>
> Hello,
>
> As you may know from my prior post, I have a homebrew 6m 3-500Z amp that I
am trying
> to get to work with a Drake L-7 power supply, so far without success. The
amp has an
> internal power supply, with the HV disconnected. I have the B+ of Drake
p/s connected
> directly to the same spot where the B+ from the internal p/s had been
connected, and I have
> the B- of the the Drake p/s connected to chassis ground.
>
> Using a high voltage probe, today I verified that the Drake p/s is
providing 2800 vdc
> directly on the plate of the 3-500Z, when everything is turned on but the
PTT is not
> engaged. But when the PTT is engaged, even with no drive, plate voltage
immediately
> drops to zero.
>
> My first thought was a B+ to B- short, but this would have caused a big
bang, and nothing is
> banging or smoking or doing anything unusual at all. No fuse or circuit
breaker pops.
> When the the PTT is pressed, a DPDT relay closes. One side activates the
T/R coaxial
> relay, and the other side grounds the center tap of the filament
transformer via the plate
> current meter. These are the only things that happen when the PTT is
engaged. In the
> absence of a filament to plate short (which would have self-diagnosed with
a bang), I can't
> imagine any reason why pressing the PTT would cause the plate voltage to
drop to zero.
>
> Anyone have any idea at all what might be going on? It almost seems like
the Drake p/s is
> shutting itself off when the PTT is engaged, and then turning itself back
on when the PTT is
> released.
>
> Thanks for any thoughts and 73,
> Bill NZ5N
>
Bill, it is a bit hard to diagnose your problem, absent a schematic diagram,
but I very much doubt your HV supply is "shutting down." More likely, you
have not isolated the B- and GND returns properly in your hookup, so the
multimeter in your amplifier is not measuring the B+ correctly. In Drake
amplifiers, the cathode current returns via a B- line which is separate from
chassis GND. When you hook up your Drake power supply, therefore, it is
necessary to make sure the B- connection is properly tied into your homebrew
6m amplifier. It is not adequate merely to tie the chasses of the power
supply and amplifier together. If I had to guess, I would venture that your
cathode current is being returned through ground. This would lead to
improper biasing of the 3-500Z (as evidenced by too high a quiescent current
when the amp is keyed up) and improper grid current readings.
And -- not the least -- it is also unsafe. Be very careful when you
experiment with your hookup, until you thoroughly understand what is
happening. My recommendation is to draw out a schematic diagram of the
amplifier cathode and meterting circuit which shows how your amp is
connected to the power supply, and trace out the B- path explicitly.
73,
Jim Garland W8ZR
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