In a message dated 6/5/2005 8:34:21 P.M. Central Standard Time,
StephenTetorka@cs.com writes:
Hi Guys:
Appreciate your generous help on my previous matters.
Permit me kindly to ask for help on this one.
I have a 1948 homebrew using 2 each 1625's in RF deck.
Its modulated by 2 each 807's.
It works.
But - I'm not sure that I have the grid bias at the 'proper value'...and the
output is lower than I'd expect for these tubes..unless they're weak...which
could be since I've been hard on the rig in my attempt to get it operating.
Presently, the grid bias is at -56 volts.
The 'Book' says -90 for telephony @ 600 volts on the plate ( I've got 700 ).
The gent who built the rig - W2PLY - apparently used -35 volts or a C
battery.
The two 1625's are in push-pull and link coupled to the output coax.
Sooo..the big question is...how do I know if I've got them at max ouptut?
Tnx,
Steve
WA2TAK
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Back in those days the grid bias was derived from the grid current flowing
through a grid leak resistor. The -35v you mentioned is termed "protective
bias" and keeps plate current in check in case rf drive is lost. To reach 90v.
an
additional 55v is needed. If the two tubes are running 5ma grid current
each, for a total of 10ma, the resistor value needed for the grid leak is
55/.01
or 5500 ohms. Enough drive is power is required to make all this happen. If
drive is higher, then the bias will be higher which will tend to make the
circuit self regulating to a degree. The power output will only increase
slightly
with higher drive because of the added grid bias voltage If the drive is too
low to reach rated bias voltage then the output power will be lower. You
might then have to experiment with the grid leak resistance to achieve best
output power with the available drive level.
The power output from a pair of 807's should be on the order of 80 to 100w
at the plate voltage you mentioned. The grid leak bias voltage will give an
excellent indication of adequate drive level.
The recommended maximum plate voltage for class C, plate modulation is
600vdc as the peak voltage will approach 1200v on 100% modulated peaks. You may
get away with 700v if your tubes can handle the extra voltage during
modulation.
There should be no problem as far as cw or carrier operation is concerned.
73/k5gw
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