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Discussion Starter #1
I have an interesting problem with a Mesa DR Maverick head. I get an oscillation (loud squeal) under the following conditions:

- on lead channel only
- master and output levels irrelevant
- oscillates when volume (ie preamp gain) is at 1 o’clock or above
- ONLY oscillates with guitar attached!
- does not oscillate with just instrument cable attached! just when you connect guitar
- squeal changes tone with tone controls
- in some but not all cases, I can suppress the squeal with guitar volume pot control ie squeal on at full or no volume, suppressed in a narrow middle guitar volume pot range

I’ve tried the following so far with no new results:
- multiple instrument cables of multiple lengths
- multiple guitars
- swapped the 4 EL84 power tubes

Lead channel shares some preamp tubes but also has a couple of dedicated preamp tubes (V2 and half a V3), so I should try those. Trouble is, it is hard to get to those — need to take the chassis out of the cabinet, so a bit of a pain.

Any ideas?

(except “don’t set volume above 1 o’clock” or “don’t connect a guitar” :))
 

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its possible you've got a preamp tube gone microphonic in your lead channel. the other thing i wonder is if the shorting input jacks are worn out/dirty. If the contacts on the input jack arent making good contact they can squeal.
 

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Discussion Starter #4
Thanks for the ideas, @WCGill and @sammyr. I did think it might be a microphonic preamp tube and as soon as I get a chance to get at the preamp tubes, I’ll check that out.

The two things I can’t fully rationalize, however, is why a guitar connection (not just instrument cable connection) would be a contributor and why guitar volume at mid level would squelch the oscillation? Those two observation would seem to point to a conclusion that lower-than-infinite pickup + volume pot output impedance is part of the effective rogue oscillation circuit and if that’s true, how would either of your suggestions fit into the theory? Genuinely asking.
 

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The two things I can’t fully rationalize, however, is why a guitar connection (not just instrument cable connection) would be a contributor and why guitar volume at mid level would squelch the oscillation? Those two observation would seem to point to a conclusion that lower-than-infinite pickup + volume pot output impedance is part of the effective rogue oscillation circuit and if that’s true, how would either of your suggestions fit into the theory? Genuinely asking.
What sources have you been studying recently in order to gain all this knowledge of electronics theory ? Very impressive!

"Genuinely asking" here also (I like that phrase).
 

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First thing to try is replace V2. It is the preamp tube for the lead channel. If that doesn't work, I suspect you might have a bad ground or a bad capacitor somewhere. Having the guitar plugged in is completing a circuit that is basically becoming an oscillator around V2. Suspect parts are V2 itself, the bypass cap on V2's cathode (pin3 or pin 8), a cap from grid to ground on pin 2 of V2, or the filter cap in the power supply that feeds V2 (and V1).
The cap on the grid of V2 is interesting just because of its value. It's awfully big, which tells me this circuit might not be completely stable.
The other interesting part is in the power supply caps. When you switch from lead to rhythm or vice versa, a group of resistors changes the voltages, and the operating points of tubes V1, V2. Normally this would cause low frequency issues, but with a circuit like this you never know.
 

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I was stumped by the OP's follow up question and didn't have a chance to look at the schematic so I was hesitant to wade in. Thanks for saving me some potential embarrassment Dan!
 

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Discussion Starter #9 (Edited)
These continue to be great comments, thanks again everyone. And thanks @dtsaudio -- once you said it, it became pretty obvious that the guitar simply completes this parasitic circuit. Oscillation and rogue signal paths can be exactly weird like that.

Not solved yet, but I can add a little more from yesterdays' investigation:
- the reverb circuit and/or tank are involved in the oscillation signal path
- when I took the chassis out (incidentally disconnecting the reverb tank), the oscillation went away (of course it would! geez)
- fully assembled again, when I disconnect the reverb tank, the oscillation disappears, so confirmed
- if I remove the reverb driver/recovery tube (V5), the oscillation goes away (and, duh, no reverb)
- I replaced the reverb tube with another, no difference, oscillation is there

Based on this discussion, I think the reverb path is just another part of the rogue oscillation signal path and not really a clear or direct indicator of the underlying issue. I will get the chassis out again and have a look at the items @dtsaudio suggests.

Update: Actually, as I finished writing above, another thought occurred. Perhaps reverb tank is a pointer to root cause. Reverb tanks often break the circular ground path by only attaching one side to ground i.e. intentionally, only one of the RCA plugs are grounded. What if that's not the case here and I have reverb tank ground loop ...
 

