Anyone Ever Do Something Real Stupid To Their Tube Amp? Fender 100T

Discussion in 'Amps and Cabs [BG]' started by AngryHammer, Nov 7, 2019.

  1. AngryHammer


    Jun 25, 2017
    Eureka CA
    Always read the f***ing manual. I did not.

    I had the amp in operation and switched the output power selector from 100w to 25w, then the switch arced, amp went silent and i blew some fuses and power tubes. I replaced said fuses and tubes but those tubes immediately blew as well.

    No warranty.

    From the schematics it looks likely to be an issue with the automatic bias control, the high voltage power supply, or maybe the output transformer.

    Anybody ever do this same stupid thing before?
    Any advice?
    Any ideas on how difficult or expensive it would be to fix this before I start unscrewing things and looking for burnt parts?

    Aaaaaand que the chorus of "Take it to a tech."

    Thanks talkbass.
  2. Did the manual say not to do that?

    I have a Vox Night Train tube amp and I learned the hard way that switching between triode (7W) mode and pentode (15W) too rapidly seems to have a malign influence on the innards. Since that first incident I now am careful to first switch it to standby, wait a few seconds, and then switch it to one of the operating modes.

    It was a floor model and didn't come with a manual. GC even wanted to gyp me out of a power cord and they couldn't find the carrying bag or the instructions, because their stockroom is a black hole where useful items apparently get sucked into a vortex to disappear forever.
  3. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    That's potentially electrically analogous to driving down the highway and then suddenly slamming your transmission into first gear.

    I couldn't find a schematic for the 100t or its output section but I did find a block diagram from the manual. Fender Bassman 100T Owner's Manual (Page 11 of 44)

    But yes, what you said: take it to a tech.

    It does not say not to switch it while in use.... you might have an argument there.: Fender Bassman 100T Owner's Manual (Page 7 of 44)

    The one linked above is the same as Fender's up to date manual on their website:

    Absolutely. I listened to cries of "turn it up" running my old V4 into a quad of '60's Ampeg SVT speakers with a 5 string bass on a holiday gig. I could hear the speakers and amp were unhappy because I was already on the limit before turning up once more. The speakers quickly started to die and took out the output transformer and tubes along with. And then they let me run through the ridiculously loud PA like I'd asked to do in the first place. Cost me like $600 in parts and hours of my time.
    Last edited: Nov 8, 2019
  4. Mingo Sanders

    Mingo Sanders

    Mar 11, 2008
    Austin, TX
    And cue the chorus of "Please turn on your video camera and do it again!" Just kidding. I've done far worse. I'm lucky to be alive. Thanks for the reminder.
    AngryHammer and mikewalker like this.
  5. beans-on-toast


    Aug 7, 2008
    Things happen. A well designed amp will anticipate things like this.

    Check the switch that caused it all, it could be locked in silent recording. Check the 5W screen resistors on the output tubes. There are a lot of other possibilities. See the schematic below.

    Attached Files:

  6. Wisebass


    Jan 12, 2017
    Lost in Space
    Anyone Ever Do Something Real Stupid To Their Tube Amp?

    I sold it! :laugh: That was really stupid!

    Maybe not a bad idea. :D (better than blowing more tubes and fuses)

    When you don' t know where to look ask someone who does. ;)


  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    The recommendation of seeking a service tech is a good one. Let me add that I would suggest a Fender authorized service center that is experienced with this particular amp because it is a very complicated amp/design with a LOT of tricky electronics that will be way beyond what a typical "tube tech" will be capable of handling.

    There is a servo bias system that looks at several operating parameters and modes, if this fails tubes may also fail and continue to fail. There are also power supply control components that can fail as well and require specialized knowledge and serious tech chops to correctly diagnose.
  8. Wisebass


    Jan 12, 2017
    Lost in Space

    The guru is back! :D:thumbsup:


  9. BassmanPaul

    BassmanPaul Inactive

    Welcome back Andy. We all missed you. :D
  10. BassmanPaul

    BassmanPaul Inactive

    I think he rather Guru on us all! :roflmao:
    12BitSlab, kesslari, Ronzo and 3 others like this.
  11. Ross W. Lovell

    Ross W. Lovell

    Oct 31, 2015

    Not sure what you want here, suggestions for more things to try?

    Just in case......

    Plugged in, volume set, started to play.......nada.

    Checked the instrument, increased volume.......still nada, but drummer did notice the quartet of tubes glowed brighter with my hitting the strings and then going back to normal as the string quieted.

    Dimed it! This time, everybody was really quiet and when I hit a chord, the tubes got even brighter and the amp head made a little noise.

    Being curious this was done several more times each time same "little noise"before...........I realized cabinet was unplugged.

    I lucked out.
    AngryHammer and Hachimitsu Pie like this.
  12. Door County Bass

    Door County Bass DC EMT 663 Gold Supporting Member

    Feb 9, 2015
    Welcome back. Hope all is well.
    12BitSlab, WillieB, agedhorse and 2 others like this.
  13. PaulCISSA

    PaulCISSA Unsweetened, highly-caffeinated Supporting Member

    Jul 10, 2014
    Northest New Jersey
    Yep. RTFM is good advice. Read it, know it, love it, live it. You’re probably not the first guy to do this - FMIC now includes a warning right on the amp as well:
    I guess they figure if they haven’t saved you from yourself at that point, then it’s a lost cause. Standby is your friend.

    Hope you get it sorted.

    Welcome back Andy! :thumbsup:
    SactoBass likes this.
  14. Heavy Blue

    Heavy Blue

    Nov 11, 2017
    Prairie Canada
    Good to know! Thx and welcome back AH.
    packhowitzer, Wisebass and agedhorse like this.
  15. Wesley R

    Wesley R Gold Supporting Member

    Anyone Ever Do Something Real Stupid To Their Tube Amp?

    Yes indeed I have! I lent a BF Showman head to a now ex-guitar friend.
    Wisebass and agedhorse like this.
  16. Ric5

    Ric5 Supporting Member

    Jan 29, 2008
    If I could only own 36 basses what would they be?
    Anyone Ever Do Something Real Stupid To Their Tube Amp? Fender 100T

    I wish I still had the old 60s Fender bassman tube amp that I bought used in the early 70s …
    Frankie Fender likes this.
  17. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Absolutely. The circuitry for user bias setup in SVTs is simpler and yet just as confusing if you don't understand it.

    Welcome back, Andy. @Wasnex has been doing yeoman's duty while you were away. He could use a break. :thumbsup:
    Last edited: Nov 8, 2019
  18. Element Zero

    Element Zero Supporting Member

    Dec 14, 2016
    Was a GC employee from ‘04-‘14. Can confirm.
    Novarocker and Spectrum like this.
  19. Medicine Man

    Medicine Man

    Apr 10, 2015
    Beat me to the punch, lent it out to a guitarist.
  20. 40Hz

    40Hz Supporting Member

    May 24, 2006
    Heeeeeee’s baaack!!!

    Good to see you again. You were missed. :):thumbsup:
    Ronzo, Wisebass and Mvilmany like this.
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