Bizarre issue troubleshooting AB763 build ... 6L6 squeal oscillation

[Diverted]

Hi,

This one is new to me, and I don't quite know how to deal with it.
I recently built a single channel, no tremolo Fender Ab763 circuit for a friend of mine. I have brought it back to my shop four times now trying to find what is causing the crazy noise and pops that he says he's gotten almost every time he's turned it on. Three of the times I didn't hear any issues at all, once I did. That time, I chopsticked around and noticed there was a lot of microphonic activity all over the board and it wasn't a repeatable noise; very random. It seemed loudest around the ground buss, which was very sensitive. Thinking a bad solder joint or ground etc. I re-soldered all the grounds and checked all other joints for good solder, checked jack washers etc. and after I did that the amp was fine again. I gave it back to him a week ago. Everything was and is tight in the amp, and I later reflowed every joint in the amp to be sure.
He called me Sunday and said it was doing it again. I brought it home today. None of the chirps or noises, though after an hour and a half of it sitting idle on the bench I noticed it doing something very weird. V4 (first 6L6) squeals when I touch it (see video below). It is not the tube, as I swapped them thinking it might be microphonic, and it does not follow the tube. It is married to that V5 position.
I checked the socket, reflowed all solder joints on it, chopsticked and tried to adjust wiring around the socket, etc., but it's still happening. It is often accompanied by a scratchy sound when I really give it a loud signal, as if something is rattling within the amp. I have swapped out every tube in the amp multiple times, thinking it might be a bad tube somewhere else contributing to it. No change.
Apart from that:
All voltages are right on the money compared to an AB763 Super Twin (I used a solid state rectifier as in the Twin). Plate voltage is around 480 per 6L6 and I'm running the tubes at 35ma. They are running fine with no red-plating.

The only changes I made to the amp apart from removing tremolo and first channel are
1. Frondelli master volume control
2. Three-way negative feedback
3. Gain boost switch that takes the tremolo load resistor in and out of circuit.

The issue is happening regardless of where I set any of these.
I would like to have faith that i can give this back to him trouble-free, but I'm stuck on this weird 6L6 oscillation issue and don't quite know how to address it.

Pix of the amp wiring
https://imgur.com/a/pH9M3rB

Video of the amp when the tube squeals
https://imgur.com/a/NgOhBUP

Layout pretty much the same as my build
https://robrobinette.com/RR763_BLACKVIBE.htm#AB763_With_Tremolo_Deleted

Schematic for AB763 super reverb. This does not have the tremolo or first channel deleted, like mine:
http://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf

[shooter]

when you built it did is scream and you swapped primary OT wires?  If not try swapping.


I worked on an ab763 once that was micro-phonic everywhere, it was biased hot, re-biased cold, that was 2 years ago, owner still playing.
other times when I get pops and microphonics it's been bad, poor, solder, arc's between socket pins, 1 tube messing up the works.


play it hard for an hour straight, monitor PA B+ tap and if you have a scope the input and out of the PI (pick a side)  something should show up

[Diverted]

Thanks. I had it biased around 70 percent but backed it off to about 65. Pitch of the squeal became higher and it got a little softer in volume. I may back it down to 60 percent but am also leery of that as I’ll be pushing490 on the plates if I do that.

As for the scope, nope I don’t have one. Wish I did!

[Diverted]

Just curious if anyone else has seen that type of squeal. I would love to get some further input.


To Shooter’s question: I actually had it wired backwards first startup a while ago, and swapped the leads.

[sluckey]

To Shooter’s question: I actually had it wired backwards first startup a while ago, and swapped the leads.

Well, have you tried swapping again since you did the MV mod? It's very easy to change the NFB phase when doing this MV mod if you are not careful.

[Diverted]

I built the mod into it originally, so it was in the circuit when I first fired it up.

Ive heard that massive phase squeal with reversed leads plenty of times and I don’t think this is it. The amp is more or less stable all the way up to 10 apart from nasty scratchy distortion way up. The tube squeal is different.. it happens only when I touch V5. Very strange.

[tubeswell]

So you swapped all the tubes and you tried swapping the OT primary.


Did you check the input jack tip switch (and FX loop tip switch) contacts are making firm connections when not plugged in. A loosely closing tip switch can have that sort of microphonic chassis vibration effect

[JayCobie]

it happens only when I touch V5

But what about using chopsticks to poke and prod wires and components on said tube socket? Is the problem not re-producable poking those connections?

