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 Post subject: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 7:04 am 
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Joined: Mon Nov 27, 2006 4:05 pm
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Location: W.L.A.
Could someone give a brief explanation. I am selling a Marshall 50 watt Plexi reissue and in my Ebay ad I stated that the amp had new tubes and had been rebiased about a year ago. The person I bought it from told me that and I am just passing on the info but I really don't know what it means!
Many thanks.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 8:23 am 
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Joined: Fri Jun 30, 2006 1:51 am
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Location: Atlanta, GA
Biasing an amplifier (any amplifier) mean setting up the DC voltages which make the active components (tubes or transistors) operate, or amplify. Amplifiers can operate in three regions - cutoff, the linear or active region, and saturation. The cutoff region does us no good for amplification, but if the active component is used as a switch, cutoff corresponds to the 'off' position. The linear region is the actual amplification region - each change in input power means an even larger change in output power. It's referred to the linear region because the output change is constant. If you have a gain of 2 in an amplifier stage, and input change of 5dB becomes 10dB output, 10dB in yields 20dB out, etc. Saturation occurs when the output can no longer keep up with the input. If the active component is used as a switch, saturation is the 'on' position. In an amplifier, saturation means either limiting (compression) or distortion (depending on how large the input signal voltage is). When someone biases an amp "hot", they've set up the operating voltages for one or more stages so that they run from the upper linear region into the lower saturation region. A larger input signal will overdrive, while a smaller signal will compress.

Different tubes have different biasing requirements. The requirements can vary for the same tube type from different manufacturers. Often when tubes are replaced, bias adjustments are necessary. This can involve tweaking the power supply voltages and/or replacing resistors with different values.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 8:38 am 
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Joined: Sun Nov 19, 2006 12:47 am
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Quote:
If you have a gain of 2 in an amplifier stage, and input change of 5dB becomes 10dB output, 10dB in yields 20dB out, etc.


Are you sure about that? A gain of 2 is 3dB. Or maybe I'm thinking backwards. I think a 10 to 20dB change is a gain of 10.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 12:55 pm 
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Joined: Fri Feb 10, 2006 2:00 pm
Posts: 59
Location: Lakewood, CA
No. It's not about volume. I'll try to explain this without math. Think about tubes for a minute. Take a good look at one. There are plates in there with electrons jumping across from plate to plate. The plates are warmed up by the "heater" which is at a certain temperature, and the other factor is the "gain" which is how much voltage is applied to the actual signal. Every individual tube has a voltage setting that is more efficient for that tube. Ok?

All tubes are different. That is why many companies charge a premium for "matched" tubes. That means two (or four) tubes that have similar gain ratings...so they are close, regarding how efficient they are at a certain voltage.

Bottom line. If you buy matched tubes and you adjust your amp's bias to a setting that is in the area of that tube's efficiency, your amp will sound better, because the new power tubes you just put in will be at, or near, a level that is more efficient.

All tube manufacturers recommend that you bias your amp when replacing your power tubes. This is (again) because all tubes are different. I'm talking about amps that are running in an A/B type configuration, which includes most Fender amps and Marshalls...etc.

I'm not going to talk about class A right now. I think you get the gist of what I'm saying. There are lots of info on the web and books written about this stuff.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 3:23 pm 
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Joined: Wed Nov 16, 2005 2:00 pm
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Location: Scotland
Basically it's like setting the idle speed on a car engine. Too low and the engine will run rough, too high and it will cause excess fuel consumption and increase wear.

Similarly with tubes - if the bias is too low (cold), the tubes won't 'turn on' properly and you'll get a nasty sound called "crossover distortion". Even if you can't actually hear it as obvious distortion, it will make the amp sound thinner and harsher than it should. If the bias is too high (hot), the tubes will wear out prematurely, may fail suddenly when the amp is really cranked, and the sound may become too mushy. There's a range in the middle where it will work fine but the sound will vary somewhat - in this range it's down to taste, and there is no 'right' value. You'll often see "70% of maximum dissipation" quoted as the 'correct' value, but in fact this is simply an upper limit for safety - and some amps, including most Marshalls, sound better and are less hard on the tubes when set a bit below this.

Your Marshall is a "fixed bias" amp, which does NOT mean that the bias is not adjustable, it means that it is not SELF-adjusting, and so needs to be set (fixed) by a control voltage. Most amps (including Marshall) have a small trimmer inside to do this, some others (like Mesa) are factory-set to a conservative value for an average tube, so you don't need to worry about it as long as you know the tubes are close to the designed spec... which they aren't always, especially a lot of cheap modern-production ones. However, some amp makers like Mesa believe that it's better to restrict the choice of tubes than risk users setting the bias to totally incorrect values and damaging the amp. Both approaches are valid.

