[Boatanchors] SB-220 with parasitics

[Rob Atkinson]

I used to run a Ten Tec Centurion on AM after modifying it for high
power high duty cycle, and it has had a pair of RF Parts (Chinese)
3-500ZG in it for about 20 years.  On AM the plates are always orange
so they are definitely de-gassed.  I had one flash on me once but it
never happened again and the amp was and is fine.  I think I was
driving it a bit hard.  My recollection was that the flash was on an
audio peak.

The SB220 is an old design and is another one of those items with a
lot of updates and mods out there for it, including soft starts, 160
m. coverage, low v. keying, putting it on 6 m., strapping the tube
grids directly to ground, the famous or infamous voodoo nichrome wire
parasitic suppressors, and on and on.  I'd look at what's been done to
it if anything, first.

My 75 m. rig runs a pair of 3-400Z in the RF PA and I swapped them out
for an untested NOS pair last summer.  I half expected them to not
have full vacuum.  The RF PA power supply has a variac on it so I
started out with a small amount of grid drive at 1 KV, gradually
increasing the plate v. until I was at 2 KV where I usually run it.
No signs of any glow inside the envelopes.  Then I started modulating
the carrier with audio to swing the plate v. higher.  Never had a
flash; the old Eimacs were still good.

Merry Christmas

Rob
K5UJ

On Fri, Dec 24, 2021 at 10:23 AM Keith Densmore <ve3gem at gmail.com> wrote:
>
> Safe Holidays to Everyone,
> I have agreed to look at a Heath SB-220 amp for a friend. The amp, tubes
> look like new but one of the 3-500's has had a parasitic bang, at least
> once and maybe twice. (On the same tube.)
> I cannot find much info on the web but the question is, can such a tube be
> reformed? Run the filament for some hours? Power it up with lower plate
> and/or filament voltage? Any hope or is it dead?
> If anyone has experience in this area, your advice is welcome.
>