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## [AMPS] G2DAF Circuit

 To: [AMPS] G2DAF Circuit W8JI@contesting.com (Tom Rauch) 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 volts. 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 peak). > ... 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 w8ji@contesting.com -- FAQ on WWW: http://www.contesting.com/FAQ/amps Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com ```
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