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Old 18th October 2011, 10:05 PM   #1
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Default A Pair of Peavey Perplixities

About a century ago I had a Peavey "Blues Junior" brought into my amp shop. It had / has an oscillation of 44kHz present all the time it's on with or withou any signal applied. I've been told that this is fairly common especially in the amps with the cream colored PCB. The problem with this one is that when the volume pot is turned up the frequency of the oscillation decreases until (at about 4 on the scale) it gets down into the audible range and makes an annoying scream like some sort of animal in heat!

I went to some forums with it and got all sorts of good ideas. I tried them, all. Make sure the HV lead going under the ribbon wire is exactly perpindicular ... purpen ...pirp ... at right angle to the ribbon wire. Nope. replace all of the ribbon wire with single wires and use shielded wire to connect all grids. Nope. Squirt silicone on all places where wire go through the PCB. Nope. So I set it aside (with the customers permission and waI have been waiting until another came i. It did and this "blues Junior" had a different problem all together. So I did some tests on it while it was there. In the second one the oscillation was again always present but on it there was no difference when the volume pot (or any other) was turned up. The oscillation stayed at the 44kHz.

Today I got in a "Delta Blues" amp with the same problem. Same frequency and scream but this one is effected by the treble pot and the reverb pot a little bit by the mid pot (chassis is on the bench with no tank connected. It was connected however and there seems to be no difference with or without it.

Obviously, well maybe, or do you think some thing is/could be making the pot "tune" whatever the source of the oscillation is. Any knowledge or suggestions??
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Old 18th October 2011, 10:21 PM   #2
Pano is offline Pano  United States
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Old 19th October 2011, 02:24 AM   #3
Enzo is offline Enzo  United States
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I think you mean a Fender Blues Junior?

I find it highly suspicious that two completely different amps made in different factories have the exact same RF oscillation. There should be no oscillation. One wonders if there is something in your electrical environment? MY shop is a half mile from an AM broadcast station transmitter. I can easily watch their modulation envelope on my scope.

When amps like this oscillate, or most anything I suppose, it is a matter of feedback through the circuit, unless it is picking the signal up. Turning the gain up and down or the EQ controls affects the freq response of the overall circuit, and so that would affect the freq of the oscillation. A common cause/effect.

Delta Blues is the same circuit as a Classic 30, so any tech tips for the C30 would apply, something to remember while searching.

But forgetting everything else, look at the Delta. Isolate the problem. IN what parts of the amp does the oscillation exist, and what parts not? Due to the series heater strings, it is difficult to pull tubes. That technique is powerful most of the time, but not here. And that folded circuit board arrangement is also inconvenient. But by grounding points on the circuit path, we can see if we can break the feedback loop. Got DC and can't ground some nose? Use a cap. A .047uf or 0.1uf cap to ground should kill any 44kHz going on. Scope the B+ nodes. Is the oscillation riding them? WHich ones? Make sure all the jack nuts are snug. A jack lacking its chassis ground can cause issues. Plug the guitar into the FX return and zero its volume. ANy effect on the oscillation?

Does the amp oscillate with nothing plugged into the input? With the zeroed guitar still in the FX return, scope the FX send. Still oscillating?
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