More Single 6V6 Speech Amp Driver: Bias

[W9ZSL]

I've started doing a chassis layout for the 6V6 speech amp/driver I've been discussing for some time now.  The plan is to run the 6V6 into a UTC S-8 with maximum output of about 4 watts.  The same module would be used to drive either of two modulators: P/P 5933s (maybe 807's) into a UTC S-20 or P/P 4-65A's into an S-22.  Then I got to thinking about bias.

The lower-power modulator, if using 807's at 600 volts, and that is close to what I have with an existing supply, calls for a grid bias of -34 volts and 300 on the screens for 56 watts output.  The 5933's are similar.

Then I dug into the 4-65A combo and found that with 2KV on the plates and 250 volts on the screens, grid bias would run around -35 volts!  Seems to me I can include a bias supply on the speech amp/driver which would work with both modulators.  Screen supplies would be separate for each.  In any case, I can build an independent bias supply into the driver feeding the center tap of the S-8 secondary and with any luck, the bias for both sets of modulators, according to specs, seems to be the same.  Of course, I never build a bias supply that I can't adjust.

In the end, I'll have to experiment, but is there any reason why I can't use one bias supply for each modulator?  It would be switched, and since the S-8 can be either 2:1 or 3:1, that would be switched as well.  The center tap of the secondary would be connected to the bias supply and if any tweaking might be necessary to get the voltage right, no problem.  I would think that might not even be necessary since the grid bias of each modulator would be nearly the same.

[KA2DZT]

Adjustable bias supply is simple to build.  Build about a 45-50 volt supply with a small xfmr.  Ground the positive side.  You need to put a bleeder resistor on the supply, maybe 35-40 ma of bleeder current.  The bleeder resistor can be a pot in series with a resistor (of some value to ground).  The pot can be adjusted to control the bias.  The resistor in series with the pot prevents turning the bias all the way to zero (ground).

Fred

[N2DTS]

Or, you can use a small variac and have the ability to vary the bias 0 to -100 (or whatever) volts.
Make a good driver and it could be used with almost any modulator tubes.
You can do the same thing with the filament transformer as well, 0 to 10 volts at lots of current.

I have a variac on my push pull parallel 100th mod deck so I can run 1000 to 3000 volts on it and still have a stiff bias supply.
If you draw grid current, you want a stiff bias supply.

A delux rig can get very complicated and busy with adjustments and meters....

My stuff tends to be universal. I can plate or screen modulate various rf decks with various modulators, I have 4 rf decks and 6 modulators, 2 rf control decks, 2 exciters, and 3 high voltage power supplies.

I can hook anything up to anything.

Variac's are nice because you are not stuck with a voltage, you can try other voltages, and you can turn things down for tuning/testing.


You can also go the other way and make something simple and fixed (and much smaller).

[W9ZSL]

That's basically what I'm saying.  I already have a bias supply good for 3-120 volts, pot adjustable.  since both modulators use nearly the same bias (34 & 35 volts), I can add it to the driver chassis, connect to the secondary center tap of the S-8 and away we go.