The importance of biasing.

A positive

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Does it really matter if an amp is biased correctly?
Has anything to do with guitarists' preferences?
Or is it fixed that you need good biasing to have decent tone in you amp?

I noticed that in my Peavey XXX I liked the tone in a hotter than normal setting but it got a bit fuzzy in mid volumes.(I run JJ tubes pre and power).
The tone was a bit clearer in o colder setting.
 

suhr_rodney

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If you like it biased hot and can deal with the associated reduced tube life, then who is to say it's wrong.
 

Blue Strat

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Too hot and you'll have short tube life and possible catastrophic damage to your amp. Too cold and your tone could suck.

Why take a chance when it's relatively easy to do?
 

A positive

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Too hot and you'll have short tube life and possible catastrophic damage to your amp. Too cold and your tone could suck.

Why take a chance when it's relatively easy to do?
I'm not so interested in long tube life.In fact I'm trying to find the colder setting that tone as you said doesn't suck and actually shines
 

Blue Strat

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I'm not so interested in long tube life.In fact I'm trying to find the colder setting that tone as you said doesn't suck and actually shines

What current readings and plate voltage are you speaking of? All we're talking now is relative settings.
 

A positive

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Ok the standard factory settings said -55vdc.The tone pretty sucked there.Peavey recommends -42 to -38.
having it on 40 it's quite clear.A bit fuzzy though.If I go hotter at about 36 I got the tone I want but a bit more fuzzy than before.
 

A positive

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53
evaluating bias, by grid voltage is like measuring weight with a ruler.........

what's important here is the plate current. (and plate voltage.)


cheers,

unk
How accurate is evaluating bias by grid voltage??

That is the setting I take from the test jack my amp provides
 

rockon1

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12,980
How accurate is evaluating bias by grid voltage??

That is the setting I take from the test jack my amp provides

It tells you nothing about the actual bias of the amp. Peavey recommends 55 vdc for 6L6's and 42.5 for El34's. A safe zone. With 6l6's (and the switchgon the 6L6 setting)set it anyhere you like with the adjustment knob. I ran my JSX with it as hot as it would go(aqbout 49 vdc at the test jacks) with 6L6's for a year without incident. If you want to bias it the coprrect way you will need a bias probe like this to do it fairly easily otherwise do it by ear.

https://taweber.powweb.com/biasrite/br_page.htm
 

rockon1

Platinum Supporting Member
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12,980
I agree, except with the "by ear" statement. Many people who bias by ear end up complaining that they're power tubes only last weeks or months, or that their amp tech charges too much to fix their blown up amps;)

Perhaps but I had a JSX and ran my 6L6's as hot as the adjustment knob would go for long periods of time no problem. Did this with Sovtek 5881's, Sovtek 6L6WXT's, JJ's and old GE6L6's without incident. I tried to use a probe but I got defective one! Almost killed the amp with it-irony! At any rate I wanted to find out how conservative their adjustment knob was. It really seems that you cant get it too hot with it. Ive even read of guys biasing this amp with a probe and switching to the hotter EL34 setting with 6L6's to get them biased correctly. Hearsay but it seems to jive with my experience. Ive got to get another probe and try my hand at it again.;)
 

A positive

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Perhaps but I had a JSX and ran my 6L6's as hot as the adjustment knob would go for long periods of time no problem. Did this with Sovtek 5881's, Sovtek 6L6WXT's, JJ's and old GE6L6's without incident. I tried to use a probe but I got defective one! Almost killed the amp with it-irony! At any rate I wanted to find out how conservative their adjustment knob was. It really seems that you cant get it too hot with it. Ive even read of guys biasing this amp with a probe and switching to the hotter EL34 setting with 6L6's to get them biased correctly. Hearsay but it seems to jive with my experience. Ive got to get another probe and try my hand at it again.;)
As I said before in another thread I had an orange spot on the plate of one stock tube(Ruby tubes actually).And the pot was at 10 'o clock
 

John Phillips

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13,040
Biasing by ear is fine if you know what to listen for. It doesn't just mean "keep on turning it up until it sounds fat enough". You either need to know when to stop by ear too, or measure the dissipation as well. Watching for plate glow is not that useful since it's a danger sign not a safety sign.

It's your amp and your tubes, but if you're trying to bias hotter and hotter in order to get a fat, compressed overdriven tone and/or more sag... you probably just have the wrong amp.

OK, it might eventually give you something roughly like what you want, but it's really a bit like lowering your tire pressures further and further in order to get your executive saloon to grip better in the snow, when what you really need is a 4x4 with snow tires.

