fuzzy/pumping tremolo

[casssax]

The tremolo on this amp has a strange fuzzy sound to it.

I made a recording of it (starts with a few clean notes then I turned the depth up just enough to hear the tremolo)
http://www.unitedsound.us/Xm12$3&4/tremolo1.mp3

It also creates a pumping sound (can be heard at the end of the recording), not like the sound of regular amp noise being turned up and down but real thumping (like a helicopter in the distance) that becomes very pronounced when the depth is turned up.

When the tremolo is engaged and the depth turned up the output tubes flash blue in time with the tremolo speed. I guessing that it's probably not good for the output tubes in the long run to run tremolo for long periods of time like this.

Is this typical of this type of tremolo circuit or does it sound like there's something wrong with it?

Thanks.

[sluckey]

That's an oddball circuit. Where did you find it?

The trem signal is way overdriving the grids of those EL84s. Can you make it sound decent simply by turning the Intensity pot down?

There is no ground path for the EL84 grids.  1M grid resistors is way higher than I've ever seen on EL84s. The bottom side of the Intensity pot should be connected to ground. Is that just a schematic error?

[casssax]

It's an oddball amp and I didn't have a schematic so I had to make one. I think it's accurate but I'll double check.

The recording I posted is with the depth just high enough that you start to hear the tremolo.

Here is a picture of the pots in the tremolo circuit. There is nothing connected to the bottom side of the intensity pot.
(it doesn't look like there's ever been anything soldered there)



I can try connecting it to ground to see if it helps.

Do you think changing the 1M resistors to 500k would help?

[sluckey]

The left side of the intensity pot is connected to ground through the wire that goes to the footswitch jack sleeve lug. So, the schematic is just missing a ground symbol.

[PRR]

There has to be a ground at the switch jack.

There has to be a few hundred pFd at the junction of the two 22Meg resistors.

Overall, you may be discovering why "Stewart" didn't become a Big Name in the guitar amp racket. There's a lot of details which probably should be done different. If the amp is generally in good shape and does *something* well, you (IMHO) should not re-design it, but leave it as-built (fix what's broke) for posterity to ponder.