red plating power tube on fender champ build

Discussion in 'Amp Central Station' started by Joe Les, Oct 23, 2019.

  1. Joe Les

    Joe Les TDPRI Member

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    I just finished my first scratch build, a fender 5f1 champ. It sounds great, but I noticed red plating on the 6v6 power tube. Unfortunately, I didn't notice it until I had played through the amp for about a total of an hour over the course of a couple days because the red plating was only occurring on the bottom side of the tube, out of sight.

    After some research (I'm new to tube amps), I realized that the bias was probably too hot, the indicators being the red plating and also the fact that the amp has a lot of headroom. However, I am also using a sovtek 5Y3 rectifier, and I read that these are low-quality tubes and that they can also cause red plating.

    so, my questions are:
    - is the red plating likely a cause of hot bias, the sovtek rectifier, or both?
    - is anything is the amp damaged other than the power tube?
     
  2. muchxs

    muchxs Doctor of Teleocity

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    No mention of what the 6V6 is.

    EH or Tung Sol "reissues" are prone to quickly drifting off, one of the symptoms being red plating.

    Your amp probably has high plate voltage and hot bias making it a torture chamber for 6V6s.

    Biased Class A means the tube has no time to cool or "rest" like it would in a P-P Class AB1 amp.

    Sovtek "5Y3s ' aren't bad tubes. They're not really 5Y3s, either. They're a Russian type closer to a 5AR4.

    You probably didn't hurt anything if you played for an hour and the amp sounds good. Don't change a thing, stick a JJ in there. It will probably be just fine.
     
  3. theprofessor

    theprofessor Friend of Leo's

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    Like muchxs said, get a JJ 6V6S and put it in there and see how it sounds. I doubt very much it would redplate. Of course we're not working with any data such as plate voltage or plate current. If you can measure those, that's even more helpful. But if not, just get a JJ 6V6S and see how it does.
     
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  4. Joe Les

    Joe Les TDPRI Member

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    Alright, thanks. It's an EH 6v6 so that makes sense. I'll try a different rectifier with a new power tube and see how it goes.
     
  5. jman72

    jman72 Tele-Afflicted

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    I put a pair of Tung sol 6v6s in my 5e3 build a few years back and they redplated almost immediately due to high voltages (my wall voltage runs 124 V or so). Replaced them with a pair of JJs and things have been fine ever since. I highly recommend them.
     
  6. Joe Les

    Joe Les TDPRI Member

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    ok, so should I just try the JJ 6V6 and keep the sovtek rectifier? or should I get a different 5Y3 as well?
     
  7. theprofessor

    theprofessor Friend of Leo's

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    You can just go with the JJ 6V6S and see what happens, or you can get a different rectifier tube, or both. The tweed Champ is spec'd for a 5Y3 rectifier, and any working rectifier that is actually a 5Y3 (this is where the problem often comes in with the Sovtek ones; they're not) will work just fine. I'd go with a 5Y3 rectifier to get the voltages down and see what happens with the power tube that's already in there. Then, if I needed to, I'd replace the power tube with the JJ. Just order the JJ 6V6S and a 5Y3 rectifier (like a JJ or something) so you can have both on hand.
     
    Last edited: Oct 23, 2019
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  8. D'tar

    D'tar Friend of Leo's

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    New to tubes.... Are you positive its not heater glow?

    New Eh 6v6, IIRC, are 14w and state 450v max plate V

    5f1 sounds good runnin hot eh!!!
     
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  9. muchxs

    muchxs Doctor of Teleocity

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    We can work it backwards.

    A classic 6V6GT dissipates 12 watts.

    An inhabitant of ampgarage torture tested a few JJ 6V6s when they just came out. At 19 watts dissipation he figured they had enough even though they weren't redplating. They were running hot enough to burn the "JJ" lettering brown.

    Couple more whisper numbers:

    I usually get bad EH 6V6s as one of a pair. It not like I buy them. They come to me in amps that don't work properly.

    I've had 'em redplate as low as 7 watts dissipation.

    Anyone messing with tube amps needs tubes, anyway. You might consider PM- ing me. I like to meet more Maine- iacs. I don't get over there as often this late in the season although I may head over to Portland to pick up some goodies.

    Seems plausible. It wouldn't sound good for an hour if it was redplating.

    Specified dissipation and actual dissipation can be two different things. Redplating is a little beyond actual max dissipation. Like I said, I've observed EH 6V6s redplate as low as 7 watts. They should go at least 12 before redplating. Some of 'em don't.

    While we're at it they don't stay matched in pairs for every long. That's where we get "orphan" hand me down EH6V6s to burn in our 5F1s.

