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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sun Dec 03, 2006 12:21 pm 
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Joined: Mon Nov 13, 2006 1:23 pm
Posts: 251
Location: Tulsa, Okalhoma
Quote:

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.

Amen brother. Especially here in the US with 110V mains, the 450-500 volts of DC Ht on the smoothers is every bit as dangerous as the Ac supply, if not more so. Plus, ironically, modern design code tends to ensure the Ac side is all nicely insulated, while the DC Ht is sitting there bare.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Mon Dec 04, 2006 12:54 pm 
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Joined: Sun Mar 26, 2006 2:00 pm
Posts: 715
Location: Dodger Town
Although the discussion prompt questions what is "rebiasing," I thought I'd refer you to the following 09-25-96 article by Randall Smith, founder of Mesa/Boogie. In it, he advocates the research, design, and production of amps that do not require rebiasing. Browse this enlightening article at ....

http://www.mesaboogie.com/US/Smith/biasadjust.html

As you delve deeper and deeper into his explanation, you begin to appreciate just what he meant by wanting ".... to build amplifiers that were individually hard wired to the correct values and NEVER needed adjustment. And for 25 years, that's how Mesa/Boogies have been built."

Best regards .... Goofyfoot.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Oct 03, 2009 12:46 pm 
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Joined: Fri Aug 12, 2005 1:00 pm
Posts: 144
Location: Cornelius, Oregon
A vacuum tube has the plate on the outside with the cathode in the middle, all inside the glass, metal, or ceramic enclosure. The cathode is heated, and is usually coated in something that when heated will emit electrons. The plate has a high positive voltage on it, and the electrons want to go to it. The grid controls how many electrons get through to the plate, and how fast they get there. The signal is injected into the grid and the tube will swing up and down from it's B+ positive voltage to zero as it reacts to your signal.

The bias in a fixed bias amp is injected into the grid, and is a negative voltage. In order for the tube to fuction properly, the grid always has to be more negative than the cathode. In a fixed bias amp, the cathode is grounded, so the negative voltage on the grid allows the tube to be biased and to function. How much negative voltage bias determines where on the transfer curve the tube will operate. If you bias to one extreme....a huge negative voltage, then the tubes will barely turn on and will be off for most of their cycle, which will result in very long tube life, but also results in poor sound usually characterized as cold, because you are getting quite a bit of cutoff distortion in with the amplification. If you bias will very little negative voltage, then the tubes will be biased very hot and will be on for most of their cycle. If you have it adjusted too hot, then the tubes will not be able to take the heat and will burn up. A happy medium usually sought after for a fixed bias amp is to bias somewhere between 60% to 70% of the maximum dissipation of a given power tube. This maximum dissipation info is on the tube data sheet for each tube type. If the tubes are very far out of spec and/or the bias is very misadjusted, then putting the tubes in without biasing at the least could cause poor sound, and at the most, could cause damage to the amp. Having said that, Fender never bothered with matched tubes and their vintage amps appear to work just fine.

In a cathode biased amp, a resistor is put in between the cathode and ground. This causes the cathode voltage to be above ground, so the grid is more negative than the cathode and the tube works. The signal still comes in to the tube on the grid, but as the tube wants to swing up and down, the voltage on the cathode reacts and after a short delay, controls the tube so that it will not run away and burn up. This delay tends to cause compression and a touch responsive amp for the player. A cathode biased amp usually does not have to be biased by the player. The tubes can be put into it and then the amp can be used.

Some tubes have two or more complete triodes, pentodes, heptodes etc., inside the bottle. In each of these each given triode, or pentode or whichever type of tube is self contained inside it's own plate inside the bottle. An example of a tube with two triodes inside the bottle is a 12AX7. There are not electrons going from plate to plate inside the bottle. The electrons go from the cathode to the plate. Sometimes they will not all stay on the plate and will bounce off and go to the grid. Pentodes, Tetrodes and other types with more grids were invented to try to stop that phenomonon which is called secondary emission, but that doesn't have to be talked about here.

Greg


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Oct 03, 2009 8:43 pm 
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Joined: Sun Apr 02, 2006 1:00 pm
Posts: 1957
Location: San Bernardino, CA
WOW !!!

And we thought you only thing you guys did up there was to wear L.L. Bean waders, Coos Bay it up and salmon fish.

I feel like I was reading the notes of Nikola Tesla!


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sat Oct 03, 2009 10:11 pm 
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Joined: Wed Nov 16, 2005 2:00 pm
Posts: 2962
Location: Scotland
It's important to remember that the 60%-70% 'rule' is only a guide, although it does work quite well for the majority of amps. The actual correct operating point and whether the tubes will stay within safe dissipation limits depends on factors in the circuit, not just the tube's power ratings themselves. For example, higher plate voltages and some transformer impedance ratios will mean that you need to bias them cooler, or they'll overload and may fail when the amp is cranked up.

