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changing tubes in my v4b-av

Discussion in 'Amps and Cabs [BG]' started by cementxshoes, Jun 28, 2012.


  1. cementxshoes

    cementxshoes

    Apr 25, 2012
    Mid set my tone goes all soft and muffled, so i peek around the back of the amp and one power tube is not lit up at all. Amp was still making noise so i finished the set. today I bought a pair of 6l6 groove tubes and im gonna have to replace them as soon as we get to the next show. Is it as easy as removing the dead tube, and popping in a new one? Do I need to replace at least two? The amp has a fixed bias so is it that simple?
     
  2. No it's not. EVERY time you replace the output tubes the idling current through the tubes has to be set. This is where a tech comes in. The tubes you buy should be a matched set of four.
     
  3. I'm going to be the contrarian here.

    There are tens of thousands of amps out there that have been operating merrily for decades without EVER having the bias checked, much less changed, when power tubes were swapped out. Thousands. I guarantee it. Most amps are biased conservatively from the factory so that a tube changeout will not push the bias into an unsafe area (where the tubes are redplating).

    Are these amps operating at optimum bias? Perhaps not, but then again, what IS optimum bias? That has been the source of much debate for the last few decades, when audiophiles started touting biasing as the way to audio nirvana. And guitar amp fanatics also picked up on the issue, claiming bias as the way to some magic mojo.

    Here is a nice writeup offering some common-sense thoughts on biasing. http://www.tone-lizard.com/Biasing.htm Do some research on biasing, you will indeed find there are several different approaches being touted. Aiken Amps has one writeup, IIRC, Duncan Amps another perhaps. Is it the magic 60% number? or 70% ??

    Or is it in that nebulous zone where it has been written "anything between 50 and 70 is good, whatever sounds best"? In that case, the factory bias is perfectly fine!

    Plug the replacement tube in, check to make sure it is operating and not red-plating. This is a "Must-Do". Taking the amp in is a good idea, I agree; since it appears something may not be kosher with the amp. Scoping the amp's output is easy and can find a problem, for example.

    But again, I know for a fact that us musicians have run amps for decades and decades without ever touching bias, in fact not even being aware of it, and amps have not blown up, good sounds were produced, and cats and dogs did not start sleeping together.

    P.S. Oh, and tubes were changed without buying matching tubes either.
     
  4. cementxshoes

    cementxshoes

    Apr 25, 2012
    As I said, I'm on tour, driving around the country playing in a new city every night, no time or funds to have a pro do this if indeed I can just switch the tube out...I do understand that it would be optimal to bring it somewhere and have all the tubes changed out and have the amp biased...once I get home and am working a day job again.
     
  5. You CAN just pop in ONE new one. It won't be functioning at its best, but will still work OK -AFAIK :)

    +1 to above post, Hopefully amps & tubes are made well enough today to handle this. They should be!

    BTW- is yr amp a V4B -AV or V4BH-AV?
     
  6. cementxshoes

    cementxshoes

    Apr 25, 2012
    V4b av...no h. Very little info and opinion on it found on the web. That's why i'm pestering talkbass with my incompetence.
     
  7. cementxshoes

    cementxshoes

    Apr 25, 2012
    So another question, since I bought the pair of new tubes, can I change out the dead one and one other, would that help the amps sound rather than only changing the one?
     
  8. demon666

    demon666

    Jul 16, 2005
    Providence RI
    +1 It's all snake oil. I've left amps on overnight not connected to a cabinet. Did it catch fire? No. Did it blow up? No. Did the out put transformer die? No. Was it the best thing to do? Probably not. Did it work fine for years after? Yes. Go look into some threads by guitarist. Most of them don't even know about ohms or matching stuff like that. Most only know what a tech tells them. "You need a whole retube" Why? Because I can make money off of that and keep your old tubes. I know I'll catch some flack for all this but whatever. Do different tubes sound different? Maybe. Will you ever notice it in a live band setting? Probably not.
     
  9. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    That's a V4BH in fancy clothes. A modern take on the original V4B amps. They use 6L6's instead of 7027s and should have a bias control IIRC? *edit: no bias control.

    Either way, since you said you're on tour, get a matched quad of 6L6s for the amp and you should be ok. Worst case, get a paid (they're aligned inside pair/outside pair). It wont be perfect, and a tech trip would be ideal, but it should be fine with a modern amp as long as there is no other issue with the head.
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    From what I've been told, the big deal about matching tubes is to have them wear more evenly and make them easier to bias. But I'd be lying if I said I never replaced one single power tube and operated just fine. I just did it to my 69 SVT and I noticed no difference in tone whatsoever. I tend to believe that it's good to do it right when you can, but if it still sounds good, don't sweat it too hard if you don't. I might put in the new matched pair, but I don't think the world will come to an end if you don't rebias it right away.
     
  11. That's correct iirc. V4BH/AV are 6L6's pairs- 2 outside ones & 2 inside ones.
    So it'd go 1,3,4,2.
    Change two if you've got 2 matching tubes, either the outside pair, or the inside pair.
    If you've just one tube-so be it. Not ideal, but should be good to go
     
  12. Sorry Rod but this is incorrect. The only amp that has pairs is one that has two output tubes. 2&3 and 1&4 are equally "pairs".

    If the amp has two bias supplies, similar to the SVT, then pairs can be considered. The SVT can take two sets of matched triples and balanced using the two bias pots.
     
  13. Matching ensures that each tube is sharing the work load equally. That is the ideal way. If your outputs are being run "cool" then you could probably replace just the one and do as N'Bill suggests. If the tube doesn't draw too much current it should get you through the tour.
     
  14. cementxshoes

    cementxshoes

    Apr 25, 2012
    Thank you for the input. I replaced the dead tube and the one next to it (inside pair). The sound was great at last nights show, and I now have a spare tube.
     

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