Two-channel > Harmonic Tremolo > Reverb > Low Watt Amp Build

Snfoilhat

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Design goals: harmonic tremolo and spring reverb, a power amp that sounds good in the work room and hits peak goodness at a volume only a little above that, and mid-sized combo cab built from some reused material.

I found this mahogany that had once been a very simple bookcase, with just a few holes that make me think it was pegged together. 11.25" wide by a bunch of feet long.
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I chose a repro Bassman chassis for the extra depth vs. something like the Deluxe Reverb, and then began working out how everything would fit.
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I have a 12" speaker in mind but want to be able to use a 15".

I looked at the source material for proven circuit 'modules' for harmonic tremolo and reverb and the rest and counted up the tube complement and their voltage and current requirements, then picked transformers.
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Then I set out adapting the Bassman chassis to work with the power transformer, tube sockets, controls, and other features of the new amp.
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Here's an idea of what the circuit will look like. You can see it draws on the brown and black Fender amps, and the only really unknown parts are the voltage dividers that join each familiar module, because I'm aiming for a smaller signal at the last stage of this amp than the Fenders were. Also, driving the cathodyne phase inverter directly from the plate of the oscillator instead of the later brown Fender arrangement of oscillator > cathode-follower > cathodyne is something I've seen others discuss but haven't yet done. I took notes from the earliest brown Fenders and think I've hybridized the 4-triode and 5-triode versions -- we'll see!
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First draft, untried

I'll post more progress (or the opposite) when I can. Thanks for reading.
 
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Snfoilhat

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If this goes according to the first plan, the farthest opening in the chassis will fit a 50+50 μF @500V capacitor. If I can't figure out how to remove the 5th triode of the 6GX-A harmonic tremolo, it leaves a bailout option for one more tube socket.
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Haven't done the layout yet but cut myself a board for the available space.
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Starting to draw the component layout.
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Cheat sheet to start cutting boards for the cabinet.
 

Snfoilhat

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Dry fitted the four sides to see if the rabbets came out ok, tried to catch any huge mistakes.
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Making sure to include the obligatory little mistakes like this router gouge.
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I find it easier to locate some of the holes while everything is still square.
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BigDaddy23

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Your cabinets are always a work of art. The natural timber looks fantastic…..great job. I’m watching your bass amp build with interest too 👏
 

NTC

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Subscribed. You had me interested with "harmonic tremolo" and set the hook with mahogany.
 

Snfoilhat

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Thanks for the kind words!

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Here's two applications of danish oil. Its beginning to get a little shine.
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After trimming the top, I decided to use the offcut as a back panel. Gotta clean up the glue line, install the speaker and take a listen.

Experience warns me that the grill cloth I think is straight enough today will look wrong tomorrow, so there's that to look forward to as well :)
 

FenderLover

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...Also, driving the cathodyne phase inverter directly from the plate of the oscillator instead of the later brown Fender arrangement of oscillator > cathode-follower > cathodyne is something I've seen others discuss but haven't yet done....
Yeah, that was mine:
post #76.

I'm glad you decided to try it out, I never got around to it!
 

Snfoilhat

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@FenderLover that was it, thank you!

Here's the last of the cabinet journey and the return to the electronics.
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I would usually just buy an Eminence, but prices have been weird. This was half what I've seen comparable speakers asking recently.
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Layout, lead dress, and however many hours ahead of trying not to have a lapse in concentration resulting in melting those heater wires' insulation with the side of the soldering iron. It's only going to get messier.
 




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