On Sun, Jan 5, 2025 at 08:42 AM, Fran?ois wrote:
I have drawn a simplified diagram of the L-7 power supply. I have drawn in red an additional bridge to establish to prevent the cathode from being isolated.
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In emission, the ammeter is shunted by the 5000 ohm resistor; this will have no impact on the measurement.
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Do you confirm this modification?
You still have it wrong.? ?The 2 x 50K? @ 50 watt? wire wound resistors in the L7PS are removed.? ?The 5K @ 7 watt resistor below the chassis? of the L7PS is also removed.?
(Originally, Drake used the +120 vdc? Voltage drop across the 5 k resistor to cut the tubes off on RX. ) This modification alone will reduce the heat in the L7PS? (66 watt CCS load removed).? 66 watts continuous.? The top lid on the L7PS will now run cool.? Each of the 8 x 200uf @ 450 vdc electrolytics in the L7PS already have a 100K @ 2 watt Carbon comp resistor across them.? ?Those are called equalization resistors? (EQ resistors)...used to equalize the V drop across each electrolytic.??
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These days, to cut the tubes off on RX, a 50k to 100k @ 2-3 watt? MOF resistor is wired into the Center tap? of the fil xfmr.? ?(The CT of the? fil xfmr has the yellow stripe).? ?Re wire the center pole of the 3PDT? TR relay...so the new resistor is in the circuit on RX.....and shorted out on TX.? ?The V drop across the new resistor? will cut the tubes off on RX.? ?On TX,? with resistor shorted out, the tubes will draw normal idle current.
Note, Drake did not use any bias at all on TX....and idle? current was very high ( 210-230 ma).? ? Wire 10 x 1N5408 diodes in series....... in series with the CT of the fil xfmr.? ?The 10 x diodes will provide for 7.2 vdc of bias.? ?This will reduce the idle current down to 100 ma.?
The 10 x diodes? are wired in series with the new? 50k - 100k? bias cut off resistor.? ?It's an easy modification.?
For safety, wire reverse connected diodes (1N5408)? between? B-? of? HV filter caps.....and chassis.? ?This will protect the plate and grid current meters.?
Does this all make sense ??
Jim? VE7RF
