Power tubes glowing and loss of power

[Bluefinger]

Hi,

What would be the most likely cause. There is a BF Bandmaster, that suddenly started to lose power. It turned out that both power tubes all of a sudden started glowing. It was running with Tung Sol 5881s. The plate voltage is almost 10% above the specified limit of those tubes but they were moderately biased and have been running great and flawlessly for half a year or so with decent use. Could it be that they finally gave in due to the high plate voltage? But is it likely that both started glowing at the same time? Wouldn't just one give in or does the other one always die as soon as the first one dies? I heard that the Tung Sols can take it if not biased too hot but I guess that 400V limit isn't there for nothing either ...

What are other likely causes? Defect bias circuit?

thanks

BF

[Jazz P Bass]

My first guess: Loss of bias voltage.

[MWJB]

Just about every amp design that Fender had from the late 50's ran 5881 at well over 400vdc, they'll take 500v all day long. It's the plate current that kills them, you shouldn't need more than 35mA per tube for them to sound right.

Once they have redplated for a while they'll be more prone to doing it in the future.

[Bluefinger]

Actually they were biased at 32mA so I assume there must be some kind of other problem ...

[Jazz P Bass]

So that is a reason to not check the voltages? (Enzo Rule #11)
Yank the power tubes, check all of the static power tube voltages.
B+steady, bias steady & at the grid pin, cathode connected properly, screen resistors healthy.
Insert a test signal into the amp & observe the signal at the (empty socket) output tube grids.
If everything checks out, put in new tubes.
And so it goes.

[Enzo]

Whatever they WERE, they ain't now. If they are red plating, then you lost your bias.

As MWJB said, it isn't your plate voltage.

Bias can go away for a number of reasons. The little bias power supply can fail - open resistor, shorted cap, open diode, broken solder connection, etc. Sometimes when one power tube shorts something to the control grid - say for example the nearby screen grid - it drags down the bias supply and that causes the other tube to lose bias as well.

Take JPB's advice, pull the power tubes and check the socket pins for proper voltages. Without tubes, the B+ will read even higher, but we are mainly interested in what is happening on pin 5.

[jmaf]

Sovtek 6L6WXT+'s can take a whopping voltage. They have a colder tone than other similar current production tubes, but they can take 500+ volts without blinking. You may want to give them a try if your Tung-Sol's have been damaged due to the lack of bias.

[Gtr_tech]

I rather like the WXT+. Put some in a BF Pro Reverb some time ago and they sounded great. However, they *don't* like ultralinear operation. Found that out the hard way in a Studio Bass head.....

I hope yer not talking about the WGC, those can take anything you throw at them but sound like crap in just about anything.

[Bluefinger]

ok, thanks for all the advice. I'll check that when I get that amp tonight.

[jmaf]

I rather like the WXT+. Put some in a BF Pro Reverb some time ago and they sounded great. However, they *don't* like ultralinear operation. Found that out the hard way in a Studio Bass head.....

I hope yer not talking about the WGC, those can take anything you throw at them but sound like crap in just about anything.

Yes, was talking about the WXT+. The Sovtek 5881's (their GC) can take a beating indeed, but they sound dead IMO. If someone asks me to install a pair for them that should last a tour and they don't care about tone so much, I'd put in 5881's.

Good tip about WXT+ and UL, had no experience with that, thanks.

Edit: I now realize you said WGC, I may be mixing things up, I meant the elephant feet ones are the ones I'm familiar with, not sure if it's the WGC you meant.

[pdf64]

Which 6L6CG types are suitable for the late 70s Fender UL monsters?
I hadn't appreciated the difference between the Sovtek 6L6WXT and WXT+ before; anyone know of an EU / UK source for the WXT+?
Or are the EH 6L6GC just a re-brand of the WXT+?
Pete.

[Gtr_tech]

Yes, was talking about the WXT+. The Sovtek 5881's (their GC) can take a beating indeed, but they sound dead IMO. If someone asks me to install a pair for them that should last a tour and they don't care about tone so much, I'd put in 5881's.

Good tip about WXT+ and UL, had no experience with that, thanks.

Edit: I now realize you said WGC, I may be mixing things up, I meant the elephant feet ones are the ones I'm familiar with, not sure if it's the WGC you meant.

Well the ones with the 2 oval cutouts in the plate (each side) are the 5881/6L6wgc and its variants (different bases).
http://www.newoldsounds.com/images/T-SOV-5881WXT-P.jpg

[Gtr_tech]

Which 6L6CG types are suitable for the late 70s Fender UL monsters?
I hadn't appreciated the difference between the Sovtek 6L6WXT and WXT+ before; anyone know of an EU / UK source for the WXT+?
Or are the EH 6L6GC just a re-brand of the WXT+?
Pete.

NOS 7581a is pretty good sounding and reliable in UL amps. I haven't tried the JJ 6L in a UL amp yet, but they're pretty stout. So I'd expect them to do the job nicely.

Generally the better sounding Sov 6L6 types are the ones with the 3 small holes in the plate. They market them under several different names, but they first came on the scene as Sovtek 6L6WXT+. I see they are under the EH6L6 moniker as well now.

[jmaf]

I see they are under the EH6L6 moniker as well now.

I hear that almost every one of the russian tubes except the SED are made under the same roof?

[Bluefinger]

I finally got to work on that amp. Everything inside is fine ... voltages and bias are ok. The only thing I have found was that the speaker caple was shortened. It read .5 Ohms from tip to ground. That was also the reason why there was almost no output. With a new speaker cable it ran perfectly for hours. Can a shortened speaker cable cause redplating???

thanks!