Peavey Studio pro 40 has squeel

[mikeskory]

Phatt: I wasn't clear. The "squeal" went away around the time I re-soldered the bass of the 4 pin reverb tank connector. But the remaining problem is that the signal at the speaker dies when the POST and REVERB controls are between 1/2 ans 3/4 up.  At the same time the signal dies, a small negative voltage appears at the speaker output (and various other places in that output circuit. I did purchase a 8EB2C1B tank and tried it.  Same results as with the stock reverb tank. The irony is that the amp sounds great and the reverb effect is good.

[g1]

Have you looked at the output on a scope?
I wonder if you still have oscillation that has just moved up in frequency beyond what is audible.  And once it starts it overwhelms the signal so it sounds like the amp is just cutting out.  And the meter could mis-read it as DC.

[phatt]

Enzo sez: Does the reverb control affect the frequency at all?  Does the post control affect the frequency?
Mike sez: yes for sure. If reverb is off then post works fine. If post off then the reverb control won't trigger the squeal. When reverb is unplugged no squeal.

If this is still the case then Reverb is suspect.

Make sure the correct pan type is used - 4EB2C1B.  It must have the input jack insulated from the pan.
Mike sez: I'll check that tomorrow.

Have you verified that?

Enzo sez: And does unplugging the reverb pan stop this or not? \
Mike sez: Unplugging reverb stops the squeal.

Ditto,,,
If this is still the case then Reverb is suspect.

I'm getting so close I can taste it!

@Phatt The wire trick did squelch squelch the problem but it reduced the reverb effect to almost nothing. Soldered the 2 resisters. Same.

Bridging the Reverb case and Amp Chassis with a wire should have ZERO effect on the sound,, but it obviously did  Soooo,,, logic sugests the reverb is your problem.
I'd be checking the ground return path of the reverb drive in the amp as well as verify the Case ground path as well as check your RCA leads for continuity.

If that checks out then  ,,What *Katoda* mentioned,, a leaky Cap,, obvious one is from the reverb return opamp.

Sorry if I've missed something ,,I read now you have a hum problem?
That also might be caused by a reverb circuit wired up wrong.

I'm actually right in the middle of redesigning a reverb drive circuit for a crappy Elcheapo Valve Amp and yes I did connect the wrong leads and it squealed as I turned up the high gain channel.  Once I realized my mistake I got full crank and full reverb.

Trying to follow ground paths is actually quite tricky so tiss very easy to miss something.
Regards Phil.

[Enzo]

Right now I am unable to wrap my head around anything complicated.  Hey, I am old...

But one thing to for sure verify is the pan itself.  The drive side MUST have the shield side isolated from ground.  The output end of the pan SHOULD be grounded to the pan.   If you measure the disconnected pan for resistance from the outside of one RCA to the other, you should see OPEN.   If you connect the amp cables to the pan, you should see 22 ohms between RCA outer sides.

[mikeskory]

Can't work on it till Wed, but I'm not explaining it clearly. Maybe I can post a video Wed.

There WAS originally a squeal. Around the time I re-soldered the 4 pin Reverb connector it went away. Enzo recommended tank 4EB2C1B but we ordered #8EB2C1B because of the small cabinet for this amp. Both the new and original reverb tanks sound great. In fact. the amp sounds great...until I turn the reverb control past 1/2 way with the post control up 1/2 way. The position of the volume control is irrelevant to the "cutting out".   This post by G1 seems logical. I'm going to post a short video on Wed so you can hear what I mean.

Posted by: g1
« on: December 31, 2017, 01:38:04 PM » Insert Quote
Have you looked at the output on a scope?
I wonder if you still have oscillation that has just moved up in frequency beyond what is audible.  And once it starts it overwhelms the signal so it sounds like the amp is just cutting out.  And the meter could mis-read it as DC.

[Enzo]

Ah, throw a scope on the output to see if it is going to RF.

[mikeskory]

Here are the scope views on the Peavey. .50 mean at 50% or 5

[mikeskory]

Here's the same scope settings normal reverb and just prior to the problem. Rev and post at .50

[Enzo]

Yep, your amp turned into a giant power RF oscillator.

Make sure the 22 ohm R66 over by the speaker output is not open.

I still worry that the reverb IC may be compromised, so do check the pan jack.  A lot of them now have a configurable jack.

When it bursts into oscillation, then check the 15v rails to see if it is riding those rails.

Is the oscillation present at the preamp out jack?  If so, does plugging something into the power amp jack stop it, or does the oscillation remain on the preamp?

Isolate even further:  when it is singing, short across the return from the pan with something.  Does that kill the osc?

Behind the gain controls is the connector to the footswitch jack,  Pull that off the board, does that change anything?  The lead over to that jack could be acting as an antenna - it is wired right to the reverb pot and preamp final stage IC.

[mikeskory]

OK Enzo...logic has to solve this. The R66 22ohm resister was burnt looking at the outset of this project. I replaced it. Just now I accidently touched. Burnt my finger. There is a wierd looking .1/100v electrolytic in front of it. I have similar. Should I change that?

[mikeskory]

Footswitch control. No change when unplugged.  RF at pre-amp out-YES.

[mikeskory]

Enzo: Yes the RF appears at Q1 and Q2 emitters. They are tied together. The resister R49 takes these emitters to the -15v. The -15 voltage budges very slightly and NO RF when I emulate the symptom. At the Q1,Q2 side of R49 we have all the RF.

[g1]

Could you clarify "NO RF when I emulate the symptom" ?

[mikeskory]

The customer complaint is that the amp squeals or otherwise cuts out when the Post control AND the reverb are both past 1/2 way up.  I verified this and can make it happen like clockwork by turning up the Post AND Reverb past 1/2 way. The far left Volume seems to be irrelevant. The Saturation control has some effect but does not seem primary to the complaint.  "Cuts" out is an inaccurate description. More like the signal is blanked out or covered up. If there was a way to post sound clips at this discussion I will do it.

[Enzo]

CLip if you can, but I totally understand what you have there, Mike.  When it goes into RF, it swamps all else.  I would never call that cutting out, but I see why a customer would.

In cases like this I always suspect grounds.  Note in classic Peavey style, there are two grounds.  The power amp and power supply have one, and the preamp another.  Look below the reverb drive IC and find C26.  That cap ties the grounds together.  On the board it is right next to Q6 output transistor.  Is that cap OK?  And is board totally installed into the chassis, so all jacks make good contact to chassis?  Chassis also ties them together.  SOmetimes I have to loosen the jack nuts, move the star washer for a fresh grip, and retighten.

But if the grounds are all good, oscillation implies a loop of some sort.  SO we can isolate the problem.  Pull U1, does it still osc?  Post and reverb controls have to be up for it, but do the tone controls affect the oscillation in ANY way?