Can I use a 5U4GB in a tweed Bassman

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by drbob1, Oct 27, 2004.

  1. drbob1

    drbob1 Silver Supporting Member

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    My tech added a pot using Torre's book to make bias adjustable in my real 59. Unfortunately the values weren't well chosen, and until I get some time with a soldering iron the idle current is just too high with the 5AR4 rectifier (470v @ 52 mA). Using the 5U4GB drops voltage to 450v and current draw to 44 mA but I can't remember if there's a problem with current draw in the rectifier tube. Is this OK? I'm planning to add a 10k resistor in series with the 10k pot which should allow me to adjust down from the above slightly too hot values...
     
  2. fullerplast

    fullerplast Senior Member

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    You'll be fine with the 5U4.
     
  3. hogy

    hogy Member

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    No you wont. A GZ34 draws 1.8A of heater current, a 5U4 draws 3A. If I'm not mistaken, the 5V wind on the original Triad 8087 power transformer is rated at 3A max. I would not consider it a good idea to run a 40+ year old transformer at its limit, especially considering that it was designed for 110V primary, not the now common 120-125V.

    If you have a tech working on this amp who needs to look at a Torres book to figure out how to install a bias pot, well, there's your problem. He needs to switch out one resistor and do a bit of math first.

    Hogy
     
  4. fullerplast

    fullerplast Senior Member

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    It's a judgement call since you are running at max and the transformers are failrly conservatively rated. I've seen it done without problems, but DrBob, take Hogy's advice over mine-it's more conservative and safer. He is quite right about the primary voltages, which is why your plate voltages are on the high side.

    The other option that may be best overall is to use a Weber Copper cap with no heater current at all.
     
  5. renico00

    renico00 Member

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    Your tech used a Torres' book on your real '59? Wow, that's ballsy. If this tech has problems creating an adjustable bias supply, then they're missing something. You might want to find someone else, but what do I know...

    Jeff
     
  6. drbob1

    drbob1 Silver Supporting Member

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    Well, the problem out here is he's the only tech within about 100 miles, and I have real trouble finding time to take my amp to the other guy, then pick it up again. Be that as it may, the PTs not original, although it'd be a pain to replace (anybody got a spare 59 PT or know where I might find one?), speakers are changed, but the basic goodness is there. He should have replaced the bias resistor with a 1/2 value one and used a pot with the same value as the original wired in series, right? Do you ground the pot or tie two of the tabs together?
     
  7. BP-Plickner

    BP-Plickner Silver Supporting Member

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    Dude
    With a little research you should be able to get your bais supplied figured out. Did your so called tech install a 10k pot or a small plastic trimmer? If you have a pot installed i think all you have to do is move the bias range resistor from the bias cap (thats the one that goes from one end of one of the 8 mfd caps to the other end of the same cap) over to the left leg of the pot, looking from the back of the pot and solder the other end to the back of the pot.
    check out this link for more info on adustable bias conversions
    http://www.hoffmanamps.com/Biascircuits.htm

    be carful and ground out your filter caps whenever you work on your amp

    :jo

    peace
     
  8. drbob1

    drbob1 Silver Supporting Member

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    Thanks, I"ll try that.
     
  9. Blue Strat

    Blue Strat Member

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    A safer, comparable, solution to the rectifier swap is a 5R4. It only draws 2 amps of heater current and has similar characteristics (though slightly lower DCV out) than a 5U4GB.

    I wouldn't risk a classic amp by using anything marginal (like a 5U4).
     
  10. Wakarusa

    Wakarusa Member

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    +1. Big fan of the 5R4 for Tweed circuits here. If you get the military version they look really scary too :)
     
  11. fullerplast

    fullerplast Senior Member

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    Thanks for the useful advice AmpNerd. If you read the posts, you'd see that we now know it's a repro PT, so any assumptions about Triad 8087 limitations are out the window anyway.

    FWIW, I based my comments on several things:

    1) Both Aspen Pittman and Gerald Weber suggest the 5U4 as viable alternatives in 5F6A Bassmans. That in itself may not make it correct, but they sure as hell have seen and worked on more vintage bassmans than I'll ever see and were willing to publish that advice in two different books.

    2) AFAIK, every bassman repro currently made will handle a 5U4. Every repro bassman PT will handle it as well. Does that mean the original did? No, but combined with the other published information in (1) I thought it reasonable.

    3) I've not seen, or been able to locate, a spec sheet for the Triad 8087 PT to verify the max heater current (and I bet you haven't either).

    4) I've seen a 5U4 used in vintage 5F6A's without trouble.

    Finally, since Hogy expressed concern, despite what other information I had, I still suggested caution and following his advice, as well as the copper cap.

    But again, thanks for your additional information and insight.
     
  12. aeolian

    aeolian Member

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    How can I get comparison numbers on these? This might be the ticket to get my DR down to BF plate voltages.
     
  13. Wakarusa

    Wakarusa Member

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    You can look 'em up in any tube manual. The RCA Receiving Tube Manual RC-30 is a good choice. While you're there take a look at the oft-ignored 5V4. Pretty sure that Duncan's Amp site will have full GE or Amperex pages for these tubes online.

    Each of the rectifier tubes will have a characteristic or operational curve showing voltage drop related to current demand. Some will have a printed rating for average plate-to-plate resistance that corresponds with the linear portion of the characteristic curve. Most will also use a curve to show max voltage and current ratings, but your Bassman shouldn't even get close to those thresholds. Keep in mind that the scales on the charts/curves can vary quite a bit.

    For your DR a 5Y3 may be the ticket as long as voltage/current stay in spec. Under any kind of serious current draw they'll sag like a wet particle-board shelf :D
     
  14. drbob1

    drbob1 Silver Supporting Member

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    Interestingly I used a Variac to feed the Bassman 110v and the bias sets up very nicely in the middle of the range for a 5881 using the 5AR4. Drops the B+ to 425 from the 470 it was running before. Sounds great.
     
  15. jetlag

    jetlag Member

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    First, I am definitely one to advocate safe experimentation when it comes to valuable and cool old amps. I want to be safe with my old iron - it's a big part of why you spend the big $$$ for those old amps. That being said, I think people are forgetting something when they get their panties in a bunch with recto tubes in 8087 PT's. 5E5A pros, 5E7 bandmasters and 5F4 supers all had 8087 triad PT's and they all specified 5U4G rectos. My '57 pro and '58 bandmaster's 8087 PT's have held up nicely for 45+ years. Don't sweat it and use a 5U4 in that bassman if you want to. The 5R4G mentioned earlier is an excellent option - so are 5V4's and GZ37's. I used to wonder if the 5V4 was up to the task, current-wise, but now think it's probably fine (not so for many BF reverb amps).
     
  16. Blue Strat

    Blue Strat Member

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    Tube data manuals are some help, but don't always use the same conditions for each tube. Therefore, the data can be confusing.

    If you're using a 5AR4, expect a 25 to 40 volt decrease in B+ voltage.

    5R4s are under $20.
     

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