Tnx for tip.

Tony I0JX

"Jim Mueller" ha scritto nel messaggio
eb.com...

On Thu, 15 Nov 2012 22:18:24 +0100, Antonio I0JX wrote:

While replacing two exhausted 6JE6A tubes in an SSB transmitter with a
pair of identical tubes, I noted that, despite the new tubes are NIB,
their idling currents are significanly different from each other. To be
fully sure that the difference was due to the tubes, I tried to exchange
tubes across sockets. Moreover, during the test, I verified that no RF
drive reached the tube grids and that no self-oscillation was taking
place: the difference was really due to the tubes.

As those tubes are not too common nor very cheap either, I would like to
try a workaround for balancing them.

Looking at the schematic diagram I determined that modifying the circuit
for having two separate grid bias controls, allowing independent bias
adjustment, would be rather complex.

On the contrary giving a small negative voltage to the suppressor grid
of the strongest tube would be very easy.

I am aware that balancing the idling current does not necessarily imply
that tubes remain balanced also when RF drive is applied to their grid
and, in that respect, I wonder whether any one has got experience with
balancing tubes by varying the suppressor grid potential.

Thanks & 73

Tony I0JX Rome, Italy

The cathode current of a pentode or beam pentode is mostly set by the
voltages on grids 1 & 2. While changing the voltage on the suppressor or
beam forming plates may or may not change the plate current, it won't
have much effect on the cathode current. In other words, if you use the
suppressor to reduce the plate current, the screen current will go up by
about the same amount. This could cause the screen dissipation to exceed
its ratings.

If you try this, watch the currents in the other elements as well as the
plate current.

--
Jim Mueller

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