Fender SF Twin Reverb red plating and mains draw

[fredcapo]

I am looking at a CBS era Fender SF Twin Reverb that has the two far right 6l6 with intense red plating (long vertical along inner center of tube) the left inner pair looks fine. The tube heat seems more intense than i would expect. The amp sounds good. What voltage node would be at fault?

Also how do I reconcile that the AC mains current draw shows 3.5 amp on a line monitor, while the fuse rating is 2.5. Thanks.

[xtian]

Fuses are not well calibrated. Don't trust them. Hopefully they will protect your amp under catastrophic failure.

Your amp is readplating and drawing a lot of current. You didn't give us many clues to work with, so here are some basic questions.

First things first: set the bias properly. Have you done this already?

If bias is set properly, but tubes are still redplating, swap the left side pair with the right side pair. Does the redplating follow the tubes or stay at the same sockets?

How old are the filter caps?

Have you done any work on this amp?

[bob p]

Output Tube Biasing Problems

[fredcapo]

So each pair of 6L6s have a common cathode connection, and both tubes of the pair have the same red glow. Seems 8nlikely both tubes would be bad...

I dont think these amps have a bias pot, but have read they have some type of bias balance pot. Maybe it is off balance...

[xtian]

Simply put, you need to check the plate dissipation on each pair of tubes, left and right. Why guess? Do you need to learn how?

The Last Word On Biasing

[bob p]

take some measurements and report back to us. then we can tell you more. until you start working your way down the decision tree there's no way to tell what's going on. it could be any of the causes already listed.

[Enzo]

The two tubes on one side are in parallel, more or less. If one of those tubes shorts its grid, it can kill bias to that tube. Unfortunately it can also then kill the bias to its partner tube.


Just pull the two tubes, and measure voltage around the socket. Got B+ on pin 3 and 4 of each? Got good bias on pin 5 of each - about -50v. That will tell you whether the tubes are the problem or if the tubes are glowing because the socket is not providing good voltage.

[Chuck H]

Since it's both tubes on one side, and it's reported as "intense", I would say DON'T TROUBLE WITH THE TUBE SWAP OR CHECKING BIAS WITH SOME KIND OF PLUG IN TOOL. Pull the tubes and take the chassis out of the cabinet. Turn it on and check for bias (-V) on pin 5 of the offending sockets. I suspect there is a cracked or badly drifted bias feed resistor, a bad solder connection for that resistor or some other fault that would ordinarily connect those tubes to their bias supply.

[fredcapo]

Thanks for the advice. Will try tomorrow. i dont do this work very often so appreciate the guidance.

[The Dude]

Since it's both tubes on one side, and it's reported as "intense", I would say DON'T TROUBLE WITH THE TUBE SWAP OR CHECKING BIAS WITH SOME KIND OF PLUG IN TOOL. Pull the tubes and take the chassis out of the cabinet. Turn it on and check for bias (-V) on pin 5 of the offending sockets. I suspect there is a cracked or badly drifted bias feed resistor, a bad solder connection for that resistor or some other fault that would ordinarily connect those tubes to their bias supply.

^^^^^^That......or a leaky coupling cap from the PI to that pair of tubes.

[Chuck H]

or a leaky coupling cap from the PI to that pair of tubes.

^^^^^^^^ Yeah! Or that.

[bob p]

... all of which are mentioned in RG's list.

[fredcapo]

There was no bias on the red plating pair. I lifted the connection from the tube pair on the balance pot and voltage was on the pot. I cut out and replaced the 2000 pf cap and reattached and looks like its ok. 50 volts bias on each pair...ok?

[The Dude]

That oughta do it........ assuming you mean -50V.

[g1]

I guess thats an AA769 with the 2000pf's from grids to ground?
I'd think it's pretty rare for one of those to short, although most of them probably get removed before they have a chance.