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Member Since: Jan 24, 2007

I just put new tubes in a Marshall JCM 2000 DSL

Do I really have to get the head Biased? (no idea how to spell that word)

What is the reason for having that done?

Anyone know?

Will it effect my tone?

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Prince CZAR-ming
Since: Apr 08, 2004

Mar 22, 2007 02:32 pm


Tone, and long tube life


and Yes.

Bias is a level of electricity going into the tube, at idle, so that the tube can amplify the AC signal correctly.

So anytime there amplification going on, the device doing so is directly related to tone.

Running tubes too hot (high bias) can sound very good, but eat tubes at a faster rate.

Running tubes too low, can lengthen tube life, but make the amp sound thin, and weak.

Some people don't bother to bias, and don't have problems. BUT, you could run the tubes outside of their 'happy' range, thereby incurring the a-fore-mentioned problems.

Mostly, if people don't bias, and the amp's working OK, then the bias setting is still in the good operating range of the new tubes. I'd guess most times you'd be in the Ok range, but I don't like to guess.

Setting bias can be done multiple ways, but a few of those are deadly dangerous, while other ways are somewhat expensive. There are a few in between, one of which I use, but it's still dangerous, as you're poking around inside while the amp is running: LETHAL VOLTAGES.

If not sure how to proceed, but want it biased, you can get a bias probe, similar device, and do it yourself, without getting inside the amp.

There's one example, from a good shop.

If you just want to play without worrying, Groove Tubes marks their tubes with a bias number. This system lets you set your tubes bias for a certain level of GT tube - say 6 - then when you retube later, you can buy GT tubes of level 6, and plug them right in. No worry about bias, as the new tubes closely match the old ones.

But you'd be stuck running GT tubes from now on, which ain't the best option, in my humble opinion.

All this bias stuff applies to output tubes, as preamp tubes are cathode biased, meaning you can't change them easily. The bias is set with a resistor. Output tubes are (mostly) fixed bias, meaning you control the DC bias with a potentiometer inside the amp.

Of course, you could get to know your local tube repair guy (if you have one) and have him bias it up everytime you re-tube. Doesn't hurt to network =).


Since: Jan 24, 2007

Mar 22, 2007 02:45 pm

Could this be the reason I am getting some Humm and buzz on my gain channel. I don't really remember if it was there before or not. Seems the more I turn up the gain the louder the hum gets.

Prince CZAR-ming
Since: Apr 08, 2004

Mar 22, 2007 02:58 pm

Possible, though hard to tell without being there.

To troubleshoot, you can either: A) replace things, or B) use a scope to find the cause, and fix/replace/tweak offending section.

Bias is a possibility, though so is a lot of other things.

Hi-gain amps are pretty much always going to buzz. THere may be ways of removing it, but I bet they're not cheap or easy.

Any amp techs around your area? Sounds like a trip to the doc might be in order.

Just to be thorough, unsmooth power in a house can add buzz/humm, as can flourescent lights, dimmers in the area, other items on the same circuit, etc. Removing those possibilities before a trip to the doc may yeild happy results.

But me, personally, I'd want to at least get the bias right, just to close that part of the equation.

Since: Aug 13, 2005

Mar 24, 2007 04:01 am

The shed dweller/amp fixer man reckons that valves are better made eg Sovtec/Svetlana (Russian)and the bias rarely needs adjusting as long as they are of the same batch.Still worth having it checked now and then.

Czar of Midi
Since: Apr 04, 2002

Mar 25, 2007 03:47 pm

I am all for making sure the bias is correctly set. If not, indeed the tone will suffer as will the life of the tubes.

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