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Thread: When to bias guitar amps?

  1. #1
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    Tube store "bias number" systems

    Hi, folks!

    I was hoping you guys could help me cut through some of the online bs to understand what I need to understand.

    My experience is primarily with fixed-bias tube amps (Hammond, Leslie) and solid-state HiFi, PA, bass amps. So I am generally concerned with amps operating in their linear range and don't usually want distortion.

    A friend recently asked for assistance replacing the power tubes in a Fender Deluxe Reverb Reissue as one has gone microphonic. He has previously ordered tubes with a "bias number of 28" - I'm assuming this is a unitless number that describes how early the tube breaks up.

    My plan was just to order him a matched set of JJ 6V6s (that's what's in it now) from my usual distributor (CE) and pop 'em in. Am I going to wind up having problems? What's the deal with ordering tubes by bias point? Is it some kind of vendor lock-in, or is there more to it than that?

    Thanks,
    Wes
    Last edited by WesG; 11-14-2017 at 05:40 PM.
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  2. #2
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    I'm sure others will have more informed opinion, but I can't see how you will run into any serious problems by replacing with same make tube and seeing how it sounds.

    I don't know that it is a vendor lock-in, more of a value-added-service. I read on a different forum that the number is the plate current at a standardized 400V.

  3. #3
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    Shouldn't be any issue replacing with same tube type and "rating" from the same vendor. Those numbers don't really mean anything except to the company that did the rating. IOW, one vendor's 38 is not necessarily another vendor's 38.
    That said, I don't trust mew tubes to stay at any rating for long...

    There is no "correct" bias setting - if it SOUNDS good to the musician AND doesn't self-immolate while doing it, then it is correct. On another note, there's no reason to just bias a guitar amp as hot as possible just because so-and-so said to. If it's not red-plating and it sounds good, it is good. If you end up biased too cold, you'll know, and for the most part, guitar players don't like that kind of distortion, either. Maybe the amp is run clean all the time - then you really wouldn't bias it at a spot where you'll be running the volume on 11 all the time...

    If you can bias hi-fi amps and Leslies, you can bias a Deluxe Reverb. Which, by the way, is a "fixed-bias tube amp," as you put it...

    Justin
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  4. #4
    g1
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    I think these numbers are of most help when you do not have adjustable bias or you don't want (or know how) to make the adjustment.
    In this case, if he was happy with those tube numbers previously, he can get the same ones and "plug & play".
    Or, if you had a non-adjustable amp, and found a number too hot, for example, you could tell the seller what number you had, and ask them to send something that would run cooler.

    In your case, if you want to use generic tubes, I would suggest you check the bias of the old ones, and set the new ones to be the same (assuming he was happy with the way the old were set).
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  5. #5
    Old Timer J M Fahey's Avatar
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    That ^^^^
    Obviously those numbers mean something to tube vendor, and you would be able to reproduce them at home if you knew the method/equation they used ... but by keeping it secret they keep you tied up.
    Many will pay a few bucks over average market price in exchange of avoiding the fuzz and expense of biasing, so in fact itīs a useful service they offer.
    Just *guessing* , there are 2 ways to do it:
    1) measure "bias voltage needed to pass "X" mA" .
    Say, in a classic Fender amp, nominal -52V bias:
    IF target current is, say, 40 mA , probably tubes needing -48V are rated "1" , -49V "2" ... -52V "5" (middle of the range) .... -57V "10" .
    I now see 1V steps look too broad, probably they use an even finer scale, say in 0.5V steps.

    2) "mA passed when set to fixed -52V"
    So "1" to "10" might go from, say, 30 to 50 mA

    Again fully guessing, but both methods look useful and repeatable.

    Of course, resellers (they are not manufacturers by any means), hide that plain vanilla description under fancy names, such as "low numbers are cleaner-bluesier-whatever while high numbers are grittier-dirtier-whatever" which is somewhat related to reality.

    As in: tubes passing less current will be colder and harder to saturate while those passing more (or requiring lesses bias voltage) will be slightly easier to saturate, but of course small differences are blown out of proportion for marketing purposes.

    FWIW and I often mention this, when I was studying Industrial Engineering in the early 70īs , in one Marketing class we were split into groups of 4 or 5 , were given bottles of greenish thick Industrial Detergent, the pure basic one, no additives , perfume, enhancers of any kind, straight from Industrial Chemist suppliers and sold for cheapest price in 200 liter drums (it was ŋSodium? Lauryl Sulphate something) and homework was: "you were all given exact same product, a useful but very unexciting one, next week each group brings *convincing* reasons as to why should I buy YOUR product instead of others, why yours is best, etc."
    Same thing.
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  6. #6
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    Hammond and Leslie tube models tended to have cathode biased power amps.

  7. #7
    Senior Member Enzo's Avatar
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    A vendor buys a 1000 tube lot. They have a test set up they have made. They apply certain conditions to the tubes, like B+ voltage, bias voltage. They measure the tube for current and maybe gm. They note the numbers on the tube, or on a chart which gives them a look-up number to put on the tube. If you see a number like 38ma on a tube label, it means the tube conducted 38ma ON THEIR TEST SETUP. it does not mean you should adjust the tube for that current, and another vendor with his own test setup might get totally different numbers, because his test setup is different.

