Super Reverb cooks Reverb Driver 12AT7

[pbabadoo]

Hello all. I'm new to the forum and looking for some help. I've got a SF Super Reverb from somewhere around 1973 - I think. No master volume. It's kind of a mutt with a mixture of speakers, a correct replacement reverb tank and beat-up lookin' grill. But it sounds like heaven.

The problem is that the reverb driver tube gets so hot with the amp just sitting there idling that I can't even touch the tube shield/cover. Then the reverb stops working after a few hours of use. A new 12AT7 tube gets the reverb working again. Obviously, the 12AT7 is drawing way too much current. What am I looking for?

I've done most of my own repair work in the past (and even pieced together a funky 50 watt version of a 50's Deluxe using the guts of an old Silvertone Twin Twelve), so I'm not afraid to get in there and work on the thing.

Thanks for any help.

[booj]

It's normal for the 12AT7 driver to get a bit warm, but you're talking nuked! Not too many parts there. Plates dirctly to Screen supply. Cathode resistor and cathode cap, 1 meg grid res. to gnd. Fed by 500pf cap. I'd check resistance values first. That done, I'd replace the cathode bypass cap. If that didn't fix it, I'd go for a new tranny.

[pdf64]

What is the circuit number on the tube chart?
The cathode resistor on V3 needs to be 2k2, some SF circuits used lower values which ran the tube a lot hotter, above the max dissipation.
After checking all the parts that booj advises, just replace that cathode resistor and it's bypass cap - it's had a long hard life.
What tubes are you putting in there? It's a 1 watt power amp and needs something beefy, NOS CV4024 are the best choice for this application. Peter.

[pbabadoo]

Thanks guys. I will have a few hours tonight to work on it and check out your suggestions. 12AT7's are usually Electro-Harmonix or Groove Tubes.

[pbabadoo]

Found my problem ... and another.

The cathodes on the 12AT7 are connected to a 470 ohm 1W resistor. Other side of that resistor goes to ground. The resistor was actually broken/burned in half - it looked like an extra Black color band around it and I missed it the first time through. The even wierder part is that there is no evidence that a capacitor was ever there connected in parallel with the resistor - the wax on the solder is undisturbed.

So it seems like I need to go with a 2k2 resistor and add the 25/25 bypass cap. Is that correct? Will that change the sound of the amp any? This thing has just the fattest sweetest sound I've ever gotten from an amp.

Now that I think of it, when I got this amp years ago, it was missing it's reverb tranny. Maybe it fried and this is why. hmmmm.

By the way, I think the amp is an AB 568 Circuit by comparing to schematics. The label inside is torn up and and unreadable.

[booj]

yup. 2.2k and 25/25. Guess you'll find out about the sound when you try it. Who knows? You might like it better stock.

[Bruce / Mission Amps]

I'm not sure if you meant there was no cathode bypass cap but if so, look to see if the old bypass cap is actually connected to the 12AT7 tube socket and grounded at the RCA jack.

[pdf64]

If the 470 was stock then it wouldn't be bypassed, see schematic 012108
http://www.schematicheaven.com/fende...-bmstr_rev.pdf
A buddy's TR had that arrangement.
But the weird thing is that those circuits were late 70's and had MV.
I can't see any way in which this will change the sound of a non MV amp.
However, if you are concerned, then bypass the 2k2 with a 22uF cap in series with a 470 resistor. That will reduce the drive signal to the reverb tank a little, and pull the tube back from meltdown underbias. Peter.

[pbabadoo]

Thanks again all. I've ordered parts to put in the 2k2 resistor and bypass cap.

Amp is not a Master Volume, but could have been built as late as 1975. Maybe there's circuitry from a later MV amp there? Who knows...