    :)
     
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  10. Urshurak776

    Urshurak776 Tele-Holic

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    I would move this discussion to Shock Brothers based on the content. Sovtek tubes are not low quality tubes (as mentioned) but the Sovtek 5Y3 is not a 5Y3 (also as mentioned.) When I was new to the game, I bought one also. My recommendation is to use the JJ 6V6's and get a NOS 5Y3. An RCA one will last you for years.
     
  11. muchxs

    muchxs Doctor of Teleocity

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    I prefer to let the audiophiles fight over the RCA 5Y3s. I'll take GE or Raytheon over RCA 5Y3s all day ever day.

    The RCAs are about average for NOS except one little thing. 1950s RCA 5Y3s are prone to loose bases. I've seen more RCA 5Y3s with loose bases than any other brand or type.
     
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  12. Urshurak776

    Urshurak776 Tele-Holic

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    Interesting. A buddy of mine has an RCA 5Y3 in his 5F1. Thats why I bought one initially. I am building a 5E3 (at the rate of molasses...) and have both an RCA 5Y3 and a GE 5Y3. I'll use the GE for this one. Thank you @muchxs.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Have you made up a voltage chart? This, imho, should be done for any amp on any bench. in doing so, one would have the information available to understand what is going on in the circuit. One would know at what plate dissipation that 6V6 was operating. if things are not as expected, one knows it. If all is proper, then one has a baseline of operational parameters to refer to in the future.
     
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  14. Joe Les

    Joe Les TDPRI Member

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    I'm pretty sure it's red plating and not just heater glow. I've seen some pictures online of red plating and this looks exactly the same. Although it is strange that the amp would sound good while doing it.
     
  15. Wally

    Wally Telefied Ad Free Member

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    Redplating occurs when a tube is drawing too much current. In a single ended amp, the amp might sound glorious......and if the tube were not being deteriorated at an advanced pace one might like to have the amp sound like that. However, the tube will have a shortened life...sometimes very much so. In a push/pull amp and if all tubes are redplating, the same may be true. This is why I advise players to give a visual check when an amp’s sonics change...whether that change is for the better or the worse. In a push/pull circuit that depends on all power tubes functioning in a balanced manner, if just one tube or one side of the OT primary is redplating, then the sound may suffer...hum might even occur.

    What are your voltages? What are your Biasing numbers?
     
    Last edited: Oct 24, 2019
  16. King Fan

    King Fan Friend of Leo's

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    A JJ 6V6 and rectifier are both good to own here. I keep 'em in my drawer for startup and testing.

    You said in the first post the red only occurs on the bottom of the tube, out of sight? Got a good phone camera? Can you take a closeup of the tube — in profile — in a dark room?

    In this shot from Robrobinette.com, the arrow shows the red plate — all the orange glow, as on the left tube, is normal heater.

    [​IMG]
     
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  17. t-luxe

    t-luxe Tele-Afflicted

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    What are your voltages? As Much said, a voltage chart will help pinpoint the problem.

    Ditching the Sovtek "5Y3" for an actual 5Y3 with the proper voltage drop reduced the plate voltage on my Mojo 5E3 by almost 30 volts. I'd start there.
     
  18. Wally

    Wally Telefied Ad Free Member

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    See at about 1:13 Into the video....
     
  19. Wally

    Wally Telefied Ad Free Member

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    re: voltage chart..... Muchxs correctly noted that the plate and bias voltages would be of interest. However, I am the one who has been adamant about a voltage chart for the entire amp being made. That should be done for a variety of reasons.
    It is not that I want the credit...but I had to reread the posts to see if I was wasting time posting what had already been explicitly mentioned.

    at this point and since the OP has not responded to any of the voltage measurement suggestions by anyone, I am going to warn the OP that there are dangerous voltages in that live circuit and that if he is not aware of the safety precautions he would do well to take Muchxs up on the invitation to visit in person.
     
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  20. muchxs

    muchxs Doctor of Teleocity

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    I worked on a late '70s Champ that happened to be on a buddy's bench a while ago. The guy knows his way around Fender amps, it just happened he hadn't looked at this particular Champ yet. I figured, "Make it take it!" I plugged it in and was instantly rewarded with a deeply authentic "Train Kept a Rollin'" tone so I bashed out my rendition of Train Kept a Rollin'. By then I'd played through my three chords. ADD set in, two of my three favorite chords. :D I figured I'd open the amp up and check it out. It had the typical melted bypass cap on the 470 ohm bias resistor. Usually they melt open or the resistor opens. This one melted into a short. That means the "bias resistor" in this particular Champ was effectively zero ohms. Red plate some? Ya think?!

    It sounded bad ass while it lasted.
     
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