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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

A question about silverface Fenders with "Hum Balance" on the back?

Discussion in 'Glowing Bottle Tube Amp Forum' started by E5RSY, Sep 5, 2017.

  1. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    Wondering how bias is adjusted on these amps...is there a bias pot inside the amp, as well?

    I ask because a few years back I had my Super Reverb serviced and they re-wired the hum balance control to be bias adjust like the older amps, instead. If there is another pot inside that was formerly for bias, was it most likely bypassed altogether? Otherwise re-purposed somehow?

    Just wondering out loud. I can't open it up and look at the moment.
     

  2. moosie

    moosie Poster Extraordinaire Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    There is no adjustable bias on the SFs. There might be something on the rear panel called "output tubes matching". The bias is fixed. This allows you to balance the tubes, at that fixed bias point. Now that matched power tubes are the norm.... let's just say I'd much rather have adjustable bias.

    The hum balance is completely different. The filament / heater wires are low volt, high current AC. Twisted tightly, as they would otherwise introduce a lot of noise, with their proximity to the DC circuitry... But, if the filament leads aren't quite at the same voltage, you'll still have hum. I guess this occurs because of inconsistencies in the wind count on the PT heater secondaries. Anyway, the hum balance is simply a pot that equalizes the voltage across the two filament wires. It silenced the hum on my '79 TR perfectly!

    If you're comfortable working in a live chassis, you can adjust the pot with a voltmeter attached, and hear the hum die as the voltages get closer. Small amounts can make a big difference to the noise level.
     
    Last edited: Sep 6, 2017
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  3. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    No, it's already been re-wired to be bias. I was wondering if there was an old bias pot on the chassis still in there.
     
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  4. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    Some techs replace the Bias Balance pot with a Bias pot and reuse the whole.

    Others keep the Bias Balance pot and use it ALONG with a Bias pot or trimpot. That way you can set the Bias and use the Balance to exactly match both tubes.

    It is certainly possible that a tech would install a Bias pot and just bypass and retain an unwired Bias Balance pot, if just for original part sake.

    So the answer is...yes, no, maybe?

    The Hum Balance has nothing to do with Bias, it's a pot placed in the filament supply to balance the 6.3VAC heater voltage feed to the tubes.
     
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  5. corliss1

    corliss1 Tele-Afflicted Gold Supporter

    Sep 13, 2008
    Lansing, MI
    In general, no. For your specific situation, as Wyatt said, you'll need to take a look.
     
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  6. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    Yeah I'd rip out the chassis and have a look. Post pictures here if you need help figuring it out.
    But only if you're comfortable pulling a chassis!
     

  7. jimbo735

    jimbo735 Tele-Holic

    567
    Sep 19, 2011
    michigan
    Mine has both the balance pot from the factory and I put a bias pot into the amp,One of the best things you do for it.
     

  8. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

    Jan 2, 2007
    Virginia, USA
    Originally, the bias on your amp was adjusted by replacing a resistor in the bias circuit. On some of the later Silverface amps, in addition to the Hum Balance (I recently learned), there was a bias balance pot. It adjusted the bias between the tubes to be the same. This is handy if you have mismatched pairs and need to balance them (although some folks believe a little mismatch can be a good thing).

    I do wonder if, when your tech installed the adjustable bias pot, he left the bias balance circuit in place?
     

  9. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    Thanks. He didn't install a bias pot. Rather, he re-wired the hum balance on the back to be bias, instead.
     

  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    That heater filament Hum balance pot is a long way from the bias circuitry. I would make very very sure that that was really what was done. I have never seen that done nor have I ever thought of putting the bias adjustment way over in the preamp section. If they did that, did they establish a virtual center tap for the heater filament circuit?
    Got pics?
    axis29, fwiw all SF Sr's have a bias balancing pot in the circuit. You are correct in that in order to change the bias voltage one has to install a different static resistor or install a bias voltage adjustment pot...in one way or another. I prefer to leave the balancing act and add the bias voltage adjustment in order to have the best of both worlds....one can run mismatched tubes and balance their operation at whatever bias point one chooses.
     
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  11. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    I'll post some pics. Thanks. May be a day, or two, but I'd appreciate y'all's input.

    Thanks,
    Scott
     
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  12. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    OK. Here are a few shots. I apologize for my photog skills. Hope they show enough.

    One pot (with adjustment access on the back):

    DSC05568.JPG

    DSC05571.JPG

    The other (internal):

    DSC05569.JPG

    DSC05570.JPG
     

    Attached Files:

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  13. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    The pot on the back of the chassis is a Hum Balance pot. It's connected to the filament wiring. It has NOTHING to do with bias. Set it for minimal hum (somewhere in the middle) and forget it even exists.

    This one looks like it was converted from the old Bias Balance to a more traditional Bias voltage adjustment pot. It is actually accessible externally as well from under the chassis.
     
    Last edited: Sep 7, 2017
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  14. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    Whoops. Double post.
     
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  15. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    that bias circuit was wired a bit differently from the way we usually see things, right, Wyatt? The results are the same.

    This amp and, with all due respect, E5RSY's misunderstanding of what was going on in that amp is an example of why one needs a tech to make sure that one understands what it is they are using.....and how to use it. Imho, anytime someone buys a new 'used' amp, that amp needs to be thoroughly evaluated. Fwiw, I also feel the same way about a newly acquired brand new amp IF the owner doesn't have a manual or can't understand the manual's explanations.
     
    Last edited: Sep 7, 2017
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  16. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    There is at least one extra wire there that I was wondering about. But it is pretty obvious it's no longer wired for balance.
     

  17. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    You are right. I just found the tech notes and they reflect almost exactly what you say.

    Thanks, everyone.

    Scott
     

  18. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    So, can y'all explain to me, as a layman, the practical diifference between the bias balance option vs. traditional bias adjustment?

    It wasn't something I asked for, but I trust him and he felt pretty strongly about it so I gave the green light.
     

  19. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    I see it now, but you'd have to pull a speaker to get to it (Super Reverb). Do people typically just pull the chassis and get it from the inside, instead?
     

  20. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    That's what I do. Tubes get hot!
     

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