[Engineer]

"François Yves Le Gal" wrote in message
...
On Fri, 09 Jan 2009 10:20:19 +0100, jaap wrote:

Is there need for replacing preamp or output tubes with lowish
isolation while emission is still strong?

Power tubes are considered acceptable with 500K HK, small signal tubes
1M.

What is the effect of low isolation on reproduction?

Unwanted AC on the cathode if you use AC heating, bad bias..
Heater/cathode
short means a dead tube.

Agreed.
Now a bit OT... The H-K short that you have to watch for is in EZ80, EZ81
and similar rectifiers - these are the ones that use the same 6.3 VAC heater
supply as the other tubes. A failure takes out an unfused mains transformer
in seconds. I was lucky enough to catch one fail in a Phillps radio a while
back - just happened to be bringing it up on a variac for some reason. I
forget why: recent quite extended use on 120 VAC with direct switch-on had
been OK. Anyway, you have to get lucky sometimes! Solution: if you want to
keep these tubes, add a fast blow 250 mA fuse in the MT centre tap to
ground, else replace the EZ80/81's with two 1N4007 Si diodes (plus a common
80 to 100 ohm resistor) - they mount easily through the tube socket (push
the leads through from the top and solder to the socket tags.)
BTW, I'm not sure if a H-K resistance reading of 1 Mohm, or so, in an
EZ80/81 is indicative of future failure but since I'm now a bit paranoid
about these tubes I won't take a chance - it's now fuse or use Si diodes!
Cheers,
Roger