71 Marshall Red Plating, blowing mains fuse.

[cluster]

Hello All, I have a 71 Super Bass that is red plating on one side. Result is: blown tube and mains fuse. can anyone here guide me through some trouble shooting on how to track down the problem? Red plating occurs when playing hard (loud). Could some DC be getting to the grid? (leaky coupling cap). Could the bias caps be leaking DC? How would you test for this. Any recommendations? Appreciate any help.

thanks,

Joseph

[cluster]

just a note to say that the red plating comes on slow and only on onside of the power amp. that is the 2 on the right side when looking at the back of the amp. also, the red plating comes on with some oscillation and noise. it can also be induced but tapping on the power tube with a chop stick. Clues??

[Chuck H]

Both tubes on one side sounds like a leaky cap to me. Testing caps for leakage is just a matter of lifting the downstream end and checking for DC there. But it's an old amp. It could still be that your biased too hot and/or have a failing bias supply.

Did you replace all the tubes or only the two on that side? Only the one blown tube?

What is your plate voltage and bias current?

Have the bias supply caps ever been replaced? They are often ignored when a "cap job" is done.

[Leo_Gnardo]

What Chuck said ^^^, plus have you tried swapping the output tubes, left pair for right pair (I'm assuming this is a 100W amp). If the problem follows the tubes, then one of the tubes that goes red is kapoot. One failing tube in a parallel pair will pull down the bias voltage and force its mate to also red plate.

[nsubulysses]

Have the bias supply caps ever been replaced? They are often ignored when a "cap job" is done.

WHy does this happen so much? Aren't these arguably the most important caps in the amp because if they don't work you will probably lose the power tubes?

So insane to see an amp totally recapped EXCEPT the bias caps but it seems to happen a lot

[Leo_Gnardo]

WHy does this happen so much? Aren't these arguably the most important caps in the amp because if they don't work you will probably lose the power tubes?

So insane to see an amp totally recapped EXCEPT the bias caps but it seems to happen a lot

True 'nuff. Early Marshalls (thru 70's) usually employed 160V rated bias caps, though they seldom if ever saw half that much voltage. And failures were rare. At this time all those amps are 40-50 years old, electrolytics of that age should be replaced as a matter of course. Unless the amp owner likes to whistle in the graveyard, and preserve his "vintage" bias filters perhaps to potentially impress a future amp purchaser. 100V caps are plenty enough for this duty.

[pdf64]

Do all 4 power tubes have grid stoppers, eg 5k6?
If not, then try fitting them; they act to suppress oscillation, which may be causing the red plating.
If yes, then check their value, verify solid connections by wiggling them a bit; decades of vibration and thermal cycling may have damaged them.

[Chuck H]

More good stuff ^^^^^^^^^^^^^^^^^^^^^^^

And... Dirty or oxidized tube socket holes can suffer intermittent contact as well.

[cluster]

Thank you so much to every one for such great advice and technical know-how. I left my studio a few hours ago but will return tomorrow to try your recommendations. In the the meantime can some one tell me how to clean tube sockets?? Thx. Also, so info on my amp. 71 Super Bass. I run it with a Variac to keep the AC coming in to 117VAC. Plates at 117VAC render about 498VDC, screens ~492VDC (have to recheck that one value). Bias voltage ~ -40 (again, I need to recheck). This red plating was happening before and I took it to an amp tech. He replaced the coupling caps, so I doubt it would be those. I'll switch up the tubes and see if it's the tubes or the circuit. Man, these old amps can be cranky. I've also heard that these old amps don't like attenuators? Is that true? Thanks again for all the help.

Joseph

[Chuck H]

Clean sockets by UNPLUGGING THE AMP and spray a little contact cleaner on the tube pins (never directly into the sockets), insert the tube and gently wiggle it around rocking in a tiny circular motion. Then do it again. Then, if the pin holes seem loose you can usually tighten them a bit with a pointy object about the size of a paper clip (like a straightened, sharpened end on a paper clip ), insulate the un-pokey part by wrapping it in tape just to be safe, and poke it firmly, but gently between the hole edge and the metal contact inside. Not into the tube pin contact, but beside it. This will push the contact metal back toward where the tube pin inserts. If you take this too far you can spring the contact or restrict it too much and the tube pin will jam against it when inserting so don't go all caveman on it.

The coupling caps are NOT the bias caps. The bias caps are the electrolytic filters in the bias supply. Further, if your tech replaced the PI coupling caps with vintage pulls or even NOS caps you may still have a leaky coupling cap.

But follow the above advice about swapping the tubes first to see if the problem follows the tubes or stays with the sockets.

And... Did you replace the blown tube, both tubes on that side or all four? Once upon a time, and if the tubes weren't too old it was possible to just replace a blown tube with one of the same brand and marking. Not so today where any two tubes, even of the same brand and marking, but different batches, can vary quite a lot in their bias characteristics.

[DRH1958]

One additional thing to add to Chuck's great advice, use the type of cleaner that does NOT leave lubrication behind. That type is for pots, etc. Make sure you get the type that evaporates completely for sockets. It's a good idea to keep both types on hand.

[Chuck H]

Right! I did indicate "contact cleaner" but many do have a lubricant in them as well and are sometimes called "potentiometer cleaner" or they say "contact cleaner" and add "with lubricant" somewhere in the description rather than "and lubricant" right in the title. Buyer beware.

[cluster]

Thanks again so much got the great help. I use DeXoit for pots. Does anyone have a link for the contact cleaner for tube sockets? Thanks again.

[Leo_Gnardo]

Thanks again so much got the great help. I use DeXoit for pots. Does anyone have a link for the contact cleaner for tube sockets? Thanks again.

My method for octal sockets is, squirt a dab of DeOxit D5 on the end of a bristle-type pipe cleaner. Scrub that into each electrode, let it sit a couple minutes, then scrub again with dry pipe cleaner. You'll see black/brown oxides & other corrosion crud collect on the end of the pipe cleaner. That will remove even more corrosion. Repeat if necessary then finish up with a squirt of non-residue solvent spray. If you haven't any of that, soak the end of a clean piece of pipe cleaner with high purity alcohol (ethyl 95% aka EverClear, or denatured alcohol) or acetone, scrub those electrodes one final time, let dry, put tubes back in sockets, ready to go.

One popular spray solvent is Caig contact cleaner wash, here's a link to one of our fave suppliers:

https://www.tubesandmore.com/product...t-cleaner-wash

I've heard a rumor it's even sold at Home Despot, but haven't seen it there myself. It's cheap, very good at what it does & no studio should be without it.

An early version had the unfortunate property of causing certain plastics to crumble & disintegrate almost instantly. The current version, I've been using at least 10 years now with no ill effects. Of course, have plenty of ventilation. It's mostly a mix of alcohol & acetone.

[cluster]

So today I decided to take the amp back to the original tech that did the initial work to correct problem. I felt that since the initial problem wasn't fixed in the first place, that he should have first crack at correcting it. Let's see what he finds. I'll report back in a few days once he gets back to me. Thanks again and hopefully this will help others with red platting issues. BTW while doing my research I found this interesting thread on red plating at vintageamps.com. Has anyone read it before? Vintage Amps Bulletin Board ? View topic - STRANGE RED PLATE ON TWO VALVES ! my Marshall is ILL . . .

Thanks again,

J