Classic 30 blows phase inverter tube

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by vbjamin, Jan 19, 2005.


  1. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    My Classic 30 blows the phase inverter tube and takex out the 1 amp fuse with it. This has happened twice now. I am not a tech at all. I this problem at all common or does it ring a bell to anyone as being symptomatic of a common problem. I replace the tube and fuse, crank it up, it sounds great, go to a jam and crank it up and bang, gone!
    Frustrating :(
     
  2. Blue Strat

    Blue Strat Member

    Messages:
    29,924
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    It's an unusual scenario. I can't think of any particular reason why this should happen repeatedly.

    Sounds like you need to get it to a tech for a full checkout.
     
  3. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Yes I have taken it in and am waiting for the verdict. Hopefully it isn,t a major issue.
    Thanks for the reply
     
  4. Blue Strat

    Blue Strat Member

    Messages:
    29,924
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Let's us know what you find out. "Inquiring minds want to know"
     
  5. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Are you using a 12AX7 or 12AT7 in that slot? A 12AT7 will usually be a bit more robust as a PI. It is an odd type of failure, so it may be something else entirely. Report back if you get it figured out.
     
  6. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Good point, I have a Jan Philips at7, that I will try in this socket.
     
  7. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Would speaker impedance mismatch be the problem?
    I say this as the only thing I have done recently that was common to both failures was, unplug stock speaker and plug into a Mesa cab with a 16 ohm Greenback in it.
    Although I did use the same cabinet at home with no problemo.
     
  8. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Don't!

    Don't waste a good tube. Don't fit anything decent until you know what the problem is. If it keeps blowing the same tube, that's the symptom not the cause.

    My best guess is that it's probably some kind of filament connection fault, since even a direct short through the plate circuit couldn't draw enough current to blow the fuse. If I remember rightly, the preamp tube filaments are in series (not the usual parallel) on that amp too, which means that quite high voltages are present if there's a problem.


    No, a speaker impedance mismatch couldn't blow the PI tube. In fact, is the C30 not a 16-ohm amp anyway?
     
  9. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Yikes!
    Good thing the amp is at the shop or I would prolly be down another tube!
    I will let you guys know what the tech says,
    Thanks again
     
  10. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    vb, my question was regarding whether or not you were using a 12AX7 when the design called for a 12AT7. I wasn't suggesting you try it as a test. Check the tube chart or a schematic.

    What other clues do you have? How long did you play it at home with no trouble? What was different at the jam? Louder, being on a longer time, using high output pedals, etc?

    What tubes are you using? There have been reports of GT12AX7M's with infant mortality. I don't really see this blowing a fuse either, but I'm sure I've not seen all failure modes.
     
  11. WailinGuy

    WailinGuy Supporting Member

    Messages:
    1,268
    Joined:
    Aug 30, 2003
    Location:
    Silver Spring
    +1

    Clearly, there's a serious problem with the amp's circuitry. There's nothing wrong with the tube.

    I remember once a customer complained that his Princeton Reverb kept blowing rectifier tubes. He must have ruined two or three 5AR4 rectifiers before finally bringing the amp to me. As soon as I opened it up, I found the cause. The tip of a 1/4" phone plug had broken off inside the amp. Of course it eventually had wound up in the lowest part of the chassis which was....

    ... that's right, the shallow round well created by the bottom of the multi-section filter can. The tip nestled between the edge of the chassis hole and one of the cap terminals, creating a direct short on the main B+ rail.
     
  12. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Actually, I don't know that we have enough information to draw that conclusion. This happened two times. We don't know if these incidents happened over the course of two years or two hours. We don't know what tubes he used or what the line voltage is both at his home and the jam. I agree it is likely a circuit problem, but not clearly a circuit problem. We just don't have much to go on. People on this board have had two 12AX7 failures in a row with new tubes.

    I just looked at the schematic. It calls for a 12AX7. B++ to the PI circuit is 275V. The 1A fuse is in front of the diode rectifier supplying B++. The filaments are DC in series, rather than the usual AC. You need to have all the tubes in place to get filament voltage down the line to all the tubes.

