[Amps] 3CV1500A7

[pegasus at mho.net]

>The few new tubes that exist are locked up in our safe-deposit boxes.

Hi Paul...
     It might make sense to take those spare CVs out of Fort Knox and put
filament voltage on them once in a while.  The below-mentioned W5GEL
had a new spare that he kept handy for some 15 or 20 years. (can't
remember exactly)  When his original failed and he put the spare in
service it failed almost immediately.  Perhaps years of slight
leakage that was never "getted" had allowed to much gas to build
inside the tube.

> Modify the amp with an 8877.  "Instant-On feature is lost."  Before Bob
> Douglas, W5GEL became SK a couple years ago,
> he modified his 70V with the 8877 and we was very pleased with the
> results.   And, he did the mod at near 90 years of age.

   As the new owner of that same amp, I'm pleased to report it is still
working beautifully.  I had to make ONE change that Bob didn't forsee. 
The filament transformer in the Alpha 70V was built with huge windings
for the 30 amp current needs of the vapor tube.  It's "battery cable"
sized heater leads are testimony to that.  The 8877 draws only 10 amps
for it's filament.  Those large windings likely provide little useful
internal resistance to limit start-up current. (I don't trust it's
step-start circuit to provide a suitable slow-rise since the HV isn't
brought on-line until the timer completes it's cycle)  It also makes it
easier for slightly high AC mains voltage to cause the steady-state
filament voltage to remain too high.  Such was the case with Bob's old
amp.  He lost the filament in his first 8877 only 6 months after the
rebuild.  He replaced it with a new one making no modifications to the
amp.  I think he believed the tube was at fault.  He passed away a
short time later.  Since I had pre-purchased the amp from him while
still alive, his daughter saw to it I got the amp.  I checked the
filament voltage and found it exceedingly high so home-brewed a
resistor using a length of small enameled copper wire wound on a
ceramic form.  This brought the filament voltage on the pins back down
to just below spec.   Anyone contemplating this mod should keep that in
mind.  Unfortunately, the compact PA-70V just doesn't have room for a
separate filament transformer.  It's a combo transformer so one can't
usefully alter the primary voltage to gain the desired result.  It can
only be treated on the secondary.

Take care...
73,
Dennis
N0SP