Bias measurement

[Paul P]

While reading up about bias adjustments I've come up with a
couple of questions :

Instead of putting a one ohm resistor between the cathode and
ground to measure the bias current (which is not quite accurate
due to the screen and the resistor itself) why not put it in
series between the plate and the output transformer ? Wouldn't
this be more accurate ?

Can I put an ammeter (a panel meter) in series between the plate
and the output transformer and get a direct reading of the current ?
Why does no one suggest doing this ?

Thanks.

[Enzo]

It would work, but then your meter, the probes, and anything else involved would be sitting at 500 volts wuth respect to ground. In the cathode leg, things are not even a volt away from ground.

Also, if you ran wires from that point to somewhere else such as a test point, you would have high voltage wires radiating signal all over just itching to get back into the prior stages and make instability.

It is simple enough to either check how much screen current is going on - simple math - and subtract it. Or just subtract a couple ma as a rule of thumb and get on with your life.

If you use the "shunt method" of measuring tube current, you are essentially doing as you suggest - your ammeter is in series between the tube and B+ supply.

No one does it that way because it is less safe, not really that much more accurate, and it is more complex. It si not like the bias being off by 2ma will have a serious effect on anything.

[Paul P]

Can I leave the 1 ohm resistor in place ? If it doesn't affect the sound in
any way I'd just as soon leave it there.

Say a 1/2 watt 1% metal film resistor ?

[Enzo]

I see I missed part of your initial question. You could mount a panel meter, but you would have to make sure the meter was OK with 500v to ground on it. Don't want tiny meter movement parts arcing to their frame. But again, it would require you running high voltage wires with very large signal levels on them out across the amp to the front panel. Just asking for instability again.

Can you leave a 1 ohm resistor in place? You can put a 1 ohm resistor just aboput anywhere you want to and it won't affect much of anything. it is only 1 ohm. Now if that resistor is across the amp somewhere, we are back to having wires flying around in there trying to become oscillators.

There are higher end hifi amps with meters, but I don't think they measure in the plate leads.

Go visit www.eurotubes.com and see Bob's 5150X with meters.

[Bruce / Mission Amps]

Paul the screen current at idle is so small compared to plate current that it is hardly worth the hassle and putting yourself in harms way by messing with a resistor in the plate supply.
Except in the case of high voltage EL34s, it is very seldom higher then an miliamp or two so just remember that when you bias your tubes using the 1 or 10 ohm cathode method.
You can always put a screen resistor on the tube socket and measure the voltage drop across that.
Use Ohm's law of, Voltage/Resistance = Current, and see what the tubes you commonly use draw and then keep it in mind when checking other tubes.

[Paul P]

1 ohm resistor specs ?

What sort of 1 ohm resistor should I use ? (I'm going to leave them in
the amp). I've seen mention of 1 watt, 2 watt and the remark that
1/2 watt is enough. Precision resistors from places like Mouser are
5% above 1/4 watt. Is this accurate enough ?

I did find the following site which appears to sell very precise 1 ohm
resistors from 1/8 to 10 watts:

http://www.micro-ohm.com/power/pwrsr.html

They don't list the prices but I only need 2...

[Shea]

What sort of 1 ohm resistor should I use ? (I'm going to leave them in
the amp).

1% metal film. I think the ones I use are 1/2 watt.

Precision resistors from places like Mouser are
5% above 1/4 watt. Is this accurate enough ?

Well, your statement isn't exactly accurate. Look at Mouser part no. 660-MF1/2CC1R00F.

I did find the following site which appears to sell very precise 1 ohm
resistors from 1/8 to 10 watts:

http://www.micro-ohm.com/power/pwrsr.html

They don't list the prices but I only need 2...

They probably won't sell just two.

Shea

[Paul P]

Well, your statement isn't exactly accurate. Look at Mouser part no. 660-MF1/2CC1R00F.

Your search skills are better than mine

I find Mouser hard to navigate.

Starting from your search I also came up with 1, 2 and 3 watts at
1% and even some at .5%

Thanks.

[Shea]

Your search skills are better than mine

I find Mouser hard to navigate.

Starting from your search I also came up with 1, 2 and 3 watts at
1% and even some at .5%

Thanks.

You're welcome! By the way, I think it's a lot easier to find parts in Mouser's print catalog than online. I had to refer to a print catalog that I keep in my office to find that for you.

Shea

[Enzo]

I think it is almost alwats easier to find parts in a catalog than online. Online searches pretty much expect you to have a pretty good idea of what you are looking for. In a catalog search, I can flip the pages until I see the type thing I want. Plus I often see other things I didn't know about, and I would never have learned of them in an online search.