Blackface Fender Rectifier Tube Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by lchender, Feb 10, 2012.

  1. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    I was hoping I might prevail upon some of the amp guru types in these parts....

    I have a 1965 Fender Tremolux Amp that I recently acquired and noticed that it currently has a 5U4GB (a Sovtek) in it. From what I understand, the amp should have a 5AR4. I'm told that the 5U4GB draws significantly more filament current than a 5AR4 and some would caution that this could be hard on the PT.

    I believe there are other options for NOS rectifiers that would have a filament current draw similar to a 5AR4, but that would be significantly less expensive. I'm not a complete and total cheapskate, but I can't help but notice that a NOS 5R4 or 5V4 is significantly less expensive than a NOS 5AR4.

    From what I understand, the 5U4GB that is currently in the amp would have significantly more sag than a 5AR4 and I would need to rebias if I were to switch rectifiers. The amp sounds great now, but I don't want to endanger the PT with using an incorrect rectifier.

    Considering the 5V4 or 5R4, which would be the more appropriate and healthy choice for the amp in place of a 5AR4? Or would either be an okay alternative? Is there potentially an advantage of using one over the other when considering tone?

    Thanks for your help!
     
  2. vibrostrat43

    vibrostrat43 Supporting Member

    Messages:
    1,688
    Joined:
    Aug 25, 2011
    Location:
    League City, Tx
    a 5R4 would give you a similar amount of sag as you currently get with the 5u4, but it will be about 20 volts more sag. The 5v4 will perform much closer to the 5ar4 in terms of sag, but it doesn't quite have the same current capability, so be sure your amp doesn't draw more than 175ma when under full load (cranked and with signal).
     
  3. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    Thanks vibrostrat! How would I determine the draw under full load? Forgive my ignorance....is that something that my tech should measure or can it be obtained from a schematic....??
     
  4. zenas

    zenas Member

    Messages:
    6,097
    Joined:
    Jul 1, 2011
    You just need to know how much each tube draws and add it up.
    But alot of guys are running 5v4s in things like Super Reverbs which have two more preamp tubes and seem to be doing fine so I'd say go for it.
    Come to think of it I've been running a 5y3 in a BFDR for awhile now. That's something no one would recomend. I need to find some 5v4s and 5r4s to try in it.
     
  5. vibrostrat43

    vibrostrat43 Supporting Member

    Messages:
    1,688
    Joined:
    Aug 25, 2011
    Location:
    League City, Tx
    It has to be obtained with math and tube datasheets. You have to add up the anode current for for both power tubes, their screen current, and the anode current of all preamp tubes.

    So your amp has 6l6gc's if I'm not mistaken. So you'd find how much anode current and screen current is drawn by 6l6gc's in class AB push-pull at the higher plate voltage given on datasheet (usually the lower voltage is like 250 volts which is really low and almost never seen in most guitar amps). Also be sure you look at the tetrode connection, instead of triode, and also use the full load specs not no load.

    Then you'd look for the anode current of each type of preamp tube (in your case it may just be 12ax7's but I can't remember) at the voltage closest to voltage that tube is fed off the rail before the plate resistor (it may be after the plate resistor I'm not sure...hopefully someone will come on here and correct me if I'm wrong, still learning myself). And of course multiply that current by the number of same-type tubes fed that voltage.

    Then you just add everything up. So for instance, I look at the datasheets for a 6l6gc and I see that a pair in class AB at 450 volts draws 125ma at the anode, and 20ma at the screen. Then I look at the datasheet for a 12ax7 and see that the anode draws 3ma at 300 volts. Then I just multiply that 3ma by the number of 12ax7's drawing a voltage similar to that (let's say we have 4 12ax7's) giving me 12ma...then I would add:

    125 + 20 + 12 = 157ma

    This is just an example, but I think it's not a wild shot in the dark number. If you get less than 100ma at the end then you were looking at the wrong part of one or more datasheets, and the same could be said if it's way over 200ma.
     
  6. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    Thanks guys - I really appreciate it!

    Considering that I like the tone I'm currently getting with the 5u4gb, maybe the 5r4 would be the safest and yield similar tone to what I have now.
     
  7. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Yes! If you like the tone with the 5U4 definitely go with a 5R4.
     
  8. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    Thanks for chiming in, Mike! It's very kind of you to offer all the free advice that you do on TGP.
     
  9. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Thanks. At least one guy appreciates it. ;)
     
  10. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    So, I picked up a few Cetron JAN 5R4WBG tubes and they're massive! In fact, too tall to fit in my Tremolux. I found one Sylvania 5R4 that isn't so massive and it fits, but I would rather put a JAN tube in there as I assume they are built super tough and would likely last longer.

    Are there any JAN 5R4s that are stubby enough to fit in my Tremolux? Also, is there any way to tell by the code numbers which ones are the tall ones and which are the more diminuitive height? It's hard to tell from the pictures on tube websites which ones are shorter and sellers don't often provide dimensions.

    Thanks!
     
  11. smolder

    smolder Gold Supporting Member

    Messages:
    13,850
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains

    Lol... You're hardly under appreciated dude. Tons of love here.
     
  12. LarryN

    LarryN Member

    Messages:
    959
    Joined:
    Jan 1, 2004
    Location:
    Santa Rosa, Ca.
    I don't know any codes, but a Sylvania should give you a long service. If in doubt, ask a seller for a measurement.
     
  13. big mike

    big mike Administrator Staff Member

    Messages:
    24,558
    Joined:
    Oct 31, 2003
    Location:
    NorCal
    It's a nos recto
    It should last 10-15 years under hard usage ANYWAY
     
  14. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    Thanks for your help, guys.

    So, the designation after 5R4 (i.e., GA, GYA, GYB, etc.) bears no relation to the height of the tube?
     
  15. shapeshifter

    shapeshifter Member

    Messages:
    507
    Joined:
    Jan 26, 2011
    I am waiting for a NOS 5R4 to arrive for my BFDR. It's really interesting how the fell of the amp changes when swapping rectifier tubes....
     
  16. lchender

    lchender Member

    Messages:
    228
    Joined:
    Nov 18, 2003
    Location:
    Toronto, Canada
    I have a Cetron 5R4WGB in my BFDR and it sounds great! I think you'll like the results.
     

Share This Page