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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. corliss1

    corliss1 Tele-Afflicted Gold Supporter

    Sep 13, 2008
    Lansing, MI
    Yes. If you have an external biasing tool you can do it from the outside, but I've guess most techs just pull the chassis to check things over.
     

  2. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    Balance = Bias balance between 2 tubes (or tube sets)
    Bias adjustment = increase/decreases bias current all together.
     

  3. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    E5RSY, these days I like to leave the bias balancing circuit intact and add a bias voltage adjustment trim pot in the circuit prior to the balancing act. That way, one can select the current draw one wants and either balance or slightly unbalance the power tubes' operation. Ex: I installed a pair of new 6L6's in a SFSR yesterday and biased the amp for the young guy....letting him play to hear what went on and choose what he liked. This amp had the mod back to a pure bias voltage adjustment circuit without the balancing act. The power tubes don't bias out quite as close as I would like.....36ma in one versus 40ma in the other....55% of max plate dissipation for the higher draw. IF the balancing pot had been left as it was and a voltage adjustment added, then I could have balanced those two tubes' operational parameters...at any current draw the player might like sonically.

    Wyatt, I had to take two looks at that one. Instead of bring the 100K resistors off of the same contact point for the bias voltage, the tech ran a wire to on end of the 100K and connected the other end of the 100K to the coupling cap/power tube control grid connection point. Same result...just looks different.
     
    Wyatt likes this.

  4. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    Oh, most definitely, Wally. I was born on a Thursday...but not last Thursday. Just to clarify, I wasn't planning on working on it, myself. Rather, I'm about to take it in (to a different tech) and wanted to educate myself on what had been done in the past and was in there before doing so.

    I've had this amp for almost 30 years and still am not inclined to service it myself. Professionals are out there for a reason. :cool:

    Scott
     
    Wally likes this.

  5. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    To (greatly) simplify...
    • With Bias Balance, the is"non-adjustable fixed" bias, you can't manually control the bias supply but you can steer how much of it goes to each tube. For example if you have one tube drawing 30mA and one tube drawing 40mA, you can use the Bias Balance to get them both the 35mA. Matched tubes are not necessary, but you can't control the final dissipation, it is what is pre-set
    • With the traditional "adjustable fixed" Bias pot, you control the about of bias supply fed to the tubes to adjust the dissipation and bias a tube colder or hotter. But tube current draw varies greatly (up to 25mA from one to the other), so you have to buy matched tube sets that draw similar bias to keep them balanced. Many vendors consider within 3mA to be match fro EL84/6V6GT and within 5mA to be matched for 6L6GC/EL34/etc.
    • If you have both pots...you can control both balance and dissipation, matched tube sets not necessary
    Of course, this assumes two power tubes, if dealing with a Twin Reverb, you just bias balance the left pair vs the right pair. If they aren't a matched quad, you can balance to matched pairs against each other.

    Rarely will a tech add a Bias Balance to the vintage amp and few new amps (including boutique) ever use them. But if the Bias Balance pot is already there, many of us prefer to just add a trim pot to it for Bias adjustment and get both
     
    Wally likes this.

  6. moosie

    moosie Poster Extraordinaire Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Adding on a comment about bias balance and tube matching...

    Bias balance is measured in terms of idle dissipation.

    It's my understanding that most tube vendors match both idle dissipation, and to some extent how the tubes will perform under load. I'm not sure how much difference it makes, but it does imply to me that if I had to choose, I'd rather buy matched tubes and forgo bias balance. And, if I already have bias balance wired in, I'll still buy matched tubes, and then tweak the balance if I wish, since it's so easy to do.

    Perhaps someone who knows more about how tubes are matched can chime in.
     

  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I buy burned and Apex matched. As for matching....rarely have I observed a set of new tubes that is an exact match. So, the balancing act might be of use even if one buys batched tubes. I buy burned in tubes because a tube's parameters can shift during the first 24-36 hours of use. Burning-in avoids this. IF they drift apart, some balancing of the circuit electronically might be useful. and....some people prefer to mismatch the draw a bit....they claim it sounds different and they prefer it. I have never gone to the trouble of doing that type of experimenting. I should, I suppose.
     
    moosie likes this.

