Peavey 5150 suddenly squeals

[LamontGrady]

I got a script logo 5150 a few weeks ago, been having a lot of fun with it.

I installed a bias pot, 470 Ohm screen grid resistors, choke, and rewired standby switch as described in this article. It removed the "pop" but it takes over 10 seconds for the standby light to go off?
Choke Troubleshooting - Premier Guitar

Everything was working fine for a couple weeks, experimented with different tubes and a MXR Smart Gate and an ISP Decimator G String. Have been running it on two power tubes with speaker switch at 8 for a 16 Ohm cab. Slowly realized that one of the power tubes that came with it would crap out after 20 minutes of heavy loud pummeling. Gave it a real long loud workout with a nice newer pair of SED 6L6 and everything seemed peachy keen.

The Problem:
I started out the next day by plugging all my pedals in which I haven't done for a while. All of a sudden, I have an oscillation/feedback squeal! It's NOT the guitar, it's NOT the tubes. I don't even think I turned on a single pedal, it was immediate and obvious.

I got rid of the pedals immediately of course and swapped all the tubes. Two or four power tubes - no difference. After a lot of trial and error, it only happens when the Decimator G String is plugged into the effects loop and the guitar out jack plugged into the front of the amp. Smart gate in the loop works fine, but of course the smart gate does not create a connection between the effects loop and input jacks like the G String does.

I'm totally baffled. The Decimator seems to work fine on another amp I have that has an effects loop, but I'm not ruling out that it's the issue. The 5150 works perfectly in any conceivable configuration except using the dang Decimator! And it worked fine for weeks! I literally changed nothing about the amp from working fine to squaling except plugging in a chain of pedals.

I've had weird oscillation issues on other amps that seemed to be caused by a Radial Headbone VT (switches between two tube amp heads and one cabinet). But they mostly went away and only happened with the Headbone. I never used it on the 5150, I'm just saying that there IS a possibility that one of my pedals is hurting my amps?? The first thing I do to troubleshoot though is go straight into the amp if I have ANY problems because I've gone through phases where I messed around with pedals a lot.

The squeal only happens if the gain is up pretty high, on either channel but it does sound a little different on each channel. It needs a certain amount of total gain from the combined pre and post, turning either one up all the way will not make it happen unless the other is also up pretty high. If I turn the pre down a bit I can get away with turning the post up a bit, if that makes sense. On lead channel, if the pre knob is set in the right place, the post knob changes the pitch of the squeal as I have seen others describe. The G String blocks the squeal, so it's happening before the effects loop send. There is no resistance between the grounds of all the jacks. With the cables all plugged into the Decimator, as soon as the ground of either of the effects loop cables contacts the ground of the effects loop jacks on the amp, it starts squealing, it does not need to have either of them seated all the way in - it happens even when the tips are not connected.

It happens no matter what position the bright, crunch switches are in. The resonance and presence have no effect. The treble control blocks it if it's turned down to 2 or lower so I guess it's coming in before the tone stack.

The only other factor I can think of is that something weird happened once or twice when I turned the mid knob, I can't even really remember what, but I had planned on cleaning all the pots, then didn't bother because it never happened again.

Thanks in advance for any help or insight!


http://www.blueguitar.org/new/schem/peavey/5150evh.pdf

[Chuck H]

I doesn't sound like this problem has anything to do with the work you've performed on the amp. That said, this seems like more of a gear question than an amp question. I don't even know what a Decimator G string is, and I doubt very many builders here do. The fact that the Decimator creates a loop between a later part of the amp and the input, combined with the treble control but no later controls affecting the problem tells me that there is a phase shift or possibly a direct positive feedback loop being caused by the Decimator when used on that amp. You mention that the Decimator works with other amps. Do these other amps have the same gain within the loop being created by the Decimator? That's hard to know. Do these amps have as much top end as the 5150 within the loop being created by the Decimator? Also hard to know. My gut is telling me that this device just isn't compatible with that amp in the manner your using it. But you could ask this question over at "The Gear Page" or some other forum where guys talk about gear. With the exception that it may have something to do with a difference in effects loop placement in the circuit (which cannot be changed on that amp) or specifics of the effects loop or preamp design, this really isn't an amp circuit question. It's a Decimator as used with a 5150 question.

Chuck

[LamontGrady]

Good points, I'll scare up a link on the Decimator, it's definitely important to understand how it fits into this. The more I think about it, and after getting all my thoughts typed up like that, it seems like it's more likely the decimator than the amp.

The only thing is, it worked, and worked GREAT, for a couple weeks. Also I've just done a lot of thinking about it and this is a great chance to increase my understanding of amps.

I tested the Decimator quickly on a 1988 Carvin X60B, and I'm actually warming up my 1993 fuzzy x100b right now to see if there is any perceptible issue w/ the Decimator. Hmm my only other amp with an effects loop is also a carvin, a 1981 x60. This is when they were very similar to the Acoustic 165 with the preamp stages in series, it's a really gainy noisy amp and really it's quite unlike the other x amps so that should be a good test.

[LamontGrady]

Ok here's a pic of how I'm using the Decimator - "G String" version. I think the Boss NS-2 is the same way. It was pretty impressive, really lowers the noise floor into blackness. It tracks the guitar signal but "decimates" the noise in the effects loop. The smart gate does not get nearly as quiet, but also allows a lot more dynamics/quieter decays and that sort of thing through. The smart gate was sort of better at reading the guitarist's mind in terms of what they wanted to come through and didn't, but had a general level of (greatly reduced) noise it let through. Since I had both on hand, I set the decimator to be first in the loop, set pretty sensitive, but that leaves it with an annoying trailoff as it kicks in a bit too slowly, then put the smart gate after it to block that tiny little bit of barely perceptible noise, also set to really sensitive. It was pretty amazing! Super loud high gain tone with utter blackness between notes. It almost makes me dizzy or something it's so disorienting to not hear that loud amp hiss!

I emailed ISP too. Keep in mind I totally assumed I had borked the amp somehow at first, and only eventually realized the problem was attached to the Decimator. And ya can't post anything about squealing and weird noises without swapping out all the tubes and that sort of thing, 90% of the time someone has some weird noise it ends up being the tubes.

I can't really totally rule out the amp yet, right?

[Enzo]

The amp works fine when this thing is not connected, right? I tend to think it is not the amp's fault. If you get squeals, Id furst suspect connections and patch cords, also check th ground connections at every jack involved, yes including the ones in the amp.

Just because the unit works in other amps doesn;t make it blameless, the other amps may have grounding that masks a loss of ground in this unit.