Power cap order in '72 Pro Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Kurzman, Jan 22, 2012.

  1. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
    (I briefly had this over on the emporium but pulled it until I'm sure it's right. My tech said it was probably fine but I think something is missing.)

    (I am NOT a tech.)


    What is the order of the power caps under the "dog house" on a 1972 Pro Reverb?

    I recently serviced and recapped the amp and it had what looked like a couple of jury rigged caps in series in the middle position. I assumed these were installed at some point because that's all that the former owner had on hand. Now I'm wonderingÂ….

    The amp is dead quiet and sounds good but it is lacking bottom. I'm thinking I maybe put a wrong cap in.

    With #5 being the cap closest to the output tranny, I have

    #5 100/350
    #4 100/350
    #3 20/500
    #2 20/500
    #1 20/500

    Should #3 be a 100/350?
    On the Hoffman site: "Larger filter caps help the bass response."

    At first I thought the 6L6's might be biased really cold but they are at about 30mA.

    Thanks in advance for any advice.

    ps I have done some tube and speaker swapping and while it does make a little difference (mostly the speakers) I still think there's something else. I'm waiting for a shipment of new pre amp tubes to try but I'm pretty sure they won't make much of a difference.
     
  2. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    Pics would help the most. Take some under the cap cover as well as the entire circuit. Also a pic of the transformers. You may have a small issue elsewhere. Also the 100uf caps go opposite in polarity to put them in series.
     
  3. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    Also the Pro Reverb actually has a small output transformer. Don't expect Super Reverb or Twin Reverb type bottom end from it.
     
  4. smolder

    smolder Gold Supporting Member

    Messages:
    13,850
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains
    For clarity.. If you are looking at the caps under the dog house, all of the caps except for one of the 100's should face in the same direction.
     
  5. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
  6. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    The only thing I can see is your 470 ohm screen resistors on your 6l6 socket look awfully black. Those need to be changed asap. As for the caps, they look fine (the installation).
     
  7. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
    Dave,
    I checked those funky looking 470's and they read fine.
     
  8. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    I would still chuck them. They are garbage. Get some 2 watt flame resistant 470ohm resistors in there. I wouldn't think about running an amp with those still in there. About 50% of the ones I have taken out literally crumble when I remove them. Those almost always read good before hand. Just my opinion of course ;)

    Also is yout Phase inverter coupling cap a .01 or a .001 ? This will make your bass act different as well.

    Aside from that I would lay down the schematic and start reading voltages. You want to see voltages within 10% of the schematic. If you think your up to measuring voltages with the amp running.

    -Dave
     
  9. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
    Dave,
    I think I'll change those 470's (they're kind of fiddly).

    Those three blue caps down by the phase inverter are all .1
    The two large ones are 600 volts and the third (smaller) is 200 volts. The two larger ones were checked by my tech and they are okay.

    I'm okay with soldering in new components but I haven't quit made the leap to sticking my hands into a "hot" chassis.

    Does 30 mA sound okay for the bias on the 6L6's? A guy on another board said it's cold and should be up around 38. Do you think this would make much of an audible difference?
    If so, I'll have to learn how to change the bias balance pot to a bias adjust.
    Thanks again.
     
  10. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    It's hard to say. Probably yes. However each individual amp is different. What plate voltage are you running? What brand name rectifier and type are you running? (Yes this does make a huge difference). There are so many factors. If I were you I would get the bias up to about 60-65% (Which is what I find best for Fenders) which is roughly 37-42 depending on your plate voltage.
     
  11. vibrostrat43

    vibrostrat43 Supporting Member

    Messages:
    1,688
    Joined:
    Aug 25, 2011
    Location:
    League City, Tx
    No you're good with the caps that are in there. You might want to move cap 3 to about 40uf (usually about 450-500v), but there's no need, and/or you might want to use 220uf (with a combined voltage rating of at least 500) for both 4 and 5. This would stiffen up the power supply more, but from experience most of the trouble that blackface Fenders have with flabby low end is from high values of coupling caps, and high value cathode bypass caps. This seems especially important on the power tubes in these amps to me...I actually like .022uf caps for the blocking caps going to the power tubes.
     
