Low power Fender Twin Reverb AB763?

[Diverted]

Hi all,

I'm going to be building a 45w or so, clean as possible amp for a friend and I've been reading a lot about the AB763 blackface circuit, which went in many (all?) of the 60s Blackface amps.
All of the various models seem to be mostly what he might want, but the Fender Twin appears to be closest.  Super clean, high B+, solid state rectification. Only problem is he doesn't need or want all of that power. And he wants it minimalist. So I'm thinking of building it with:

1. Two 6L6GCs instead of four.
2. Single 12" speaker
3. Single channel; no vibrato channel.

I'm going to take the twin schematic and modify it to reflect these changes and will post it here, and hopefully you experts can critique it for me. Until then, has anyone built something similar? I would love to hear from others who have tried this, and what they think about the end result.

Thanks!

[sluckey]

Once again...

I would build the Hoffman AB763 One Channel. Just delete the trem circuit.

[Diverted]

Thanks Sluckey. Sorry for the redundancy.
I'm just trying to make sure I build the best version based on what he wants. With so many AB763 circuits (Bandmaster, Showman, Twin, Pro, Tremolux, Vibrolux etc) and all being slightly different with various preamp and B+ voltages, etc., I wanted to get one as clean as possible from within that list and from all I read it seemed the Twin is cleanest.

That said, if the Hoffman AB763 layout is the way to go, that'd be great. Just one question: Apart from the correct output and power transformers, is there anything else I need to do with the schematic to accommodate 6L6GC tubes, that's not shown in the literature here? Thanks!

[sluckey]

I know you are interested in a single channel model, but look at the documentation for the two channel model. Some parts are different between models. Hoffman shows all the model specific parts in blue on the schematic. Then he refers you to the BOM for particular info on these parts for each model. I only mention Hoffman's AB763 because it is a proven circuit/layout and is widely known. It would be very easy to adapt to your specifications.

In your first thread about this project you said you wanted reverb, but there is no mention of reverb in this thread. And you specifically said no vibrato channel. Reverb lives in the vibrato channel, so be specific. Do you want reverb? If yes, then you will need to build the vibrato channel but strip out the stuff for vibrato. That's very easy to do on the Hoffman single channel but is a little more difficult on a real Fender AB763.

Now if you don't want reverb or tremolo, you may be interested in my AB763 Deluxe Lite. Just use transformers for a 2x6L6 amp such as Super Reverb or Concert. Put in your ss rectifier and change two resistors in the power supply. Maybe change the 100Ω feedback resistor to 47Ω depending on your speaker configuration.

http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf


The Fender Concert comes closest to matching your specifications. Well worth a look-see.

[jjasilli]

In addition to sluckey's good suggestions: clean also comes from the preamp.  The 1st gain stage sets-up the rest of the amp.  E.g., an 820 Ohm K resistor in the 1st stage enables it to take up to 1V of signal voltage without distorting.  The point being that if the 1st stage gets overdriven, or it easily overdrives the next stage, then all other attempts at clean tone are futile.  (Of course, this limits the versatility of the amp, unless other measures are taken.)

[Diverted]

Do you want reverb? If yes, then you will need to build the vibrato channel but strip out the stuff for vibrato. That's very easy to do on the Hoffman single channel but is a little more difficult on a real Fender AB763.

Thanks Sluckey. Sorry, yes it's got to have reverb. As far as removing the tremolo from the tremolo channel, I'll look at the schematic and work on it. Thanks! I'll be back.

Ted

[92Volts]

With so many AB763 circuits (Bandmaster, Showman, Twin, Pro, Tremolux, Vibrolux etc) and all being slightly different with various preamp and B+ voltages, etc., I wanted to get one as clean as possible from within that list and from all I read it seemed the Twin is cleanest.

FWIW, voltages in actual Fenders of that era weren't consistent. Voltages on schematics should be taken with a grain of salt because of normal variation between amps, not to mention differences between production parts vs. what the design team was using.

With that said, higher voltage equals cleaner, including in the preamp. With larger caps you could use smaller power supply dropping resistors, even a lower-resistance choke to achieve the same ripple reduction with less voltage loss.