Emergency help with Wes' amp

[mashaffer]

Help!, I had Wes' amp running very well but the bright channel was a bit too bright and the first filter cap seemed a bit large so I made the following changes to the attached schematic.

Replaced 30uf first filter cap (C1) with 22uf unit.
Replaced plastic input jack with switchcraft.
Replaced 500pf C11 and C12 in bright channel with 250pf.
Replaced 500pf C6 treble cap with 150pf

When I first powered back on I got a terrible oscillation but found that I had failed to solder one of the 250pf cap connections and failed to connect the jumper from S1a to the input jack.I fixed those errors and the squeel went away but now I have a moderately loud crackle and hash for a couple of seconds whenever I flip the standby switch on or off.

What could be the problem? I don't know where to look and I will only be here for a couple more days.

mike

[kevinkr]

Look for obvious errors then undo one part at a time starting with the 22uF cap, until you find the cause of the problem....

Another possible thought is the filament voltage thing you are doing. I have not seen that commonly done in guitar amplifiers, and am not sure how happy any of these tube filaments are operating on halfwave pulsed dc during standby.. (probably somewhat less than 50% of nominal supply voltage across them, so about 1/4 heating power as a result.) Part of the problem might be due to filament warm up, I would expect them to take some time to come up to full operating temperature from standby. Series string tubes used in instant on TVs were presumably designed for this sort of operation, standard audio tubes really weren't.

Normally you wouldn't leave the amplifier on continuously in standby mode when not in use so I am not sure I see any point to this feature?
Cathode poisoning might be the result either way.

[mashaffer]

It turns out the the squeal is still there it was just hiding. I tried a few things and have narrowed it down.

I pulled all of the preamp tubes and the squeal stopped. Putting the PI tube in brought it back. I put a shorting plug into the power amp in jack, which shorted the PI input to ground, and it went away again. I am guessing that the 150pf value of C6 (treble cap) has turned the tone control stack into a tank circuit making the PI an oscillator. Do you think this is a good possibility?

mike

[kevinkr]

Does the position of S1 play any role in the squeal problem you are having? I'm thinking capacitive feedback through that switch.. You might want to rethink the stage switching scheme if that is the case.

[mashaffer]

I have only tried it in the parallel mode however I still had the squeal with only the PI tube in place so the switching circuit would have been isolated at that point.

mike

[mashaffer]

Actually I haven't implemented the low heat standby and probably won't.

I was able to fix the problem by putting the 500pf cap in the tone stack back. I didn't bother trying any intermediate values. We tested the mods and the changes in the bright channel itself was adequate. I will be tidying up some of the wiring and buttoning it up tomorrow morning.

I appreciate the help and moral support. I was near panic at first.

mike

[mashaffer]

While we are in there cleaning things up I had thought to clear up one minor problem. The gain pot for the bright channel makes a rather loud click when turning all the way down. I was thinking of putting a 1M resistor between the wiper and ground side of the gain pots so that a glitch in the wiper would not allow the grid of the last gain stage tube to float.

I figure that the possible side effects include a change in the taper of the gain controls and lowering the input impedance of the following stage. At first glance it seems that we should have enough wiggle room on the impedance and it is easy to try so we will probably give it a shot unless someone thinks it might be a bad idea.

mike