How do you bias Your slightly un-balanced Power tubes

Discussion in 'Amp Tech Center' started by Platefire, Jul 21, 2018.

  1. Platefire

    Platefire Friend of Leo's

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    For instance, I'm currently re-biased my DIY AB763 DR with JJ 6V6S tubes. The tubes are running about 3 to 4 mA apart depending on the setting.

    So my approach has been in going for 70% dispensation Calculated:

    Watts/Plate Voltage X .70
    14/418=.0334928 or 34mA x .7 dispensation = 23.8 mA bias setting

    So I was previously running the lower one at 22mA and the higher 24.8 mA to obtain a balance between the two.

    I got concerned that it was a little to hot so I reset it to 16.9/20. Now I'm concerned that it my be a little too cool for my taste.

    How do you balance your bias in these cases???? I always order a balanced set of power tubes but they are always at least this far off. Platefire

    BTW---no red plating was occurring on the higher setting but it was mighty hot to the touch
     
    Last edited: Jul 21, 2018
  2. Inglese

    Inglese Tele-Meister

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    I think you feel the problem just because you can measure the current.
    I guess 90% of the amps in the world runs at worst conditions than both of yours.
    Keep the hotter around 70%.
     
  3. VintageSG

    VintageSG Friend of Leo's

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    I set my Cub to ensure the hotter valve doesn't run away with things and relish the slight imbalance. I've got a pair of JJ which are within a couple of mA and a pile of 6p6s which are all over the shop. I've played about and found I may actually prefer the sound of an imbalanced pair. Enjoy the sound and leave perfect matching to hi-fi buffs?
    Providing the hotter one doesn't run away with itself, no harm will be done.

    Note: the above is purely my opinion and may not reflect any sort of actual reality.
     
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  4. Dacious

    Dacious Poster Extraordinaire

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    I would sit it right in the middle. If you meaamea them again after some hours of operation it'll have moved around some.

    As far as too hot, too cold, what sounds good? If you run your amp flat strap all the t
    If you're measuring overall current you're 10% up on what's actually happening, because your grids are pulling a ma or two. I'd set your bias exactly between. You won't be going over current. Unless you're running dimed with lots of boost in front constantly.
     
  5. D'tar

    D'tar Friend of Leo's

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    I set bias by ear and assure the hotter tube is within a safe mpd. Also have been ordering tubes "burned in". I've had matched tubes off as much as 7ma.
     
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  6. BobbyZ

    BobbyZ Doctor of Teleocity

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    That's my take on it too.

    Tubes are like relationships, start off a perfect match, then drift apart.
     
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  7. Wally

    Wally Telefied Ad Free Member

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    You could install a balancing adjustment?
    To your question, I bias the hotter tube to a safe point that sounds good to my ear.....+1 with d’tar. I also make sure that I know what the wall AC is. Ex: I fired up a Blues Deluxe yesterday after replacing a bad power transformer. The wall voltage was 113VAC. This morning when everyone’s Ac is not working dull time to keep the heat at bay, it will be about 10 volts higher than that....so the bias will be set for operation at the higher wall voltage. If a player plays one a regulated and controlled AC voltage supply, the. I will bias to the situation present3d by that voltage...making sure that a higher voltage will not present problems if the player 1)runs at less than normal wall AC and 2) gets caught having to ply the amp without his voltage supply equipment.
     
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  8. Doctorx33

    Doctorx33 Tele-Afflicted

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    2ma difference is bupkiss. Forget it.

    23-24 ma seems kinda cold, I usually run my bassman heads hotter.
     
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  9. Platefire

    Platefire Friend of Leo's

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    I've always heard 25mA is kind of standard setting for 6V6's and 35mA for 6L6's but it all depends on the plate voltage. If the plate voltage is running 400+ on a 6V6, you going to have to come down off the 25mA on your bias setting to get any kind of life out of your tubes.

    I'm not really concerned about a 4mA imbalance. I just have never seen a discussion before on the best way to set the imbalance with a single bias pot. Maybe it's so minute of an issue it don't deserve recognition as a problem to be discussed but since I was in the process of biasing, thought I would bring it up. Do You:

    1-Set the hottest tube at the intended dissipation and let the cooler one fall where it may?
    2-Set the hotter one slightly above your intended dissipation to bring the cooler one up closer to the intended dissipation?