  8. Wyatt

    Wyatt Tele-Holic

    748
    Nov 3, 2004
    Yeah, but the Apex-matched tubes I got from AES off by 4mA (at 460V on the plates) as well. Good thing they were priced competitively because their matching sure doesn't beat others.
     

  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The APEX matching process is proprietary to CE Distribution(AES). I don't suppose it matters from whom you buy, you are buying AES-sourced tubes??? I don't like 4 ma mismatches and don 't often see that from CE Distribution, which is where I buy my tubes.
    Hopefully, the burn-in process for which one pays extra adds some margin of assurance that any mismatch one sees in the beginning will not get worse.
     

  10. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    I didn't realize that was available. For $1.50 per tube (Tube Depot), I'm definitely going to do that in the future. Is it just for power tubes, or pre-amp tubes, as well?
     

  11. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    E5RSY, that burn-in is for power tubes only. tubesandmore...Antique Electronic Supply....charges less than that for burn-in.
    IF a person starts getting serious about triode matching in preamp and driver tubes, one starts paying for that. Check out some of the prices in the high-fidelity stereo world where that matching of preamps and phase inverters is demanded. LOL.....a just saw a pair of very slightly used Telefunken ECC803/12AX7 tubes with very closely matched triodes for....$1,698.99! Yes..... $1,698.99.
     
    E5RSY likes this.

  12. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    TE="E5RSY, post: 7856950, member: 29126"]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.[/QUOTE]

    The "bias balance" evens out the bias between the two power tubes (or two pairs of power tubes in a 4 power tube amp).

    The bias adjustment changes the bias setting up or down for ALL power tubes at once. For it to work well you must have well-matched power tubes AND a way to measure both plate voltage and the bias current.

    Unless you understand amp it's extremely dangerous opening the chassis, and it sounds like you don't. The filter capacitors can contain deadly voltage even if the amp is unplugged - touch the wrong wire and you can be seriously hurt - or worse.

    There are tools that allow you to read the plate voltage and bias from outside the chassis, but if you don't understand the process it's best to have a tech do the work.

    Buying burned-in and matched tubes has nothing to do with bias other than a group of tubes will be the same. each time power tubes are changed the bias should be checked, even if they are the same brand.

    Some tube companies sell their tubes with "ratings" that they claim save you from having to rebias. In reality I find them to be just about as inconsistent as every other brand of tubes, with a check still needed and a rebias needed most of the time.
     

  13. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    I appreciate your concern and the reminder. Don't worry, in spite of my readily confessed degree of relative ignorance, I've been around them long enough (30+ years) to know that I just don't touch ANY wires. Sorta like the first rule of gun safety: "Treat all guns as if they are always loaded." That pretty well avoids 99.9% of the risk. Also, to be safe I do run the amp and turn it off without using the standby switch prior to opening it up (which I very, very rarely do, anyway).
     

  14. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    FWIW some say it only works by pulling the plug; others that the power switch works fine, and some that you don't have to play it. Or do. And I've seen enough residual voltage in some cases to not use a single one of those methods.

    It's smart to always discharge them and then test with a multimeter. And if you don't know how to do that safely to not take the chassis out at all.
     
    BobbyZ likes this.

  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States

  16. E5RSY

    E5RSY Poster Extraordinaire

    Age:
    50
    Mar 5, 2009
    Georgetown, TX
    Yes, sir, I can run a VOM. I'm a home inspector so there's always a voltage detector pen nearby, as well. But your concern is well-placed and worth repeating many times. Just like in the shooting world, even if we have a group of experienced shooters at the range, a safety briefing covering the basics should be given every time. It never hurts to hear it and be reminded of it.

    I appreciate all of you amp gurus' input.

    An old BillM video:

     
    Last edited: Sep 8, 2017

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