Does anyone know the Kenwood TL922 amplifier? I will try to explain here:
I visited Russ Stevens, K5HAT. Russ is a blind ham but that does not stop him from pounding the brass and doing plenty of SSB work with his 3-band beam and modern station with instruments that speak to him and have braille markings on the panels. He is also a QCWA member and has been in ham radio for over 50 years. His first transmitter was an AM rig with two 807s' modulated by two 807's and used a carbon mike from an old style wall-mounted crank-phone.
The reason I visited Russ was to look at his amplifier. Since he is blind, he did not want to poke around
inside the high voltage compartment by himself. Russ said that the amp had made a "bang" sound and then the power had dropped from 1400W to 1100W.
The amplifier is a Kenwood TL922. It uses two Eimac 3-500Z triodes in AB2 grounded grid operation with 7.5V Zener bias and so it is a fairly typical design. Upon removing the tubes, I did not notice anything wrong with the tubes, but after he whacked one against the palm of his hand, the plate became askew. I could see where the welded joint at the top of the plate had been very hot before. Maybe it would have failed soon. I replaced it with a good used one he had, a Chinese one with a graphite plate. Some people don't like the Chinese ones, but I use a pair in a modulator and have never had any problem after about 3 years. I looked at the other Eimac tube again more closely, but could see nothing wrong. I inspected the electrolytic filter stack and did not find any that had vented except in a very minor way due to age. The high voltage regulation was as expected for a good full wave voltage doubler with capacitor input filter for no-load 3200V and full-load 2700V/500mA conditions, indicating that the filters, rectifiers, and transformer are functioning well and the line voltage is steady. The manual agreed with the measurements. There was only the transceiver, and no extra receiver, so it was not possible to check for hum or other artifacts on the air.
I observed these conditions with 100W of RF drive on 80 meters:
2700V/500mA DC input single tone (SSB position), 1100W RF output
2000V/400mA DC input single tone (CW position), 500W RF output
I believe these are normal steady state conditions for this amp, although the manual allows up to 630mA in CW mode and 750mA in SSB mode for adjustment. It is rated 2KW PEP input, but we had no way to measure peak power. the metering was a talking power/SWR meter. The SWR was said to be 1:1 to the load after some tuning of the antenna matcher. There was no oscilloscope to check PEP levels. I say this because Russ claimed the amp was putting out 1400W RF (not PEP but wattmeter-RF), and had produced even more, once before. I will not discuss here the amplifier's capacity or the results one might get with a new pair of Eimac tubes, but the manual is available here for study:
http://bama.edebris.com/manuals/kenwood/tl922/He asked me to inspect it for any other problems, and upon removing the bottom cover, the bandswitch for the pi-network (KW part number S01-2406-05) was found to have been damaged. It is probably not available any more, but could be taken out, disassembled, repaired, and re-installed. The carbonized and burned away portion of a nylon spacer could be replaced by a piece of tubing or a section of "Bic" pen.
From the examination I believe this damage is not the result of the "bang", but is a result of another past problem such as an RF arc lasting for several seconds. It should be repaired. I advised him to run the amp in the CW position (reduced plate voltage of 2KV) to stave off the possibility of this part arcing again or completely failing till it can be repaired. I know people hate to run large amplifiers at low power, but we have to do what we have to do. This was the end of the work on the amplifier.
It is not known what the loud bang was caused by. Russ mentioned that in the past, one of the 3-500Z tubes had failed by having a hole suck in on the glass envelope. I suspect he is running the amp pretty hard on CW with the obviously well-employed Vibroplex and may not always have it loaded up to the best efficiency, being unable to see the amp's metering. I speculate that the "bang" was caused by a layer of dust buildup coating a high voltage component until conduction took place.
Has anyone got experience with this amp to say whether it can put out 1400W single tone in the SSB position? The iron looks too small for this (amp only weighs about 60 LBs) but what do I know. I don't want to tell him to buy a new set of tubes only to find the power still won't do as he stated, putting me in the middle of some kind of mess.
