Rick, if the big resistor drops 100v at 2A then it must be 50 ohms, right?
This is a typical value used in series with the high voltage lead to act as
low frequency parasitic suppression circuit.
An 8877 has mucho gain at low frequencies and some really nasty things can
happen if an audio frequency oscillation were to take place. The power supply
filter capacitors and the plate rf choke and bypass capacitors don't
necessarily cause the tube plate to "see" a low enough Z to kill the circuit
gain at
lower frequencies.
Installing the 50 ohm resistor can lower the Q of these nuisance resonance's
enough to keep things stable and trouble free.
There are probably some that would say this is balderdash but my personal
experience with the 8877 on 2m surely supports the story.
73,
Gerald K5GW
In a message dated 3/21/2007 8:23:33 A.M. Central Daylight Time,
rstealey@hotmail.com writes:
Tom, W8JI, says:
>The small glitch resistors in your amp don't do a darned thing
I am in the process of finishing up my 8877 amp, and the plans I am using
show a 0.6 ohm 1 watter and a 50 ohm 50 watter in series. So in the event
of a flashover (milliseconds ?) the 0.5 ohmer is supposed to blow open? I
can't see the value of the 50 watter. In case of extremely high plate
current, such as hitting it with full drive and no load, say 2 amps of plate
current flows and even then the big resistor only drops 100 volts, and heats
up 200 watts. I wouldn't expect a wirewound power resistor to fail
immediately under these circumstances, and I can't see how it is protecting
anything.
What am I missing here? Is the 50 watter the wrong type?
I have other topics to discuss relating to control circuitry, soon.
Rick K2XT
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