[Diverted]

Yes, tried swapping tubes, no effect. When I noticed it I touched the tube while chopsticking around V5 and 6, and PI. No noticeable difference. When I get back down to the basement I'm going to back off the bias and see if I can get it to go away. But that's just masking the problem.
If I have weird oscillation going, would it do me any good to put a small snubber cap between the PI plates to try to kill it?



Coincidentally, Shooter asked yesterday if I had a scope. I do not, but saw one locally on Craigslist that looks quite good and has been serviced, and will pick it up this morning. The only issue there is I have never used one and will have to learn it.

[tubeswell]

Did you check the input jack tip switch (and FX loop tip switch) contacts are making firm connections when not plugged in.

?

[HotBluePlates]

...
1. Frondelli master volume control
...
Video of the amp when the tube squeals
https://imgur.com/a/NgOhBUP
...

The crazy-long wire to & from your master volume control is almost certainly contributing to the oscillation issue.  Grid wires are an antenna for RF, and the longer they are to more they pickup stuff.  Shielding might help, but if I understand correctly there was no crazy-chirping until after the MV was added.

[Diverted]

The MV was built from the start.
Chirping still going on, but it only starts really after the amp’s been on for 1 1/2-2 hours.

I’m going to shield the MV leads and see what happens. Thanks.

I noticed yesterday that when touching the chirpy tube, both outputs flash deep blue.

[Diverted]

I think I found a way to kill the weird oscillation/squeal that's happening on V5 6L6 but want to run it through more knowledgeable minds.
The power rail B node (right after the choke) had been supplying the screen grids. It was within 1-3 volts or so of the A node. I moved the screen supply from the B to the C node, which also feeds the PI plates. This brought 6L6 screen voltage down to about 473 (plate voltages currently at about 482 or so, biased around 60 percent). This (so far anyway) has stopped that instability.
Is this a safe operating configuration?

I did this after:
1. running shielded coax for all MV connections (no improvement)
2. Removing MV altogether (no improvement)
3. Removing feedback switch and returning feedback circuit to stock (no improvement)...

Thank you!

[Diverted]

Scratch that. Squeal returned with grids on the C node.
I've just placed that supply back on the B node where it was originally.

A few more observations:

1. The squeal doesn't start immediately, I have to wait for the amp to warm up a little before I can get the tube to squeal.
2. I don't need to touch the tube to induce it; just putting my finger a half inch or so does it.
3. When I touch a grounded point on the chassis, or the guitar, I cannot induce the squeal in V5.

I've tried pretty much everything I could think of to throw at it, but the science is what's holding me back. Now that I have the MV removed (at least temporarily) would it be a good idea, or would it help, to run shielded cable from the 220K grid resistors to the V5 and V6 grid stoppers? Would changing the value of the grid stoppers help?

Thanks for your patience and help.

[shooter]

just putting my finger a half inch or so does it.

I touch a grounded point on the chassis, or the guitar, I cannot induce the squeal in V5.

sounds like a bad connection.  it doesn't have to be V5, V5 might just be a symptom.
keep looking at grounds and signal path. 
set it on metal surface (to "seal" the bottom) any help?
move it to the neighbors house and re-test

[Diverted]

Thanks. I asked him to bring by his pedal board and guitar, to see if I can recreate the problem.
I was hoping it might just be a shielding issue on the back of the amp. No dice though :( my back panel has a metal sheet across all interior surfaces that contacts the chassis and should be a good ground. I put it back on the amp during testing to see if it knocked it out, but it didn't.

[tubeswell]

Did you check that the input jack tip switch contacts are closing firmly? (Intermittent closing can cause microphonic chassis problems)

[Diverted]

Yes, as far as I can tell every joint and connection point in the amp is solid and all have been tightened just in case. Switches and jacks are all brand new switchcrafts, using star washers, and all pots (CTS) also use star washers. The preamp ground bus is a 16g length of copper wire running from one of the input jacks to the other end of the circuit board. It's solid too.
Really scratching my head on this one!

[HotBluePlates]

The MV was built from the start.
Chirping still going on, but it only starts really after the amp’s been on for 1 1/2-2 hours. ...

Unless you're using a scope to look at the signal in the amp, the oscillation could have always been there & going above/into the range of hearing.

