[Amps] Amp plate voltage drops to zero when PTT pressed, help please

[Bill Dzurilla]

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