Tube Bias

MillerLiteScott

I do not know what to set my multi-meter on to read the tube bias?

Also which inputs do I put the probes in the multi-meter?

Picture below.

The biasing instructions say 330mV +/- 5%.

Thanks

MLS

Ricardo

Black probe in Common.
Red probe in V
You should select the DC Volts; I think it's the black one (looking at the scales), but I'm really not sure; the manual of the multimeter should tell you.
You need to measure 330mV, so select the "2".

Edit: when you're ready to roll some tubes, try the JAN-Philips 6L6WGB..they are expensive, but the difference is worth it. If the AV-25 is like the AV-20 (which I think it is), you can also use EL34's, and I think KT66 can also be used. Lots of possibilities.

Edit 2: not sure how many decimals that multimeter will display when you select 2 in the scale; if it displays only one decimal, you'll be limited to 0.3 vs. 0.33; if this is the case, maybe for $20 or so you can look for one that can measure more mV's (vs. 200mV max on that one).

And yet one more edit...I see you have the monoblocks with the M5's as the "budget 2 channel"; maybe it is, but music will sound so much better than in your RTi rig; you'll love it

MillerLiteScott

Thanks HTrookie.

That was it and I have both tubes in the one amp at 334 and 336.

The manual says to wait 10 minutes and check it again.

At the "2" position it was .334 Is that right?

MLS

mhardy6647

.334 VDC is 334 mV. Sounds fine to me.

MillerLiteScott

Thanks

There is a sheet of paper that is for the Wave AV-20 that says 330mV and in the manual for the Wave AV-25DT which is what I have says 350mV.

I ave both tubes in both amps set at @ 341-342mV after being on for 10 minutes. If I remember correctly Mark said that if I set them anywhere @ 330mV-350mV I should be fine.

I am going to give them a try in about an hour after the kids finish watching
A Bugs Life.

mhardy6647

after the kids finish watching A Bugs Life.

A perfect time interval to let an amp warm up before checking the bias :-)

dorokusai

You're ok Scott, that's perfect. As aforementioned, check them again after the movie but you should be fine after that. It's always good to recheck them every now and then.

After the first few rounds you'll have a better perspective on what interval to check bias in the future.

MillerLiteScott

What does the bias(ing) affect?

audiobliss

The voltage going to the tubes, no? I'm not sure, either...

RuSsMaN

MillerLiteScott wrote:

What does the bias(ing) affect?

In a nutshell, it is the negative voltage applied to a tube's control grid which allows the tube to stay within its operational parameters for your specific amplifier. Too much bias, the amp will sound like poo. Too little, tubes can overheat and fail, taking output transformers and other parts in the circuit with them.

Cheers,
Russ

mhardy6647

We ask a tube to amplify an AC waveform, which (typically) varies both above and below zero volts. The problem is, electrons are always only negatively charged. The bias shifts the zero point of the waveform so that the tube can linearly amplify both the "+" and the "-" component of the waveform.

http://www.pcmag.com/encyclopedia_term/0,2542,t=bias&i=38587,00.asp

MillerLiteScott

Thanks.

To much bias and sound like poo poo.
To little is bad.

MLS

dorokusai

If you haven't seen a tube flare up, glow fire engine red/orange and then POP!....you haven't lived.

pblanc

Sounds like cooking hot dogs over the campfire in Boy Scouts. How do those popped tubes taste?

dorokusai

Crunchy & painful with a hint of terragon.