MoiInAust ha scritto:
"MoiInAust" wrote in message
...
"Piero Soldi" wrote in message
...
MoiInAust ha scritto:
"MoiInAust" wrote in message
...
"Piero Soldi" wrote in message
news Hi,
probably a gassy 6H6 ?

Cheers,
P.
Thanks. Yes, I thought that possible but according to my AVO 160, no
gas on any valves! I'll try that again with some replacements...
Further to that Peiro, there is no provision on either of my valve
testers to test diodes for gas! I always understood that was a problem
if there was grid current but of course in a 6H6 there are no grids!
Also a gassy diode can make havoc in a very high impedance circuit like
the AVC of AR88 ( note resistor's values ! ) .

If slightly ionized inside, the *reverse* resistance is much lower than
normal, as some positive ions are bombing 6H6's cathode.

Try exchanging it using the limiter tube, also a 6H6, and redo
measurements, if something different, try another 6H6.

Sometimes old *new* tubes need to be "formed" before use activating
residual getter inside applying filament only, for a very long time,
like 48 hours and after that some other time wth plate voltage on !

HTH,
Piero.



Well that's something new I learned! I've got a few and will try
different ones! Thanks heaps!
Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same
results. So, still puzzled...



OK, nothing useful.

When you have restored the capacitors and resistors, substituted any
valve ? If so, check for a *slightly* gassy one !
( RF amplifiers, mixer, MF amplifiers )

Cleaned carefully where soldered new components ?

Capacitors are of the *best* type ? ( low loss types ).

Impedances are very high, a current of 3 *microAmp* is nothing with a
grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable.

Wires are well insulated ? No cracks or degrading rubber ?

Cheers,
Piero.