Biasing an amp...is it critically important?

[cmm71]

I know, you're fingers can't type fast enough "OMG YES...stop what you're doing and bias it now...N O W ! ! !" Just hear me out before you lecture me, there is a deeper question here.

I have a Marshall 4501 JCM900 that I use for home use only. I don't play out with it; in fact, I don't even really play it too much. It runs for a maximum of an hour a week...and the Master Volume rarely goes above 3. I got a great deal on the amp and it was in "like new" condition when I bought it. The only problem is that it needed new tubes. Once I installed new tubes it really improved the tone. I called the only place in town that I know of that does biasing and they estimated a 4 week wait...apparently they are really busy. They will charge $50 to do the work. If I want it done sooner they will charge me $30 extra for the "rush" fee. I could buy a Weber Bias Rite for $140ish...but is that really worth the trouble. Do all of you bias your amps when you change tubes? I have to believe that the general tube amp buying public is not doing it if there is only one place in town doing the work. Is it really all that important for me to bias if I use the amp so rarely?

Honest and thoughtful opinions are welcome...condescending "your an idiot" responses can be deposited elsewhere. Remember, an hour a week playing time and it's in the closet for the rest of the week. Thanks.

[beep.click]

I suspect some amps benefit more from biasing than others.

One day my AC30 was making weird "whooshing" noises. I decided to change the power tubes, mostly because I had some in a drawer -- new tubes, but a different brand from the ones in the amp, and not matched at all. Just random tubes, basically.

Well, the "whooshing" went away, and the amp sounded great. No biasing required.

I read somewhere that there is no "right" way to bias an amp, and if it's sounding good, it is good. Bear in mind, there was a time when most players had never even heard of biasing an amp...

[gmann]

Originally Posted by cmm71

I know, you're fingers can't type fast enough "OMG YES...stop what you're doing and bias it now...N O W ! ! !" Just hear me out before you lecture me, there is a deeper question here.

I have a Marshall 4501 JCM900 that I use for home use only. I don't play out with it; in fact, I don't even really play it too much. It runs for a maximum of an hour a week...and the Master Volume rarely goes above 3. I got a great deal on the amp and it was in "like new" condition when I bought it. The only problem is that it needed new tubes. Once I installed new tubes it really improved the tone. I called the only place in town that I know of that does biasing and they estimated a 4 week wait...apparently they are really busy. They will charge $50 to do the work. If I want it done sooner they will charge me $30 extra for the "rush" fee. I could buy a Weber Bias Rite for $140ish...but is that really worth the trouble. Do all of you bias your amps when you change tubes? I have to believe that the general tube amp buying public is not doing it if there is only one place in town doing the work. Is it really all that important for me to bias if I use the amp so rarely?

Honest and thoughtful opinions are welcome...condescending "your an idiot" responses can be deposited elsewhere. Remember, an hour a week playing time and it's in the closet for the rest of the week. Thanks.

OMG! Stop what you're doin' now and bias it you idiot! I couldn't resist, sorry man. Well to sound at it's best it should be biased. Who knows, it could actually be biased for the tubes you have in it. I've bought amps and changed tubes to what I like and went to bias the amp (I have a Weber Bias Rite) only to find out it was already really close. This doesn't happen very often tho. Given your playin' situation I would say it's not really necessary. The amp sounds good to you and as long as the tubes aren't glowing red I'd say you're good to go.

[jhundt]

you will soon hear from both sides of the issue, I suspect.

My own opinion in your case is - if you replaced with the correct type of tubes and it sounds great, leave it alone. An amp is designed to operate with a certain type of tube and it should not require biasing to operate safely if the replacement tubes are correct. Bias can affect the sound and it can affect the longevity of the tube, but your bias would have to be extreme in either direction to make a great difference.

[Green Lantern]

If you're fine with the sound, and if you put in the correct type of tubes that the amp requires, then...

Leave Alone...

[Ricky D.]

Biasing is one of the reasons why two apparently identical amps can perform differently. Here's a good non-technical article from Randall Smith at Mesa Boogie:

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

[DeepSouth]

It's critically important if you change your tubes to a new brand or type because it could suddenly run too hot and fry your new tubes (or affect their useful lifespan) and cause other probs when they fail.

I don't think you'll do much harm running them too cold - except you aren't doing your amp justice.

Personally I run mine just a little hot - but nothing extreme.

I do my biasing with a good quality multimeter on my Hot Rod Deville. I haven't tried it on my DRRI because they are a bit trickier and beyond my current level of incompetence I'd really want a proper bias meter for that one.

