Bias question

[SacDave]

I’m trying to learn and understand Biasing an Amp. I know the reasons for Biasing but the how too in getting confusing I watched quiet a few videos, read way to many articles.  So what I’m biasing is a 64 Bandmaster I think I’m around 60% the Amp sounds good where I have it set. Still want to try some of the bias methods I’ve watched and see what I come with always seems the end results is a little deferent with each method but pretty close. I’ve readjusted it by ear but haven’t checked the numbers yet.  So during all this experimenting with deferent methods I bought two Bias Scouts the Bias scout give you plate voltage and Cathode current. I can’t find any information on what to do with the voltage readings the instruction have no formula. Hears the voltages I get plate voltages 420VDC cathode current .070  (I believe is 70 milliamps)  The Amp has two JJ 6L6GC tubes  the plate voltage is the same on each tube  The cathode current is within one milliamp on each tube. So any ideas what to do with the numbers?  So to me 70 mA seems really high or dose that divide by 2 two tubes? Do I multiply the .070 X 430 = 30.1 divide that by 2 = 15.05 ? 15 watts plate dissipation?
The bias scout  is from Tube Depot I sent them an e-mail no response .

[sluckey]

You simply multiply plate voltage times cathode current. In your case, that would be 420 X 0.070 = 29.4 watts. And that's for one tube. Max plate dissipation is 30 watts, so you are biased at about 100%. I'd back it down to about 70%.

[SacDave]

Thanks  I'll do some adjusting with the bias pot.

[PRR]

If you bias too cold, small sounds are rough.

If you bias too hot, the tubes melt.

There is a w-i-d-e range between which is perfectly acceptable.

Yes, most over-volted guitars amps biased "by ear" will "sound best" when biased to 90%-150%. Tube life this way will be poor. Turning down to 70% is usually so very-very similar that you could not tell the difference one day to the next; meanwhile daily/weekly tube replacement is avoided.

There is no deep magic in "the 70% rule", yet it works out for essentially all fix-bias amplifiers.

[SacDave]

I did some adjustments so now my plate voltage is 430  the cathode is set at .048  so that gives me 19.35 W  I believe 70% would be 21W  so I'm right in there.  Also I did do all the measurements with Volumes on 0  and warmed up for a couple minutes.

[sluckey]

I'd say you're good to go. Play it for a while and see if you are happy.

BTW, I like the fact that your bias checker has a 1000:1 voltage divider for the plate voltage. Nice and safe!

[SacDave]

I have a handle on it now at least I know how to use the Bias Scout.  Sluckey  I do like the fact your hands are out of the Amp for the most part and the voltages are in millivolts .  I did have my old multimeter blow on me while testing an Amp  it just shorted out. After  doing a lot of research I bought a nice Fluke 115 built like a tank.  PRR I did read a lot of post on various forums were some suggested biasing by ear.  One person suggested biasing by ear then check with the meter he said I'd be surprised.  Anyway thanks for the help

[pompeiisneaks]

I've got the Fluke 117 and love it, yes built like a tank.  The only thing I don't like is that it only goes down to nF in the capactiance range, so I use a cheapo 40$ meter for the pF range measurements.  Other than that, its stellar.

~Phil

[PRR]

> Fluke 117 ... only goes down to nF in the capactiance range

It is sold as an "electrician's meter". Electricians rarely care about even nFds. They might look at uFd motor caps. pFds are everywhere (perhaps 900 of them in a typical run of power wire) so are not relevant to electricians.

Fluke probably has another model for "electronics". These days "electronics" may not run over 500V which is a zone tube-amp geeks need. Fluke makes too many meters for me to figure out. (They should make an app so your cell phone watches you at work and refines a suggestion based on what you are doing...)

[PRR]

Fluke 87V is more electronics-oriented. Goes to 1,000V which covers much tube-work. Hits 0.01 nF which is 10pF? (Meaning it may not know 9pF from 14pF, but probably will sort your 100pFd from your 220pF with resolution.)

I have a Fluke 83, very old and varnish-stained, and have never used all its features. Had to look to see if it did C (yes, to 0.01nF). NO idea how the 117, 87, 83 compare on price and stray features. 83 is clearly old enough you can get good-used on eBay, but a clean well-presented Fluke never sells far under retail price.

[pompeiisneaks]

The difference is in the price.  The 117 is 174$ whereas the 87 is 400$ BIG difference for a pF range :)

[PRR]

83 is also in the $400 zone, I see. (I got mine much abused and unloved.)