Opinions on Bias - Amp Builders / Techs please share

[Glide]

I have an old plexi that runs around 410 plate volts and I want to put in XF2 single or double getters.

From purely a technical standpoint what are the pros' opinions on running bias up in the range of 30 to 40 vs. in the range of 15 to 20?

I don't mean from a tone standpoint but purely a technical one - life of the tubes, any other technical issues...etc..etc.

I have heard guys say, you never want to run up in that range. At plate voltages of 450 you want to be around 18ma, at 475pv you want to be around 17ma, at 500 around 15 to 16 ma.

What are your thoughts on bias? How does the tube death curve increase running at 32 vs. 18. Is it huge? Is life halved, or more?

I appreciate your opinions on it.

[mark norwine]

EL34 has a plate dissipation limit of 25W....at quiescent operations, 70% is a good target limit. 25 * 0.7 = 17.5

410 * .040 = 16.4

Where's the problem?

[PRNDL]

Running tubes hot reduces crossover distortion, which is a way to get better numbers for a HiFi amp, which makes no sense for a guitar amp. (Most likely they drop the bias once the test is over.)

Mark is correct in that 70% is a good target limit, although many prefer to set the bias on the cold side (50%), which extends tube life with usually little loss of tone.

The best way, IMHO, is to simply plug in the guitar and see how the amp sounds with different bias settings, and decide for yourself what sounds best.

Also, I'm wondering if I have this right.
For the first album, EVH ran his amp with the tubes extremely hot (by increasing the line voltage), but the "brown sound" has them running cool.
In my experience, however, you get that great sound with a good, older Marshall 100 watt amp turned up, with a great guitar and an awesome guitarist.

[phsyconoodler]

Quote:"At plate voltages of 450 you want to be around 18ma, at 475pv you want to be around 17ma, at 500 around 15 to 16 ma."

What the heck are you talking about here?
Do you mean milliamps?If so,you are way cold. EL34's want about 32ma at 450 plate volts.Are you talking about using 6V6's?

Or are you meaning 70% plate disipation?If so,mention it rather than using ma in your description.

[skipm45]

Quote:"At plate voltages of 450 you want to be around 18ma, at 475pv you want to be around 17ma, at 500 around 15 to 16 ma."

What the heck are you talking about here?
Do you mean milliamps?If so,you are way cold. EL34's want about 32ma at 450 plate volts.Are you talking about using 6V6's?

Or are you meaning 70% plate disipation?If so,mention it rather than using ma in your description.

I'm confused as well. Use this:
http://www.ionpool.net/arcade/tech/Weber_Bias_Calculator.html

+1 psyconoodler. EL34's (AB) @ 450v should be biased to 38.8mA (70%)

Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

[Glide]

Thanks for the link to the calculator, Skip.

And I too am confused, and that is why I posted this, hoping there was a pro amp builder or tech here that could shed some light for me and any others that may have some confusion with this. I am not an amp tech or builder. I just play through them.

So to avoid any confusion I have Mark Norwine that says that 410 Plate volts on an EL 34 tube should bias to around 16.4mA

And I have phsyconoodler and skipm that says that is WAY too cold and it should be biased around 32 to 38.8 mA. Although they actually used 450 plate volts in their example, you get my point.

That is what I am trying to determine - Are there two schools of thought here? Is it a terminology dis-connect?

Why is it so difficult for an amp person to explain why there is such a disparity in proper bias numbers? Again - remove tonal considerations from the equation - strictly life and care of the tube.

Or, does it really make a rat's ass whether you bias EL 34's at 16mA or 40mA from a tube life standpoint?

Thx.

[FrankieSixxxgun]

An EL34 running at 410vdc on the B+ biased to 16.4mA will last probably an entire lifetime as it'll be idling at ~6.7 watts. You'll also have close to no sustain and probably sizzle out a lot of your harmonics. The amp will most likely not sound good. That's idling at 27%, most Plexis are biased to 65-70% idle. I personally run mine with a pair of RFT EL34's in that range and they sound tasty.

If anybody knows amps it's Velvet George and this is what he says:
http://metroamp.com/wiki/index.php/Bias_Formula

[mark norwine]

So to avoid any confusion I have Mark Norwine that says that 410 Plate volts on an EL 34 tube should bias to around 16.4mA



You do?

Re-read it.

What I said was:

410 [plate volts] * .040 [milliamps] = 16.4 [quiescent plate wattage being dissipated]........

....and 16.4 [watts being dissipated] is below the 70% figure of 17.5 [maximum quiescent power dissipation], inferring that, at 40 [milliamps, i.e. the value you suggested] you'll be fine.

[Glide]

An EL34 running at 410vdc on the B+ biased to 16.4mA will last probably an entire lifetime as it'll be idling at ~6.7 watts. You'll also have close to no sustain and probably sizzle out a lot of your harmonics. The amp will most likely not sound good. That's idling at 27%, most Plexis are biased to 65-70% idle. I personally run mine with a pair of RFT EL34's in that range and they sound tasty.

If anybody knows amps it's Velvet George and this is what he says:
http://metroamp.com/wiki/index.php/Bias_Formula (http://metroamp.com/wiki/index.php/Bias_Formula)

Man that is beautiful Frankie - EXACTLY what I am looking for.

So you run yours up around 32 to 38? And if you are running a pair of $350 NOS tubes what kind of life would you be haapy with getting from them?

Thx again.