    The tube numbers are used for matching tubes to EACH OTHER, not to anything in the real world. The numbers will only help you match tubes bought from the same vendor. 38ma on a MAgic PArts tube won't mean the same thing as 38ma on a CEDist tube.
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  8. #8
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    Thanks folks, great feedback!

    pdf64 is correct, I meant to say that was I used to Hammond/Leslie amps were cathode-biased. Slip of the brain!

    I guess the reason (many) guitar amps use adjustable fixed bias is that the bias point must affect how the amp sounds when the tubes are operating outside of the linear range. This also different from HiFi....I don't do a lot of tube HiFi, but in Hifi, juke boxes, etc, I just drop in new tubes, measure the voltage drop across cathode resistor, etc, and twist the pot until I get the desired numbers. In guitar land, there seems to be a bunch of "you gotta listen to the mojo" stuff going on. How do guitar techs return amps to customers? Do they have the customer come in and play the amp while they diddle with the bias every time they change power tubes? That seems completely insane.

    How do most guitar players change power tubes? I know most people don't buy tubes with "bias numbers" on them. I also know that most guitar players are not capable of safely working on tube amps. Surely they are not going to their amp tech every time they change tubes? Some of these guys change tubes like underwear!

    I wrote CE in the off-chance they knew how to match again thetubestore - of course, they did not, but mentioned I could order by plate current. So if The Tube Store's numbers really are plate current at 400V, I should be able to order a tube which biases the same, or close enough...? Or maybe I should just pass. I don't mind helping out a brother, but I don't really want to monkey with his amp. g1's suggestion of measuring the performance of the current power tubes DOES seem to be the best way to go without better information about The Tube Store is doing.

    I suppose the worst case scenario is that I wind up with some extra 6V6s on hand. I need some for M3s anyhow.

    Wes

    PS - Enzo, those numbers may be meaningless, but they /are/ being used to market tubes within a given vendor; as Juan says, the idea is that if you stick with the same vendor, you can avoid the risk of bad tone and expense of re-biasing. I'm starting to understand why from a guitar-player POV this makes sense.
    Last edited by WesG; 11-17-2017 at 04:31 PM.
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  9. #9
    Senior Member Enzo's Avatar
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    If your amp is all biased up to your satisfaction with #28 tubes, then sure you can order more #28 tubes from the same vendor, and whatever 28 means, the new ones will be the same. But CE will never know what Tube Store numbers mean and vice versa because they are arbitrary numbers.

    Imagine you and I each have a big bag of peanuts, and we each sort them into small, medium, and large. You and I might not agree on where the draw the lines, even if we are consistent with ourselves. Show me a marginal peanut, and I won;t be able to say whether you called it small or medium.

    If your amp doesn't sound like trash, and the tubes are not getting red hot, then as far as the amp is concerned, the bias is fine. If you hear a tonal difference, within those limits, you can adjust by ear. Your bias varies all the time anyway. If you have 480v B+, that means every time your mains 120v moves one volt, your B+ moves four. 120v to 125v raises your B+20 volts. That in real time affects your tube conduction, affect the bias. yes, bias voltage also goes up and down, but they don't "track". The 70% rule is just internet lore.
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  10. #10
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    Quote Originally Posted by Enzo View Post
    ...Your bias varies all the time anyway. If you have 480v B+, that means every time your mains 120v moves one volt, your B+ moves four. 120v to 125v raises your B+20 volts. That in real time affects your tube conduction, affect the bias. yes, bias voltage also goes up and down, but they don't "track"...
    Surely, as the bias supply voltage and the HT voltage have a proportional relationship to the mains, then it seems reasonable to think of them as tracking the mains / each other; so there should be a degree of self regulation in regard of mains fluctuation?

  11. #11
    Senior Member Enzo's Avatar
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    That was my point, that I don't trust them to scale up and down together. I don't assume that a 20 volt increase in B+ is exactly offset by a two or three volt change in bias voltage. Might or might not.

    And underlying point being that bias is nowhere near as critical an adjustment as the internet makes it out to be.
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  12. #12
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    I have some (thankfully rare) customers who bring me tubes and ask me to bias them to a specific mA draw. Why? Because they read it on the internet. My policy is to bias tubes so they run cool and prolong their life. Biasing on the cool side also gives additional protection to random tube swapping by subsequent owners. Perhaps a reason why many amps without bias adjustment are set to run cool from the factory.

    An acquaintance runs a garage and he did a service for a guy. Half an hour after picking the car up the owner rang the garage to say he had pulled in to check the oil and it was fractionally low. He turned back and insisted it be topped up to the full mark. The next day the garage got a call to say the oil now measured high with a cold engine. The customer bought the car back and had the oil dropped out and replaced while he was watching, and supervised the level. Later on the same day the garage got a call to say the oil now measured lower than when it was filled. In the end the garage gave him a refund in full and recommended he went to a main agent.

    I had someone just like that who had a bias probe. In the end after numerous calls I lost it and told him to fuck off. Which, thankfully, he did.
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  13. #13
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    Some people never quite get the idea of "acceptable range." Personally, I bias between 50-60%. Since I like to use "Scavenged Old Stock" tubes.

    Justin
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    "Are you practicing in the lobby of the municipal library? It's still a guitar amp and it SHOULD make some noise (!!!)" - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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