So in your shoes I'd either use a scope to see where the oscillation is happening, or assume it could be anywhere/everywhere, and shorten all grid wiring as much as possible.  Also keep any large/late-circuit signal wiring away from small/early-circuit wiring especially where the latter has a high impedance to ground.

[Willabe]

Switches and jacks are all brand new switchcrafts, using star washers, .......

We've had guys post with problems before that turned out to be brand new switchcraft jacks that were not closing. It can happen.

[Diverted]

Verified the switch jacks as working fine.
I put the master volume and NFB switches back in, using shielded wire, since the issue was happening without them as well, so I figured I'd eliminated them as a variable.

While I was fiddling around I checked out the tone stack. The pitch/intensity of the squeal goes up markedly with the treble pot, as well as (to a somewhat lesser extent) the mid pot. Bass pot has no effect on it.

When the treble knob gets close to zero, 90 percent of the squeal goes away.

The only thing that is different layout-wise about this tone stack is that he wanted it configured bass-mid-treble, instead of treble-mid-bass. So I reversed the positions of the pots when building it.

I just bought a new oscilloscope last week but won't be able to use it until the weekend ... waiting on an audio generator and hefty 8 ohm resistor to serve as a dummy load.

[Diverted]

It also increases in intensity when I switch off the tremolo load resistor from ground.

[Diverted]

Hi all,
Wanted to see if my thinking is correct here.

So, the weird tube squeal was happening when I brought my hand close to V5 with guitar plugged in. The amp is in front of me on the bench, chest height, and I'm using a Fender strat sitting on my lap.

I bought an oscilloscope last week, and though I really don't know how to use it, I was able to hook it up to various points in the circuit and look at a 1khz sine wave injected into the low input with an audio generator. I was using an 8 ohm dummy load in place of the speaker.

With the 1khz signal, I was getting a clean sine wave at various points from V1A grid all the way up to the phase inverter. I was not able to cause the squeal, or at least see anything on the scope that looked wrong. The sine wave stayed steady and looked good and uniform throughout.

Switching back to the guitar, I plugged it into the low jack and was able to "see" the squeal as early on as the V1A grid. I could not hear it, but the scope trace went haywire when I brought my hand close to the tube. Subsequently it happened on the high jack as well.

Given that this happened at the beginning of the first gain stage, am I right to assume that the guitar itself (bad shielding or some other issue?) is the thing that's causing the squeal? Some kind of nasty feedback from the guitar circuitry?

The only time I was not able to get a clean and still sine wave with the audio generator was at the speaker terminal. I was not sure how to properly set the scope and as a result the trace was bopping around.

I'm hoping I'm on to something here, and that my friend has some other issue with his stuff, or there was something else I have since been able to eliminate with reflowing solder joints, etc.

I was pretty intimidated using the scope but it was very cool to see a sine wave and be able to "see" the crazy squeal on the screen.

Any input would be welcome ... I have much to learn with respect to the scope but want to learn. Thanks!

[shooter]

am I right to assume that the guitar itself (bad shielding or some other issue?) is the thing that's causing the squeal? Some kind of nasty feedback from the guitar circuitry?

I'd say it's worth a deeper dive
cable swap, guitar swap, I'll use an MP3 sometimes that has volume control, to "see" if I can upset the apple cart

[Diverted]

I really hope I can tip the cart over, just to pinpoint it! The guitar's never been an issue before, but who knows if something farted out on it. Cables have been swapped and don't make a difference, so I'll keep digging.
The MP3 thing is a good idea too. So much to learn with the scope, but I'm pretty pleased that I was able to get useful information out of it on the first time I really tried playing around with it.
Thanks.

[nandrewjackson]

Any updates?


You aren't by chance using any new production mullard preamp tubes are you? They are the most microphonic tubes I've ever come across

[Diverted]

Nope, not using Mullards. I've been using a mix of a lot of good pulls (Amperex, RCA, GE, etc etc etc) and some JJs.
As for the chirping, I think it's the guitar as it does not happen with an injected sine wave or two other guitars. So the wiring in the guitar is suspect!
I am wondering if something is going on with my friend's rig. Because I've had the amp on 2 hours a day, at least, every day for the past week and a half, and have yet to find an issue other than the weird squeal which appears to be external.
Very weird.