[AMPS] RE: 8877 conditions

[Peter Chadwick]

Rich says:

>The TS-830S uses an unregulated screen supply for the 6146Bs.  It appears 
>to have the lowest total IMD of any amateur radio transceiver.  

6146's are pretty good with unregulated screen and bias until/unless you
start getting negative screen current - which in 4CX250's and the like is
normal. In 6146B's especially, age can lead to negative screen current. If
as in some yaesu rigs you have an 'anti-flashover diode' in series with the
screen, negative screen current leads to a soaring screen voltage and run
away. Under those circumstances, a shunt regulator of zeners is a good idea.
Then you need to stabilise the bias as well. A separate bias pot for each
tube is a good idea. I haven't done the last bit, but the zenered screen and
bias suppplies mean that I'm still running the same 6146B's in my FT102 that
were fitted by Yaesu in 1993. There's a bit of plate current creep if you
hold it key down on CW, but not much.

In my HW101, I did the same  regulating trick, and ran the 6146B's with 1100
on the plates for about 200+ watts of fairly linear (-28dB 3rd order IMP)
output.

Bottom line: not all tetrodes need a real fancy screen supply for linearity,
but many of them last longer.

73

Peter G3RZP


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm