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Power tube bias only at 8 mA

Discussion in 'Glowing Bottle Tube Amp Forum' started by jslo1269, Oct 7, 2016.

  1. jslo1269

    jslo1269 TDPRI Member

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    Just got a 1968 Pro Reverb AA165. All original and I put in new filter capacitors. The amp sounded very quiet, weak, and badly distorted. I checked the bias on the power tubes and was getting 6 mA and could only get a max of 8 mA. Tried two different sets of brand new 6l6s and got the essentially the same result. Any ideas what's going on here?
     
  2. slider313

    slider313 Tele-Afflicted Silver Supporter

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    The tubes you have are low current draw. Replace the 27k resistor on the bias pot with a 15k; unless it's really an AB668 which would have the dread balance bias circuit.
     
  3. RLee77

    RLee77 Friend of Leo's

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    How are you measuring current... and did you measure on both tubes separately, or the sum of both. I'd suggest measuring the plate voltage, and also measuring the voltage on the middle wiper of the bias adjust pot. Could be many things, including a bad output transformer, but those two measurements may help narrow it down.
     
  4. jslo1269

    jslo1269 TDPRI Member

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    hi there- bias checked with Eurotubes bias pro.

    Plate votage 336

    Bias adjust pot wiper -46
     
  5. slider313

    slider313 Tele-Afflicted Silver Supporter

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    The plate voltage is much too low. You should have about 440v on the plates and at 8 ma it would be even higher. Something's not right there. Check the battery in the bias probe.
     
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  6. jslo1269

    jslo1269 TDPRI Member

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    Checked batteries and same result. Sound is very quiet and distorted, and power tube bias current on both tubes is at 8 mA.

    What could be causing such a low plate voltage and bias current?
     
  7. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Was it working correctly before you replaced the capacitors?
     
  8. jslo1269

    jslo1269 TDPRI Member

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    No - was the same before I replaced the capacitors
     
  9. peteb

    peteb Friend of Leo's

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    The PT is suspect

    What voltage on pilot light/filament?

    What tube as rectifier?
     
  10. jslo1269

    jslo1269 TDPRI Member

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    Rectifier is new JJ GZ34. Will check the voltage on the pilot/filament when I get home- thanks for all the help!
     
  11. RLee77

    RLee77 Friend of Leo's

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    Sounds suspiciously like the bias current you're reading is just the screen grid current, as if no plate current is flowing, which makes me suspect something with the output transformer, since the bias voltage seems correct. But that's just conjecture. It could be another component.
     
  12. jslo1269

    jslo1269 TDPRI Member

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    Right. How can I confirm/rule out the output transformer? What other components could it be?
     
  13. RLee77

    RLee77 Friend of Leo's

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    Just reread your original post. It might help to frame the problem from a wider perspective. I assume the amp was working ok when you first got it? Did the problem start right after you replaced the filter caps, or were other changes made, or did it always have this issue?
    If the former, I'd suspect that maybe the OT centertap connection to b+ was not good.
    Double-check the polarity of those new filter caps, even though you've probably already done that, and a backwards cap would likely be blowing smoke by now.
    Otherwise, with the amp off, unplugged and all filter caps safely discharged, you can measure resistance of the OT primary windings.
     
    Last edited: Oct 8, 2016
  14. jslo1269

    jslo1269 TDPRI Member

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    Hi RLee77 -

    I was given the amp by a friend who hadn't used in quite sometime. Before changing the filter caps, I only verified that the amp would power on as the original filter caps were bulging and leaking so the first thing I did was replace them. Double-checked the polarity and it's good.

    I've never had to measure the resistance of the OT primary windings - would you be able to walk me through the correct way to do this? Thanks once again.
     
  15. RLee77

    RLee77 Friend of Leo's

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  16. jslo1269

    jslo1269 TDPRI Member

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    Great- Thank you! Will check today.

    I noticed that the B+ line from the OT had a bad solder connection- enough so that I was able to wiggle the wire loose very easily. I re-soldered and now I have a plate voltage of 356 V (up from 336) and the highest bias current I am able to get now is 12 mA. A bit better, but there is definetly something else at play here. I will check the OT resistance and get back to you - thanks so much once again!
     
  17. jslo1269

    jslo1269 TDPRI Member

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    Resistance of B+ to output tube plates: 78 and 81 ohms

    Resistance B+ to speaker jacks: 115k

    So, looks like the secondary winding is burned open.

    Guess its time for a new OT?
     
  18. peteb

    peteb Friend of Leo's

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    Maybe the OT is bad, I don't see the evidence.

    Since the supplied voltage is low, I would look at the power transformer, the source of the power and the high voltage.

    One job of the PT is to provide 6.3 VAC to the pilot light and the tube filament heaters. An easy health check for the PT is to measure this voltage. Even with today's high wall voltages, the heater is supposed to be below 7 for good tube operation.

    Also, check the high voltage secondaries on the PT, the red wires. Each should be 340 VAC above ground, per the schematic.

    There could be a short somewhere dragging down the voltage, but if it was a bad short, the amp would be blowing fuses.

    Not sure why you think the OT is bad with an open.

    The resistances across the primary's are nicely matched. There should be negligable continuity between the b+ and the speaker Jack. The 115k could be too low of a resistance, no continuity would be best.

    Measure the secondary of the OT, unplug the speaker, and measure the resistance between the two wires going from the OT to the Jack. Should be very low resistance, less than one ohm.
     
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  19. Ten Over

    Ten Over Tele-Holic

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    8mA with a plate voltage of 336V and a bias voltage of of -46V sounds about right to me. A bias voltage of -30V would probably be necessary to get 60% dissipation at that plate voltage.

    Pay attention to peteb -- he is on to what is wrong.

    115K from primary to secondary is completely plausible with the B+ connected to the primary because you are measuring the resistance of your power supply. You can get wildly different values depending on the meter you use and the polarity of the probes. If you really want to check for continuity, you need to either disconnect B+ from the primary or disconnect both secondary leads.

    When you measure the resistance of the secondary, the speakers must be disconnected and an open phone plug needs to be inserted into the speaker jack (not the ext. speaker jack). Otherwise, you are measuring the resistance across a shorted phone jack.

    I would also like to know what the voltage is at the standby switch with the switch open (the amp on standby).
     
  20. jslo1269

    jslo1269 TDPRI Member

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    Got some numbers for ya:

    High voltage secondaries on the PT: 360V
    Heater: 3.26V
    Standby switch, open: 110V

    Just re-checked the plate voltage and I'm getting 107V

    ... now I'm really confused
     
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