red-plating 6l6's?

[chuckb]

hi guys, i have a traynor custom special YBA-3 with 4-6l6's and it's red-plating. I checked the voltage at the plates and i got a reading of 568v. the voltage on the cathode is -55v. is this to high? shouldn't it be around -21 or so? i inspected the bias caps and one of them appears swollen on the pos end, could this be the problem for the bias being to high? I tried adjusting the bias and the lowest i can get it is -47v. Any help would be great!

Chuck.

[chuckb]

oh and the fuse keeps popping when the amp is driven?

[Jazz P Bass]

First off, the bias pin of a 6L6 is pin 5.
That is not the cathode pin (pin 8)

Is the redplating occurring while being driven or at idle?
You do have a speaker or dummy load attached, right?

Anyway, if the bias cap is bulging, simply replace it (maintain the proper orientation of the + lead to ground).

Check the screen resistors for proper resistance & wattage rating.

I believe you are confusing the bias voltage reading in how it applies to an output tube.
The higher the negative voltage reading, the Less the tube conducts.
At -51Vdc the tube should be biased a tad cold but it will not allow the tubes to redplate.
-21Vdc is not a normal bias voltage for a quad of 6L6 tubes.

Redplating tubes are conducting full on, so it's possible that the current draw will blow the fuse.
What fuse was installed?
The factory calls for a 7 amp slo-blo-.
That's a lot of current.
You may have other issues with the amp.

[Mick Bailey]

You may have other issues with the amp.

Like the rest of the caps...

Finding one blown cap in an old Traynor is like finding one rabbit with Myxomatosis.

[Chuck H]

Just to be clear on this, more -V bias = cooler and less -V bias = hotter. So going to -21V instead of -55V won't cool the tubes it will heat them.

Since you DO have bias voltage I have to ask, is the red plating happening on all the tubes? If it's only happening on one tube, you likely have a bad tube.


As noted, if you have one bad cap you may have others. Time for new electrolytic caps all the way around then. It sucks, but it happens.

[The Dude]

I could be mistaken, but if memory serves, some of these had flyback diodes. If this amp does, you could have a leaky diode also. You can temporarily unsolder one end of each of them to see if the problem goes away.

[Chuck H]

Good catch! I admit I didn't look on the schem. I wasn't aware that some models had the diodes.

I'd replace the caps anyway.

[Enzo]

Sure, but the diodes are parallel the tubes, so if the diode goes leaky the transformer current goes up, but current is shunted past the tubes by the leaky diodes. so leaky flybacks ought not be the cause of redplating.

[The Dude]

At any rate, my first order of business would be to insert a bias probe and see what the tubes are actually drawing for current. If you don't have one, buy one. It's one of the best investments you can make for troubleshooting these sorts of things and they don't cost that much. They also work pretty well for biasing an amp.

[The Dude]

Sure, but the diodes are parallel the tubes, so if the diode goes leaky the transformer current goes up, but current is shunted past the tubes by the leaky diodes. so leaky flybacks ought not be the cause of redplating.

My thought is that, if the diodes are leaky, it would be similar to an OT with a semi-shorted primary (since the diodes are across the primary and not to ground in this case) causing excessive tube current. The symptom "the fuse keeps popping when the amp is driven" especially makes me think leaky diode. Not so? It wouldn't be the first time my logic was flawed.

[The Dude]

There are different versions of this amp. Schematic at this link shows one with the diodes.

http://www.prowessamplifiers.com/sch...Version_3.html