Just because Peavey give you a pot with a given range doesn't necessarily mean anywhere in that range is safe for the amp or the tubes. The whole point of putting an adjuster on the amp is to let you set it correctly for different tubes, which vary quite widely. If you start with a set of tubes that already run hot, setting them at the hot end of the range may well put you into the danger zone.

Beware of biasing only by idle current or dissipation, with some amps too. For example, old 100W Marshalls are very hard on the tubes, and if you set them to 70% of maximum dissipation at idle (or even close, often) you will be WAY over the safe dissipation when the amp is cranked up.

Just my opinion...
 

rockon1

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12,980
but if you're trying to bias hotter and hotter in order to get a fat, compressed overdriven tone and/or more sag... you probably just have the wrong amp.
Yes it was. Then I bought the wrong amp to replace it!:jo


As I said before in another thread I had an orange spot on the plate of one stock tube(Ruby tubes actually).And the pot was at 10 'o clock
There are exceptions for sure but I have to think that tube is very weak or bad. As I stated Ive run many types of tubes in mine at hotter setting with out incident.Besides the range on the pot isnt that great. At any rate if you run them at factory 55vdc and they are OK then dont sweat it. You can always run them cooler. That certainly wont damage the amp!
 

A positive

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Beware of biasing only by idle current or dissipation, with some amps too. For example, old 100W Marshalls are very hard on the tubes, and if you set them to 70% of maximum dissipation at idle (or even close, often) you will be WAY over the safe dissipation when the amp is cranked up.

Just my opinion...

So if I want to crank the amp I may set it colder right?

Anywat I took my amp from my tech and I noticed that the sound is good at low volumes(2-3) and high volumes(above 6).In the mid section I have a little bit of fuzz.
 

jbltwin1

Member
Messages
272
I call them fiddlemongers. Learn, understand, or leave alone. Not only can you kill yourself if you don't know what the he!l you are doing, it can endanger a nice amp if you "fiddle" to the point of destruction. Yes, a lot os us learned by destroying our own stuff in the past, but back then, it was very common and cheap stuff. We used to get 6L6's for $2.25 and they were good ones. We also didn't know back then about tube conservation because they held up better and we didn't worry about biasing. Pop them in and see if it lights up. We fixed what broke and lookd for the cause. Now, stuff costs' a LOT more and parts are a little more scarce. I used to learn on stuff other people threw away and I tried to fix them. Still not a bad way as long as you can get parts. Biasing is improtant now because if you were to buy NOS tubes, you sure as he!l don't want them going up in smoke on your firts power up. Bias cool and they will live longer.
 

John Phillips

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13,040
So if I want to crank the amp I may set it colder right?
Depends on the amp, but possibly. The reason you can't bias old 100W Marshalls as hot as is often recommended without risking tube failure is because the plate voltage and OT impedance ratio will mean that if you do, it then goes well over the maximum when the amp is cranked (have a read of the 'Super Lead Build Plate Glow' thread if you can, there's a lot of useful info in there) - they're just trying to push the envelope too far really. I've never worked on a JSX, but if it's like other Peaveys it will be more conservatively designed.

This is a very dangerous quote since EVERYONE who doesn't know where to stop will take it to mean that they're an expert;)
True :). What I meant was that you aren't actually listening just for "good tone", surprisingly. That does come into it, but you need to know what good tone means in a technical context relating to bias... it doesn't just mean "if I want a fat sound, I should keep turning it up since it keeps on getting fatter".

What I should have said was that you need to know what an overbiased amp sounds like, and what an underbiased amp sounds like, and bias for the best sound within those two limits... and if you do prefer the 'hot' sound - everyone thinks they will, because that's become the 'received wisdom', but actually you may be surprised, personally I prefer a cooler setting in most amps - then you really do need to check the dissipation too.

I always do it that way round BTW - the dissipation is a check, not a target.


Ever wonder why Randall Smith designs Mesa amps without a trimmer? :)


One thing that strikes me as odd is why Peavey give you a bias voltage test point. If you're going to bias an amp by voltage, you only ever need to do it once really - that's actually the reason Fenders were fitted with them originally... to compensate for poor-tolerance resistors in the bias supply, not for tube variations. The idea was to set them at the factory to a fixed voltage, more accurately than the resistors did. This is also what Mesa does with hardwired (better tolerance) resistors.

If you're going to fit an external test point and a trimmer, it's only worthwhile if it's to measure current...
 




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