Stupid bias question

[HiGain]

Hi all,

Here's a question from someone who is armed with just enough information to be a danger to himself.

Question is, if I connect both power tube (6V6 in this case) cathodes through a single bias 1 ohm resistor do I double the mV (mirroring the current across the resistor) reading on the DMM?

Thanks, and happy holidays.

Jake

[DonMoose]

Yup. Or, use a 0.5 Ohm Rk to get the same voltage for doubled current.
Just remember what choice you made. (Write it next to teh bias pot or measurement terminals).

Hope this helps!

[martin manning]

If you want to measure average current use a 0.5 Ohm like Don says. If you don't have a 0.5, parallel two 1 ohm resistors.

[Phil_S]

The problem with using a single resistor is that you will read the sum of the two tubes and the measurement will tell you nothing about either tube individually. If you are looking to see if the tubes are reasonably well matched, you don't get the answer. So, for the extra few cents it costs to use two resistors, the quality of the information rises probably 100-fold.

You'll see debates about the merits of matched pairs, so I don't want to get into that. What I don't think you'll see arguments about is that a significant mismatch isn't desirable and will often manifest as hum.

[HiGain]

Thanks muh, guys!

Just making sure ohms law still exists!

Btw, this is wiring I did several years ago, and involves a fixed/cathode bias switch. I don't think I could have had individual bias resistors because of the cathode bias network, but I probably didn't consider it either.

Regards,
Jake

[martin manning]

You can certainly make it work- bring the cathodes to a common point, each through it's own 1R, then switch that point either to ground or to the top of the Rk.

[HiGain]

Great suggestion! Thanks again.

Jake

[DonMoose]

With only a single bias control, if you find a mismatch, your only recourse is a frenzy of tubeswappy, though. Assuming you have a bucket full of spares.

I'd think anything much under 20% mismatch is a 'live with' in that case.