I received a lot of interesting posts when I asked about repairing a grid
bypass choke fire on a customer's SB-220.
The most suggested fix I received was to strap all grid pins to chassis
ground, after removing all of the 200 pf bypass caps and the two 1 uh RF
chokes.
Another fix was to replace the 200 pf bypass caps with at least 500 pf
units, and replace the RF chokes with fusing resistors.
Absolutely nobody suggested returning the circuit to original factory specs.
On the far-end of the spectrum from direct-grounding of the grids has to be
the Henry 2K-4. No RF chokes are utilized, but there are 18- .003 mfd bypass
caps, 12- 10 ohm resistors, and a 250 ohm grid ma adjust potentiometer, all
hanging from grid pins to chassis! Grid pins (pin #4) of each tube are
strapped together. Pins 2 and 3 on each tube are strapped together, but not
strapped tube-to-tube. Also, Henry chose to use 2- 150 ohm resistors in
parallel with the coil on the parasitic suppressors.
The question of the day is why Henry chose a more expensive approach than
any of their competitors? I would love to hear the reasoning and experiences
when they built up the prototype of this amp.
The direct-grounding camp all reports no problems since their mod. and swear
by that solution. The other three solutions speak for themselves, as there
must be thousands of 3-500Z's operating in the field with long track
records. The problems seem to increase when replacing Eimac tubes with
knock-offs.
It is obvious to me that some kind of grid fusing should be included in any
modification one chooses. Using an RF choke (Heath, Kenwood, et al) for a
grid fuse can result in a hole burned in the grid, and a choke fire.
I just wanted to pass along my experiences FWIW. Mods seem to be in order;
the choice is up to you!
(((73)))
Phil Clements, K5PC
