Peavey Valveking feedback troubles

[bobshbob1]

I posted on here a little while ago about trying to improve the sound of my Peavey Valveking head, basically to make it a little less muddy. Since then I have made some minor changes to it. I changed C149 from 1 nF to 220 pF, put in new preamp tubes (2 Electro-Harmonix 12AX7s for the preamp and a Tung-Sol 12AX7 for the PI), and added a bias pot (swapping the 39k bias resistor R205 for a 15k resistor in series with a 50k pot). It's now biased a little hotter and I feel like the sound has definitely improved.

The issue I'm having now seems like a microphonic tube, but I'm not sure. After I had adjusted the bias to about 35mA per tube, I started getting a high-pitched squeal on both channels, even with no guitar plugged in, starting at about 5 on the lead volume and 8 or 9 on the clean volume.

I had 2 spare 12AX7s, used but definitely still working normally, and went about swapping tubes trying to isolate the problem. With both these spares in place of V1 and V2, I only got this squeal when the guitar was plugged in with its volume turned all the way down, on the lead channel, with gain at 10 and volume at 7, or about 5 when the gain boost was engaged.

Then I thought, what the heck, and put the original tubes back in. Now I can't reproduce the squeal with the guitar unplugged, but it still comes back as before, with fairly high gain and volume settings. The tapping-on-the-tubes-with-a-guitar-pick test didn't produce any unusual sounds either, at least with the guitar unplugged.

I'm wondering if this indeed sounds like a microphonic tube problem, or if it might have been caused by something else. I had initially set the bias to a cold 15mA per tube just to see how it sounded, and played with that setting for about an hour before running it up to about 35mA per tube. So my other thought was that this could be an effect of the tubes settling into this new operating condition.

I'll post a schematic in case it helps; any thoughts on this issue would be very much appreciated as I'd rather not have to take this amp into a tech if it's just a minor fix.

http://music-electronics-forum.com/a...ng_100_212.pdf

[52 Bill]

Seems more like your amp is unstable and is breaking into oscillation at higher gain settings. This may be a result of your mod of the cap value or it may be something that was moved or broken when you changed the cap out.

Retrace your steps and see if there are any wires out of place or if any have come loose, or have bad grounding, etc.

[bobshbob1]

Thanks for the tip; I've never actually heard a microphonic tube in person so I wasn't sure if this was the cause or not. I played the amp for a little while earlier and it sounds normal otherwise, so I will re-check my connections and maybe try returning that cap to its original value, or putting a resistor in series with it as someone on here suggested. It's definitely an issue that comes and goes like a bad connection, like once it starts feeding back I have to turn the volume almost all the way down before it goes away, and then it doesn't start again until the volume is returned to almost max...hopefully it's something I can find fairly quickly.

[oc disorder]

Just thought I should point out that a microphonic tube is a physical vibration
ie the valve innards can vibrate acting like a microphone so thats acoustic feedback
as the speaker reproduces it and it gets picked back up again by the sensitive pre tube
acoustically whereas what Bill 52 is mentioning is induced electrical oscillation (feedback).
eg If you have a poorly shielded cable and you drape it over a speaker cab connected
to a very hi gain amp its possible for the signal to couple from the speaker through to the inner core of the lead.Although this is a rough explanation the speaker and input lead are acting like a primitive transformer the two are not physically connected but
the radiated field from the speaker is being picked up by the sensitive input.
This can happen inside an amp the output being picked up again by the input hence oscillation.
When the guitar lead is removed the input jack should close shorting the input or connecting the hot or tip to ground.
Check that is the case and that all stray wires lie close to the chassis and that all grounds are good.
I notice also that the Peavey ValveKings don't appear to have a ground shield under the chassis like Fender and Marshall do. This is usually a thin sheet of aluminum (sometimes foil ! ) stapled or glued to the bottom of the case (or top if its a combo)sheilding the guts of the amp.
Doubt they use conductive paint as its more expensive but shielding could possibly solve the problem. I use thin pieces of aluminum I get from a mate who works at a printers they are chucking/recycling lots every week used in the printing process often has some logo or printing on it .
Doubles as a wobble board although this is an instrument peculiar to Australia and made famous by our culture ambassador Rolf Harris whom is probably not that well known in the States as compared to Europe!!! (eg "Tie Me Kangaroo Down Sport" and other great hits ! )
Lettuce no how it went :-)

[bobshbob1]

Interesting; I didn't see anything out of whack inside the amp but I also haven't put it back in the head cabinet, either. And if I set the amp so it's just starting to oscillate, I can move it farther away from the speaker and the oscillation goes away. I like the sound much better with a smaller C149 so maybe adding more shielding inside the head cabinet will do the trick...I'm actually pretty sure I have some copper shielding left over from putting inside a guitar a while back.

[bobshbob1]

Well, I added shielding to the bottom of the head cabinet, but that didn't help very much. Still oscillates at high volume and gain (the amp pretty much lives with these almost maxed out), but does it with some guitars more than others, and does almost nothing when a cable is plugged into only the effects return (not that I expected it to). Next step is to restore C149 to its original value of 1nF and see if that helps; the local RadShack doesn't carry this value, so I'll have to wait until I have a chance to go to a better electronics supply store (the nearest one has the exact same hours as my work schedule unfortunately).

I'm kind of a n00b when it comes to modding amps, but I was thinking...since the resistor going to the bias pot and the wire going from there back to the board are the only places where the signal really goes out of the plane of the board, so to speak, where it didn't before, could that contribute to the circuit picking up excess noise and feeding back? I was thinking about switching the bare wire for a shielded wire, grounded to the body of the pot, but I don't know if this would actually make a difference...would it?