[AMPS] G2DAF Circuit

[Tom Rauch]

> All the proponents of grounded grid are violating Rich's Rules - that the
> voltage on the screen grid of a pentode must have constant voltage to
> produce an acceptable signal.

I prefer triodes in grounded grid, although some tetrodes work OK 
in GG.
 
>  Yet, in a grounded grid amplifier, the screen grid voltage varies with
> drive - just as the control grid voltage varies.  It is not zero volts
> simply because the screen voltage is grounded.  Heavy grid and screen
> current flow - contrary to Rich's Rules.

Heavy screen and grid current do not flow, unless the operating 
parameters of the tube require it to be so. It is quite possible to 
have a cathode-driven tetrode with zero mA of grid current, and a 
wide range of screen current.

The thing I keep stressing is the linearity of the screen voltage, and 
the phase of any variation in screen voltage (the same would be 
true of grid bias).

If the tube is cathode-driven, the phase of the change in screen 
voltage is exactly opposite of drive voltage. It exactly tracks in 
phase, it's just always very close to 180 degrees out-of-phase. If 
the screen voltage is reasonably high compared to the cathode 
voltage swing, and if the phase is correct, all we have is 
conventional negative feedback of limited voltage. Negative 
feedback is generally a very good thing to have.

What we don't want is grid bias voltages following the envelope 
(rather than the individual RF cycle) at a low frequency (audio) rate, 
especially if there is phase difference. The DAF circuit does 
exactly what we don't want the system to do.

The saving grace of the DAF circuit is the heavy negative RF 
feedback (from cathode drive) helps correct for the "half-fast" 
manner the circuit developes screen voltage.

> And, in grounded grid, the tetrode or pentode tube is being used in a
> manner other than how the factory intended.

So what? The "factory's" circuit recommendations are just typical 
ideas, not the law. They offer some typical applications along with 
some guidelines on limits. There is a considerable degree of 
flexibility depending on end-use.
 
> Lastly, many commercial sweep tube amplifiers put out a  -22 db distortion
> figure on a good day, if operated according to the factory.

And you know that because?

The 6LQ6/6MJ6 Ameritron sweep tube PA measured around -33 
dB below one tone of a two tone test for third order. That would be -
39 dB according to ARRL standards. 
 
> Using the same reasoning as Rich, may I proclaim that all grounded grid
> ampllifiers which use tetrodes are bad since I have measured a -20 db
> distortion level of a commercial grounded grid amplifier?
 
What amplifier measured that bad? It must have had problems.

The Ameritron used a 22 ohm cathode negative feedback resistor 
to clean the PA up and reduce gain. The Amp Supply was a bit 
worse because it used a system that tied the grid back to the 
cathode, but that was an engineering flaw copied from a west coast 
Handbook.

Even so, the Amp Supply (and Dentron) PA's were about 8 dB 
better than the -20 dB (below one tone of two tones) that you 
measured.

Does the fact you measured one bad sweep tube amp suddenly 
mean the DAF is considered good engineering? (I don't think so.)


73, Tom W8JI
w8ji@contesting.com

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