5Y3 Red Plate Special

[dkevin]

Hi All!
I am working on a Gibson GA-5 Skylark and my 5Y3 seems to be red-plating. The other tubes are not even warm to the touch but the 5Y3 is pretty hot. All of the electrolytic caps are original (Astron) and I read on a radio repair website that a red-plating 5Y3 is a sign that the input capacitor is shorted out. Anyone here have a similar experience?
I am prepared to replace all the electrolytics. It's just that I have never seen a 5Y3 behave like this.
Appreciate any advice/help
dkevin

[techineer]

The red plates on the 5Y3 are telling you that it is carrying too much current. Any short circuit or very low resistance across the power supply could cause this, including shorted filter capacitors as you mentioned. It could also be a shorted bypass capacitor, shorted tube, or just about anything else.

As a reference point, use a clamp-on ammeter to get a reading of the AC line current or the transformer primary current. Leave the ammeter in place as you troubleshoot - that way you won't have to wait for the tube to redplate every time you make a live test.

Remove all the tubes (except the 5Y3) and check current. If the current drops, one of the tubes was shorted. Re-plug one at a time to find the bad one. If the current is still high, leave the tubes out and keep testing.

With the AC cord unplugged, and (just to be safe) the filter capacitors discharged, use an ohmmeter across the caps. If you get a steady low resistance reading, a short circuit exists. If the reading starts low and gradually rises toward overrange, then it's less likely to be a cap (although they can sometimes break down only at high voltage.)

If they are axial caps, disconnect one end of each cap (one at a time) and do a power-on current test. If the AC current drops dramatically, you've found the shorted component.

If it's not the filter caps, you'll have to try isolating the problem by opening the B+ line at various points. Use the 'split-half' method.... open it halfway down and see which side the short is on. If the short still pulls too much current, it's located between the break and the rectifier. Move halfway back (or forward) and break again. It shouldn't take more than 2 or 3 tries to locate it. This is much easier with point-to-point wiring than with PCBs. If it's a PCB, it's probably better to unsolder component leads one at a time from the B+ trace, testing the AC current each time. You could cut the trace, but I personally don't like to do that.

[Dave Curtis, dB AudioTech]

Just replace all the 'lytics. They need it, anyway. Leave the signal caps alone for now. Check for drifted resistors. I usually replace the power supply resistors with flameproof when I do a "cap job".

[Enzo]

And a defective 5Y3 can certainly behave this way. Have you tried another?

[dkevin]

I tested the tubes (5Y3 included) before I attempted to power up the amp. None of them were shorted. All tested strong. I am going to replace the electrolytics, as a matter of fact. If that does not solve (or make progress toward solving) the problem, I will re-read the previous posts and perhaps consult an exorcist.
Most of what I read on the internet is focused on the red-plating effect found in power tubes. In my (admittedly limited) experience, a cap usually signals its weakness by a loud, low-pitched hum. However, this red-plate problem and the possible cause of a shorted cap is a new one for me.
I'll let you know.
dkevin

[Dave Curtis, dB AudioTech]

A bad rectifier can do this, too:

[Enzo]

Ooooohhh, shiny.

[dkevin]

I began this thread on a quest to discover why my Gibson GA-5 Skylark had a red-plating 5Y3 rectifier. I did as suggested, (reasoning that I should take the advice I solicited ) and tried a "known good" 5Y3 in its place. However, there was no change in the outcome.
Since I was 99% likely to replace the electrolytics anyway, I went ahead and finished up that task. When I was all done and fired up the amp- lo and behold, the beast emerged! All the heaters functioned normally and the voltages were well within reason. The amp sounded good and I felt good about learning something new.
I want to thank all of you for offering me your good advice!
Until next time,
dkevin

[Dave Curtis, dB AudioTech]

On behalf of the group, You're welcome.

Y'all come back, now. Y'hear?

Enjoy the Toy,
Dave