problematic PI tube or not

[kaiomenos]

hallo
i have a fender super reverb silverface which had hum.
after some tube swap i found out that the hum came from the phase inverter tube
i replaced with a new one and voila problem solved
BUT after 40 hours of use i had the same hum...
it was the pi tube again
replaced again with a new one and ok now..
but i'm concerned. I don't get it. Will i have to change the tube every some hours of use.I know this is not normal.
So what could be wrong?
where should i check?

[stokes]

These amps have a "bias balance control" instead of an actual bias set control.It was used so you dont have to use matched pairs of power tubes.If the bias voltage is not balanced between the two tubes it will induce hum.There are other places to look as well but this would be a good place to start.Check the negative voltage on pin 5 of each power tube,if they dont match use this pot to adjust if need be.

[Rob Mercure]

"Stokes" are you sure? As best I understand the circuit it alters the voltage between the output tubes to "balance" the idle currents - unmatched output tubes will, by definition, have different voltages on each control grid (unless the screens are truly doing something "interesting"). I think that you meant idle current - this can be "matched" by ear on this anp - as the designer's intended - or but soldering cathode current sensing resistors between pin 8 on the output tube sockets and ground - I like 1 ohm, 1% cuz the reading is mV reading is directly proportional to the mA reading.

But of course you know this, the info is actually for Kaiomenos but I'm still wonder why replacing the 12AT7 has "fixed" this in the past unless it's just been fortuitous coincidence as a random pick of "unmatched" 12AT7s would "swing" the problem one way and then the other (softer, louder hum) while a matched section dual triode would just preserve status quo.

Hopefully this is a "screwdriver" fix.

Rob

[Mars Amp Repair]

any chance the tube socket has corroded connections? Replacing the tube could just be stirring things up in the pin connectors, to just have them get poor connection again after going through heating & cooling cycles a number of times.
Try wiggling the tube around & see if the hum changes. If so, you could either have the corroded pin connectors in the socket or pehaps a badly soldered from the factory connection on the tube socket. I've seen this in 35yr old amps. Somehow the connection was fine for 35yrs & today decides to give up.
To remedy the corrosion issue, you can take a used tube & score the pins with a fine file...& then work it in & out of the socket effectively cleaning off the corrosion. Then spray a fine coat of contact cleaner on the same old tube pins & work it in & out....
Of course you could just replace the socket if it's too far gone....glen

[stokes]

Rob,yes the circuit does "alter" the neg voltage between the two output tubes.In a normal bias supply,even if you have unmatched power tubes there will be equal amount of neg voltage on both grids but the current draw will not match.There is a fixed amount of negative voltage feeding the balance pot,by adjusting the pot you balance this voltage between the two grids thereby balancing the current draw in an "unmatched" pair of tubes.In a push-pull output if the two sides of the output are matched it will cancel the hum that is induced from the ac filaments,which is present to some degree no matter if you have a CT on your heaters or a balnced pair of 100ohm resistors on the heaters,the push-pull output will further cancel this hum,but the tubes must be balanced.This is why a single ended output has more hum,it doesnt have the hum cancelling properties of a push-pull. As I said there are other places to look for the hum,but if the tubes are not balanced you will go in circles trying to find it.So I am not offering this as an absolute fix,but as a necessary step to trouble shoot the problem.It is possible that as the PI "burns in" or wears to some degree it is inducing some hum that would otherwise be canceled by the properly balanced push-pull output.So it is possible that may be all that is happening here.

[stokes]

Rob,reading your post again,I think you are looking at the neg voltage on the grids and the current draw in reverse,sort of.If you have two unmatched output tubes they will have the same negative voltage on the grids of each,but the current draw wont match.Even if you were to pull one of the tubes and measure pin 5 of each socket the tube and the empty socket will have the same or very close volts on each.There is little if any current in the actual bias supply so the volts going to the grids wont change with unmatched tubes or even with only one tube in place.

[Rob Mercure]

Stokes,

Guess I was unclear but doesn't the bias balance pot provide, if desired, differing control grid voltages to establish more or less identical plate currents in each output tube to "buck out" the hum? But perhaps I misunderstood your statement that I was replying to: Check the negative voltage on pin 5 of each power tube,if they dont match use this pot to adjust if need be. which seems to imply that identical negative bias voltages for each output tube is the goal and not current balance between each output which may or may not require identical voltages.

But as Churchill stated concerning the USA: "Two great peoples separated by a common language." <grin> So perhaps we're just "circling" each other.

Rob

[stokes]

Rob,sorry,that was a misstatement by me.It is the current that you want to match,not the bias voltage.Somehow I got the idea in my head that we were looking at a matched set of tubes.Its just a case of age causing the mind not to keep up with the mouth.Looks like we are on the same page again.Sorry.

[Mars Amp Repair]

Good issue to bring up Stokes, I've wondered why the result of mismatched bias current produced a HUM. Hum bucking in the OT sounds plausible.

Also makes sense that the more power tubes you have, the more hum is induced when the current is mismatched. The loudest hum I ever heard was with an old Ampeg SVT when bias was mismatched. It almost sounded like bad filters.
glen

[stokes]

Anytime you have two components working together,out of phase with each other there is "phase cancelation" and certain frequencies get canceled,that is the idea behind "humbucking" pickups.In a push-pull output you also have most odd-order harmonics canceled,that is the reason a single ended out put has a somewhat richer sound,odd order harmonics are amplified-as well as some of the undesireable hum.A good way to get rid of the hum associated with single end is to connect the heater CT to the cathode of the cathode biased output tube,puts the ground reference at 25 or more volts above ground and keeps the hum out of the signal.But I'm getting off topic here.

[Enzo]

Adding a stage to the B+ filter helps. The hum in the SE amp stage is B+ ripple. That is what cancels out in PP amps since the ripple on each side is 180 degrees out.