  12. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    It's not advisable to up the capacitance in a tube rectified amp. The tube has to work a little harder than normal. A solid state rectified amp should be fine.
     
  13. vibrostrat43

    vibrostrat43 Supporting Member

    Messages:
    1,688
    Joined:
    Aug 25, 2011
    Location:
    League City, Tx
    For some reason I was thinking he had a Twin, maybe that was from a different thread. Davemccarthy707 is right, don't go any higher than 50uf total for any leg with 5ar4 or 5u4 rectifier.
     
  14. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
    I'm going to let my tech take another crack at adding a bias adjust pot with the bias balance pot. The two 6L6's are now balanced at 28mA.
    I swapped the orig. 5U4 for a newish Mesa branded 5U4, swapped out the orig. Sylvania 6L6's for newish Sovteks and swapped out the orig. RCA preamp tubes for new EH. This is the second time I've swapped tubes around and it made a huge difference.
    I think if I can just get the bias up around 35-38, it will be perfect. It sounds fine as it is now. I just think it could sound a little better.
    I appreciate all of the help.
    BTW...my tech says to NEVER bias a Fender at more than 35mA which is different info than I get on the web. My tech is a really good guy and helps me out alot (I have about 15 old tube amps) so I'm usually in the middle with his opinion and the general consensus web opinion.
     
  15. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    Get a 5Ar4 in there. Put back the RCA pre's if they are not microphonic or noisy. The Sovtek 6L6WXT is actually a great tube in Fender amps. The 5881wxt is not. Lastly, get a new tech as he does not understand bias. To say not to go above 35ma is wrong. Simply because it is only one part of a mathematical equation. 35ma is a generally safe baseline in 6l6 Fenders, but that is it. Without knowing plate voltage, it means nothing.

    http://www.webervst.com/tubes1/calcbias.htm
     
  16. smolder

    smolder Gold Supporting Member

    Messages:
    13,850
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains
    that's a great link Dave... I use it nearly every time I bias... and I almost never bias at 70% but it gives me a good ceiling. I like to start there... and then drop it until the amps starts feeling cold... then tweak back upwards just a hair.
     
  17. davemccarthy707

    davemccarthy707 Member

    Messages:
    1,064
    Joined:
    Oct 4, 2008
    Location:
    Newfoundland
    I usually bias at what sounds best, then I check the math. :)
     
  18. smolder

    smolder Gold Supporting Member

    Messages:
    13,850
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains
    you have just a tad more experience under your belt ;)
     
    Last edited: Jan 24, 2012
  19. LarryN

    LarryN Member

    Messages:
    959
    Joined:
    Jan 1, 2004
    Location:
    Santa Rosa, Ca.
    True. Allen makes a drop in upgrade.
     
  20. Kurzman

    Kurzman Supporting Member

    Messages:
    989
    Joined:
    Jan 9, 2007
    Location:
    Mid - Va.
    The amp is now sounding like a million bucks.
    My tech installed a 5k trim pot and a 12k resistor across the balance pot. After dicking around and dicking around trying to get the old Russian 5881's to bias up and sound good, I finally just ditched them and installed a nice set of USA Fender (probably Sylvania) 6L6GC's. They biased up perfectly at 39mA and the plate voltage is at 450.
    I had bought this amp just for re-sale as I don't need another amp. It sounded okay as it was but I wanted to make sure the next owner would go WOW! when he/she plugged a Strat into this thing. That's what's supposed to happen and that's the way I like it to work when I buy a piece of gear.
    Thanks for everyone's help and advice. Another 40 year old Fender is ready for another 40 years.
     

Share This Page