Hum in Super reverb

[ampchimp]

I have a super reverb(SF) that has a hum on with the volume all the way down, then is clear at about 2-4 , then the hum returns and goes up with the volume, although the guitar signal is stronger. In the the clean spot, the amp sounds fine (low volume practice). I've switched a few tubes but no change.

Thanks

[txstrat]

Hello and welcome to the forum,

when the hum goes down with turning down the volume pot the source of the hum lies BEFORE that pot.
Which tubes did you change? The first two of the tubes in your SF Super Reverb are the preamp tubes. The first tube is for the normal channel, the second is for the vibrato channel. That's what you affect with the volume control(s).
Does the hum go up with both channels volume?
When you pull the tube(s) does the hum disappear?

[loudthud]

Lets be clear here. Is this a Fender Super Reverb built in the late 60's early 70's or something you built? Regardless, check for corrosion around the pot bushings and the brass grounding plate and input jacks. Clean and tighten.

[ampchimp]

The Fender super reverb is a silverface, not sure of the year(belongs to a friend). Talking out the second tube it still hums with the volume all the way down. removing the fourth tube the amp is quiet and the normal channel works ok. I tried other tubes as well. So I'll check the connections of the pot and input and visually inspect when I can pull it out.

Now, coincidently I have recently made an amp, a 2 12ax7, 2 6ak6, that has the same symptom. Maybe I should make another thread. It has hum at zero volume and then at about "2" it has a clean spot and sounds good. It sounds loud for the low volume setting and then turning up the volume the hum starts again and increases although the guitar signal remains louder. I have checked the grounds and moved wires around with no effect. Without the first tube it still hums, taking out the PI tube it is quiet. It is a longtail-pair PI.
This is a lower voltage amp(160-180v) and I am using low value resistors (100r.200r,390r) between the filter caps(22uf). Perhaps the resistors need to be higher or maybe the filter cap(s)?.
Thanks

[Enzo]

WHen the hum is least - nulls - at some point above zero on the volume, it means you have two sources of hum. They are 180 degrees out of phase. The volume control is turning one of them up and down. When the volume control turns its hum so it is the exact level of the other hum, then they cancel out. One would be from before the control, and the one there when it is at zero is after it.

[ampchimp]

There was a poor solder connection on the ground sction of the input jack(insulated type) and after cleaning up ground wires, bother solder and locations, the hum was still there, maybe a little less. There are a lot of good threads in this forum I might add. Ok the main hum came from too low a value of the resistors between the filter caps. They are now 100r, 1k and 2k2.
I was concerned about dropping the voltage more than nessesary but these new values only dropped a few volts. very quiet now. Circuit is similer to 6g3 circuit but lower voltage and lower wattage.

Now to get to Super reverb....Thank you for the info

[pdf64]

Re SF Super Reverb, have you confirmed that the heater circuit has a good ground reference? If it's being done via 100ohm resistors, or a pot on the back panel, then a bad output tube could have damaged one or both of them. This would allow hum into the high impedance grid circuits, causing this type of symptom. Peter.