The AM Forum

Howdy the forum,

I have a recently acquired but well worn GK500C that has low output, and I am hoping that someone can lead me to the next step in troubleshooting. The following is what I have measured so far (all readings under load):
- Input voltage 115 VAC per manual
- CW Mode
- 3885 KHz
- Dummy load with 1:1 SWR
- Power meter Mirage MP1, checked against another PM of different make
- 4-400 final tube
- Plate voltage 1700 VDC
- Plate current 290 mA
- Final grid current 15mA
- Final  screen current 38mA
- Input DC power to final calculated as 493 Watts
- Measured output power 250 Watts

This is only 50 % efficiency. I have another GK 500 that does much better. I assume that the 4-400 is good since it loads to the book values. What do you suggest I try next? I will  appreciate any help.

Jim

Jim,

  It has been a while since I've run a GK 500. Seeing  the screen current at 38ma suggests to me that the RF tuning is lightly loaded.

I'd look at the metering circuits first. First check for accuracy. Is the 'Plate Meter" readout actually cathode current? If it were, then you are only running 400 watts DC plate input, and your efficiency is around 62%.

Two things I remember about the kings is that the final RF tube sometimes runs at "brown-out" filament voltage due to voltage drops in connections, undersized filament transformer(s), the socket, and tube pins. I used a bigger transformer on mine with a tiny 1A variac in the RF deck to set the filament voltage.

I also recall that the 4-250's I had were always better performers than the 4-400's I tried. My theory was that the average distance of a 4-400 plate from the filament was greater than that of the 4-250. So the 4-250 had an advantage at lower plate voltage.
Maybe I had a bad lot of 4-400 pulls, and good 4-250 pulls, and my theory is all wet..

Jim
WD5JKO

Definitely the 4-400 0r 4-250 filament voltage on the actual tube filament pins.

Just went through this with Carl WA2UJX several years ago.

I checked the schematic for the "plate" meter, and it is in the cathode circuit. So, that explains that the rated power input is 540W with 1800V at 300mA plus the screen current of 30mA. They recommend loading to 330mA.

I will definitely check the filament voltage for 5.0VAC when I get time to pull the chassis and rig some test leads.

Thanks for the suggestions.

Jim W5COA

Just for grins, pull the final out of the gooder GC500 and see ifn you get the higher power outta this one. If so you found the answer.