[Boatanchors] Audio drive needed for 813 (x 4) AM modulator

[GGLL]

Hello Chris:

chris at yipyap.com escribió:
> Greetings!
> 
> I'm still working on the restoration of the 1-KW AM transmitter
> I inherited from the former W6MSC  (see http://www.yipyap.com).
> 
> The modulator uses four 813 tubes, two pairs each side of a push-pull.
> There is an audio transformer, but no other active devices in the
> audio chain before the 813s.  So I will need to provide whatever
> audio drive is necessary by other means.
> 
>>From my RCA TT book, it says the grid circuit resistance is 30k ohms max.
> And the peak AF grid#1-to-grid#1 voltage is around 180.  (is that the 
> total swing of voltage or the positive peak?)  Assuming thats the

Total swing

> total swing, I'm calculating about 1.1 watts drive per tube.   I need to
> have an audio amp will will put out somewhere around 5 watts.  Since
> volume controls come cheap a little extra would be better, maybe 10 watts.
> 
> One line in the RCA TT book has me puzzled, it says
> 
> "Maximum-Signal Driving Power(Approx.)   0 watts."
> 
> What does that mean?

You are working them AB1 class. There's no grid current at any point of drive 
cycle, no power consumption to drive the tubes, but voltage; you must be sure 
to obtain the proper voltage swing to drive them accordingly.
The "power" is that from resistive losses in the interstage transformer and 
grid circuit. Also, if you overdrive somewhat your output stage, a voltage 
swing past the operating bias then there will be a certain amount of grid 
current (you step into AB2 class, you'll obtain more audio power output but at 
the expense of more distortion). So in this case, also to resistive losses you 
add tube's driving power (plus a safety margin) to the amplifier drive power 
requirements.
Take in care the impedance ratio of your transformer.

Best regards
Guillermo - LU8EYW.

> 
> Chris
> kc0atc
> chris at yipyap.com
> 
> 
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