Antonio I0JX wrote:
Does any one have experience on using 6DQ5s in place of 8236s in the
Hallicrafters SR-500? Not easy to find 8236s new.

Tubes are electrically identical, but the 8236 has a carbon plate.

I can't imagine the carbon plate would make a huge difference in ham
radio service where people will often transmit for a minute or two at a
time. It might help somewhat, but it would not be the huge improvement
in power handling that you would get in pulsed service.

Perhaps not pushing the transmitter to maximum power and cooling the 6DQ5
with a fan could be OK.

Another possibility would be to replace the 8236s with larger sweep tubes,
such as 6KD6, 6JE6 or 6LQ6. But apart from changing sockets, I should check
whether the bias and screeen voltages are compatible.

I would consider the EL509, since it's currently manufactured and pretty
reliable. Maximum ratings including the power dissipation are all MUCH
higher. Socket will have to be changed, or you can make an adaptor, but
it's a far more advanced tube design.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."