    Since we don't have the amp in front of us it's hard to guess what components or traces around the PI may have failed. A visual inspection may make it obvious. My best guess at this point would be a PI coupling cap failure allowing DC to pass between the power tube bias circuit and the PI.

    Vb, please get back to us when your tech discovers what the actual problem is.
     
  13. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    I will let you guys know 4 sure.
    I played the amp at the same jam in consecutive weekends. Played there many times before with other gear and no issues. Other amps ran fine. I do remember a few months ago a guy blew up a DR a Mesa F30 in one night. I was playing a DC-5 next to him with no issues. I think it was just his bad day.
     
  14. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Have you had the amp a long time and had you been playing it consistently with no troubles prior to this happening or is this amp a new acquisition and you are just finding this problem? Did you play the amp at home between jams after changing the tube and fuse? When you play at the jam, what are your volume settings relative to your home settings? How long did it take before the amp failed? What tubes failed (manufacturer)?

    Sorry for all the questions, but long distance debug can be a difficult thing, especially for a "non-classic" type problem like yours...
     
  15. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Have you had the amp a long time and had you been playing it consistently with no troubles prior to this happening or is this amp a new acquisition and you are just finding this problem? Did you play the amp at home between jams after changing the tube and fuse? When you play at the jam, what are your volume settings relative to your home settings? How long did it take before the amp failed? What tubes failed (manufacturer)?

    Sorry for all the questions, but long distance debug can be a difficult thing, especially for a "non-classic" type problem like yours...

    Had the amp one month only
    Cranked it at home for hours on end , no issues, put the Greenback in myself.
    First tune at jam, dead ,
    Replaced tube and fuse, cranked again and used OD-3 AND other pedals , no issues
    First tune at jam in a differant ac outlet than the first time. dead

    Volume settings not really differant at the jam as I had an empty house the same day at home and really let her rip.
     
  16. WailinGuy

    WailinGuy Supporting Member

    Messages:
    1,268
    Joined:
    Aug 30, 2003
    Location:
    Silver Spring
    The fact that the fuse blows along with the tube makes it, in my mind, extremely unlikely that the problem is being caused by the tube alone. It's not easy for a 12AX7 in the phase inverter or in the preamp to take out the mains fuse.

    Now it might be helpful if we could find out exactly how the tube "blows". Does it stop lighting up? Do the plates glow red? Does it lose vacuum (getter flash turns white)? This info might give us a clue.
     
  17. vbjamin

    vbjamin Member

    Messages:
    1,116
    Joined:
    Jan 19, 2005
    Th first tube I did not notice until I checked/ replaced the fuse. It has a cloudy white residue near the top and does not light up at all. The second one does light up at the bottom of the tube only/ faintly, has no residue and no sound.
    I did not bother putting in another tube and or fuse , just took it to a tech I know.
     
  18. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Vbjamin, you've added quite a bit of new information in your last two posts. The odd part is that you were able to play it at volume for a long time at home with no problem. It also sounds like you may not have lost a second PI tube, just the fuse. I strongly suspect the problem will be obvious with visual inspection of the boards. The Classic 30's are good sounding amps, but are not built especially well from a durability and maintenance point of view. Again, let us know what he finds and good luck.
     
  19. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Jim, I have to agree with you there. Even a straight short plate to cathode would draw much less than an amp, given the plate and cathode resistor values in the circuit. What I don't get is the fact that he played it hard after replacing the PI and all was well.....I wouldn't be surprised if it was something rolling around in the chassis, much like your princeton problem.

    Edit to add: as I mentioned earlier, a shorted coupling cap between the PI and the power tubes would draw an amp or more, based on the values in the schematic.....
     
  20. Blue Strat

    Blue Strat Member

    Messages:
    29,924
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Using the incorrect tube type is not generally recommended unless you're going to change the circuit (the cathode bias resistor) to operate the different tube correctly.

    Not changing the resistor in this case poses no danger however.
     

Share This Page