Need help identifying this amp

[The Dude]

I really can't take credit for the design. It's been around for quite some time. IMO, adding that resistor is just a quick and easy way to "fix" what I would call poorly designed bias circuits.

Edit: Silly me, I just now noticed you already had resistance measurements on your last drawing. Sorry, Tom!

[TomCarlos]

Team....

Just a follow up on the two mods I made on this amp - what I call the Version A plus circuit. As mentioned previously, this amp seems to have changes that puts it between a Version A and B.

I was able to utilize the pc board to update the bias circuit. But I needed to mount two additional resistors (see circuit). I used a very small drill (a mini hand drill) bit to add some holes for R2 and R3 (see my circuit diagram). For the bias, a setting of -55v with 488 on the plates yields 38ma for the tubes I am using – 65%. Worked like a charm.

I also abandoned the canned cap idea and opted to use Mod Caps for the Power Supply. I had a 20uf and 40uf on hand. I ordered two 30uf caps and put those in parallel for the 60uf.

I changed out all electrolytic caps.

In the Power Supply, I used a 1K and 68K for the dropping resistors. The (loaded voltages – with tubes in) I get 494v, 488v, and 302v. Close enough. The amp did have an 82K for the second dropping resistor but the Version B schematic calls for a 68K.

The amp is working and sounds ok. But I will go back to clean up the wiring and lead dress, I am going to open a new thread to get some help on the Tremolo. It's working but I have a couple questions.

Thanks!

[DrGonz78]

Check out this thread as it talks a lot about that tremolo circuit. Post #13 is Enzo explaining the circuit very well.

[TomCarlos]

Check out this thread as it talks a lot about that tremolo circuit. Post #13 is Enzo explaining the circuit very well.

Got it.... thank you Doctor G !!