Tom Rauch wrote:
>> ? In the DAF circuit, it seems to me that there is no RF neg. feedback.
>> Am I missing something, Tom?
>
>Yes, or maybe I am.
>
>The G2DAF circuits I've seen might have been modifications of the
>original design, but they were all cathode driven with screen voltage
>robbed off the exciter power.
If it's cathode-driven, it isn't a G2DAF - it's somebody else's even
weirder creation.
I can see why someone might want to try the G2DAF idea to modify a
typical cathode-driven "triode-connected tetrode" amp that has both
grids grounded to chassis; but G2DAF himself never did that.
The original G2DAF amp was grounded-cathode, grid-driven (across a 300
ohm resistor) with a voltage-doubler (2 * 6U4GT) to generate the screen
voltage. G2DAF's stated operating condx for two 4-125s at the UK power
limit of 400W PEP output are:
Single-tone Two-tone
Va = 2500
Ia 250mA 175mA
Vg2 105 75
Ig2 38mA 22mA
Vg1 = 0 (no DC bias)
Ig1 70mA 45mA
RF in (PEP) 30W 35W
Note the screen and control-grid voltages. These are so far "off the
map" for the 4-125 that you're completely on your own.
Note also that the input loading resistor was 300 ohms, not 50, so the
original G2DAF design was only suitable for tube exciters with a
flexible pi-tank output. Some people have modified the circuit for a 50
ohm load using a step-up input transformer, but that may not develop
enough screen voltage, especially at 30MHz where stray capacitance may
be a problem.
Also the efficiency of the RF rectifier-multiplier diodes may become
poor at higher frequencies. Many people have used more modern rectifier
tubes, but a low heater/cathode capacitance would be critical.
All in all, it's probably quite difficult to develop enough screen
voltage. In that case, there's a very strong temptation to increase the
RF input to get more screen volts, which grossly overdrives the control
grid... with the results that Rich, Tom and others have noted.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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