Ampeg V4B Conversion Question

Discussion in 'Amps and Cabs [BG]' started by degroove, Oct 21, 2003.

  1. degroove


    Jun 5, 2002
    Wilmington, DE
    So I bought the V4B on Ebay and its on the way. My girl friend's dad (tube radio hobbiest)has a tube tester. If things are fine, no need to make any changes.


    If I am getting new power tubes, I read in old threads that you can have a tech mod the V4B from 7027As to 6550's. Would this increase output to 200 Watts? Can any repair shop do the mod? How much does the mod cost? Any further advice?


  2. No, you might gain 10 watts overall. The tone will change significantly to a tighter feel with better headroom, but your overall power won't increase to any great degree.

    Yes, all it involves is a re-bias. 6550's are a direct replacement in the amp. Even thouogh they draw substantially more heater current than 7027's, most old Ampeg power transformers can deliver it with no problem. 6550's are even listed as subs in old Ampeg service manuals. If you find that 6550's run the power tranny too hot, 6L6GC's are the same tube as 7027's as far as that amp is concerned.

    There are no parts involved unless you decide to add a separate filament transformer, so all you're looking at is a re-bias.
  3. Just to add one thing to what Psycho said:

    My V4B did not have a bias adjustment pot, so I had to add one. If a repair shop does this, they'll probably charge you $25 or so.

  4. degroove


    Jun 5, 2002
    Wilmington, DE

    So, a rebias is all? I was thinking a new tube socket or something like that. That should be a piece of cake!

    Thanks, so much! Can't wait to get the amp as its my first forray into the realm of tube! :cool:
  5. degroove


    Jun 5, 2002
    Wilmington, DE
    Cool, Chris!! :)

    Again, here I am projecting its going to be major work!!


  6. Psycho and Throb are right on the money, as usual. All I'd add is that I believe JJ Tesla (or someone) is making 7027s again, and if the amp sounds fine with the existing power tubes there isn't really any need to replace them.
    Check with Bikehorn on HCBF about 7027 availability.
    (I'm L. Ron Hoover BTW...;))

  7. Pluking too hard on meeeee..... :D

    Oh yeah, Mark's right. 7027A's notoriously last for 25+ years if they haven't been knocked around. The quad in my V4B are about 12 years old now and still put out full power.

  8. LRH is my user name on hcbf. :D
    I'm a pretty big Zappa fan.

  9. I think the 6550's will need different retainers though but I haven't tried 'em to see.
  10. degroove


    Jun 5, 2002
    Wilmington, DE
    Sweet. I will have to check out the Website.

    I remember you L.Ron!! Its funny to see HCBFites wandering around other places.

    I have talked with Bikehorn. Man, he is an ENCYCLOPEDIA of tube knowledge. Like Rain Man or something.

    I will PM him about 7027s.
  11. degroove


    Jun 5, 2002
    Wilmington, DE
    Sweet. I am hoping that when the amp arrives, all the tubes are working. If thats the case, then no worries. Luckily my Girlfreids dad has a tube tester. This will set my mind at ease.

    I do have a question about testing the tubes though. If the power tubes are failing, will you hear anything when playing? Also, when testing, what would you look for as signs of tubes on the verge of failure or not putting out their max power?

  12. Well, tube testers are neat, but don't ever trust them. :D Really the only way to test a tube is to put it in the circuit where you want to use it and see if the circuit works properly. In your case, you'd want the amp to put out a full 100 Watts into an 8 ohm resistive load. If you don't have an oscilloscope, 8 ohm dummy load, sine wave audio generator and voltmeter, then you can't do this. You can do the next best thing which is to listen to the amp and see if it sounds good. Most Ampegs reach maximum "clean" volume when the Volume knob is at about the 11 o'clock point, anything beyond that just adds distortion, which sounds good too, just not classified as clean power. There shouldn't be much hum or sizzle or crackling or anything. Also take a look at the power tubes and make sure the plates (the internal metal box inside the glass) are not glowing red hot. The filaments which are in the center of the structure are supposed to glow, but the outer metal plates should not glow red from overheating. A little blue glow near the outside of the glass is OK too.

