Infamous 6505+ 112 Combo squealing with high gain

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Creamymemedude
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Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Sun Jul 23, 2017 9:25 pm

I know this is a topic that's been visited on here before, but upon viewing every single thread on every forum that's even slightly related, I've still come up short.
The issue I'm describing is as follows: When the gain is turned up past 6 or so on the lead channel, I get a squealing microphonic sound. The sound is muted when a noise gate is put through the fx loop. It also persists when a guitar isn't plugged in.
After browsing the forums I tried a couple different things. I replaced the preamp and power amp tubes, all properly biased and whatnot. I also cleaned out all the sockets with contact cleaner. I'm positive that there is nothing wrong with the tubes that I put into the amp. This did not work. I also went and replaced the filter capacitors as provided by Peavey to no avail.
As one can imagine, this has been very frustrating. If anybody has any experience with this sort of issue or knows a lot about this amplifier, you input would be greatly appreciated. In essence, I've narrowed it down to an issue with the preamp system. However, it's not the tubes that are causing the problem. My next best guess would perhaps be a faulty socket or something wrong on the motherboard. I know a bit about electronics but not too much, so if you can dumb it down as you attempt to assist me it would be very nice of you.
Thanks a bunch in advance,
Max

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Enzo
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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Enzo » Mon Jul 24, 2017 3:37 am

Since everything is on the boards, it is a good guess the problem lies there.

It can be frustrating to change lots of parts hoping to hit upon one that solves a problem, but what if the problem is a broken connection? Change every part on it and the break remains.

So the amp oscillates when turned up without a guitar. Does plugging the guitar into the FX return stop this?

WHen it does this, reach in and grasp each preamp tube firmly, one by one. Does holding any of them affect this at all? Push down (in?) on each tube, to put a little flex on the board, any affect?


This only happens on the lead channel? Or will it do it on the clean as well?

Lead volume matters, but how about lead gain?

Creamymemedude
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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Mon Jul 24, 2017 2:03 pm

Enzo wrote:Since everything is on the boards, it is a good guess the problem lies there.

It can be frustrating to change lots of parts hoping to hit upon one that solves a problem, but what if the problem is a broken connection? Change every part on it and the break remains.

So the amp oscillates when turned up without a guitar. Does plugging the guitar into the FX return stop this?

WHen it does this, reach in and grasp each preamp tube firmly, one by one. Does holding any of them affect this at all? Push down (in?) on each tube, to put a little flex on the board, any affect?


This only happens on the lead channel? Or will it do it on the clean as well?

Lead volume matters, but how about lead gain?
Hey Enzo, thanks for the reply. Upon trying these things out I came across some very, very weird things.
First off, I should note that this problem only occurs when the gain is turned past 6, not the volume. This also only happens on the lead channel, not at all on the rhythm.
When I plugged into the return, the signal was clean and without the issue whatsoever. When I fiddled with the preamp tubes I didn't get anything different out of it, however when I played with the board it did change.
Since I unscrewed the motherboard from the chassis, I was able to lift the corners a little bit. When I lifted a corner, which in turn disconnected a screw with the metal chassis, the pitch changed. I could go back and forth and it would alternate in pitch. Not only that, if I would touch the 6th pin on the v3 preamp tube, it would also change pitch. The weirdest thing, however, was directly behind the gain knob that I'm having issues with. There are a few reisitors and a capacitor behind the knob, these being C30, R63, and R64. As I would move my hand closer to these resistors/capacitors, the sound would also change pitch. Without me even touching them, though! This effect is very much like a theremin, which is very cool but very annoying and I wanna find the cure for this. Could perhaps one of these small parts be causing the issue? When I actually touch them and cause a short, I get a static-y sound but it actually stops the oscillation noise.
Very weird indeed...
Thanks again for the response, I really appreciate it.
-Max

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by ScottMarlowe » Mon Jul 24, 2017 4:48 pm

Yeah sounds like a bad connection to me. Try poking things with a wooden chopstick (not a pencil they can conduct) and see if any components you poke affect the sound. Could also be a connector acting up. Peaveys tend to have a lot of failures of their connections.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Enzo » Mon Jul 24, 2017 7:19 pm

The amp is oscillating, and your hand is acting as a capacitor in that oscillator.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Mon Jul 24, 2017 10:51 pm

Thanks for the replies guys, upon my "poking around" it appears that the R64 resistor is making the most noticeable change in noise as I poke it on the right side (facing from the back of the amp).

You guys brought up that it may be a bad connection, would you mean any connection with these resistors and capacitors in the area? Or could you mean virtually anywhere on the board?

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Enzo » Mon Jul 24, 2017 11:22 pm

COuld be anywhere, the main job of troubleshooting is to isolate the problem.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Tue Jul 25, 2017 12:06 am

What would you suggest I try next? The resistor I'm looking at appears to be attached securely but maybe it just doesn't work anymore. They're cheap so it's worth a try, but if anybody knows with any certainty what could be causing this it would be great.

I saw a different thread which seemed to describe the same exact problem that I'm having, however the only person who gave a reply owned the head version rather than the combo so when they referred to the schematic it didn't line up properly. Good to know I'm not the first to have the problem, but the solution doesn't seem to be easy to define. I'll keep poking with it and keep you posted.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Enzo » Tue Jul 25, 2017 3:13 am

I might resolder the existing resistor, and I would keep in mind pushing in it also flexes the board and nearby connections.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by ScottMarlowe » Tue Jul 25, 2017 4:28 am

If you've got a loose or broken component causing this, you'll usually see a LOT of change moving it around, or none because it's effectively out of circuit. Crackling and popping etc are what we're looking for. If the pitch or volume vary a little that's likely just you affecting the capacitance of the circuit etc.

And you're not trying to prybar components. Just a light tap will tell you if a connection is iffy.

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Tue Jul 25, 2017 11:50 am

With a lot of poking around, I haven't noticed change other than a slight change in pitch around that one resistor. Perhaps this would mean whatever is causing the problem is, as you said, out of circuit. Any other possible suggestions?

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Wed Jul 26, 2017 2:35 pm

Hey guys, little update here.
I believe I've found the source of this error. It appears to be a smaller capacitor that's causing all the hubbub. I have a feeling it's either poorly connected or is just dead. The problem is, it's covered in some silicon glue and I have no idea how to get that out of there, or really how this capacitor was supposed to be installed in the first place. When I put a voltmeter up to it I got some cracks and hiccups, and when I touched it with my hand (yes very stupid of me), the noise would actually stop. The signal from the guitar would continue to go through as normal, which I believe to be a sign that this capacitor is dead. I'll attach an image for you all to see.
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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by ScottMarlowe » Wed Jul 26, 2017 3:08 pm

It looks like it's not connected on one end...

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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Creamymemedude » Wed Jul 26, 2017 3:35 pm

Should I try to take out this glue stuff and reattach it? Another odd thing, when I touch the 6th pin of the v4 preamp tube, the oscillation stops completely. See the video attached:
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Re: Infamous 6505+ 112 Combo squealing with high gain

Post by Enzo » Wed Jul 26, 2017 7:13 pm

Your body capacitance is affecting the oscillation.

The glue LOOKS like hot melt to me, see if a hot iron near it melts it. A way to remove hot melt is to spray freeze spray on it, that makes it brittle and you can shatter it away.

The loose cap looks like C11, and I suspect the floating end was to be soldered down to the copper spot, just to the right of the wire lead.

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