    I've previously tried to hit the dissipation goal right in the middle of the imbalance but now thinking that the #1 method is probably the best approach. I do like the sound and feel of it set slightly on the hot side of normal without setting it so hot I feel I need to be peering into the back of the amp to make sure no red plating is occurring.

    I hadn't even considered the wall plug AC voltage. Just checked the voltage where I set the bias the other day and it's 121VAC. Now the amp is where I play at, so when I go to reset the bias, I will check the wall AC there just out of curiosity. Platefire
     
    Last edited: Jul 21, 2018
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  10. JD0x0

    JD0x0 Poster Extraordinaire

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    I don't. 3-4mA is minimal. These aren't HiFi amps, and sometimes a little bit of asymmetry helps the tone. I'd consider trying to balance a 3mA difference a waste of time, personally. This is also why I don't bias Class A/B push/pull fixed bias, to the max rated 70% dissipation, in case there's some drift, or the wall voltages go up, it doesn't push you over max.



    The only way to balance with a single pot, is to have a Bias balance pot wired in, like some Fenders and other amps have. Other than that, you could maybe try swapping tubes around in the slots to potentially achieve a slightly better balance, I still don't think it's worth doing. If the tubes are severely unmatched (and causing issues) it's time for a new set, and if you're really critical, get tubes with 24hr 'burn in' and then have them matched post 'burn in'
     
    Last edited: Jul 21, 2018
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  11. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Math has NO tone. None. Can you tell me what 70% dissipation sounds like? If you set bias by math you have NO idea what the amp will sound like - and not all tubes sound best at 70%.

    Bias your tubes for an average centered on 70% if you want. or use 60% or 75% Its really doesn't matter as it's only a STARTING point, not a "final setting".

    THEN play the amp cranked up a bit. Tweak the bias a few ma in each direction. Find the "sweet spot", which will rarely be at 70% (or the average of 70% if the tubes are mismatched).

    Check and make sure you're close to a safe setting and leave it there - where it sounds best in a safe range.

    Having tubes that are slightly mismatched is perfectly normal. And chances are your phase inverter isn't balanced anyway - the driver tube would have to be preciosely balanced, as would resistors and other components. And in most amps they are not.

    Math is an electronics guide. It is not a tone guide.
     
  12. dogmeat

    dogmeat Tele-Holic

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    I stick in an unbalanced inverter tube & put the low power tube on the high side of the inverter. gotta know what values the inverter has though. it may be meaningless but seems to work for me
     
  13. Old Tele man

    Old Tele man Friend of Leo's

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    Uh, you are aware that OT windings are NOT perfectly "balanced" resistance-wize (due to one winding having slightly more turns) so that SOME idle current imbalance might be caused by the OT and not by the TUBES! Way to find out, is to simply swap tube locations, and if the imbalance roughly stays the same, the imbalance is due to the OT and not the tubes.

    And, IF this happens, simply leave the tubes in the location that where the lowest imbalance occurs.
     
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  14. Platefire

    Platefire Friend of Leo's

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    What folks seem to be mis-understanding is I'm not concerned about the imbalance in the tubes. I realize there are a lot of reasons for imbalance of tubes.

    My question was centered around how do you??? Handle the imbalance when you adjust your bias?
    Some say in not worth even thinking about, others say adjust hottest running tube to your intended dissipation and let the cooler one fall where it may, one said that's too cool and previously I had biased subject amp to put the center on the imbalance right at 70%----which is running the hotter tube a little over 70% but also pulls the cooler tube up closer to 70%.

    So after reading everybody's post what I decided was to bias it where the hottest tube is a 70% and let the cooler one land where it may. So I went and re-biased it just while ago. Checked the wall voltage which was 121.5 VAC. I reset the bias at 20mA/23.8mA.