[FrankieSixxxgun]

You do?

Re-read it.

Glide, he said 16.4 watts, not mA. 16.4 watts is what you get with a bias of 40mA against 410vdc, which is about perfect.

[Glide]

Glide, he said 16.4 watts, not mA. 16.4 watts is what you get with a bias of 40mA against 410vdc, which is about perfect.

Great - I didn't see a "w" in his post. That is what I meant - maybe it is a terminology disconnect.

Thanks again for clearing it up Frankie.

[FrankieSixxxgun]

Man that is beautiful Frankie - EXACTLY what I am looking for.

So you run yours up around 32 to 38? And if you are running a pair of $350 NOS tubes what kind of life would you be haapy with getting from them?

Thx again.

Well, what kind of life are they going to get? Bedroom jamming or rough gigging and high volumes? Obviously they'll last a lot longer in the former condition than the latter. NOS tubes generally last close to a lifetime unless you beat the everloving crap out of them, and even then they still can last. I've got a JCM 800 running old GE 6550's that's never given me grief as well as old RFT's in my Plexi since I built it a few months ago, and these are amps that are gigged almost every weekend with practices in between.

Edit - The only amp to ever eat up NOS tubes on me is my AC30, and that's because it's cathode biased and the Sylvanias just couldn't take the abuse. It gets JJ's now. ;)

[Glide]

Well, what kind of life are they going to get? Bedroom jamming or rough gigging and high volumes?

I would say playing 1 to 2 hours every day at high volumes - 2 o'clock to 3 o'clock on the dial.

So at a plate voltage of 410, NOS Mullard single or double getters should be biased around 40 to 43mA and that is not considered over-driving them in any way, shape, or form?

And if I bias them colder I lose clean headroom, or is it the other way around?

Thanks again.

[mooreamps]

From purely a technical standpoint what are the pros' opinions on running bias up in the range of 30 to 40 vs. in the range of 15 to 20?

I don't mean from a tone standpoint but purely a technical one - life of the tubes, any other technical issues...etc..etc.

.

Issues, there should not be any. It's the difference between running the PA Class A/B and Class A. The pro's would be a warmer tone running class A ; and would bring the output power from 50 watts to about 25.

-g

[rockon1]

Man that is beautiful Frankie - EXACTLY what I am looking for.

So you run yours up around 32 to 38? And if you are running a pair of $350 NOS tubes what kind of life would you be haapy with getting from them?

Thx again.

You could listen to them at 32ma which would be a tad below 55% and at 35ma (60%) then a bit hotter too. I'd go with the lowest setting that sounds good. Bob

[FrankieSixxxgun]

I would say playing 1 to 2 hours every day at high volumes - 2 o'clock to 3 o'clock on the dial.

So at a plate voltage of 410, NOS Mullard single or double getters should be biased around 40 to 43mA and that is not considered over-driving them in any way, shape, or form?

And if I bias them colder I lose clean headroom, or is it the other way around?

Thanks again.

They should last a good long time now. I've found my tubes more prone to dying from mechanical abuse (i.e. my head in the back of a pickup truck), or some faulty building power than from them just going kaput.

Colder the bias, within reason, the sooner they'll overdrive, the longer they'll last. Hotter bias will give you less power section distortion and more clean headroom, also to a point. Too hot and the amp'll be muddy and grainy, too cold and it'll be glassy and lifeless. 50-70% are nice operating ranges IMO.

[Glide]

You could listen to them at 32ma which would be a tad below 55% and at 35ma (60%) then a bit hotter too. I'd go with the lowest setting that sounds good. Bob

You guys are freaking awesome.

[FrankieSixxxgun]

Yeah, there's no set rule on bias actually. If they sound good and are at a normal operating range, then have at. It's not that an exact of a science.

[Fred Farkus]

Be careful about throwing around numbers a la the 70% rule and etc. Here's a good article on setting bias using a scope:

http://tone-lizard.com/Marshall_bias.htm

Also a good general article about biasing myths and etc here:

http://tone-lizard.com/Biasing.htm

[FrankieSixxxgun]

Be careful about throwing around numbers a la the 70% rule and etc. Here's a good article on setting bias using a scope:

http://tone-lizard.com/Marshall_bias.htm

Also a good general article about biasing myths and etc here:

http://tone-lizard.com/Biasing.htm

Because we all have a scope available. 70% max is a great rule for 99% of the amp jobs out there, man.

[phsyconoodler]

I try 50,60 and 70% and see which sounds the best.I never exceed 70% on fixed biased amps because I hate tube failures.
Modern production tubes cannot handle too hot a bias.

[Glide]

I try 50,60 and 70% and see which sounds the best.I never exceed 70% on fixed biased amps because I hate tube failures.
Modern production tubes cannot handle too hot a bias.

And you bring up a good point phsyco, in that not only will the NOS tubes possibly improve your tone, but they last longer, more durable...all that yadda yadda.

I think a lot of people's reasoning for investing in them is strictly tonal. But there are utilitarian advantages to outlaying that much dough for them too.

You guys really made this a good read for me and I appreciate it.

Checking out Fred's links....thx Fred.

[skhan007]

As a slightly humorous aside, for a couple months recently, I had guys continually bringing me marshalls that were biased up REDICULOUSLY hot, and burning up the tubes.

Some of these amps I was seeing were drawing over 70mA at idle!!!



Holy Crap, does that sound familiar!!