The Amp Garage

Were you able to rebias the amp?
Does the amp have any distortion when the volume is low?

ViperDoc wrote: ↑Fri Jan 01, 2021 9:18 pm I noticed on another forum that the Silver Jubilee preamp tube plate voltages ought to be about 450 VDC. That sounds high, but what do I know? How do you raise it at the first filter cap without replacing the transformer?

What?

Can it tap off between the PI and preamp? Another resistor/cap...can split one of your last dropping resistors to the preamp into two resistors that add to the same resistance. Cap-to-ground and loop tapping off the joint of the two resistors.

sluckey wrote: ↑Tue Sep 01, 2020 3:28 pm

A high voltage FET or BJT set up as a follower at the input would be the easiest thing.

But a follower will not flip the phase.

you're right, my bad. Could do 100kOhm source and drain resistors and have an inverting buffer. Could get really exotic with that stage.

A high voltage FET or BJT set up as a follower at the input would be the easiest thing. If you use a N type DFET (like the LND150) the input doesn't need any DC bias, it can be DC coupled like the typical tube input so just an series gate resistor and a source resistor (100kOhm or so).

From the photo it looks like the resonance control is tied right to the speaker jack (resistor inside heat shrink tubing). Then the orange lead goes to the PI, I see a second orange lead connecting to the same node. Looks like the presence control has an orange lead going under the board also, very ...

You're right Roberto

One commonality to what constitutes a CS (cold gain stage, "cold clipping" stage, etc) is that there is no cathode bypass. So the gain stage is biased at a cold Q point but also there is heavy cathode degeneration. With the typical attenuation between stages these "CS" stages are not used for voltag...

One thing no one mentioned is your tonestack switch relay is right at you cathode follower, which has a reasonably large dc voltage. If you try to switch there, it will pop like a 44 magnum, and your relay isn't going to be happy with that dc either. You could use a large blocking cap before your r...

Sounds like a short inside one of those tubes; screen to suppressor since the screen drawing a ton of current but no B+ fuse blow. Could multimeter your good EL34 pair between the G2 and G3 and compare to the 'bad' EL34 pair, maybe one of those will give some wack reading on your meter. Really sucks...

In a 50W head I recently built with quite a bit of feedback (typical Marshall output stage with 33k from 16Ohm tap). With a DPDT (ON/ON/ON) switch I've got the resonance cap value switchable: 14.3nF/4.3nF/1nF. 1nF - starts to exit the resonance range (midrange boost) 4.3nF - where I tend to leave it...

Whipping up a batch of pedals for my bandmates and myself. The circuit is loosely based on a preamp with some compressor fun. Let me know what you think, ideally I would run this in front of an amp on the edge of break up.
https://www.youtube.com/watch?v=G3pe4k6TClI

Did some math just on the ratio of preamp to PI output gain. The PPIMV changes the open loop gain of the power amp and the only way to get similar power amp response with low open loop gain would mean crazy NFB. I believe it is unrealistic and would require cascaded long tail pairs where the preamp-...

Here's that page for ease for more people to jump into this thread.

How much modding do you want to do? What speaker is being used with this amp? Initially I see that the photo cell input has a level control, would be cool to have that pot used for a divider between the halves of V201 (R203 in place of R207). The tone circuit is active about those two triodes. Havin...