It appears that a new filament transformer will be needed for the 3CX800A7
tubes as they are 13.5vac. If this is the case, then why not use the normal
high voltage transformer primary step start circuit with appropriate delay
timing and avoid the high voltage switching issue altogether?
73,
Gerald/K5GW
In a message dated 9/7/2005 6:44:04 A.M. Central Standard Time,
w2cqm@juno.com writes:
I'm retrofitting an Alpha 76A with 3cx800A7's. Eimac states
unequivocally, that "...heater voltage be applied for a minimum of 3
minutes before anode voltage and rf drive are applied...". Rather than
completely reconfigure the Alpha start up circuitry with their step
start and 1 minute delay sequence, I've decided to disregard that delay
function and add a 3 minute solid state delay relay and to switch in one
leg of HV from the plate transformer. Here's the questions: After the
delay,
1. Is it more prudent to switch in one of the HV AC legs before
the full wave bridge rectifiers or to switch in the B-?
2. Is a completely enclosed mercury displacement* SPST relay a
better choice than a standard P&B DPDT
relay reconfigured for SPST to accomplish the task?
* Can anyone familiar with the relative merits of a mercury
displacement relay explain its design?
Any guidance would be greatly appreciated. Ron W2CQM/3
PS: If I could ever locate the elusive SPST vacuum relay I have
somewhere in inventory, the questions would be academic!
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