> >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
> 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
FAQ on WWW: http://www.contesting.com/FAQ/amps
Administrative requests: amps-REQUEST@contesting.com