Hi Francois.
I would suggest you leave it as zero bias, without any diodes. As
you say, with just 500W PEP output, the tubes will not get hot
enough to de-gas.
Your solution of running the amp at 220mA without drive is very
good and it will ensure that your tubes are always healthy.
So keep it as it is and don't change the bias. It will work fine
for your application.
73, Alek VK6APK.
On 3/03/2025 11:39 pm, Fran?ois via
groups.io wrote:
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So
I went to full voltage tests with the 2 tubes in place. With
zero bias I measure
?
·????????
CW
voltmeter 1700 V measures 236*8=1888 V intensity 175 mA; the
tubes do not redden 298 W (2 tubes)
·????????
SSB
voltmeter 2500 V measures 329*8=2632 V intensity 220 mA; the
tubes redden 550 W (2 tubes)
?
In
low voltage tests 231 W (plate current 390 mA under 592 V)
for one tube redden. The result therefore seems consistent.
?
In
my country (France) the power is limited to 500 W.
·????????
I
could operate the L-7 on the CW position and by limiting the
excitation 500 W RF output.
·????????
Periodically,
run the amp in SSB position, without RF, with just
the 220 mA quiescent current to make the tubes glow and thus
maintain optimal degassing.
?
I
am concerned that by placing diodes in the cathode circuit
to bias the grid slightly negatively, the quiescent current
in SSB mode will not be sufficient
