>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
>With it present, the tube would
>never go totally into cutoff.
--True, but the current would be minimal. How much is 25v across 100k
ohms? --
>With it out, it would. Why is that
>important?
--It does not matter either way. The resulting current with a 10k cutoff
bias resistor is so low you can't read it on the typical anode/plate
ampere meter
>. Many older low power rigs (6146 etc) have the key directly
>in the cathode circuit with no apparent problems. With an 8877 for
>example, wouldn't the cathode voltage simply rise to the cutoff point
>and stay there?
--Yes. Typical potentials are in the 25v range.
>I have the cathode tied to the heater at the socket,
>so cathode-heater voltage differential wouldn't be a problem. The
>heater transformer insulation is rated at 535 working volts, so as
>long as the cutoff voltage is less, that shouldn't be a problem
>either.
--as long as you use a filament choke there should be no problems.
However, I would install 2 or 3 series 200a-peak (1N5400 series) glitch
diodes between the HV negative and chassis ground.
Rich---
R. L. Measures, 805-386-3734, AG6K
--
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
|