The opposite of fixed bias is called "cathode bias", which is where the bias voltage is generated by the tube current itself, so is largely self-adjusting. (This has become called "Class A" by many people for some reason, but it is NOT even remotely the same thing - there are almost no true Class A guitar amps except single-tube ones like Fender Champs. The Vox AC30 which is often quoted is a perfect example of a Class AB cathode-biased amp.) You may ask why all amps aren't built like this since it seems less trouble... but actually it has its own set of limitations and different tonal characteristics, and so isn't always desirable.

Setting bias on an amp like a Marshall is easy, but usually involves working inside the chassis with the amp fully powered up, so is potentially DANGEROUS if you don't know what you're doing. If you know it's been set by someone who knows what they're doing, you shouldn't have to worry about it unless you change the power tubes - and even then, if it's been set sensibly and not right up to the hottest possible point (as has become fashionable), you probably don't really need to. Sticking to the same brand as what's in it now will make it more likely that it won't need adjusting - although individual tubes do vary too, the better quality brands tend to fall within a narrower range.

If your amp is one of the more modern Marshalls (DSL/TSL etc), the bias adjusters and voltage test points are actually on the outside of the chassis, behind the metal grille on the back. You can bias these amps with nothing more than a cheap multimeter and a small screwdriver, completely safely - there are instructions in the owner's manual.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Thu Nov 30, 2006 5:03 pm 
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Joined: Mon Nov 27, 2006 4:05 pm
Posts: 309
Location: W.L.A.
Thank you very much for all of your good info.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Fri Dec 01, 2006 5:50 am 
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Joined: Sun Oct 08, 2006 6:07 am
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I must reiterate the safety aspect. As much as I like to tinker with guitars, I do not have the appropriate grounded workbench and isolation transformer power supply to properly work on amplifiers. Not just bias currents, but residuals from filter capacitors with high amperage and plate voltages that average from @ 350 for 6L6's to @ 500 for 6550's and EL34's can be DEADLY. I've been in a friend's shop when he discharged filter capacitors of an amp that had not been turned on or plugged in for a week, and even after that much time the spark was about an inch long, and bright blue. So unless you really know what you're doing AND have the appropriate equipment and work area to work on amplifiers, please leave that aspect of things to those who do.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Dec 02, 2006 1:44 am 
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Joined: Thu Dec 01, 2005 2:00 pm
Posts: 158
Location: california
Quote:
I must reiterate the safety aspect. As much as I like to tinker with guitars, I do not have the appropriate grounded workbench and isolation transformer power supply to properly work on amplifiers. Not just bias currents, but residuals from filter capacitors with high amperage and plate voltages that average from @ 350 for 6L6's to @ 500 for 6550's and EL34's can be DEADLY. I've been in a friend's shop when he discharged filter capacitors of an amp that had not been turned on or plugged in for a week, and even after that much time the spark was about an inch long, and bright blue. So unless you really know what you're doing AND have the appropriate equipment and work area to work on amplifiers, please leave that aspect of things to those who do.



I absoultly appreciate the voltage warnings, and it is important, but when changing tubes, grip them by the glass and remove them. There is NOTHING to worry about here. Bringing an amp to a tech when you can do it yourself makes no sense. Use common sense and save yourself some money.

This is advice on all newer amps that have either self biasing circuits or can be end user adjusted using the manual.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Dec 02, 2006 3:12 am 
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Joined: Wed Nov 16, 2005 2:00 pm
Posts: 2962
Location: Scotland
As a professional tech with over 20 years experience... (and yes, I have had one or two shocks, as most or all of those of us who do this have)

A grounded workbench would be a dangerous idea for working on a tube amp that's powered up. If any part of you (eg your arm, elbow or even your leg, clothing is not an effective insulator at these voltages) is touching the bench, and you contact a live part of the amp with your hand, the circuit will be completed directly to ground via you, resulting in a high probability of a fatal shock.

You want to keep yourself and anything outside the amp UNGROUNDED, so that if a part of you touches something live - and it should be only ONE part, the practice of keeping one hand behind you can be a life-saver - no significant current can flow because no circuit is completed. (This is exactly why birds aren't fried on high-voltage overhead cables.) Grounded workbenches are to eliminate static build-up which can damage sensitive solid-state components, not for safety.

I do not use an isolating transformer either. It's totally pointless - the danger in a tube amp is greater from the internal DC supply, and an isolating transformer does absolutely nothing about that, it just isolates the incoming supply - which is better-protected by a Residual Current breaker. It also does nothing about the stored charge in filter caps.

The way to avoid a shock is to learn safe working practices, not to rely on equipment which gives a false sense of protection. The shocks I've had have all been avoidable and due to carelessness, I admit... it happens given enough time and work pressure. And a couple of them really hurt. But I'm still alive, because I understood how to minimise the resulting current draw, which is what kills you (not the voltage, actually).


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Dec 02, 2006 10:49 am 
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Joined: Sun Oct 08, 2006 6:07 am
Posts: 3811
Oh, good....on rereading, I was typing way too quickly. I was, of course, referring to the wiring and the sockets being up to code 3-prong and, if required, GFCI, not the actual bench itself! On rereading that first line of my post, it does look a little scary, doesn't it. Sorry!


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