Tim Shoppa wrote:
On Sep 20, 7:38 am, cliff wright wrote:

Hi guys!
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.



Whoa, whoa. I know that tube ratings can be bent a little bit, and
while there is a tap on the transformer for "800V" this is intended to
be 750V to 800V after rectification, with load being drawn.

If you're measuring the voltage without load, you're right, it will be
higher than the voltage with load.

You might check that you're using the right tap on the input of the
transformer, too. IIRC there are 100V and 120V taps on the input.


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!!!


See, if you're transmitting then you draw current and there's no
longer quite so HV there. 6146B's will stand up to a good amount of
abuse, and usually other components in the final compartment will arc
over and give up their smoke before the tubes.


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.


Well, if you really want to, sure. But 6146B's served fine in these
radios for decades. Yeah, Kenwood pushed some of the numbers past a
strict reading of the tube limits but everyone pushed their 6146B's a
little past ICAS. (Remember, those were the days when Swan claimed to
get 600W out of a couple of sweep tubes.)

Tim.

Well Tim OM.
Don't worry, I checked the voltage both on and off load. Since I've been
a ham for 50 years+ and a professional electronics engineer (now
retired) I'm pretty familiar with PSU regulation problems.
BTW the secondary is labelled 800 volts both on the transformer and in
the service manual, so there is no possibilty of error there.
With 60 mA of anode current for the 2 finals the voltage is 970 and
when putting out 75 watts (CW) it is still 890. (using 3 different meters)
However as you say it was made in a time of wishful thinking!!!!
Looked up an old ARRL handbook from the 70's while searching for ideas
and found a 4 sweep tube final allegedly putting out nearly 1KW.
However call me an old conservative (engineering wise anyway) but I
still like a factor of safety.
I'm off to a local junk sale in a couple of weeks and if I can get some
more 6146B's (or 12 volt equivalent) then I'll stock up and flog them to
death! But if I can get another TT21 (KT88 with a top cap anode) then
I'll drop down the tube bases, enlarge the chassis hole and replace the
6146's altogether.
As you probably know the TT21 has a 45 watt dissapation rating and can
be run safely up to 1.2KV in class AB1.
Then I can get the rated output without "blowing a gasket".
BTW what I said about a grid current meter is VERY important!!!
Nothing tells you if a class AB1 final is working properly as well.
In actual fact a pair of 6146B's would be VERY hard pressed to deliver
more than 130 watts ouput, even with an optimally designed final tank
circuit in AB1. Very few transcievers have such a good Pi network on all
bands either!
In conclusion I should point out that I mostly work CW on 14MHz, with
the odd excursion into SSB.

73's Cliff Wright ZL1BDA ex G3NIA