Hi Bill,
> Most designs for GG amplifiers show a relay in the cathode
circuit to
> switch the tube from standby to operate and there is usually a
> resistor across the contacts, typically 10K or so.
>
> Is this resistor really necessary?
No, it does almost nothing. It makes only the slightest change is
quiescent current and open circuit relay voltage.
There was even a rumor going around (it was on an amplifier Web
page) this was a cause of 8877 grid cathode shorts. The real cause
was Eimac was having material problems with a heat dam in the tube.
By the way, if you are building an 8877 PA, avoid tubes from the mid
to late 80's. They were VERY prone to grid-cathode shorts, and gold
sputtering. I used to reject about 60% of the new 8877 tubes tested
in bad production batches. The failure is related to filament
operating time and number of off and on cycles.
73, Tom W8JI
--
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
|