Hi all,
I came back to this thread to ask a question, but started by just re-reading the whole thing as I think I know a bit more now than I did back then, I'd like to say thanks again to everyone who helped me build up a bit more knowledge, little by little. An Happy New Year everyone too!
So on to the questions
Recently I built a couple amps from old tube stereo consoles. My more recent one is from an Admiral stereo which has the power amp and supply in a separate chassis from all the remaining AM FM, turntable and input selection stuff. That made for a cleaner chassis to start from. And I was pleased the power amp is a push pull EL84 pair (per channel) with a cathodyne phase inverter, instead of single ended like my other stereo project.
I grafted a clone of my Mark1 to that power amp, using the closest value components I had in my leftover parts from dismantling the previous stereo consoles. I simplified a bit, combined the two gain knobs into a double pot (stereo consoles are full of double pots, to affect the two channels at the same time), and eliminated the second input, having all 4 stages always in use. I like a lot how the amp sounds, the controls have quite a drastic range of control but I can find a lof of very good lead tones. And like my real Mark 1, the presence control is of great use to fine tune the tone (which is more like a high frequency roll off control as we discussed previously).
But now I'd like to add an effect loop to add some reverb. I plan to use my Mk1 circuit, but looking at it now, I wonder if I should eliminate the 0.12uF cap at the start of the circuit. It is in series with the existing 0.047uF coupling cap, so it seems like it would do nothing, since it is already coupled, and moreover with a cap of lower value. As I understand it, the 0.047uF already blocks the too low frequencies, while the 0.12uF lets pass more (but the frequencies are already removed).
Maybe it is there because the person designing it as a stand alone FX loop circuit, without consideration of what is connected before and after it? Having high coupling cap value to pass the frequencies mostly unaltered. I re-drawn the 4th stage of the Mk1 circuit on the right, but the FX loop circuit really starts between the 220k and 0.12uF cap.
The rest of the circuit I am pretty happy with, so I plan to reuse as is, located between 4th stage and master pot.
I added some pictures of my frankenamp
Since those pics I added some shrink tube to exposed wires and solders, replaced the old filter caps. The wooden box I made out of the wooden panels from the stereo consoles, made it pretty rough just so it is not open chassis anymore.
Thanks all!
Fred