I see a bunch of postings about getting audio out of old comm. sets,
proper bias, and so forth.  Many of these sets use a 6V6,
single-ended.  

You can get into a lot of folderol setting up a 6V6, and reading the
literature doesn't simplify things.  I'll cut through some of this
with my super-simple approach.  This assumes that B+ in the set is 250
volts (285 on the 6V6 is essentially the same), and that the output
transformer is around 3000-4000 ohms.  

For good hard core class A operation, set the bias around -12.5 volts.
Remember that this voltage is measured grid-to-cathode.  This will
give right on 50 ma. cathode current, of which 45 is plate current, 5
screen current.  This "10% Ik" for the screen is standard for beam
tetrodes.  Pentodes like 6F6 and 6K6 have a higher percentage in the
screen.  Audio drive should be 20 volts peak-peak, maximum.  With a
plain vanilla 8-inch speaker in an open box, that is a ton of audio
power.  That gives a range -2.5 volts to -22.5 volts on the grid,
which is about all you can put on a 6V6 without approaching the
near-cutoff distortion point too hard.  Note that a 6V6 is essentially
cut off at -25 volts on the grid, but is still conducting current at
-30 volts.  For a self-bias arrangement, 240 ohms is the correct value
to use for this current level.  A 25 mike bypass cap across the
resistor will give you low degeneration down to reasonable frequencies
for speech

Now, if you want to reduce drain on the power supply, reduce heating,
and so forth, you can bias the tube down as far as about -16 volts and
still run class A at modest power output levels.  This will still be
genuine class A operation at audio drive up to 10 volts peak-peak.
Anything between is a compromise, and generally gives quite acceptable
performance for the type of listening a communications set is used
for.  Push-pull class AB1 is generally set up with around -17 volts of
bias, giving Ik per tube around 34 ma., zero signal.  

Class B operation of a pair of 6V6's is with about -21 volts bias.  If
you run a single-ended tube that way, you are going to get
wall-to-wall distortion.  Remember that a tube does not cut off
suddenly, like a transistor, and as I mentioned, a 6V6 still has
transconductance between -25 and -30 volts, but it falls off so badly
that you are going to see tremendous lopsidedness in the output
current waveform between positive and negative current peaks.  

One way to linearize the output is to remove the cathode circuit
bypass cap, which essentially puts the transconductance of the tube in
series with the conductance of the resistor.  This is a standard AA5
gimmick, howver, you will want to remember that the transconductance
of a 50L6 is much higher than a 6V6, and the cathode resistor used is
typically 150-180 ohms.  The unbypassed resitance gives degeneration,
so you need enough additional audio drive to compensate.  The typical
6SQ7 drive circuit only has about 30 volts of headroom in the usual
zero-bias detector/avc/audio amp circuit, so if you need more audio
drive, you need to rig up something else.  

So far as voltage charts in Sams and Rider manuals go, I sometimes
wonder if these measurements were taken with a roulette wheel instead
of a voltmeter.  On a single-ended beam tetrode circuit, cathode
current should be between 40 and 50 ma. zero signal, whether the tube
is a 50L6 or a 6L6.  Things don't change until you get into the
807/6146/813 class.  If it is very far different than that, you are
going to get distortion and/or red-glowing plates.  

-- 
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Hank van Cleef 
E-mail vancleef@netcom.com or vancleef@tmn.com  
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