FT101 Power Amplifier Valves
(that's "final tubes" in American)
from Ham Radio Today (RSGB) April 1998
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Beware of changing the 6JS6C power amplifier valves
in the venerable old FT-101
Clarence came in with a long face and an FT101.
He explained that the rig had been running perfectly,
but he thought he would treat it to a new set of valves.
He found the 6JS6C power ampli?fier valves a little too
expensive in the UK, and from a friend in the USA
he had obtained a pair of genuine RCA 6JS6C tubes.
His friend, who had been in TV service, told him that
they were really the best that could be got, and that
he had held them in stock for many years as they were
no longer made. They were specially constructed to give
long life in colour TVs, as they had extra heat
dissipating plates added to the electrode system.
Clarence had fitted them, but as the rig had warmed up,
he had noted a strange noise on receive followed by
the sound of arcing in the PA compartment.
He had switched off quickly and refitted his old valves,
but now found that the rig had PA current, but no RF output.
"What has happened, Harry, were the valves duff and
can you do anything about the rig?"
I had to explain that all valves are not equal,
and that some are much better for one purpose than
an other. Many valves used as driver and power
output devices in Amateur Radio equipment were
actually designed for use in TV sets.
Line output valves such as the 6JS6C, 6KD6 and 6HF5
were designed to run at around 15kHz, and so have no
agreed standard for interelectrode capacity
(at 15kHz a few pF one way or other isn't going
to make much difference). Also, these valves were
intended to be used only to pass pulses,
and so once again the characteristics of the control grid,
as regards to how much bias was required,
seems to differ widely between brands.
When Yaesu, Swan and KW were designing modestly
priced transceivers they found that using certain brands
of mass produced TV valves worked well, were cheap,
and therefore gave them a competitive edge.
What must not be forgotten, however, is that
these valves are being used outside their maker's
specification, and hence the valve which is best in a
TV can be a disaster in an Amateur Radio transceiver.
Differing interelectrode capacity can make them
impossible to neutralize, and, as in Clarance's case,
differing bias levels allows some to conduct in the receive mode.
Clarance's rig had gone into violent oscillation
in the PA stage. No load was connected and so high
RF voltages had been produced.
This had caused the PA anode RF choke to arc over,
making it go short circuit. Fortunately this was the
only damage that had occurred, but it still lost Clarence
a lot more than if he had purchased a pair of the correct
valves in the first place.
Before using television valves in a transceiver,
do check that they are suitable for use in the equipment
you have. It is much better to be safe than sorry.
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