    The cool thing about tubes is that if it sounds good, it is good. Kinda like music!

  13. degroove


    Jun 5, 2002
    Wilmington, DE
    I have a volt meter. Thats it! Guess I will go by sound. 11 O'Clock it is. That doesn't leave a lot of space for further cranking. I will check to see how its working. I just can't wait till Friday when it arrives!!

    So much for "scientifically" testing the stuff.

  14. For what it's worth, I've got an early 70's Magnavox-era V4. I've thought long and hard about replacing the 7027A's (which, by the way, were still putting out 100 watts as measured on my scope) with 6550's. I'ver read all I can find on the subject on the 'Net and in the amp repair books. But I still have a hangup with the extra heater current draw. Yeah, I know people say it's no problem, but that's an extra two and a half amps. And power transformers aren't cheap.

    So the next logical thought: shoehorn a filament transformer in somewhere, Mouser's got one for about $20. And put on spring retainers, then rebias it...oh, and maybe make a ventilated back panel since the 6550's are bigger and will almost touch the wooden panel........All for an extra 10 or 20 watts??

    Finally I concluded that it just isn't worth the extra work. So I sprung for a new set of Sovtek 7027's (yeah, I know, i coulda put in 6L6GC's, but...) and it puts out exactly 100 watts at 1KHz when I visually observe the waveform starting to clip. (And I've completely recapped it too, so everything's up to snuff). Tone galore!! But too darn heavy (and perhaps underpowered) to use as my main gigging amp.

    Now here's a question for Psycho: the Pitman book says to change the screen resistors from 470 to 1K. (should I change my mind and go for the 6550 changeout) But I've also read that there's drawbacks to increasing the screen resistors. Do you feel they should be changed, or left at 470?
  15. 6550's can handle the extra screen current that 6L6GC/7027's can't. 1K is roughly double the 470 ohm in there right now. Measure your screen voltage, and as long as it's not above 250 volts by a bunch, leaving the 470 will be fine. 6550A's (nobody makes a true to spec 6550 anymore) and all their modern copies have equivalent screen ratings to a KT88 which takes 600 volts screen and 800 volts plate. You can go over that as well. My Trace VR400 runs its Sovtek KT88's at 728 volts UL, which is equal plate and screen voltage. That's over 21% above rating for the screens.
  16. Screen voltage according to the schematic is 530, last I measured the actual reading was within five or ten volt
  17. Sub the 6550's without changing the resistor. I screwed up before and got it backwards. As long as your reading isn't above the 600 volt screen limit for 6550A's, you don't need to reduce it. Old 6550's listed a 250 volt screen limit, hence the 1K recommendation.
  18. Thanks, Psycho, for the great info.

    But after taking another look at the amp, I recall the REAL reason I didn't pursue the 6550's (aside from the filament tranny...I could shoehorn one in if I wanted to)

    No space for the bigger tubes!! I've got the Sovtek 7027's in there now, and they measure 37 mm diameter. I've onlu got about 4 or 5 mm clearance between one tube and the PT. So the biggest I could go would be a tube that's less than 45 or maybe 47 mm. I don't know about the Svtek 6550's but I seem to recall the EH tubes and I think the Svetlana's were 52 mm diameter.

    Any suggestions for a tube that'd fit?
  19. slugworth

    slugworth Inactive

    Jun 12, 2003
    So. Calif.
    I ran a few different tubes in my V4b over the
    years: 7027's, EL34's 6L6' and 6550's. All
    produced a slightly different tone, but not
    a huge increase in power. The 6550's won't
    fit in the standard retainers, you have to
    modify. Also, the 6L6 and 7027's are really
    the same tube except for pinout and cosmetics.
    Uncle Ned has a good write-up about this very
    topic. You can find it here:
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