Marshall Plexi Squeal - Experts: Please help

sws1

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My 68 Super Bass (converted to superlead specs) squeals when the volume gets to about 7 or so.

Here is what I've done to debug, but i think I need some expert advice.

The amp squealed when there was NO MV.

Changed out PI tube. Still squeal.

I put in a MV (Rich-mod) and it still squealed, even when the master was close to 0. That tells me the problem is before the PI coupling caps, not in power stage.

It seems to be sensitive to the EQ settings. For example, if I turn up the treble or mids, the squeal comes sooner on the volume dial. That now tells me the problem is post-EQ.

I tried moving some of the wires around that are connected to the pots to see if that made a difference. While I was never able to get rid of the squeel, it was obvious that the position of the wires altered when the squeal came on. I'm not sure what this told me, but maybe it says that position of the wires is what is causing it. However, there was no manipulation of the wires that got the squeal to go away.

So, it seems the squeal might be one of the following things: arrangement of wires from EQ, the cabs / resistors that form the PI circuit, of the PI coupling caps.

HELP!!!
 

Swarty

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1,130
Try repositioning the input lead (between the jacks and the 1st preamp tube). If squeal persists, change 1st preamp tube. If squeal persists, replacing input lead with shielded lead grounded at one end. One of these will almost certainly kill the oscillation.

Oh, and does it do it on the dark channel as well?
 

Mark Kane

Gold Supporting Member
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3,746
If the amp doesn't have grid resistors(pin 5 of output tubes) put them in.
 

sws1

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10,784
Originally posted by Swarty
Try repositioning the input lead (between the jacks and the 1st preamp tube). If squeal persists, change 1st preamp tube. If squeal persists, replacing input lead with shielded lead grounded at one end. One of these will almost certainly kill the oscillation.

Oh, and does it do it on the dark channel as well?
It does do it on the dark channel too (I'm almost positive).
 

sws1

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10,784
Originally posted by Mark Kane
If the amp doesn't have grid resistors(pin 5 of output tubes) put them in.
It does have grid resistors. Plus, it even happens when the master is almost off, so the power tubes shouldn't be causing it. (It's still squeals at this point, just not as loud because master is down.)
 

sws1

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10,784
Originally posted by Swarty
Have you tried a different input preamp tube?
Yes - swapped them all except power tubes. But those wouldn't be the problem since it squeals with MV down low.
 

John Phillips

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13,040
Squeal is almost always preamp tubes (if it's tubes at all) anyway. Power tubes rumble or howl, but don't squeal.

Something to remember is that the Rich Mod MV (if it's what I think you mean) effectively removes the negative feedback loop from the circuit as it goes down, like all post-PI MVs.

But it seems more likely that the squeal is being generated internally in the preamp to me. The fact that it's sensitive to EQ settings does not mean it's post-EQ - just that part of the internal loop that's causing the trouble is. The main problem can still be before it. In fact, the source of the signal pickup must be at or before the volume controls since running them below 7 stops it.

I'd look for bad grounds, or wildly high-value plate resistors - something that could be causing either positive feedback or simply far too much gain. That is, if there isn't some old mod to do exactly that...

Has it had a cap job?
 

sws1

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10,784
Originally posted by John Phillips


Has it had a cap job?
It has had a cap job at some point as there are different brands of caps in there. The 4 filter caps grouped together are all yellow Dalys (one of which has a small pimple). The cap near the PT is an RS, and the cap near V1/V2 is a Chemicon.

I suspect all of them are old.

All of the caps inside, with the exception of those change to convert to lead specs are original mustards. There is also an addition cathode bypass cap on the 2nd half of V1 to the split circuit.
 

sws1

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10,784
Originally posted by TheAmpNerd
It Squeals,

When you pull v1
and crank everything (10)
does the amp still squeal?
VERY good question. I had not tried that. Will do that tomorrow.
 

sws1

Member
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10,784
OK - Here's an update.


I turned on the amp last night and didn't plug anything into the amp.

The bright channel squeals at 7 even with no input.

The dark channel does NOT sqeal. Even with an input going to it.

Playing with the presence knob alters the tone of the squeal. (Would be a cool special effect if that's what I was going for.)


So, it would appear that the squeal is being generated somewhere BEFORE the volume controls.

Possible options:
V1 tube
V1A cathode cap (It does have the original Superbass cathode cap/resistors. This might be higher than super lead specs)
V1A coupling cap
Input wire needs to be moved

What else?

(Thanks - I think I'm getting close)
 

WailinGuy

Member
Messages
1,275
Here are some suggestions:

- Double check lead dress. Make sure no wires carrying a signal from later stages in the preamp are placed near wires carrying a signal to an earlier gain stage.

