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Classic 30 retubing problem

Discussion in 'Amps and Cabs' started by StompBoxBlues, Jan 21, 2008.

  1. StompBoxBlues

    StompBoxBlues Member

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    I just got a matched quad set of EL84's from TAD, to put into my Peavey Classic 30. The current ones were old, and had tendency to rattle.

    As far as I knew, and can see in the schematic, there is no adjustable bias on these tubes it is set.

    I started playing and it sounded LOUD...almost overly so. I checked the tubes at power up and they glowed a light red top and bottom, and blue in the middle of the tubes. After a few minutes playing the second tube was glowing like an ember from inside the middle of the tube...the first tube (after turning down lights in the room) also was glowing slightly red, the 3rd and 4th aren't showing any red.

    It also had a slight burning smell.

    I turned the amp off. Looked up the schematic on the blueguitar.org site and don't see bias (in fact one mod he has is for adjustable bias).

    So what is the next step? I probably can put the old tubes back in and see if there is a difference in glow in the same positions. That might tell me something.

    Also might be good, because the preamp tube that feeds the power section is not glowing much at all...I could see if it does with the old tubes?

    Any help appreciated, I may try the old tubes, but will wait for some advice before turning it on with the new ones.

    Is it just likely I have not got matched tubes, or something else going on?

    I also have a new set of 6BQ5's I could try, that are matched, but bought also at the same time.
     
  2. Blue Strat

    Blue Strat Member

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    Aside from glowing plates (plates are the largest gray, metal object inside the tube) , which is BAD, the amount of glow inside a tube doesn't tell you much at all unless there's no glow which would mean that the tube is dead.

    Probably best to try whichever EL84s you have and see what happens with each set and then draw some kind of conclusion based on the results.
     
  3. jamison162

    jamison162 Member

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    The Classic 30 and 50 are cathode biased, plug & chug.

    If you blow a tube or continue to have issues/doubts, put in a set of JJ's from a reputable dealer.
     
  4. rockon1

    rockon1 Member

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    No they are not. They are fixed bias without an adjustment pot.
     
  5. phsyconoodler

    phsyconoodler Member

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    Indeed.The classic 30 is fixed bias.Not adjustable.You have to get a matched quad and hopefully they are the same rated as the old tubes.A good tech can install a bias pot,but it is a little complicated inside the classic 30 so beware.
     
  6. StompBoxBlues

    StompBoxBlues Member

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    It's not on the schematic as far as I can see (http://blueguitar.org/) there is something that looks a little like a pot but is a marker to point to the bridge rectifier output that gets connects up to that point.

    Also, (I'm not arguing, just mentioning what I see) blueguitar has a mod for making the bias adjustable.

    In any case, after it cooled down I put in the original tubes (from when I bought it used) and played at the same volumes, and for MUCH longer (I must be nuts..the thing sounds really nice as it is) and the tubes all glowed the same, and not red.

    This leads me to believe I don't have a matched set. I will contact the Tube Amp Doctor and see what they say, but I am really dissapointed. Can there be any other explanation? There is no balance adjustment either as far as I can see.
     
  7. The Pup

    The Pup Supporting Member

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    I don't care for perfectly matched output tube sets...I like a little 3D swirl in my amps.
     
  8. StompBoxBlues

    StompBoxBlues Member

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    I don't mind a little either, but glowing bright red (V5) and slight red (V4) even if i didn't have a "burning smell" worries me...

    I mean, I believe also that this is "plug and play" fixed bias, but they should all be within spec and not glowing like that. That the old ones back in don't I think does mean this is not a matched (and in fact VERY unmatched) set.

    Waiting to hear back from TAD now.
     
  9. The Pup

    The Pup Supporting Member

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    I'm comfortable with as much as 9ma with AB1 and AB2 amps...maybe a little less with fixed bias.
     
  10. rockon1

    rockon1 Member

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    Two things - You may have gotten a set of tubes that was way out of your bias range or you a bad tube. Bob
     
  11. teefus

    teefus Silver Supporting Member

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    swap the power tubes around in their sockets to check if the tube is the problem or the socket/resistors, etc. if the same tube always has the problem then swap in a spare. a resistor associated with one of the power tube sockets could have gone down, hence the burning smell. sometimes the paint that tubes get branded with cooks a little when they are fairly new too. you can pull the chassis and check for baked resistors but be careful of the circuit board jumpers, they are very fragile.
     
  12. jamison162

    jamison162 Member

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    Bob's right; fixed not cathode. I meant and was thinking fixed just said the wrong thing and it didn't even phase me.

