[Amps] TL922 mods

[Vic K2VCO]

I have my TL-922 on the air. I added a soft-start relay, vacuum relay QSK, and a bias 
switching circuit to cut the tubes off in between keyed elements. I also strapped the 
grids directly to ground (W8JI convinced me), added a 5 ohm series resistor between the 
input circuit and the grid, and a copper strap from the chassis piece that the tuning 
capacitor is on to one of the grounded grid terminals.

I also replaced the VHF choke in the plate circuit with a 15 ohm glitch resistor. To 
improve bypassing for 160m, I added a pair of 0.0024 uf 6kv bypass capacitors after it. I 
added 0.02 uf capacitors from the cold ends of the filament choke to ground.

This TL-922 is the 220-240v model. My line voltage is very close to 240, and with the amp 
wired for 240, I measured 5.25 volts at the filament pins (with a true RMS Fluke meter). I 
put a 15 ohm resistor in series with the primary of the filament transformer, which 
reduced the filament voltage to 4.92V. This also will reduce the voltage on the pilot 
lamps a bit.

I added an operate/stby relay which opens the key line on stby and also switches the 
stby/operate lights. This allows me to remotely enable/disable the amplifier.

Some interesting facts:

The designers apparently INTENDED the original antenna relay to hot-switch on semi-QSK CW 
or with VOX! The little spark gap thingy on the relay is a protective device. This is 
horrible, since it means that the spark gap is all that stands between you and 
transmitting into an open circuit. No wonder bandswitches burn up! If you don't modify the 
922 to have faster properly sequenced relays, then you should always use PTT.

The fan is really quiet. Like they say in the horror movies, too quiet. I wonder about the 
cooling of the seals around the tube pins.

Kenwood provided slotted setscrews for the plate caps. It can be very hard to get a 
screwdriver in there to tighten them. Allen heads would be much better. The setscrews are 
some Japanese metric thread. But the setscrews in the knobs are the same thread but have 
Allen heads! I switched them.

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/