- Check for microphonic ceramic caps. The ones to concentrate on are the .0047u or .005u (4700p or 5000p) bypass cap on the brite channel volume pot and the 470p cap that bypasses the brite channel's 470K mix resistor (which then feeds into V2A). Tap on these with a chopstick and listen for noise.

- You might be able to get rid of the squeal by either removing that 470p cap and/or reducing the value of the .005 cap on the brite volume pot (which I think is overkill anyway - 220p to 500p is a more sensible value). Of course, you may or may not like the change in tone that would result.

By the way, for the Superlead setup, the normal channel should have a 820 ohm cathode resistor bypassed by a 250u/25v electrolytic cap (on V1A) and the bright channel should have a 2.7K cathode resistor bypassed by a .68u film cap (on V1B).
 

sws1

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10,784
Originally posted by Jim Salman
Here are some suggestions:

- Double check lead dress. Make sure no wires carrying a signal from later stages in the preamp are placed near wires carrying a signal to an earlier gain stage.

- Check for microphonic ceramic caps. The ones to concentrate on are the .0047u or .005u (4700p or 5000p) bypass cap on the brite channel volume pot and the 470p cap that bypasses the brite channel's 470K mix resistor (which then feeds into V2A). Tap on these with a chopstick and listen for noise.

- You might be able to get rid of the squeal by either removing that 470p cap and/or reducing the value of the .005 cap on the brite volume pot (which I think is overkill anyway - 220p to 500p is a more sensible value). Of course, you may or may not like the change in tone that would result.

By the way, for the Superlead setup, the normal channel should have a 820 ohm cathode resistor bypassed by a 250u/25v electrolytic cap (on V1A) and the bright channel should have a 2.7K cathode resistor bypassed by a .68u film cap (on V1B).
It's definitely not the Vol1 bypass cap as I don't have one. I put one on to see if I liked it, and it squealed like a pig. Made things much worse.

Let me try the lead dress and mixer bypass cap.
 

Jimmy Somma

Member
Messages
154
Sounds like you have an oscillation problem due to lead dress. Looking at the front of the amp, make sure the purple wire going to the presence control is as far to the right side as possible and the wires to the volume and tone controls are far left.

www.sommatone.com
 

Swarty

Member
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1,130
Did the amp squeal prior to the SuperLead conversion? Anyway, at this point I'd be inclined to sheild the input lead.
 

sws1

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10,784
Originally posted by Swarty
Did the amp squeal prior to the SuperLead conversion? Anyway, at this point I'd be inclined to sheild the input lead.
I got the amp after the conversion so I don't know.
 

John Phillips

Member
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13,040
Originally posted by Swarty
Did the amp squeal prior to the SuperLead conversion? Anyway, at this point I'd be inclined to sheild the input lead.
You shouldn't have to, since originals aren't shielded and don't squeal. That's addressing the symptom, not the cause, really.

It's difficult to guess more accurately at this stage, but I'm still inclined to think it may be a lead dress problem. Or a bad ground - the jacks and pots (including the presence) are grounded together, so a poor connection at the actual ground for this can cause a loop from the presence control (post-EQ) right back to the input stage... which would possibly agree with all the controls affecting it.

Unless it's simply a bad tube, have you tried swapping V1 yet?
 

sws1

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10,784
Originally posted by John Phillips
You shouldn't have to, since originals aren't shielded and don't squeal. That's addressing the symptom, not the cause, really.

It's difficult to guess more accurately at this stage, but I'm still inclined to think it may be a lead dress problem. Or a bad ground - the jacks and pots (including the presence) are grounded together, so a poor connection at the actual ground for this can cause a loop from the presence control (post-EQ) right back to the input stage... which would possibly agree with all the controls affecting it.

Unless it's simply a bad tube, have you tried swapping V1 yet?

I thought I had swapped V1, but now I can't remember.

Incidentally, where are the pots grounded? Aren't the pots themselves grounded against the chassis, or is the wire run somewhere else?
 

Swarty

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Messages
1,130
Originally posted by sws1
I thought I had swapped V1, but now I can't remember.
This would be the first thing to do. I have some older Marshalls. The squealing is common in the lead amps when you dime them. Sometimes just moving away from the amp will cure it. Sometimes you just have to go through a few preamp tubes until you find a quiet one.

Much of the problem is that the "Lead" circuit that is usually installed is the ultra bright late plexi/early metal panel circuit. These amps were designed to pump treble into the back of large halls without the help of a PA system. IMO these amps are way too bright and sound much better (fuller/rounder) w/ a stock SuperBass/JTM45 preamp.

Shielding can be a good option because not only does it often clamp down the oscillation (squeal), it also knocks the hiss level way down.
 




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