SF DeluxeReverb ugly distortion

[Zouto]

Hi, my 70s SF Deluxe Rev. sounds terribly distorted in both channels when cranked past 3 or 4 on the Vol.knob's scale.The amp is correctly recapped and retubed with EHs and RI TungSol 6V6s and has a new Eminence 875/4 Ohms speaker.Since it sounds like a blown OT I put in a new one (TAD)for no bettering, same with different Speakers. All "defarting" strategies descripted in the old AMPAGE forum did not fix it.
Any help much appreciated
Zouto

[MWJB]

Are you sure you have a Delux Reverb/Eminence 875 @ 4ohms? One OR the other maybe?

What does a voltage check show? What plate current do you have?

[Zouto]

Originally Posted by MWJB

Are you sure you have a Delux Reverb/Eminence 875 @ 4ohms? One OR the other maybe?

What does a voltage check show? What plate current do you have?

It's a 8",75W,4 Ohms Eminence Guitar Legend Speaker.
Plate voltages: on 6V6s 396V, on V1 206V, V2 204V,V3 387V, V4 223/220, V5 384/331, V6 52V/56V, cathode 84V, bias 30mA/6V6.
With another GZ34 plate voltage is 406V on 6V6s .
I checked all the plate and cathode resistors but couldn't find any fault.
What to check next?
Thanks for help
Zouto

[Zouto]

I posted a speaker mistake: I got an 8ohms Eminence Cannabis Rex in my SF Deluxe, the 875 is in my Vibrochamp.
Zouto

[MWJB]

Assuming that you have a 12AT7 in V6 (confirm), your plate voltages are wrong. I would be expecting to see 200-220v at pins 1 & 6. Measure plate resistor values.

V5 - pin 6 should read the higher voltage compared to pin 1 (though this shouldn't affect the tone of the amp).

[Zouto]

Yeah, the plates of V6 (12AT7)have now 196V/206V, I changed the 82K/100K resistors.The amp is louder now, but that nasty distortion is still there, sounds like a blown speaker.Must be some bug in the first stages, probably blocking distortion, reducing the values of coupling caps did not fix it.
Zouto

[MWJB]

Don't start modding the amp, until you have cured this. These symptoms are not typical of a Deluxe with the original value coupling caps.

Does this distortion affect both channels? If so, it is unlikely to be in the earlier stages.

[Zouto]

MWJB, thanks a lot for your replies.That distortion occurs on both channels.Volumes from 1-3 give a great clean tone, sometimes with a light crackling on higher frequencies when using humbuckers(higher output).Higher volumes create this §$%&%$§ distortion instead of a nice breakup.
Zouto

[MWJB]

What are your screen voltages at pin 4 of the 6V6s, grid voltages at pins 5, grid voltages at pins 2 & 7 of v6?

Triple check 6V6 sockets for shorts/arcs.

Sounds like your problem is later than the preamp, more likely PI or output.

You haven't clipped off the grid to ground caps from the power tube sockets?

Generally, your voltages seem a little low for a SF Deluxe with an original PT & GZ34, I would normally expect you to be struggling to get under 425v at the 6V6 plates with 30mA. Do you have the original PT, are you in an unusually low voltage AC supply area? What are your AC heater voltages measured from pin 2 to 7 of a 6V6? I would be concerned that something may be drawing additional current?

Did you replace filter caps with exact uf & voltage ratings?

[Zouto]

Originally Posted by MWJB

What are your screen voltages at pin 4 of the 6V6s, grid voltages at pins 5, grid voltages at pins 2 & 7 of v6?

Triple check 6V6 sockets for shorts/arcs.

Sounds like your problem is later than the preamp, more likely PI or output.

You haven't clipped off the grid to ground caps from the power tube sockets?

Generally, your voltages seem a little low for a SF Deluxe with an original PT & GZ34, I would normally expect you to be struggling to get under 425v at the 6V6 plates with 30mA. Do you have the original PT, are you in an unusually low voltage AC supply area? What are your AC heater voltages measured from pin 2 to 7 of a 6V6? I would be concerned that something may be drawing additional current?

Did you replace filter caps with exact uf & voltage ratings?

