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[AMPS] G2DAF Circuit

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Subject: [AMPS] G2DAF Circuit
From: (Tom Rauch)
Date: Thu, 26 Oct 2000 23:24:59 -0400
> >Can you tell me how you got that number, for a zero bias 4-125A 
> >with a voltage doubler hanging off the grid driving the screen at 
> >some unknown voltage?
>    The original article states that the amp required 25 watts of drive. 17
> watts, if I recall correctly, went into the 300 ohm resistor and the
> balance went into the two grids.

Table 5 on Ian's web page says the drive power is 30w PEP in a 
single tone test with the 125's. It also says RMS grid voltage is 65 

65 v RMS grid voltage across 300 ohms is 14 watts PEP in a 
single tone test.

It appears the data for drive power or grid voltage is wrong.  
>    I also took the Eimac 350V charts for the 4-125A and scaled them
> according to the 3/2 power law by a voltage factor of ½, as per "Care &
> Feeding... 1967 edition" p.98, just for a quick check.  This gives a chart
> for 175V screen V which should be very close to the voltage on the screen,
> at the peak of the SSB waveform, in the DAF amp at full drive -- i.e.
> voltage doubling the peak drive V (100V), less some drop in the two
> thermionic rectifiers; close enough for ham discussion.

Table 5 claims screen voltage is 105v in single tone. I assume 
G2DAF measured the voltage with a properly working meter. 105v 
seems more reasonable to me, from what I recall of tube type 
rectifiers given the voltage across the resistor is 65v RMS (92v 

>  ... with 175volts on the screen the 160V (chart) line will now be the 80V
> value and the current will be .35 of that corresponding 80 mA
> (interpolated chart) grid value, which is about 30 mA.   Figure 60mA for
> two tubes which is 1333 ohms with your numbers.  Close enough for radio
> amateur work.

I'm missing something here. That's the instantaneous grid current.

The constant current characteristic charts I have only show control 
grid currents in the range of 30-40 mA at that grid voltage.

Table 5 claims dc grid current is 70 mA, which would be a hell of a 
lot more than 60mA peak grid current.  
> >With less screen voltage, control grid current is much higher in a
> tetrode.
>    That is NOT a general rule.  Some of the larger tubes run the other
> way, plus there are the secondary emission effects which come into play in
> the positive grid V region.

Table 5 seems to indicate they are driving the pants off the grid. G2DAF claims 
70 mA grid current.

>    From the above example, it doesn't appear that the control grid
> current vs voltage of the 4-125 changes significantly with a variation in
> the screen V.

Not according to what I see.
>    The doubler takes about 5 watts.  The details are all in the article on
> Ian's website.

Screen current is 38mA. Screen voltage is 105 volts. That's 4 watts 
at the screen, with another 3 watts in the rectifiers (assuming they 
are the major power loss). Of course peak current will be much 
higher than 38mA since it is a voltage doubler and capacitor input. 

Shunting a 300 ohm grid resistor with a voltage doubler that 
delivers 38mA of current (the doubler probably draws 200 mA or 
more at peaks) is a bad idea.
>     ... and of course the impedances of both the control & screen grids
> are going to vary with the applied screen voltage but, I don't care enough
> about this "project" to figure those numbers at several points. The
> maximum was sufficient for the input circuit loading discussion.

Thanks. It's enough to convince me the grid circuit has a lot of load 
resistance change over the RF cycle. Looks like on the peaks the 
grid circuit resistance drops quite a bit.

The high grid current is worrisome. It looks like the circuit really 
overdoes the control grid. 
73, Tom W8JI

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