Re: Bias for 6L6?

On Sun, 10 Jan 2010 22:10:54 -0500, "Phil_S"
<psymondsnospam@xxxxxxxxxxx> wrote:

Well, fellas, looks like those pulls from the Sears organ chassis are going
to be the ticket. I've got them in the amp. Va=~360 and Ia = 61mA on one
tube and 62.4mA on the other tube. I'm making this for about 22W per tube
or 44W for the pair. Grids are about -26V. I can move grid voltage down to
almost -22. Where do these "belong"? The OT is a Stancor A-3801, the
fabled Trainwreck iron, 6600 ohm primary.

Phil, It sounds like you have those biased hot. The lower B+ will
allow you some slack, but 61ma is still warm, and -26v is down around
EL34 territory.

Screen voltage is around 350V.

mA are calculated based on the voltage drop between the plate and CT of the
OT. I know the Ohms on each half of the winding. I'd call this the
modified shunt method.

You may have some error in that measurement. Why not just go with the
conventional 1 ohm or 10 ohm resistor in series with the cathode?
You'll be able to approximate screen current easily, so that shouldn't
be a concern. (That's the major difference between plate current and
current through the cathode resistor)

You'd also be able to put pin jacks on the back of the amp for
monitoring cathode current without worrying about high V or shorts.

I freely admit that I'm too chicken to put the meter
in amps mode. Last time I tried doing it, I slipped and it cost me both the
OT and the PT. I had tip jacks installed in the chassis and darn if I
didn't slip while putting the probe in the tip jack. Some things are very
unforgiving and defy idiot proofing.

See above.

I have the idea from looking at the RCA Receiving Tube Manual that a pair of
6L6GC might be happier with a more mA. But all the tabled data show a big
spread between plate voltage and screen voltage.

Thanks.

Phil
.

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