Question on validating large amplifier metering.

GM List:

I am in the testing phase of renewing a basket case Amp Supply LK-800 ISM version. This is the one with the 17 position band switching and the external 3KV at 1.5Amp transformer package. It came to me with no docs and apparently after a meltdown of some type. Partially disassembled. I have recently gone thru several LK-800s but this one is giving me fits. Making low power out but showing very small amounts of grid current.

The series scaling resistors for both Plate and Grid meters were way off value and the main shunts in the floating B- type scheme were different than the normal values in the LK-800. So I set about trying to evaluate whether the meters were showing the true values.

If I use an external controlled current source and an accurate mA meter, I pass a metered current thru each shunt with the amp cold. Both Ip and Ig meters read properly now using this external current source. When I run the amp live, into a dummy load, I can easily drive it to an amp indicated with 50-70W from the exciter but output power is well under what that amount of plate current should produce. Also, while I see only a tiny amount of grid current, it peaks on plate resonance but is nowhere near what it should be. it might be 10-15mA at best. Two differnet known good tube sets react the same.

My question is basically is the method of externally testing the metering valid or am I missing something?

The analyzer sees what I expect of the output pi net. It just looks like the metering is not indicating properly when operating but shows correctly when the shunt alone is passing current.

Any thoughts? Thanks,

Curt
KU8L