    Like I said, Plug & Chug. You will eat up some tubes; that's why I say go with JJ's, they're known to take the heat and they sound good to boot.
     
  13. Blue Strat

    Blue Strat Member

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    There's no such thing as "plug and play fixed bias". It doesn't matter if there's a pot or not, you need to rebias (by swapping resistors or having a pot installed) or find a set of EL84s that biases correctly for wherever the amp's bias is permanently set.
     
  14. StompBoxBlues

    StompBoxBlues Member

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    Thanks...I was thinking about that (swapping positions) so I did mark the two that were red so I can put them in V6 and V7.

    Also, that was what I was hoping to hear, that new tubes (especially when they get as hot as those two) might burn off a little and have generated that smell. The old tubes still work well, and it sounds okay.

    To Blue Strat...that blows my mind...I thought that setting fixed as they do would adjust (like preamp tubes) for differing gain, etc. so EL84's in spec wouldn't have a problem. What is the range of values (and IS it gain, or mu, or something else that differs in power tubes?) for them...how off could they be?
     
  15. Blue Strat

    Blue Strat Member

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    Sorry, no. "Cathode biased" (AKA "self biasing") is how preamp tubes are configured and SOME power amp sections are configured.

    Fixed Biased ALWAYS means that adjustment is required whether there's a pot or not.

    Power tubes vary a lot in their conductance which is why you order matched pairs or quads and don't just pick 2 or 4 at random.
     
  16. John Phillips

    John Phillips Member

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    If one tube glowed bright red (and affected the other one on the same side), that's not a mismatched set or the bias being wrong, that's a bad tube.

    (Or just possibly a bad contact between the socket and the grid pin.)

    A mismatched set would not produce results as extreme as that, and if the whole set was biased wrong they would all glow equally, but probably only slightly. While I agree with Mike that in a sense all fixed-bias amps need adjusting, in fact if the bias is set conservatively (which is it, on these amps), you can plug and play, usually.


    FWIW the last set I had that did that were JJs from Ruby Tubes (supposedly tested), they're no more immune to it than any other maker or vendor.

    The vendor should give you a new tube (with the same matching parameters as the other three) or replace the whole set.


    You can conclusively identify which tube is dead - it might not be the one that glowed most red, but the one that glowed a little pulling the bias down on that side so the other one ran red - swap the one that glowed slightly red with either of the other two that didn't. If the same tube now glows bright red, that's the dead tube. If the glow transfers to the other side, it's the one that you moved.
     
  17. StompBoxBlues

    StompBoxBlues Member

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    I was hoping I'd hear from you :D on this!

    Thanks John. I have contacted TAD, and am waiting for an answer.
    I see what you mean, but am wondering if it is safe to try to isolate using
    the amp as a test bench? Could it actually do damage (I know, I wouldn't in any case leave it on for any length of time, but it takes a few minutes to get glowing) using these tubes to test? I'm not asking for a hard and fast answer, i.e. not holding you responsible let it be said, but just best guess, and knowing there always is a chance of damage from this...best guess if it is likely?
     
  18. John Phillips

    John Phillips Member

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    If it takes a few minutes to start glowing, it's not drawing enough current to be a serious risk to the amp. It's if they glow bright red within seconds or arc inside that you need to be careful.
     
  19. Doug's Tubes

    Doug's Tubes Member

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    Even though we order all our TAD, RUBY and Groove Tubes power tubes in factory matched sets, we have found from experience that everything still has to be checked for matching integrity. Fact of life. We often find mistmatches and rematch accordingly.

    Doug
     
  20. bmoring

    bmoring Member

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    Odd to see you post on this subject. I got a matched set of jj el84 tubes along with 12ax7 preamp tubes and an eminence cannabis rex as a christmas gift. After installing the tubes the 2 el84s on the end were glowing like a pumpkin. Like a dummy I continued to play knowing it was not right,but I wanted to hear the new speaker. After about 5 minutes the amp
    shut down. After taking it to my tech it was determined that the thermal fuse in the power transformer had blown due to bad tubes. These were a matched set of tubes from a well know supplier. If I had cut the amp off right away instead of continuing to play I could have saved a $65.00 repair bill and another $35.00 for more jjel84s, but I'll just chalk it up
    to lesson learned.
    I will say that the new tubes and and the c rex in a classic 30 sound great,but you need to take the glowing tubes out right away or it can lead to other problems.

    Bob
     

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