[Captain Bb]

If you've already got a multimeter then you only need a bias probe to go with it to bias your DRRI.

Check out the Eurotubes demo on Youtube.

[DeepSouth]

Originally Posted by Captain Bb

If you've already got a multimeter then you only need a bias probe to go with it to bias your DRRI.

Check out the Eurotubes demo on Youtube.

Will do thanks. Always intended to work out how to do it but I've been a bit lazy. It's a newish amp and I assume Fender normally biases them conservatively at the cold end of the spectrum - so I'm trading tone for laziness in the short term

[donh]

It works like this:
If you have not enough bias voltage, the tubes fail to overlap enough and there is bad distortion. Really bad. Icky bad. You may like icky bad, who knows?
If you have too much bias voltage, the tubes get all red and burn up.
In between is a reasonably large useful area.

So. If you see your tubes getting all red and really hot, you may wish to consider getting the bias correct. If they are looking and feeling ok, then you get what you get . . . .

[flyingbanana]

Is there a certain safe temp range tubes can be in, in case you want to bias them a bit hotter?

[tazzboy]

It does help in the performance yes.

[donh]

Originally Posted by flyingbanana

Is there a certain safe temp range tubes can be in, in case you want to bias them a bit hotter?

The temperature range is a side effect of the tube current draw. The higher a current draw, the higher the tube's temp. The joy spots are all under the max current. Each tube design has a maximum current draw that you exceed at your own risk - it's in the tube specs, look it up.

[flyingbanana]

Originally Posted by donh

The temperature range is a side effect of the tube current draw. The higher a current draw, the higher the tube's temp. The joy spots are all under the max current. Each tube design has a maximum current draw that you exceed at your own risk - it's in the tube specs, look it up.

Does that mean you could potentially up the bias to try for a better tone up to a point, or are there other components that could suffer damage before the limit is reached? Or are we mainly talking about tube damage only?

[Riffraff12571]

We are mainly talking about tube damage however when tubes have a catastrophic failure (which happens more when they are running too hot) they can take other components out with them when go.

[PinewoodRo]

Originally Posted by beep.click

I suspect some amps benefit more from biasing than others.

One day my AC30 was making weird "whooshing" noises. I decided to change the power tubes, mostly because I had some in a drawer -- new tubes, but a different brand from the ones in the amp, and not matched at all. Just random tubes, basically.

Well, the "whooshing" went away, and the amp sounded great. No biasing required.

I read somewhere that there is no "right" way to bias an amp, and if it's sounding good, it is good. Bear in mind, there was a time when most players had never even heard of biasing an amp...

An AC-30 is cathode-biased so does not need manual adjustment when you replace the valves. Other amplifier designs will benefit from bias adjustment. How critical it is depends on how far out of adjustment it is!

[Captain Bb]

If you have pcb mounted tube sockets you run the risk of board damage if the tubes run very hot.

I tend to bias mine around the 70% plate voltage dissipation point and then fine tune it by ear from there.

Too cold and the tone is weak - too hot and there is unwanted hiss and noise.

[gwjensen]

I've been lambasted for this before, but it works for me. I turn the bias up until the tubes start to red plate, then dial it down until it goes away. This way the tubes are running on the hotter side vs the colder side (lousy tone), but not too hot. I do this in dim lighting and check them every 15 minutes or so over the course of an hour. I used to do it by the numbers with a bias probe and multi-meter, but now I use my eyes and ears.

[donh]

Originally Posted by gwjensen

I've been lambasted for this before, but it works for me. I turn the bias up until the tubes start to red plate, then dial it down until it goes away. This way the tubes are running on the hotter side vs the colder side (lousy tone), but not too hot. I do this in dim lighting and check them every 15 minutes or so over the course of an hour. I used to do it by the numbers with a bias probe and multi-meter, but now I use my eyes and ears.

This makes perfect sense to me. You are taking full responsibility for your own actions and also taking reasonable precautions.

[BobbyZ]

Originally Posted by gwjensen

I've been lambasted for this before, but it works for me. I turn the bias up until the tubes start to red plate, then dial it down until it goes away. This way the tubes are running on the hotter side vs the colder side (lousy tone), but not too hot. I do this in dim lighting and check them every 15 minutes or so over the course of an hour. I used to do it by the numbers with a bias probe and multi-meter, but now I use my eyes and ears.

I've done that myself for years. However posting it on a forum will get you lambasted and blaimed for all the worlds problems.