[Heathkit] Grounding the grids of 3-500Z tubes

[John King]

While I am not an engineer, I take advice from Engineers very well.

Several years ago, I had a 3-500Z in ONE of my SB 220 amps short and take out grid caps and chokes. I had NO original components to use, so I sought advice from W8JI AND Rich Measures without telling either that I was picking the brains of either. They were very friendly and helpful.

W8JI told me that he was working for Heath, as an Engineer, when the SB 220 was designed and disagreed with Heath's decision to use the caps and chokes on the grids of the SB220. He told me that Heath told him that they were using the caps and chokes on the grids BECAUSE BILL ORR W6SAI recommended that practice to limit IMD. W8JI told me he didn't agree with that practice at the time and did not when I talked to him. He recommended that the grid pins be grounded through a low value resistor  from each pin to ground. So each tube pin ended up with a 3 ohm 1/2 watt resistor to ground. One OR BOTH of the two Engineers told me that if there was some type of momentary arc in the tube that it would open the small resistor like a fuse and protect other components. As you may know, W8JI has been the design engineer for Ameritron and other companies and none of those amps I have seen have anything except wither direct grounding the grids or grounding through a
 low value, low wattage resistor.

Collins 30L1 seems to have started the cap/choke design with Heath, Omega, Henry (2K series, which I own one of)and Kenwood TL 2000 following the design. Yes, I know the 30L1 uses 811As but grounded grid design was just copied. My Amp Supply 3-500 amp was designed with directly grounded grids at the socket also. My GSB 201 MK III has directly grounded grids on the four 572B tubes. My experience has been very good with the amps using directly grounded grids, however I like the suggestion of using the small value resistors to protect against momentary shorts.

The foregoing is not my CLAIM to being an expert and isn't attempting to evaluate the engineers who have so kindly assisted me with information. The information supplied to me by BOTH was good and WORKED!!

Several years of use supports the SB220 grid grounding scheme with success. 73, John, K5PGW