A question about a grid leak biased V1 and hum level

Discussion in 'Amp Tech Center' started by Viejo, Dec 9, 2019.

  1. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    I am working on a Gibson GA-20T from 59-60. The non tremelo channel is a grid leak biased 12AY7. Voltages are right were the should be and the signal passes well but the 60 cycle hum is very loud. Is this just the nature of this type of circuit? I have read they are prone to picking up heater hum. The heater filaments are just daisy chained not twisted pairs.



    http://schematicheaven.net/gibsonamps/ga20t.pdf
     
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  2. Wally

    Wally Telefied Ad Free Member

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    The GA20T from that era that I had did not exhibit any such loud hum. If this was heater filament voltage hum, you would not be able to pinpoint it to one certain channel, it seems to me. Usual suspects...tube, cap, ground point.
     
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  3. D'tar

    D'tar Friend of Leo's

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    Try a different tube. I have a lovely GE 12ay that just humz 60hz no matter what. Almost got to the point of mod to dc heater supply but just retired the tube instead.
     
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  4. King Fan

    King Fan Friend of Leo's

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    Excellent idea. FWIW I just saw an MEF thread where a bad tube caused hum in a grid-leak bias stage.

    Wally, I know tons less than you. I'm sure you're right: That amp seems unlikely to have had massive hum as built. As far as hum on just one channel, tho, it looks like only that channel has a grid-leak bias. Yes no?

    schem.png


    No real insights here, but of course you're right; grid-leak input tubes are hum-prone. It couldn't hurt to twist-pair the heater wires and lay 'em down nicely on the chassis. Also, I may be missing it on the schematic, but if there's no heater CT, would an 'artificial, elevated' CT with 80-100 ohm resistors tied to a power tube cathode help?
     
    Last edited: Dec 10, 2019
  5. Wally

    Wally Telefied Ad Free Member

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    1)I would not take it for granted that my electronics knowledge is greater than yours.

    2)yes, that 12AY7 channel is as the OP notes a grid leak bias circuit on the preamp input while the 5879 is cathode biased.

    3) that heater filament circuit....when one end of the winding goes to ground, there is no need for a CT whether real or artificial. I would think that establishing an artificial center tap with a twisted pair supplying the heater filament voltage to the tubes would be advisable even if this particular hum/noise is not being generated by the original heater supply.
    I am thinking that the noise that the owner is objecting to is a new noise.
     
    Last edited: Dec 10, 2019
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  6. King Fan

    King Fan Friend of Leo's

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    Thanks, Wally. I kinda figured I might be missing something about the need for a heater CT. I told ya you knew more than me. I'm not sure about much, but I'm sure about that. :)
     
  7. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    First, thanks for the replies and taking the time to help. I am adding some things I should have put in the OP.

    I have substituted the 12AY7 the hum remained unchanged.

    The hum is dependent on the volume control it gets louder as the volume is turned up

    I just replaced the electrolytic caps but the same cap feeds the 5879 channel and it is very quiet.

    I did have to remove the board to replace the electrolytic bypass cap on V2, this required unsoldering and resoldering the connections where the V1 grids connect to the .01 caps. I could have over heated one of those caps. Would that cause the hum? Both inputs work and are grounding well through the shorting jacks when nothing is plugged into the first channel. The hum is the same whether or not anything is plugged into either input
     
  8. Wally

    Wally Telefied Ad Free Member

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    Yes, a bad cap could exhibit the problem you are having. Easy to check that....
     
  9. timfred

    timfred TDPRI Member

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    Things I would check first:
    1) Remove input jacks to check that if they are grounded to the chassis that their mating surfaces are clean and making good contact. If they are isolated from the chassis, make sure of that. Clean the contact surfaces on the jacks and test that the tips are well grounded when no connector is inserted.
    2) Evaluate and experiment with the grounding of the 12AY7 cathode. Try (a) close to the tube, (b) at the input jacks, and (c) at the PS ground or AC mains ground lug if you've converted to a grounded power cord.
    3) Evaluate the lead dress between the input jacks and the 12AY7. Push the wire around to see if the hum changes, or convert to shielded cable if it doesn't have it already.
    4) Check the values and grounding of the 10Meg grid leak resistors.

    As an aside - this might be blasphemy for some - but if this were my amp I'd have a hard time resisting the urge to convert that tube to a 12AX7 with "normal" elevated cathodes and using one of the triodes as another gain stage before the phase inverter. That would make it a different amp, though.
     
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  10. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Thank you for your time all. I have pulled the two caps and they are right on value at .01 they had measured .05 in the circuit. I am at the point of deciding to do as @timfred suggests and just wire V1 to cathode biased. The grounding on V1 is a mess they have just pushed the socket tabs for both cathodes and pin 4 and 5 filaments up to the center of the socket and soldered them to a wire running to the jack grounds. The shorting jacks are definitely shorting to ground. It would sure make the amp more usable converted to cathode bias V1 as far as using pedals. I guess I'll think on it over night. I know many don't like them but I'm half a mind to make a brass grounding plate like a Fender . I dislike grounding buss wires. The amp is player grade nothing mint.
     
  11. The Ballzz

    The Ballzz Tele-Afflicted

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    What the heck kind of phase inverter is that called? I don't think it's a "para-phase?" Looks pretty bizarre to me! Maybe my perspective is just off.

    Just Curious?
    Gene
     
  12. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    I put it back together with two new .01 caps and the hum is still there so it must be a grounding issue. I'll pull the jacks in the morning and clean under them and if that doesn't cure it redo grounds until I find it or give up and convert V1 to cathode bias
     
  13. vintagelove

    vintagelove Tele-Meister

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    I have a beautiful sounding grid bias octal that is very quiet.
     
  14. Wally

    Wally Telefied Ad Free Member

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    http://www.valvewizard.co.uk/paraphase.html
     
  15. tubelectron

    tubelectron Tele-Holic

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    You said it all... If all the rest of the amp is checked and OK, of course.

    That said, a more or less defective 12AY7 is possible too : a filament to cathode more sensitive leak appearing with this poorly designed and unbalanced heater circuit (very common on old radios and amps)... :confused:

    Here's what I do on all my amps fro the heater line - that said, I invent nothing new ! o_O

    [​IMG]

    For powerful or hi gain amps, I replace the 2x 100R balance resistors by a 100 to 470R balance pot (wiper to GND close to the 1st preamp), to fine tune the heater line hum cancellation (that's notably what was factory done on post 1975 SF Twin Reverb amps). ;)

    -tbln
     
  16. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Well I finally found the hum . It was indeed a ground issue Wally. My thanks to everyone who took the time to help
     
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  17. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Working on one of these Gibson amps sure makes you appreciate Leo Fender
     
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  18. Wally

    Wally Telefied Ad Free Member

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    Side note....I just modded a ‘64 GA17RVT....hot amp now. Think 5E3 with Reverb and trem. No, they are not as easy to work on as the old Fenders. They have other attributes, though.
     
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