal change etc so it might be a while before I actually get on this thing. I have all the parts except I need to build a chassis and a cabinet

thanks all... keep it coming of you think of something

[tubeswell]

I've got a bunch of AX84 schematics - here's some of them

[tubeswell]

Some more

[tubeswell]

Still more

[tubeswell]

Knock yourself out

[Beezerboy]

as of now my plan is to go with this, since it pretty much matches whats on the board:

[Beezerboy]

this (above) is 2 different amps grafted together. values in red are changes for the application. they almost exactly match what is already installed. I intend to add a Hoffman type bias circuit. tubes will be 6BQ5s. I've looked at dozens of AX schematics and there are none that look like what I have. again... I didn't build this thing, and there was only the board layout that came with it. I have verified the value of every component on it. it looks like it should work right?

[tubeswell]

Yeah it’ll ‘work’ (although way too much preamp gain for EL84s methinks . YMMV)

[Beezerboy]

sooooo.............. is this even worth foolin with or should I strip it for parts

I'm a good technician with a lot of electrical experience in other areas. but, not an engineer. if I have a schematic I can build it. if there is a fundamental design flaw in a tube amp, I prolly wouldn't know that. I assumed this thing would work since somebody went to a lot of effort to design and build it. but having second thoughts since this seems to be the only one in the world as far as I know. on the other hand, I don't feel bad about it.... I've shopped it around a couple good web sites and no one else seems to know what it is either.

if you were going to "fix" it... what would you do?

[Willabe]

EL84 tubes are very easy to drive, it takes a little more to drive 6V6's, even a little more to drive 6L6's and so on.

That preamp has a lot of gain.
 

[Merlin]

if you were going to "fix" it... what would you do?

It looks fine to me, two proven amps bolted together. Build it

[Beezerboy]

update:  welp... I built it. it kina sucks. too much gain, harsh and brittle. I been fartin with it for months. I messed around lowering the voltages in the preamp and the output stages... there isn't a clean sound anywhere to be had. I made it sound less than awful at medium volumes but really... not a useful amp in the real world. I guess thats why it is such an orphan. I never did find anybody that built THIS amp. so now... what do I do with it. my plan is to strip it and build something else with the residue. . give me suggestions based on:

2 power tubes - EL84 and B+ voltage at 400. TMB and a Master Volume

[TIMBO]

Marshall 2144 Combo
This one rocks

[Merlin]

I mean, it was called the high octane for a reason, clean was never really an option!

[AlNewman]

I had good luck with a build similar to yours, except the second triode had a 10k cathode resistor, and no bypass cap.  I also made the bypass cap on the 3rd triode switchable, (with a lesser value), and I bypassed the 100k cathode resistor at the cathode follower with a 500pf cap.  I also used a 12AT7 in the PI position.
It isn't messy at all, lots of headroom, preamp and poweramp distortion can be controlled by their independent volumes.

[JPK]

I built a stock SE High Octane and never liked the sound. Horrible tone. Sitting on a shelf.