For example, 70s Marshalls will go into heavy screen and sometimes plate overload when fully distorted, if biased to 70% of 25W (17.5W) at idle. They work reliably - and sound far better too - biased a lot cooler than conventional 'wisdom' says, at around 13.5W, or only 55% of their maximum dissipation. You certainly *shouldn't* just set them to 70% as seems to be the current fashionable advice, without knowing more about the amp. (Let alone to a certain current draw, which in some amps with extreme voltages like MusicMans, will cause almost immediate failure.)

Mesa's amps (apart from their cathode-biased ones) are biased fairly cool, too - which is one reason they can allow the user to replace tubes without needing to rebias, because it's unlikely that *any* set will then run too hot. Cathode-biased amps can be set hotter at idle because the bias voltage rises under load and self-limits them to an extent - although most cathode-bias designs still do run the tubes too hot, which is why amps like AC30s have a reputation of being hard on tubes, and EL84s for having a short operating life. They don't really, it's just that they're most commonly used in over-hot cathode-bias designs...


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Sun Oct 04, 2009 11:15 pm 
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Joined: Mon Feb 18, 2008 9:10 am
Posts: 618
Location: CT
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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.


...Unless the little spring tube clamp (Marshall JTM) springs into the socket and hits the high voltage and another part of your body is touching ground, That left me wondering "how long until I die"! The golden rule is to keep one hand in your pocket. My first time was a Fender Champ in 1972. I always sell the amps that try to kill me.



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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Mon Oct 05, 2009 12:15 am 
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Joined: Wed Nov 16, 2005 2:00 pm
Posts: 2962
Location: Scotland
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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.
But to check or adjust the bias on most amps requires opening the chassis, and measuring and/or adjusting things with the power on.

You *can* flip the standby to off to be safer every time you put your hand inside, but it's time-consuming. You should *not* power off the amp fully - it will cause inaccurate readings because it takes tubes a little time to warm up and settle in before the bias stabilises, and it's also bad for them to have to go through too many heating and cooling cycles. (And it still isn't 100% safe, since filter caps can store charge after the amp is off.) If you're not confident enough to learn how to work safely with at most the standby turned off, you probably should find someone to do it for you. It isn't actually *that* dangerous, but fear or lack of confidence will actually make an accident more likely, I think.

Quote:
...Unless the little spring tube clamp (Marshall JTM) springs into the socket and hits the high voltage and another part of your body is touching ground, That left me wondering "how long until I die"! The golden rule is to keep one hand in your pocket. My first time was a Fender Champ in 1972. I always sell the amps that try to kill me.
Both those are the more dangerous type of amps that can retain very significant voltage in the filter caps even after the amp is switched off too - not all do, and in fact probably the majority don't. It depends on the arrangement of the filter caps, and where in the circuit the standby switch is (if there is one). There is a myth that you should turn the amp off by flipping the power switch with the amp playing, and that this will drain the caps - well, sometimes it will and sometimes it won't, it depends on various factors and there can still be dangerous - or at least painful - voltages present even if the caps have partly discharged.

Be careful, and assume nothing.


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Mon Oct 05, 2009 10:32 am 
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Joined: Sun Oct 08, 2006 6:07 am
Posts: 3848
Quote:
Be careful, and assume nothing.


Words to live by. They should be repeated before starting any project.

This section of the forum is supposed to be only about Rickenbacker amps, but because this thread was started before this stricture, I feel fine to relate this: after all the discussion, especially on this thread, I finally got the urge to customize an amplifier to my liking. I took an early Blues Jr, checked out the billm mods website, and went for it: set up a bench properly, proper tools, research the mods before hand, access the parts before hand, and plan the appropriate time plus leeway to deal with unexpected situations. I am so pleased. Yes, it took more time, effort and money than the average mod job on a guitar, and ten times the concentration, especially when trying it out live on the bench before bolting everything back together to make sure nothing is touched improperly, and my, was it worth it!

The main reason for the post is to amplify BA's comments (pun intended) about biasing. Early Blues Jr's are biased way too hot. My old EL84 pair of power tubes actually have scorch marks on glass envelopes. I could have put in the fully adjustible mod, but, I just wanted a plug-in-and-jam amp. So instead of going fully balanced, adjustible bias, and still wanting some of the "hot" edge, I did the math on the bias resistors, made an educated guess as to values, and hardwired an auxillary resistor into the bias circuit that was in the ballpark to cool the tubes a little bit to get more tone. After all, we're only talking about a pair of EL-84's here. The difference in the bias is one or two watts at most, on a 14 to 17 watt window. At this low power, tone is much more important than watt-squeezing. With the other tone stack mods, I'm in tone heaven for a cash-and-carry lightweight jam amp to go with everything in the stable, including the 12-string, for the neck pickup on Ric-O-Sound and the bridge pickup going through the Vox Pathfinder (See--for those of you who didn't think I could work a legitimate RIC-related aspect into the post!)

But again: plan in advance; take safety precautions; allow for the unexpected; and ENJOY!


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 Post subject: Re: What is "rebiasing" a tube amp?
 Post Posted: Tue Oct 06, 2009 2:30 am 
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Joined: Tue Jan 09, 2007 7:39 pm
Posts: 1123
Location: Berlin, Germany
Enlightening stuff! Thanks everybody for that.


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