On Sat, Mar 1, 2025 at 05:48 PM, Fran?ois wrote:
?
The removal of the 50k + 50k +5k bleeder resistors worries me.
1.???? It quickly makes the high voltage disappear
2.??? It regulates the high voltage a little
3.??? It avoids stressing the amplifier with a no-load HT which will be equal to the peak voltage
?
Here are the measured data:
?
With the bleeder resistors and a zero anode current I note:
·???????? CW: 1500 V DC
·???????? SSB: 2500 V DC
At the secondary of the transformer, depending on the transformation ratio and at no load:
·???????? CW: 230 x 2.95 = 678.5 V (960 V peak)
·???????? SSB: 230 x 4.14 = 952.2 V (1347 V peak)
?
The high voltage at no load, without bleeder could be (estimated):
·???????? CW: 1920 V
·???????? SSB: 2694 V DC (we are therefore 200 V above the voltage without bleeder - Is this serious ?
Francois,? I have owned the? 1st L4B since 1977...and bought 3 x more since then.? ?The 1st? thing done to? any? L4B or L7? is to grnd the grids.? 2nd thing done is TOSS the 50k + 50K + 5k? 'bleeder'.?
I was expecting the B+ to increase with their removal,? ?but the B+? DID NOT BUDGE !? ? 2650 vdc? on SSB.......and? 1900 vdc on CW...same as before.? ?Removal of the 50K+50K+5K had? NO effect.? ?Drake used the scheme simply to obtain +120 vdc / 90 vdc? for their ALC circuit.?
There are NO ham amps that use bleeders.? They all use a? single 50k / 100k? across? each electrolytic.? ?Who cares how long it takes? ?for the B+? to drop to zero ?? ?There are a ton of screws on the top lid...+ a HV interlock.
B+ comes down to zero after 1 min.? ?Plenty good enough.?
Put 10 x 1N5408 / 6A10? /? PM-600? diodes in series? on some perf board.? ?It goes in series with the CT of the fil xfmr.? ?UN-banded end of the diode string faces the CT of the fil xfmr.? ?This will reduce the idle current down to 100 ma? on the 2650 position......and 40 ma on the 1900 position.?
A? 50k to 100K? ?@ 2-3 watt? MOF resistor is wired in series with the new diode bias string.? ?Re-wire contacts of? the center pole of the TR relay? to shunt the resistor on? TX.?
Done.?
Jim? ?VE7RF