    I just wanted to get an idea of all you pickers/amp techs thoughts and attitude toward the subject. You have provided me some good insight into how you adjust around the "balance difference". So I've make my choice and very much thankful for everybody's input. Thanks, a bunch! Platefire

    BTW---Even though I think it's mission accomplished with me---we can continue to discuss the subject
    because it's always interesting where everybody is coming from. I've learned some stuff!!
     
    Last edited: Jul 21, 2018
  15. Old Tele man

    Old Tele man Friend of Leo's

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    If the imbalance is within ±10% of what you'd like, simply split the difference between the two tubes.
     
  16. Platefire

    Platefire Friend of Leo's

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    That's actually what I was doing previously at 22/24.8. My amp blew a GZ34 rectifier, which probably had nothing to do with the bias setting but when I got the new rectifier in, I decided to check my bias.
    So when I realized I was running about 75% dissipation, I started thinking of backing off a bit for safety.

    Seems like everytime I re-bias I have to re-educate myself---I forget all my stuff in between biasings :>) Platefire
     
  17. Wally

    Wally Telefied Ad Free Member

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    Platefire, since you asked again, I’ll answer again. I haven’t changed my mind since my post #7. I want to know at what pate dissipation factor the hottest tube is operating no matter where my ear wants the amp biased....or no matter what number marks where my ears and the math find common ground. S
    +1 with switching tubes in the sockets to see what goes on. So...your first option for me.
    As for ‘safety’ at 75% of MPD, that is no concern, ime. I have not done the experiments because I don’t regularly put power tubes into redplate, but I have seen fixed biased amps running 85-100% of max plate dissipation at idle and NOT redplating during operation. For sure, those power tubes will live shorter lives than if they were biased at 65%mpd; but they were working safely and sounding great. The 100% situation I changed immediately, though, with a different set of tubes that ran at 85%. I did not have a trim pot to install, so that is where I left it...and I was not alarmed because my Super Champs, which is what 5E5A amp was, have been running over 70% MPD for decades. Hot, lively, harmonically rich.... the range of ‘correct’ biasing is fairly wide, but cool and cold don’t work for me. Warm to hot does. Ommv...
     
    Last edited: Jul 21, 2018
  18. Platefire

    Platefire Friend of Leo's

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    Well Wally with your running that hot and doing well makes me feel I'm way in the safety zone. As I recall the previous time before this time I switched the tubes around for the best balance and the position they are in was the best balance. I didn't even think of re-checking it this time, because tubes do drift over time. I really like tubes to last years and years and years, so 70% is about the hottest I think I may obtain that. I agree with you, a cold bias amp is like a sick puppy--if blue is cold, green is warm and red is hot---I like it just barley in the red just past past green. Thanks, Platefire
     
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  19. misterdontmove

    misterdontmove Tele-Meister

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    Start with the hot one near 70% (you can't do anything about the cooler one anyway and 2ma is negligible). Play a bit and if you need to, adjust down from 70% till it sounds good. Of course if it sounds good at or near 70% you can leave it alone.

    I just went back and read the posts and saw you already came to this conclusion, so take this as affirmation.
     
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  20. Platefire

    Platefire Friend of Leo's

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    Yelp, decided that would be my SOP from now on. Glad I got that settled:rolleyes:

    Another thing I'm working on regarding bias, is to have an external test point for plate voltage.
    I've installed external test points with 1 Ohm resistors to cathode pins plus Mar Vac external locking bias pot on a lot of my amps. It is kind of useless without the plate voltage which normally would require pulling the chassis which is defeating your purpose for external check points for bias adjustment to start with. My first thought was to run full plate voltage from pin #3 to a test point but the thought of a short out in the test point to chassis could cause a major burn out. So I got with some tube gurus for a solution.
    .
    So this is what they came up with---see picture. In my power tube socket for a JJ 6V6, run a wire/1Meg resistor from plate pin #3 to test point on back of amp with also a 1K resistor to ground. Use you multi-meter set to mV to get a reading. Say your plate voltage is 417VDC. It would read out 0.417. Then you would multiply your reading by 1000=417VDC Plate voltage. Then do you bias calculation based on that. This method was designed by PRR on the Hoffman amp Forum and I'm going to try it. platefire
    B-test.gif_thumb.png
     
    Last edited: Jul 27, 2018
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