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long shot as I'm not looking at any schems but have you tried cleaning the reverb tank connections and the other jacks with de-oxit? even if it doesn't solve the problem it can't hurt and will help rule out weird stuff like gremlins in the fx loop jacks.
 

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Discussion Starter #11 (Edited)
O...M...G... fixed the problem (note I say *fixed*, not *fully found*). You are never going to believe this ...

Here is what the bottom of the Maverick looks like:





And here is what it was sitting on top of:



Any clues yet? Well, here is how I *fixed* it:



How about now, any ideas?

So to break it down ... the bottom of the reverb tank was touching the metal handle holders of the amp below it. That created a ground loop of some sort for the oscillation rogue signal. I found it by accident when I tilted the cab forward off the handle for better access.

The battle is already lost ... Chaos, Entropy, and Mother Nature will be the end of us all ...
 

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good find! sometimes things like that can lead to a frustrating search through schematics and components that can become a real time sink. thats why I always try to rule out the simple things before getting the iron out.
 

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I found it by accident when I tilted the cab forward off the handle for better access.
Easy and inexpensive fix!
Congrats on both the discovery and knowing that it was the cause of the problem.

My amp was doing some 'snap, crackle and pop' stuff last evening.
Turns out my pickguard was creating static through my strumming hand.
A wipe of an anti-static Bounce 'tissue' and all was well.
 

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Discussion Starter #14
My amp was doing some 'snap, crackle and pop' stuff last evening. Turns out my pickguard was creating static through my strumming hand. A wipe of an anti-static Bounce 'tissue' and all was well.
My collection of techniques that intermittently and variably “work” for the pickguard crackling static issue are the following so far:

- apply conductive metal tape to entire back of pickguard and ensure it is electrically connected to some other ground point on the guitar
- apply a strip of packing clear tape to a section of pickguard where your strumming hand lands; I can’t believe this works, but it does
- used Bounce rub; but it only works for a short period of time

What others have said but I haven’t tried:

- sand the front of the pickguard to a rough surface (I’ve never got the guts to do that to a pretty pickguard yet )

Like I said, entropy will get us all in the end ...
 

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Not that it makes much difference now, but for future reference, these kinds of reverb issues are often due to physical or magnetic coupling rather than actual electrical connections. In this case, that handle should not be making any electrical connection so there is no electronic path for a ground loop. I think if you would have physically isolated the tank but still kept it connected, it should have also cured the problem.
Sometimes with combo amps, replacement speakers with bigger magnets can set up tank oscillations. Moving the tank further from the magnets sometimes helps, sometimes the tanks need to be physically isolated from the cabinet with foam. Even over-tightening the tank mounting screws can sometimes cause the tank to oscillate.
It can be a high frequency squeal, but also a low frequency rumble or a howl that slowly builds.
 

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Agreed, reverb springs can do the strangest things. I once had a Vox AC30 Reissue Head that had an oscillation when the reverb was turned up a bit. Lifting the amp a couple of inches off the speaker cabinet would stop the oscillation. I can't remember how i ended fixing it, but i think the springs got replaced with a different configuration and impedance.
 

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Discussion Starter #18 (Edited)
Actually, I just measured the amp below the Maverick handle mounts and they are, in fact, solidly grounded to that amp’s chassis. So it was conducted, not inducted, from the Maverick reverb tank to the amp below chassis when Maverick was sitting on top of and touching the handle mounts.

I also did a direct conductivity test —one side of the clip on Maverick reverb tank, one side touching the other amp’s chassis ground points. Maverick squeals as soon as I touch both. Can transmit Morse code with it :). The ground loop was definitely conductive.

The amp below is a stock 1964 Blonde Bassman 6G6, with only caps and grounded power plug as mods. Is it not expected that the handles would be chassis grounded?
 

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The amp below is a stock 1964 Blonde Bassman 6G6, with only caps and grounded power plug as mods. Is it not expected that the handles would be chassis grounded?
No, the fender handle cap screws will be going in to t-nuts or similar. The top side of the chassis is open. There is sometimes a metal plate stapled to the topside of the headshell which contacts the chassis. They do not make a point of connecting the screws to the plate so it will be inconsistent. In this case it sounds like it was making contact.
 

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Discussion Starter #20
No, the fender handle cap screws will be going in to t-nuts or similar. The top side of the chassis is open. There is sometimes a metal plate stapled to the topside of the headshell which contacts the chassis. They do not make a point of connecting the screws to the plate so it will be inconsistent. In this case it sounds like it was making contact.
Hm, ok, thanks. I’ll need to open up the Bassman and see what’s going on in there.
 
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