1965 Super Reverb Noise At Idle

[keithb7]

Your experience on noises from a 1965 Fender Super Reverb are appreciated:
A local musician asked me to have a look at his amp. The amp has been serviced over the years and modded.
Someone added reverb and vibrato to both channels. The cathode resistors on V1 and V2 second stages were upped from
stock of 820 ohm to 1.5k ohm. Nots sure why, I assume to increase gain? All electrolytic caps have been replaced, unsure of date.
All coupling caps were replaced with orange drops.
Complaint was the amp was making noises, and tone was dropping off and sounding anemic.

Here is what I did:
Pulled and tested all tubes. Appear good on my thermionic tester.
Tested all resistor values. Found one under dog house more than 10% out of spec.
Removed death cap.
Chop stick test over entire board and connections while live. All good.
Inspected all pots and all other connections, grounds, hardware, etc.
Spray clean all pots. Vibrato intensity pot over 10% out of spec. Will replace.
Checked all voltages. Good.
Set bias at 63% plate dissipation
Lifted B+ caps and tested with ESR meter. Within spec.
Found very poor speaker connections. Removed all push on clips and soldered all speaker wires in place.

The amp now sounds good. It appears the speakers were cutting out from such poor connections. That is fixed.

The amp was tested for a couple hours and sounded great. Then it started making noises at idle. Humming and frequency wave type noise.
Tapping on the cabinet top, stops the noise. I am suspecting B+ caps, as the dates on them are unknown. They could easily could be 15 years old.
Am I on the right track with thinking the noise is due to the B+ caps? I hate guessing and throwing parts at an amp. I like to learn. How could
I properly trouble shoot this symptom? My understanding is an ESR is a partial test of the caps, but not under high voltage operating conditions.
The caps may act up when in use, yet now show up on an ESR meter. Am I correct here?

This is a hobby, I am no expert and learning every day. Thanks, Keith.

Here is a sound sample of the current symptoms.

https://youtu.be/Fj_FEIWnjPY

[Jazz P Bass]

Not too sure that 'tapping' on the cabinet would make a capacitor fail in a noisy manner.

If in doubt, measure the Vac ripple on all of the power supply capacitors.
ESR meters are fine but the proof is in the circuit.
If clipping a known good capacitor of the proper voltage rating lowers the ripple or eliminates the noise, then that cap is bad.

[Tom Phillips]

...Vibrato intensity pot over 10% out of spec. Will replace...

Some comments:
The original parts were ±20% tolerance. New current production parts are also usually loose tolerance too. What value did you actually measure and was the measurement made with the pot isolated from the circuit?
Unless there is an operational problem that has been demonstrated to be caused by the pot I wouldn't replace it at this time. Note also that the taper of the original part is "RA" = "Right Audio Taper" which can be difficult to find. POP20POP20POP20

[keithb7]

Thanks Jazz, I have a scope. Should I use it to measure and monitor AC ripple, or just a use DMM set to AC?

Tom, thanks I'm sure you are right. The RA pot in there is the clicking index type and the owner would like it swapped out anyway, so I will proceed there.

[Randall]

The 100K carbon plate resistors can make quite a bit of creaky noises at idle. Hit them with cold spray, if they go nuts replace them.

[g1]

This could easily be a tube fault of the type a tube tester would not find. Start pulling tubes one at a time to see if you can isolate the problem. Tube testers can reliably find faulty tubes, but can't really guarantee good ones. Substitution with known good tubes is the best test.

The cathode resistors on V1 and V2 second stages were upped from
stock of 820 ohm to 1.5k ohm. Nots sure why, I assume to increase gain? All electrolytic caps have been replaced, unsure of date.

The original 820R is shared by the 2 tubes. If someone wanted independant resistors there, doubling the value would be correct.

[Tom Phillips]

...The RA pot in there is the clicking index type and the owner would like it swapped out anyway...

Oh. So it isn't the original part anyway.

[Chuck H]

There may also be intermittent connection at the tube sockets due to oxidation or grime.

[Justin Thomas]

Define "tapping." Usually, jarring the cabinet in any way while the amp is in operation indicates a faulty connection. Freeze spray and a chopsticks or similar arenyour friend. Not just the plate load resistors, but just about any other carbon comp resistors can make noise too, but only as a last resort. Just don't go shotgunning all of them. One at a time. Check the other items above first!

Next time you have it out, run a stiff straight-edge across the top of the chassis to check for wires that are sticking out above the top edge. Maybe something is intermittently shorting to the metal screen on the top of the cab and causing hum and noises. It doesn't take much contact. Maybe that's why just "tapping" fixes it.

Justin

[keithb7]

By tapping I mean a thump on the top panel of the amp. Yes I agree it often means loose connection somewhere. I have supplied the owner with 2 good known power tubes to test this week. He will report back.
I checked every connection I could see, and tapped everything with a chopstick, nothing loose found. There was one small pF mica cap in the reverb recovery section that seemed to amplify the chopstick tap, more so
than anywhere else in the amp. I am unsure if this is normal? Thx.

[Tom Phillips]

...There was one small pF mica cap in the reverb recovery section that seemed to amplify the chopstick tap, more so
than anywhere else in the amp. I am unsure if this is normal? Thx.

That is a ceramic type cap and they are known to be microphonic. We can't tell if yours is worse than normal without hearing the tap test but it is very unlikely that it is causing the problem you are trying to fix.

[TigerAmps]

Also chop stick test the connections on the power and rectifier tubes, in particular the grounds soldered to the power tubes.
I had a Bassman that one of the 6L6 tube grounds looked good, but you could move the solid wire thru the solder on the tube socket.
Power cap hum would be there on startup; I don't think it would get louder with time.
Also, freeze spray might help find the issue. Did you tension the tube sockets and try removing tubes systematically to isloate pre-amp vs power amp source of the hum?
Did you verify the bias (just to rule it out)?

[keithb7]

Well after two days of heavy use with replaced 6L6 power tubes, symptoms are gone. Thanks for the suggestions everyone. Sometimes I tend to overthink things. All it was was a bad tube or two. All good to go now. Cheers!