AB763 to Trainwreck

[tsl602000]

Hi all,

I have this AB763 amp that I built quite a few years ago, but just doesn't do it for me anymore.
First I considered transforming it into a dumble clone, but after seeing/hearing the youtube video I'm completely hooked on that Trainwreck sound.

I'm pretty much a noob, so please excuse my ignorance...

I compared the schematics of an AB763 and version A1a of the wreck and noticed the pre-amp section of both schematics is pretty close.

The big difference is in the power amp section and I was wondering if anyone has experience with a wreck pre-amp and an AB763 (Bassman) power amp section.

I'd also like to know if I could/should use El34's. The OT is a Hoffman 018343 Bassman replacement transformer that's designed for 6L6 tubes.
I'm aware the tube sockets have to be rewired to accept EL34's, but I have no clue if this OT is appropriate to hook it up with EL34's.

Thanks in advance!

Coen

Edit: I just found out this transformer has a Primary impedance of 4200CT.

[Rick]

I don't know much about the AB763 regarding conversion to TW, but I just last night switched out a Hammond 5K primary impedance output tranny for a universal 4K ohm primary impedance unit, and the amp became entirely more controllable with out all the problems I had with redplating output tubes, and feedback from the negative feedback loop causing squealing in the input circuits or whatever was going on. ???

It has lost some of the violin like tones it had, but the improvement in power handling may be worth it. I may try the 5K in a TW circuit again before too long, but the apparent mismatch problems I was having really cause me to wonder if it's worth it. I guess this mismatch is somewhat the source of the unique TW tone, but I really had some unfavorable reactions to it, especially regarding the power tubes.

Some early TW's also used 6.6K primary impedance in the OT too I believe. This really is a experimental/learning type of amplifier building process with the TW. Well worth it though. You will learn a lot getting one of these going.

I'm sure you'll hear more fromt the old hands regarding this, I hope, and good luck!

[tsl602000]

Thanks for your reply, Rick!
I'm sure other people have noticed, but the Trainwreck's pre (A1a version) is almost identical to a AB763 pre-amp. Only a few values are slighty different.

Anyway, I hope some of the "old hands" come in and can answer my questions. I really appreciate the help.

Coen

[doctord02]

The big thing to consider (IMHO) is reducing the PT plate voltage to the Wreck's sweet spot of +/- 10V from 400V. The OT should ideally be either 5.2 K or 6.6 K (depending on who you talk to, I have both, and both sound fine...), but dropping the voltages will make all the differance in the world... I'd also stiffen up the filtering like the TW does while I was at it.

The A1a schematic is mighty close, but take a look at the Kelly/Franchesca revisions for the dead-on reverse engineered accurate versions.