Cathode Resistor Biasing - Fender Hot Rod Deville

[samato]

I've learned quite a bit on this forum, enough to build my own AB763 from scratch, but I've been away for a while and forgotten a lot!  Actually, I don't think I ever really understood everything, I just learned enough to be able to follow instructions.

I have some very basic questions about biasing the 6L6's in my Fender Hot Rod Deville.  I want to use the Cathode Resistor Method, as I've done before on my AB763 build.  Looking at the schematic for the Deville has me confused.  It doesn't seem like I can connect a 1-ohm, 1W resistor between pin 8 and ground on these tubes without messing up the circuit that is already there.

The instructions I've found on biasing this amp tell you to just use the test point provided on the circuit board.  That is easy enough but this means I wouldn't be able to look at each tube individually, as I seem to remember doing back when I did my AB763 build.  I guess that would be fine if I was buying a new pair of matched 6L6's but I have a lot of tubes around and I'd like to choose a good pair from what I have.

Any help would be greatly appreciated.

[punkykatt]

Build one  of Doug`s bias probes.  I built two so I can see what both tubes are drawing at the same time. You will need two DMM`s though.

[samato]

I guess that would work.  I might be able to get my hands on a Weber bias probe.

Is that the only way to measure both tubes though?  Nothing that can be done without the probe and with just one multimeter?

EDIT:  Oh, I see.  With just one bias probe and one multimeter I could find out what's going on with one tube and subtract that number from the reading I get at the test point.  I guess I do need a bias probe then.

[punkykatt]

With one bias probe and one DMM, I would check each tube individually. Then there is no second guessing.

You also need to measure the plate voltage of each power tube to determine what % the plate dissipation is set at.

[samato]

If I understand correctly, the Hot Rod Deville has a test point where you measure the cathode current of both tubes combined (well, you're actually measuring voltage across a resistor but for the same purpose).  The only problem with this is I can't tell if the tubes are significantly mismatched this way.

So...

Can I just measure at that test point with only one 6L6 in at a time?  Once I find 2 tubes that are close enough I could do the final biasing with both tubes in.  Would this give me accurate readings?  I don't think it would damage the transformer since I'm not actually pushing signal through the amp, but I don't know?

[punkykatt]

If you pull one tube the plate voltage should go up. But if you are just trying to test currant draw of the tubes you have to find the best matched pairs, that should work. 

[samato]

Cool, that will work then.  Thanks.

I'll just find my matched pair that way.  Then I'll put them both in and get my true plate voltage and do the final bias.