The screen voltages at pin 4 of the 6V6s are 416V,grid voltages at pins 5 are -36V, I clipped off the grid to ground caps and removed the pull Boost cct.
Grid voltages at pins 2 & 7 of v6 are 45V, new voltages at pin1 194V and at pin6 199V.I have the original PT,OT, heater voltage is 6.1V AC.I've recapped w/22u/500V Atoms.Meanwhile I found out, when removing V4 (reverb recovery), the Normal channel sounds fine without any nasty distortion.Replacing the reverb tranny did not remove that
fizzy break up.I've also checked the OT,it's ok.Next and last thing I'll do is to replace the PT with a TAD 126P33A. Any further help still much appreciated,
Zouto

[Zouto]

BTW I resoldered all tube socket pins, chassis grounds and lots of obscure looking eylet board connections, which changed the voltages a little bit but did not remove the fizzy break up distortion and hum.
Zouto

[Jag]

You've got 396v - 406v on the plates and 416v on the screens? Have you checked the screen resistors? What voltage do you have at the CT of the OT? The screen voltage should not be that much higher than the plates and, preferably, less than the plate voltage. Sometimes you might see screen voltage a volt or two higher than the plates but 10 - 20 volts is not right.

What's the voltage across the tail resistor of the PI?

[Jag]

Removing V4 souldn't have much, if any, effect on the normal channel if the amp is stock. Have you modded the amp for reverb/vibrato on both channels?

I agree that your voltages seem a little low overall but I don't think that is the root problem. What is the AC at the wall?

Double check the voltages at Pins 4, 5 and 6 on the output tubes and post the values at those pins for each output tube.

[jrfrond]

The description "Volumes from 1-3 give a great clean tone, sometimes with a light crackling on higher frequencies when using humbuckers(higher output).Higher volumes create this §$%&%$§ distortion instead of a nice breakup" sounds like one power tube is working. If you pull one power tube in a two-tube Class AB push-pull amp, will get a few watts as the amp operates in a lame sort of Class A mode. Once you exceed the "A" range dialed in by the bias, and enter "B" mode, the rest of the waveform gets clipped HARD, and would sound as you describe.

Pull each power tube individually and try it. IF this is the case, one tube will not affect the sound at all, and the other will kill the output signal.

I would suspect arcing and/or carbonized sockets, but without the amp in front of me, it's just a wild guess.

[Zouto]

Originally Posted by jrfrond

The description "Volumes from 1-3 give a great clean tone, sometimes with a light crackling on higher frequencies when using humbuckers(higher output).Higher volumes create this §$%&%$§ distortion instead of a nice breakup" sounds like one power tube is working. If you pull one power tube in a two-tube Class AB push-pull amp, will get a few watts as the amp operates in a lame sort of Class A mode. Once you exceed the "A" range dialed in by the bias, and enter "B" mode, the rest of the waveform gets clipped HARD, and would sound as you describe.

Pull each power tube individually and try it. IF this is the case, one tube will not affect the sound at all, and the other will kill the output signal.

I would suspect arcing and/or carbonized sockets, but without the amp in front of me, it's just a wild guess.

Thanks, that was a good hint.Pulling the outer 6V6(100K/0.1)gives a lame but
good sound,pulling the inner 6V6(82K/0.1) gives screeching distortion+oscillation even at lowest volume, regardless which one of the 6V6s I use.Should I change the socket?
Resoldering all socket pins did not fix it.
Zouto

[Zouto]

Good news:a 47 pF Mica across the PI's plate resistors plus swapping the 220K grid leak resistor on V4 (on Rev.pedal RCA connector) removed all that sick distortion and oscillations completely, the amp sounds finally fine.The remaining bug is a loud hum as soon as I turn on the Reverb pot.I swapped V3,V4, cables and Rev.transformer, tried some other reverb tanks with long cables at quite a distance from the PT for no bettering.Hums like some grounding problem.The reverb sounds ok, but the hum is much louder than on another SF DX Reverb and a SF Twin I tried last week.Any ideas for some final dehumming?Thanks,
Zouto

[jrfrond]

OK, so placing a snubber cap across the PI plates stopped the distortion. So, what you had was most likely blocking distortion caused by supersonic oscillation, which is leading me to believe that, together with the reverb hum, you might have lead dress issues. Not uncommon in SF amps.

[Bruce / Mission Amps]

Originally Posted by jrfrond

OK, so placing a snubber cap across the PI plates stopped the distortion. So, what you had was most likely blocking distortion caused by supersonic oscillation, which is leading me to believe that, together with the reverb hum, you might have lead dress issues. Not uncommon in SF amps.

Supersonic means faster then the speed of sound, ultrasonic means you can't hear it.

[jrfrond]

Originally Posted by Bruce / Mission Amps

Supersonic means faster then the speed of sound, ultrasonic means you can't hear it.

OK, OK..... I can't hear anything above 13kHz anyway!

Problems like this are the reason that everything I work on gets a look-see on the 'scope. On a 'scope, it would be readily obvious watching all that IM distortion do it's thing.