Seems to me this problem may not be so troublesome. The dc
resistance of the hot filament is 7.5V/51A=0.15 ohms. If the B- return
is on one side of the filament, and if we assume the plate current is
1ADC, then the maximum DC voltage drop across the filament would be
only .15V
Furthermore, the filament emission is distributed across the
entire length of the filament and not just at one end, so the average
resistance is actually less than .15 ohm. What this means is that the
variation in operating bias from one end of the filament to the other
by using a switch mode power supply is at most a tiny fraction of a
volt. For practical purposes, I think it could be neglected, which
means a user would see no difference in performance, compared to
using a center-tapped filament transformer.
73,
Jim w8zr
Sent from my iPhone
> On Feb 6, 2020, at 3:52 PM, Steve Thompson <g8gsq72@gmail.com> wrote:
>
> DC switch mode PSU is not good with directly heated valves such as 3CX3000,
> 3-500Z etc. One end of the filament ends up with different bias to the other
> so all the emission happens at one end. It might be ok if you reverse the
> polarity regularly.
>
> With a transformer with centre tap, the average bias voltage is the same
> everywhere on the filament, so the emission happens evenly over the whole
> length.
>
> Steve G8GSQ
>
>> I have just acquired 2 x 3CX3000A7 and I would like to put one on 2m.
>> I have a few designs for but before I start I have 2 questions.
>> The Heater current is 7.5V @ 51A - does anyone use a switch mode PSU? Not
>> sure if Meanwell have anything suitable but that is where I would probably
>> start. Otherwise I have no idea where to get a custom transformer wound in
>> the Netherlands, which is where I live these days.
>> I do not want to run in AB2 all the time am I correct to assume that I can
>> just switch in a Zener with a pass transistor to bias to class C? I have
>> never used a zero bias tube before.
>> I could use a GM3SEK triode board of course!
>> I already have a 4.5kV supply that will drop down to 4.2kV on load, if
>> anyone is using a 3CX3000A7 on VHF I would also like to know how much gain
>> to expect. I do not expect the 144MHz performance to differ from that of 110
>> MHz based on observations with a YC-156.
>> Regards
>> Conrad PA5Y
>
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