Ampeg PF-50T tube issues are driving me insane! Please help!

Discussion in 'Amps and Cabs [BG]' started by mas_akala, Jul 14, 2021.

  1. mas_akala


    Sep 1, 2018
    Hey all, long time mostly-lurker here. I got a great deal on a PF-50T, totally dig it, except…

    It shipped with a really microphonic 12AU7 and the power tubes make this weird fizzy sputtering noise when certain notes are played (like 14th fret E on the D string).

    I took it into the local authorized Ampeg guy but he’s been really shady through the whole deal and seems like he’s trying to make a buck off me by insanely over pricing some Groove Tubes instead of just following through with the factory warranty’s replacements.

    This had been going on since April and I finally got sick of it and just ordered a bunch of matched Tung-Sol tubes with the 6L6GC STR for power.

    So now the amp sounds awesome and the sputtering went away, but the red biasing light doesn’t ever turn off. Once you warm the amp up and turn everything on with no instrument plugged in both the green and red lights are constantly on. When I switch back to the Ruby ones it can bias as written in the manual, but sounds like garbage.

    Did I get mismatched tubes? Is there some rating difference between these Tung-Sols and the factory tubes that would cause this to happen? The matched set are guaranteed so should I ask for replacements?

    Hoping @JimmyM or @beans-on-toast or someone with some experience could give me some advice. Ampeg support has not been very easy to deal with as you have to use their stupid ticketing system and responding to their emails seems to go nowhere.

    Thanks in advance!
    DJ Bebop and EatS1stBassist like this.
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Is the guy you are dealing with an AUTHORIZED Ampeg warranty service center, or are you taking his word for it?

    IIRC, the factory warranty covers tubes for the first 90 days.
    Mvilmany, DJ Bebop and EatS1stBassist like this.
  3. mas_akala


    Sep 1, 2018
    Yeah, he is. There’s another one I guess I can go to on the other side of town, just after months of dealing with this dude I figured I’d just pay for replacements myself to not have to deal with him anymore.
    EatS1stBassist likes this.
  4. mas_akala


    Sep 1, 2018
    I think it was happenstance from the Yamaha purchase.
  5. Wasnex


    Dec 25, 2011

    Here's what the manual says about both Red and Green Bias LEDs on:

    "Both on all the time Possible bad Bias Control or bad tubes Have unit checked by a service technician."​


    "If the green and red LEDs are lit, the amp is under-biased (clockwise) and too much current is flowing to the power tubes. "​

    Since the amp biases fine with the original tubes, there is a good chance the bias control is fine. However the circuit may not have enough range to dial to new tubes down into the proper current range. This is something that may be fixed by swapping out a bias ranging resistor, so the bias can be set to a higher negative voltage. But a better solution IMHO is to get tubes that are graded to work in the circuit.

    Not exactly sure how the bias monitoring circuit is set up, but I believe the green LED will not come on until both tubes reach their minimum target current. This is an "AND" logic function. If A and B satisfy the minimum current target, you get green.

    Then I believe the red LED comes on if either tube exceeds the max current limit. This is an "OR" logic function. If A or B satisfy the max current target you get red. The Green lights stays on, because both tubes are above the minimum current target.

    Is the amp humming? The reason I ask is if the tubes are not matched it will produce sort of a dull roar. With this problem it may be possible to get red without green. I believe red without green means one tube is over the max current target and one tube is under the minimum current target.
    EatS1stBassist, jw23mind and nbsipics like this.
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Give Ampeg's customer service department a call and explain what's going on. Maybe they can help you by recommending a better choice for getting this fixed. You shouldn't have to pay for a service tech's inability to fix any product that's under warranty.
  7. Wasnex


    Dec 25, 2011
    Something I would check with original tubes. Does the sputtering occur if the amp is not placed on the cab. It's possible the pins in one or more of the tube sockets is loose, so vibration creates an intermittent connection.

    The size of the pins in the tubes varies somewhat from one company to the next. In particular, I have seen lot's of posts that suggest the pins in some or all JJ tubes are smaller than other brands.
    EatS1stBassist likes this.
  8. mas_akala


    Sep 1, 2018

    The only time a noticeable sort of hum comes on is if the bias dial is turned toward fully clockwise. If it’s in the middle or lower it sounds pretty good. Both lights are constantly solid though. I guess I stupidly assumed all 6L6GCs would be interchangeable, but maybe that’s not the case?
    EatS1stBassist and Wasnex like this.
  9. mas_akala


    Sep 1, 2018
    Calling just led to some Line 6 voicemail, but I can try the ticketing system again.
  10. mas_akala


    Sep 1, 2018
    I thought I tested this, but I’ll try again. That may have been before the microphonic 12au7 was sorted out.
    ficelles and Wasnex like this.
  11. Wasnex


    Dec 25, 2011
    It depends how wide the range of an amp's bias control is. Some amps can run pretty much any tube of the proper model, while other amps need tubes graded to a certain range.

    It's also possible the new tubes are bad.
    EatS1stBassist likes this.
  12. mas_akala


    Sep 1, 2018
    Also, what happens if I continue to use the amp incorrectly biased?
  13. Wasnex


    Dec 25, 2011
    At the very least the output tubes will wear out faster. At the worst, the amp will be damaged.
  14. beans-on-toast


    Aug 7, 2008
    A new amp comes with a tube warranty. I don’t see why the service center was trying to sell you new tubes. Try another service center. Document in a ticket all that you’ve been through in attempting to get the amp repaired through their system.
  15. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Replacing my 12au7 with a low microphonic 12au7 helped a bit. But the other tubes are under warranty and pretty good other than the 12au7.
  16. Cave Puppy

    Cave Puppy "Humph Bo, he's wond!" - John Lennon

    Jan 13, 2015
    I sent my PF-50t back to Sweetwater to get the "hum mod" installed. Something about a filter cap being too close to one of the transformers (?). It was a factory authorized mod so it was covered. All I had to pay for was shipping, which was discounted.
    JimmyM likes this.
  17. Greg75

    Greg75 Supporting Member

    I have the Tung Sol 6L6GC STR tubes in my PF-50T. Sounds fantastic. And, I had no trouble correctly adjusting the bias. Also, I had no sputtering problem with the Ruby tubes. Betting there is something else wrong with the OP's Ampeg.
  18. ficelles


    Feb 28, 2010
    Devon, England
    I have the original power tubes in my PF-50t at the moment... retubed the preamp with reissue Tung Sols but even with the originals it works fine. Do you have a recording of the sputtering noise?

    I had a great response from Ampeg the one time I emailed them for support, that was a while back though.
  19. mas_akala


    Sep 1, 2018
    Actually called the correct phone number and got to their customer support and they were super awesome. They extended my tube warranty for a different service center. He also suspected that the later issue may be the tubes so I’m working on getting ahold of the reseller, West Coast/East Coast thing is slowing me down.
  20. mas_akala


    Sep 1, 2018
    Ok, so they also told me to try one of each to see if I can narrow the biasing issue down to 1 of the Tung-Sols, and indeed one of them will appear to bias correctly with a Ruby and the other immediately goes to red… so I think I’m sorted.
    Thanks for all the help folks! Such a rad community.
  21. Primary

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