Silverface Twin Reverb Blackfaced with Scratchy Buzz

[Obejuan]

I blackfaced by Fender Twin Reverb 100w with Master Volume which of course resulted in some bad parasitic oscillation. So I did the following to resolve the issue:
* shortened and shielded the grid wires on the first 2 preamp tubes.
* shortened all the grid wires.
* shortened all the reverb wires.
* moved the screen filter cap ground from preamp to the main filter cap ground.
* rewired the filaments so they are parallel.
* checked all grounds and solder joints.
* replaced all the power resistors and any resistors that drifted.

There was improvement but a heavy buzz was still there. The parasitic seemed gone or very minimal (I think its gone because the amp sounds rich and full). The buzz is on both channels and increases when the master volume is turned up. If I keep the master volume at 1 and increase the preamp volume the buzz is almost gone and sounds very good.

So then I did the following:
* shielded all the master volume wires.
* shielded all the reverb wires.
* replaced all the 100k plate resistors.
* replaced the output tube screen resisitors.

There was quite a bit of improvement but the annoying buzz is still there. Its a scratchy noise now that while lower is too loud to be usable. I rewired the amp to remove the master volume and that did not help. I removed the vibrato (unhooking the intensity pot wire) and that did nothing. Could it be the grid wires still?

My next plan includes shielding all the grid wires on the preamp and output tubes. After that I'm at a loss.

Any other suggestions? What about the wires that run across the preamp board to the PI? Sound I shield those? Thanks for the help.

[PRNDL]

Often the problem is due to grounding, which can be very difficult to find.

Many old techs change the ground scheme so that it matches the vintage amp exactly.

[Bruce / Mission Amps]

Does it have a hum balance pot or is there two 100 ohm resistors across the filament supply.. etc?

[loudthud]

Is there still a buzz when the master is all the way down? That would indicate a problem in the phase inverter or power amp. Where are all these shielded wires grounded? The usual practice is to only ground the shield at one end to prevent ground loops. Have you tried moving the ground you moved for the screen filter back? The Choke does a marvelous job of removing ripple from the B+ and by moving the ground, you may have defeated some of that.

[Obejuan]

Thanks for the responses.... it has the hum balance pot. I did ground all the shielding on one end everywhere. Yes when the master is all the way down the buzz stops... also when I remove the PI tube the buzz stops. When only the vibrato tube V4 the buzz becomes really lite but still there. I'll try moving the ground on the screen supply back to the preamp and see what happens but from what I remember I think it had no affect. Moving the ground helped remove a lot of the hum.

Tonight I grounded the grids on all the preamp tubes with no success.

Should I move to start putting resistors on the preamp tube grids or capacitors across the plate load resistors? I would only go there as a last resort since the purpose of blackfacing was to remove all that tone deadening circuitry. I'd appreciate any other suggestions.

[pdf64]

Does the problem go if the reverb driver tube V3 is removed? I've been through a similar thing with a friend's silverface twin, it ended up being caused by something around there. If it's the same with yours I'll check back through my notes - Peter

[Obejuan]

Actually the issue goes away when the PI tube is out (which makes sense). If I remove V3 there's no change. Removing V4 reduces the buzz to a whisper. Thanks for checking...

I'm wondering if replacing the balance pot with two 100 ohm resistors on the pilot light will do the trick... I think Bruce was suggesting that with his question. I may have to address the multiple grounds throughout the amp. That should have been an earlier priority but I don't have the soldering iron hot enough to create those star grounds on the chassis. Maybe time to invest.

[Bruce / Mission Amps]

Actually the issue goes away when the PI tube is out (which makes sense). If I remove V3 there's no change. Removing V4 reduces the buzz to a whisper. Thanks for checking...

I'm wondering if replacing the balance pot with two 100 ohm resistors on the pilot light will do the trick... I think Bruce was suggesting that with his question. I may have to address the multiple grounds throughout the amp. That should have been an earlier priority but I don't have the soldering iron hot enough to create those star grounds on the chassis. Maybe time to invest.

The reason I mentioned the hum balance pot is because very frequently an amp will come to me with many years of use and that pot has been flashed over from a power tube socket shorting lug 3 to lug 2 some where in it's life time.
With respect to the power tube socket's lugs 3 to 2, the next path to ground is through the 100 ohm pot and resistor and pot will ignite with +400vdc-475vdc across it to ground.

[Albert Kreuzer]

The reason I mentioned the hum balance pot is because very frequently an amp will come to me with many years of use and that pot has been flashed over from a power tube socket shorting lug 3 to lug 2 somtime in it's life
With respect to the power tube socket's lugs 3 to 2, the next path to ground is through the 100 ohm pot and resistor and pot will ignite with +400vdc-475vdc across it to ground.

Had this yesterday with a SFTR while the chassis was on the bench. The flash and the smoke were very impressive.
Oh well, now trying to get the smoke back in

Cheers,
Albert

[Obejuan]

Unfortunately removing the hum balance pot from the circuit and adding the 2 100ohm resistors on the pilot didn't solve the issue. I checked the voltages on the filaments and was all good. I'm going to try moving the screen filter ground back to the preamp.

I've gone back through my work again to check all grounds and solder joints expecially in the power section. I'll finish up tonight with the filter caps.

I tried removing tubes last night and its clear that most of this oscillation is happening in V4. I may try shorting the plate resistor with a cap or adding a resistor on the grid to see if that cures it. I could follow the silverface circuit and put a .002 cap grid to cathode (7 to 8) but trying to avoid all these tone killing mods.

I have a thought... could the fact that the V3 and V4 cathodes are grounded on the vibrato and reverb pedal jacks be the problem? The preamp grounds on the filter caps that supply those tubes are on the other side of the chassis on the grounding strip. That could cause the oscillation right?

[Lee_ranaldo]

i had the same issue with a home built super reverb that got here.

I rewired everything based on the AB763 specs and the blocking distortion (caused by parasitic oscilations) didn't stop. I tried everything including adding filtering caps in the power section just like the later super reverbs.
In the end.. The old STR 6l6GC mesa power tubes were old and the cause of the problem.

Try with a trusty set of power tubes and see if that makes the problem dissapear. because that's the only thing that worked for me in that time.

[Bruce / Mission Amps]

That was four years ago... I would think by now the amp is either gone or it got fixed! ha ha