Loud hum in a Super Twin Reverb

[JC@]

This amp was working one day, then suddenly it begun making a very loud hum as soon as it is switched on.
And it is drawing lots of current, 3A or more.
On the tube tester there's no short in any of the power tubes, but they are all the original ones that came with the amp.

With the power tubes pulled out of their sockets the current draw is normal.

Any help please?

[Rob Mercure]

JC,

Well, you've obviously identified the output tubes as the culprit so you might start pulling output tubes in pairs - that is, pull the tubes farthest to the right and left and see whether anything changes. If nothing, replace these and pull the next one on each side - if nothing replace these and pull the innermost two tubes. Unless you've got a leaky coupling capacitor or bias issue (or even, a heater cathode short) I suspect that you'll find that one of the output tubes is drawing too much current and unbalancing the currents in the OT primary thus eliminating it's ability to buck out common B+ hum. Are any of the outputs getting particularly hot? Maybe look at 'em in a darkened room.

If this doesn't provide any information you might carefully put a DMM set on AC volts on pin 5 of the output tube sockets and see if you've got hum on the bias rail - then switch the meter to DC and measure the bias as long as you're there.

While I suspect a bad output tube if this monster hasn't been recapped I'd at least replace the bias filter "fer the hell of it" cuz you've got more stuff to "lose" with six outputs and the large trannies.

Rob

[JC@]

Rob,

I will try that. Thanks for the procedure.
The part I couldn't understand at first is why the power tubes test low, but good, on the tester. I would think that an internal short in a power tube capable of drawing 3 amps should show up both in the tester and the amp, which is not the case.

[Bruce / Mission Amps]

In additon, if the tubes really check OK... I go after the bias supply and see if it is failing, causing the tubes to super conduct when plugged back in.
Or bad tube sockets or wiring to the tube sockets from the bias/grid load resistors.

[JC@]

May very dirty tube sockets (and loose pin contacts) be the cause?
After putting some strong contact cleaner the amp looks almost ok.

I'm going now to retension them and change all the little electros including of course the bias supply ones.

[tortoto]

...change all of electrolytic condencer
20 uf 500 volt dc

[Bruce / Mission Amps]

...change all of electrolytic condencer
20 uf 500 volt dc

No, these amps need a pair of 220uF/350v caps (in series) on the first node to get any real filtering for a high powered amp.

[tortoto]

No, these amps need a pair of 220uF/350v caps (in series) on the first node to get any real filtering for a high powered amp.

......super reverb 40 w...super twin reverb..160 w
i'm sorry

[JC@]

The amp I was looking for help was a Super Twin Reverb, not a Super Reverb.
This amp had maybe three 20uF 500V caps there, but that was not the problem.
The main filter caps were indeed 220uF/350V though..

Anyhow after retensioning the tube sockets, refurbishing the bias circuit and plugging in a new set of tubes, the amp left the bench sounding nice and loud

[mykey]

This amp was working one day, then suddenly it begun making a very loud hum as soon as it is switched on.
And it is drawing lots of current, 3A or more.
On the tube tester there's no short in any of the power tubes, but they are all the original ones that came with the amp.

With the power tubes pulled out of their sockets the current draw is normal.

Any help please?

this is usually caused by a shorted diode in the power supply.
or perhaps a capacitor in the power supply has a broken lead.
whoops forgot to mention could be shorted output transformer
primary winding, probably is, don't try to work on this yourself-
shock hazard.

[Rob Mercure]

Why would a shorted power supply rectifier draw less current when the output tubes were pulled? You might want to review the "clues" to this mystery again <grin>.

Rob

[mykey]

a real common problem in these amps was burned tube sockets: the screen grid
resistor flames and melts down onto the control grid resistor, the socket
then starts arching, but the amp turns on ok after the tube is pulled out,
in the process the diodes are damaged, usually from replacing the fuse
over and over again.
more than likely, if the output tranny is not shorted in the process.
of course problem like this may be past self help point and its time
to visit the fender service center.
i hate to encourage self help in this case since someone could shock themselves pretty badly in the process of working on it.
problem could be solved by replacing tube socket and a couple resistors, but remember this persons safety- is it worth dying over?

[Bruce / Mission Amps]

Maybe you didn't read any of the other hints from other techs... like mine to check the tube sockets and the bias supply caps.
He fixed it over 5 weeks ago with a socket tweak, new bias filter caps and a set of new tubes.
Its always a good thing to read the entire thread before posting.