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Thread: biasing pre-amp tubes?

  1. #1
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    biasing pre-amp tubes?

    I have my amp in the shop that was referred to me by the local guitar shop. I wanted to have someone with experiance take a look at it to find out how good it could sound. They said that they need to replace and bias the 6SL7 preamp tubes.I have never heard of anybody biasing preamp tubes is this normal. I asked another shop and they said that was BS. Should I be concerned with the competancy of the shop or that they are going to try to charge me for work I don't need? btw the amp is a 57 ampeg M12.

  2. #2
    Old Timer Tom Phillips's Avatar
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    You are correct.
    Pre-Amp tube circuits are self biasing. There is no bias adjustment required when you change a pre-amp tube.

  3. #3
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    Depends what they charged you, if it's just a nominal cost/handling charge I wouldn't lose sleep over it. Not that many shops have a tech permanently situated in-house, so there might be a slight case of Chinese whispers going on - because 6SL7s in Ampegs, like all preamp tubes in guitar amps, are self biasing - either 'grid leak' biased or 'cathode' biased (the M12 schematics - no dates - I have seen both show cathode biased 6SL7s). Cathode resistors rarely fail so replacing/rebiasing is rarely necessary unless you are setting out to change the character of the amp. So, if they have hit you with tens of dollars, specifically to rebias your preamp tubes, then that is NOT normal.

    Old cathode biased Ampegs typically didn't run their power tubes at very high plate dissipations, so these also would't really need rebiasing unless they switched 7591s to a different type like the GT/EH7591XYZ which requires a new cathode resistor.

    I love the sound of old cathode biased Ampegs, but don't really enjoy working on them. The old can cap values are not often available, in any case I prefer to rig up discreet caps which can provide mounting headaches. Hum/noise reduction can be time consuming too.

    So if you really want it to sound as good as it could I wouldn't expect it to be that cheap as I'd expect the filter caps to be replaced, preamp & output cathode bypass caps, the power tubes and at least the driver tube. Pots, jacks and tube sockets should be cleaned/replaced as necessary.

    Ideally, you want them to itemise the work done so that you can query it with an independant party in the event that you're not happy with the sound of the amp. If it ends up costing you maybe $150-$200 and it sounds great then that wouldn't necessarily be out of the norm (maybe more if you're particularly bothered about hums/noises or if you needed other major parts).

    With any tech/shop you are really looking to them for guidance as to what work "needs" doing, they are supposed to be the experts after all. That said, they should keep you informed, prior to hitting you with a big bill, so that you can make the decision as to whether or not to go ahead, or take it somewhere else.

  4. #4
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    It is not BS.
    All tubes need to be biased.
    What you need to do is make sure the preamp tubes have proper bias voltage because some of these old timer amps have an electrolytic cathode bypass caps on them that have gone bad... this shorts out the cathode bias voltage, turning the tube on too hard or running that stage into what is called mu gain, wrecking the circuit and B+ rail by drawing way too much current.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  5. #5
    Senior Member Enzo's Avatar
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    I think the argument here is what the shop meant. All preamp tubes have to have some sort of bias to operate properly - bias as a noun. All preamp tubes do not have to be "biased" as a verb. There is no adjusting to do. Certainly one should check that the circuit is working - ie no shorted bypass cap - but I would never describe that as biasing the tube. SO if the guy said he needed to install and bias a new 6SL7, I would be suspicious

    On the other hand if the guy said "The amp needs a new 6SL7 and bias" he might have meant you need a new tube, and the amplifier needed rebiasing, meaning the power tubes. Pretty reasonable when trying to get the best sound.

    Ask the guy what is involved with biasing a 6SL7. If he tells you he meant the power tubes or something like that, then he's off the hook. ANd if he says it just means a check for proper voltages, that's OK. If he offers some baloney about some adjustment procedure, he's a crook. At that point then ask him what adjustment spec he recommends and will set it to. I am dying to hear.

  6. #6
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    I think he is talking about the voltage now that I think about it because when I was looking over the amp myself all voltages seemed low. another factor here is that there is a language barrier involved as it is a German shop. If the amp sounds good and the price isn't outragious I don't mind paying. Thanks for your replies.

  7. #7
    Better Tone thru Mathematics bob p's Avatar
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    Some of the confusion could be explained by differences in terminology. as Bruce noted, all tubes need to be biased properly, but as Enzo noted, preamp tubes don't come with adjustable bias. From what you've described, there could be some component of a linguistic barrier between you and your tech. If for example, he says that the amp needs to be biased, that might just be his way of saying that it needs all of its electrolytic caps replaced. If that's the case, its a communication problem, the repair won't be too expensive, and he's not a crook.

    One thing that you'll notice with old PA amps (and the like) is that the years do take their toll on the little electrolytic caps, and some people tend to overlook them. With parts changes being so common over the years (sometimes with the right value of parts and often times not), and with many amps receiving "partial maintenance" at best, its not at all uncommon to encounter an amp whose B+ rail is all buggered up - for any number of reasons.

    As an example, I picked up a 1955 RCA PA amp that used a pair of cathode biased 6L6. What was odd about the amp was that it was manufactured by RCA and used RCA 6L6G beyond their rated voltage specifications. Getting back to the point, it had a somewhat high voltage PT (560-0-560) and a choke-input filter. At some point the PT failed, and it got replaced with a 400-0-400, and the PS was reworked as a cap-input filter in an attempt to get the voltages back into the "ballpark" using "available parts." Then the main cap can was replaced so that the 20-20-20 was replaced with a 40-40-50. The result was a totally half-assed repair of the amp; the voltages were wrong everywhere and the amp sounded horrible.

    There was a particular problem with the phase inverter. It was a paraphase type, and because the voltages had changed, the balance was completely thrown off and the amp sounded horrible. Tracing a signal with a scope showed that things really went bad at the PI. To fix the problem would require rebalancing the PI, which would involve some math, some resistor exchanges, and a bit of trial and error fine tuning with a signal generator and a scope.

    To put things into perspective then, "rebiasing" every stage of the amp could be as simple as a simple cap job if everything else is normal, or it could be as complicated as a complete redesign/rebuild of the PS rail and re-scaling of individual gain stages, if a significant change has been improperly imposed in the circuit, and someone with skill is needed to fix the problem of a less than optimal previous repair. That could be a time consuming job, and anyone would want to get paid extra if they had to address something that complicated -- especially if they don't have a schematic and specifications.

    Depending upon the extent of your language barrier and what the exact problem is with your amp, you may or may not be getting a good deal or a rip-off. Personally, I decided not to try to spend the time attempting to "restore" the old RCA PA amp to its former glory, as the consensus of opinion seems to be that old RCA PA amps sound like ass as guitar amps. For me the bang for the buck is going to be to gut the amp, and rebuild it as a Pro, Super, or Bassman with vintage parts. But if you want to have someone else go through your amp with a fine tooth comb, don't balk at the repair bill if the time charges are adequately substantiated.

    HTH.

  8. #8
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    Could it be that since the original question to the tech concerned "how good it could sound",the tech has his own ideas about re-biasing the preamp to make it sound better?I know this would amount to a "mod",and if that is the case,the tech should explain it better.The best thing to do would be to ask the tech exactly what he meant before condemning him.

  9. #9
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    all resolved... talked to the real tech.

    I will take this as a lesson. The facts are plain now after simply calling at the right time. The other times I called I had talked to the shop owner and not the tech. The shop owner felt it was his job to answer questions that he didn't know enough to answer (about several aspects of the job). I talked to the real tech and it was straight forward, made sense and he answered everything I asked and then some.

    I learned a good lesson from this and maybe someone else will to.

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