#1
hey
i need help with my friends tube amp
can any1 tell me how to bias the power amps in a tube amp? and can u tell me all the things that ill need

thx
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#3
its a peavey valveking 112 combo
im also thinkin of buyin it
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#4
You don't need to bias the Valveking, I own it.. just drop in a new set of matched 6L6s and it'll work fine
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#5
so all i need to do is replace the busted tubes with 6L6's and it should work fine
what about the tone, would it still be as good as a biased tube?
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#6
The thing is, you can't bias the Valveking, it's fixed bias..
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#7
Fixed bias means that you need to set the bias every time you change power tubes.

Some fixed bias amps come biased very cold from the factory such that you can usually just pop in a set of tubes and not have any problems, but for optimal performance, you want to set the bias on any amp.

If the amp has an external trimmer and test points, then all you need is a multimeter. However if it doesn't , then you should take it to a tech if you don't know what to do.
#8
so when you say 'your friend' needs help its really just you and your amp isnt it...
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#9
Quote by al112987
Fixed bias means that you need to set the bias every time you change power tubes.

Some fixed bias amps come biased very cold from the factory such that you can usually just pop in a set of tubes and not have any problems, but for optimal performance, you want to set the bias on any amp.

If the amp has an external trimmer and test points, then all you need is a multimeter. However if it doesn't , then you should take it to a tech if you don't know what to do.

This.

You could also have the tech add an internal bias trim pot for future tube changes. All it entails is setting a correctly-valued potentiometer instead of the static resistor the amp already has.
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#10
Quote by weetadsftw
so when you say 'your friend' needs help its really just you and your amp isnt it...
what's the difference? All he wants to know is how to bias an amp, I'm pretty sure it's already been established that he doesn't know how, hence he's asking... what difference would it even make if he said it was his amp or his friend's?

And to elaborate a little bit on what I said earlier. "Fixed" bias does not mean that the bias is not adjustable. In fact, even "cathode" bias does not mean non-adjustable bias. Biasing is just setting an idle current. You can bias an amp in either case, whether it's fixed bias or cathode bias.
Last edited by al112987 at Jul 1, 2009,
#11
The VK doesn't have a bias pot, though, so I'm not sure how it would be done without installing one (which is actually a very beneficial mod for the amp so you might as well do it).
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#12
Quote by al112987

And to elaborate a little bit on what I said earlier. "Fixed" bias does not mean that the bias is not adjustable. In fact, even "cathode" bias does not mean non-adjustable bias. Biasing is just setting an idle current. You can bias an amp in either case, whether it's fixed bias or cathode bias.


i still dont understand the difference btw a cathode or fixed bias, and i also still dont know how to actually bias the amp, cos u said that for optimal performance i had to bias it, so if u could give me more details that could really help.
a link would also help cos google isnt really helping me
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#13
^ Don't worry about it, the VK are biased very cold, it will not hurt the amp what so ever as long as you get a matched pair/quad. Some times people make things to complicated, fixed bias amp are nonadjustable unless you are willing to do some tech work and solder in a new reistor or trim pot.
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#14
Several fixed bias amps come with trimpots already. Biasing is setting the idle current. Fixed bias does not mean that the idle current is fixed. It refers to a negative DC bias voltage used to set the idle current, but it is a fixed value (but NOT unadjustable) which you need to adjust when biasing the amp to ensure that the idle current on the tube is not too high or too low.

If you want a tip on how to actually bias the amp yourself, you need to have a trimmer installed in the bias circuit of your amp before you can adjust it if your amp does not already have one. In my amp I have a 1 ohm resistor installed between the tube cathode and ground which I measure voltage across (and using Ohms law) to find the cathode current (which actually lets you bias slightly hotter as it's going to be higher than your actual plate current). This is what most people usually do as it's easy and relatively safe, and you just set the current so that you have 70% of maximum plate dissipation at idle.

Here is the problem though, if your amp is not meant to have bias adjusted, it may not have the 1 ohm cathode resistor between the tube cathode and ground which means you would need to install it yourself as well as adding a trimpot to the bias circuit (which should be on the board). If you aren't comfortable doing this, then I suggest bringing it to a tech, and watch him do it as well as show you how to bias the amp.

edit: and also, before you do ANY of this, pick up some basic safety about this stuff, only one hand in the amp, the most dangerous aspect of the amps are the big filter capacitors (those plastic coated cylinders) which store charge after the amp is turned off and unplugged, the negative terminal is always grounded to the chassis and placing your hands on the chassis and something that is connected to the positive terminal (which means it doesn't even necessarily need to be near the filter caps) will give you a nice shock and can kill your ass.
Last edited by al112987 at Jul 1, 2009,
#15
^ I am not dissagreeing with you, what you recomend is right on the money, but its really its not nesisary with the VK, everyone and thier brother just swaps them out, same thing with Mesa, just plug and play, right?
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#16
Quote by Johnbryant
^ I am not dissagreeing with you, what you recomend is right on the money, but its really its not nesisary with the VK, everyone and thier brother just swaps them out, same thing with Mesa, just plug and play, right?


Chances are it'll sound like an MG though.
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#17
Quote by Johnbryant
^ I am not dissagreeing with you, what you recomend is right on the money, but its really its not nesisary with the VK, everyone and thier brother just swaps them out, same thing with Mesa, just plug and play, right?
Sort of, but Mesa recommends their "own" tubes which they test before rebranding them to make sure that they run at the correct operating point when put in their amps.
#18
Quote by DLrocket89
Chances are it'll sound like an MG though.

BS, a tube swap is the one of the first things people do to impove the overal tone of the VK's, usually followed by a speaker change and an OD to tighten things up a bit.
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#19
Quote by Johnbryant
BS, a tube swap is the one of the first things people do to impove the overal tone of the VK's, usually followed by a speaker change and an OD to tighten things up a bit.

*Shrugs* if he has the stock tubes, true. But without a bias to make sure they're good and right, you can end up with an amp that sounds great, one that sounds eh, or one that sounds like total crap. Not saying it's automatically in the crap category. Just saying it's a possibility.

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#20
^Peavey fixed Bias amps, always seem to biased way to cold, and for this reason is why JJ tubes usually seem to liven them up a bit because they are typically a hotter tube, and ends up makeing the cold biased amp sound about right.
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#21
Quote by Johnbryant
^Peavey fixed Bias amps, always seem to biased way to cold, and for this reason is why JJ tubes usually seem to liven them up a bit because they are typically a hotter tube, and ends up makeing the cold biased amp sound about right.


I have a XXX in for repair right now that blew up because someone threw a set of JJs in without biasing. waaay over dissipated it, poof!

Amps need to be biased, end of statement. There's too much variability in today's production tubes.

The one area where you have a chance of being OK is if you have something like tubedepot.com's "bias point" where you can order another set of tubes that need the same bias point. Otherwise, you're gambling with some really expensive repairs.
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#22
^ the Triple X has a Trim pot that is ajustable buy the User, the Vlaveking, or the Classic for that matter do not, someone jacked thier Triple X because the were screwing around with the Bias knob without knowing what the hell they where doing. The Vk is a different animal, but I understand that biasing an amp to hot can be hard on the amp and the tubes, but the VK are biased so cold that even the hottest tube should not be that big of a problem.
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Last edited by Johnbryant at Jul 1, 2009,