Highland Ham wrote:
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need
a bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?

=========================
Cliff , Does the TS-820 HV transformer have just 1 HV output or is there
also a lower voltage tapping.
I recall that the early Yaesu FT400/500 series (valve)transceivers had a
transformer with 600 and 850 V tappings.
The TV line sweep valves (tubes) in the PA worked well with the lower
voltage .....but 850V was toooo much especially when you used the PI
tank for matching the antenna (no external matching unit)
I used an old 1970 era FT-400 that way as my first TRX from 1985 - 1989.

Frank GM0CSZ / KN6WH
Hi Frank!
You know some years ago I had a similar Yaesu rig. I actually replaced
the sweep valves with 6146B's because of just the problem you describe.
When it was converted I had to pull all but one plate off the
neutralising cap beacause the interelectrode capacity of the "real"
TX valves was so much lower!!! Sold it when I got my more "advanced"
Kenwood a few years back.
However the 820S only has an 800 volt secondary with a bridge rectifier
and about 100 uF of smoothing caps.
I have discovered that valve testers are quite useless for testing
powerful tubes like 6146's and larger. They simply don't have a high
enough supply voltage built in and valves that test Ok can be a bit
"soft" and flashover at once with 1KV on the Anode.
However I hope to get some more 6146B's or their 12 volt version
the 6883B at a junk sale coming up soon. If not I'll look out for
another TT21 and modify the final box to take it.
At least then I'll be loafing along with 90 watts anode dissapation.
I have a "soft spot for the TT21 as I used a single one back in the 70's
to get DXCC and WAC. That worked with 1KV on the anode fine for years
and was dead stable!

One last point about Jap transceivers. They never seem to monitor grid
current. My home brew final did, and it was by far the best way to be
sure that your final was working properly in class AB1. In the 820s
there is even a spare switch wafer so I'm thinking of installing it
using the existing meter!
Regards
Cliff Wright ZL1BDA ex G3NIA.