[Amps] Amps Digest, Vol 90, Issue 2

[Jim Garland]

> 
> 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