Good day all!
I've been having some problems with my 820s. It was fine for several
years and then I had a flashover in the finals. This may have been due
to a faulty ground on a "UHF" coax connector.
Have replaced the finals with 12 volt 6146B's (putting the heaters in
parallel of course). However although it now works, its effeciency is
damned awful. When it is loaded up to give 60 watts RF into a dummy load
the dissappation in 6146 anodes makes then glow cherry red!!!
Now my info tells me that the 6146B is rated at 30 watts anode
dissappation per tube (total 60 watts) so based on my 50 odd years of
electronics wouldn't expect a red hot anode at anything like the
theoretical 66% efficiency.
Yes, I have replaced the anode coupling cap (with an appropriate 3Kv rf
type) checked all the tube voltages set up the neutralisation and tried
the rig on all bands to ensure that the padding caps are OK. Same result!
My thinking is that my 6146B's may be in poor condition. Unfortunately
My old Heath valve tester does not do a very good job on big power
valves so although it says they are OK I don't really trust it.
So far I can't locate any parasitics up to about 120 MHz either.
Anyone had a similar problem? I am getting so fed up with it that I'm
thinking of running with 1x6146 and building a small linear amp
~400watts to drive the antennas.
Regards Cliff Wright ZL1BDA ex G3NIA.