Fender Super Reverb silverface Hum

[Ampzone]

So this 1973 Silverface Fender Super Reverb has been rec-capped power/preamp, new tubes installed, but it has a loud harsh hum when you plug into the Vibrato channel two section (plugged into input one). Also the signal is very very weak in the vibrato channel, and low, but when you turn the volume up to 5-6 it hums like crazy.


Channel 1 inputs are fine and normal, the bass and treble pots behave as they shouldl. No on off pedals or reverb tanks plugged in.
Any ideas?

[nickb]

Are you saying that the vibrato input #2 works OK? If it's just the vibrato input #1 that hums then I'd be looking at the continuity of the jack to ground.

[Ampzone]

Are you saying that the vibrato input #2 works OK? If it's just the vibrato input #1 that hums then I'd be looking at the continuity of the jack to ground.

Thanks for the reply, so to clear "Normal" channel 1#2 inputs behave normally. "Vibrato Channel" input 1 is super, super low volume at 1-4 then at 5-6 it's nasty hum city almost like a feedback loop. I did not check the 2nd input of the vibrato channel yet.

And as I said the vibrato, reverb tubes are in but no rca connectors (re-verb tank) or vibrato foot switch are inserted.

[nickb]

Short the grid i.e. pin 2 of the vibrato input 7025 to pin 3 or ground. If the hum goes it's probably the input wiring or jack grounds. Failing that, it could be a bad tube - try swapping it.

You mentioned a re-cap. It seems unlikely unless some rewiring was done, but have you checked the plate supply for that vibrato input tube? Visually inspect and/or measure the AC Volts at the junction of the two 100K plate resistors. It should be just a few mV.

[Ampzone]

Short the grid i.e. pin 2 of the vibrato input 7025 to pin 3 or ground. If the hum goes it's probably the input wiring or jack grounds. Failing that, it could be a bad tube - try swapping it.

You mentioned a re-cap. It seems unlikely unless some rewiring was done, but have you checked the plate supply for that vibrato input tube? Visually inspect and/or measure the AC Volts at the junction of the two 100K plate resistors. It should be just a few mV.

Good ideas, I will check in the AM and report back

[Ampzone]

Good ideas, I will check in the AM and report back

I did as you suggested, with the amp on and a guitar plugged in the viibrato channel I, it did not pass any signal from the guitar and there was a very low mild buzzing that got slightly louder the more I turned up the volume. But the hum was completely gone.


I will undo the jumper from Pin two and three and put in a different tube and see what gives and report back

[nickb]

I did as you suggested, with the amp on and a guitar plugged in the viibrato channel I, it did not pass any signal from the guitar and there was a very low mild buzzing that got slightly louder the more I turned up the volume. But the hum was completely gone.


I will undo the jumper from Pin two and three and put in a different tube and see what gives and report back

If that killed the hum then it's most likely the input wiring.

[Ampzone]

If that killed the hum then it's most likely the input wiring.

A replacement tube solved it from howling at low volume but if I adjust the bass above 3-4 it will groan/howl and it will sound like high pitched whistling wind if you adjust the treble high. So it's better but still odd. I will try and post a quick video clip...

hold on.

[Ampzone]

here is video with a guitar plugged into the vibrato channel. You can hear the noise

https://vimeo.com/131974824

[nickb]

here is video with a guitar plugged into the vibrato channel. You can hear the noise

https://vimeo.com/131974824

So not hum at all! More like a microphonic tube. Try a known good one. The 7025 was a designed for low microphonics and so has a short plate.

[Enzo]

Yeah, that is feedback of some sort. We generally reserve the term hum to mean the 60hz sound like touching the tip of a guitar cord with your finger.

Flick each tube with your fingernail, like flicking a bug off your sleeve. See if any tubes are sensitive to it.

It sounds like microphonics, but it COULD also be related to a loss of decoupling caps in the power supply.

[Ampzone]

Yeah, that is feedback of some sort. We generally reserve the term hum to mean the 60hz sound like touching the tip of a guitar cord with your finger.

Flick each tube with your fingernail, like flicking a bug off your sleeve. See if any tubes are sensitive to it.

It sounds like microphonics, but it COULD also be related to a loss of decoupling caps in the power supply.

Thank for the heads up, well I tried other tubes in V2 and it calmed down now. Now when I remove V3 (reverb) tube the sound tone gets a little sweeter. Also this V3 position tube is super super hot! Almost too hot to touch! Is this normal?

As far as decoupling caps I replaced all 7 25uf 25v caps on the board and all of the power supply caps in the top can compartment.

I checked all of the .022uf and .1uf wax covered caps with my Anatek ESR meter and got under 20 readings on all of the .1 caps and 40-68 on the .022uf and .047uf caps.

Any particular coupling cap in the power supply I should re-check?

[Ampzone]

could a higher resistor value around V3 cool things off for this tube it's scalding hot?

[Jazz P Bass]

V3.
Is that a 12AT7?

What are the plate & cathode voltages?

Vdc & Vac.

[g1]

V3 is the reverb driver (12AT7) and it will run quite hot. It idles at higher current than the other preamp tubes and loads down the B+ to a certain extent. When you pull it the B+ will increase a bit to the other preamp tubes and that may be responsible for the tone difference you are hearing.
But maybe it's running even hotter than usual, so check those voltages Jazz mentioned.