Can OD/Boost screw up the amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hawkeyeinexile, Jan 27, 2006.

  1. hawkeyeinexile

    hawkeyeinexile Silver Supporting Member

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    had a '61 Ampeg Reverberocket retubed & capped, etc. about a year and a half ago. used it weekly for a couple hours, often w/a V-Stack Tweedy or Pharaoh Amps Class A Boost. with the latter i'd run the amp around "3" or less and crank the Pharaoh (maybe 20+ dB?). then recently, mid-gig, the little Ampeg went belly up. solution: re-tube, incl. the preamp tubes (6SL7 & 6SN7 - ouch). amp is stock, except for a 50w speaker (Emi Private Jack).

    is it possible the boostage did the damage or contributed to it? i expected the 18month-old tubes to last considerably longer.

    thanks in advance,
    :cool:
     
  2. hasserl

    hasserl Member

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    Boosting the front end of an amp with distortion pedals or clean boost can cause the amp to run a little hotter than without, due to increased signal voltages. But any increased wear on the tubes should be practically immeasurable. It sounds like you had pretty short tube life to me, but I don't think it was caused by the boosts. Since you ran the volume at 3 or below the output tubes should not have been stressed at all.

    But tubes, like any mechanical device, can fail at most any time for lots of reasons. I doubt though that all your preamp tubes failed at the same time. Preamp tubes can last a very long time. Did you get your old tubes back? If so, give them a try and see how the sound. If not, why? I always give the tubes back to my customers, unless they tell me they don't want them. Even then I tell them to take them and keep them in case they need them later, if just for trouble shooting.
     
  3. John Phillips

    John Phillips Member

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    +1 on what Hasserl said.

    I find it surprising that all the tubes should need replacing, unless there is something more seriously wrong with the amp.

    Boosting the front end can't really stress the preamp tubes more than normal since they are always run in single-ended Class A and so are actually running full-on all the time - the signal level makes very little difference to the dissipation. Those old octal preamp tubes can be a bit prone to microphonic failure from the cabinet vibration though... and you really can't tell from control settings how loud the end result will be. Depending on where the distortion occurs first in the circuit, that could have been pushing the power stage pretty hard. It wouldn't be as unlikely for power tubes (especially modern-production ones) to fail in those conditions.
     
  4. hawkeyeinexile

    hawkeyeinexile Silver Supporting Member

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    thanks, guys. i'm gonna go back over my work sheets from Evan (first tech to re-cap & re-tube) and Scott (recent work). Scott Kinkel puts out a pret-ty full report on each tube & how it measures for this&that, and i do have the old tubes.

    :cool:
     
  5. TubeAmpNut

    TubeAmpNut Member

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    Nope.

    BK
     
  6. bobgoblin

    bobgoblin Supporting Member

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    ya know, i have a '60 ampeg mercury & had a very similar thing happen back in '04. in my case the preamp tubes were fine, but i blew out the cap can. it literally exploded, my tech thought it was funny. the bill wasn't...it's up & running again, although i had really short preamp tube life (also 6sl7's) when they went down in june of '05. i attribute this to using a weber minimass w/tons of attenuation & the amp full up for about 6 mos.
     
  7. Matt H

    Matt H Guest

    i can think of a worst case scenario that'd cause all the tubes to go... but it's highly unlikely...

    power tubes u nder the best of conditions are "about to blow"- running way too much signal (square wave style, baby) causes one of the screens to fail... screen shorts to heater string- the heater line of the rest of the tubes is suddenly exposed to b+... yowza! (or cathode has too much current through it, runs way over spec, melts down, you've some cathode-heater bleed through, etc)


    that being said- too much voltage/signal gain CAN kill a tube... physically. I was working on this design (which became the p1 "super eXtreme" on the ax84 board) with Brian (who now posts as gainmaster). anyway- in early designs, we were, er... put diplomatically running the 12ax7's way over what they should take. (four cascaded stages, minimal to no attenuation between each one, biased super warm). Anyway- he found that the amp would fire up and play, and then when he shut down, the "click" of the standby (or power, can't remember now) switch would get amplified... well- because it was a larger signal (in dB) than what the amp had been designed to take (hot humbuckers at most)- he found he was literally blowing up preamp tubes.

    the workaround involved changing the standby switching and adding some inter-stage attenuation. no more exploded tubes.

    again- we're not talking redplating leading to meltdown- he was reporting physically broken glass. we're both sure that "something else" was going on... but...

    the reason i bring this up, a +20dB boost is a lot- especially if it's hitting an amp that's already pushing it's limits. i find it unlikely that all the tubes went "at once"... but it's possible that one went, and caused some other problem that took the rest out.

    (just thinking out loud here- don't take any of this as gospel)
     
  8. hasserl

    hasserl Member

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    Matt, I'm not following your scenario. How does a "click" from the standby switch end up as a signal voltage to the preamp tubes? I'm not sure what was going on with that amp you guys were working on, but to result in physically broken tubes, hmmmmm, well, interesting is all I can say.

    Regarding boosting the signal voltage, you could drive the hell out of the signal, if the grid is driven too far to the point of going positive you will get grid blocking or blocking distortion. But no damage to the tube results.
     
  9. TheAmpNerd

    TheAmpNerd Member

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    I will beg to differ. I've had to replace a fair number of preamp tubes for that reason: Driving the hell outta them with a pedal.

    If that isn't the reason, then I don't know what is.

    No it was not all the preamp tubes either, just the first stage.
    Popped in a new tube, and crank the amp up with the pedal
    cranked and it sounded great again.

    Don't ask me why or how, I can't explain it. No, I don't
    have another explanation for it either. If anyone cares
    to take a shot at it I'm all ears.


     
  10. Fifthstone

    Fifthstone Member

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    I've wondered about this too. I use a Fulltone Fat Boost and or OCD, Fullboost, etc, when I need to have an extra channel of gain, but I'm always afraid of hitting the input too hard with too much of a boost. How much is TOO much? I rarely go more than a few clicks above unity.
     
  11. Matt H

    Matt H Guest

    hasserl, i have absolutely no idea... well- that's not entirely true. i mean- you can hear it the click, so it's definitely being pushed "as signal" somehow... but at the same time- if you pull your PI tube, the standby doesn't click as loud, at least going from memory here last time i fired up an amp without having a pi tube in it (haha silly mistake). but yeah- it was definitely the on/off of the b+ sending a spike that killed the tubes. the voltages in the amp were pretty tame (B+ of 350, all downhill from there, as it were). by the time you got to the preamp node- pretty low.
     
  12. avwalker

    avwalker Member

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    I boosted my old 18 watt clone with a Reverend Drivetrain one time. The 18w was on full tilt and the pedal was at 1/4 gain and 3/4 output. I blew out three resistors in my amp circuit (screen grid resistors and cathode resistor i believe).
     
  13. Fifthstone

    Fifthstone Member

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    Scary stuff. I guess I'll have to keep an eye on the overall gain I'm pumping into the amp.
     
  14. nag hammadi

    nag hammadi Guest

    hmmm. i saw an ax84 get beat up by a dha tube purist. the amp was cranked, and the dha was cranked just enough to shred. sounded amazing, then the 6l6 smoked, taking with it some resistors.
     

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