Proper SB-1000 output.

[MVD@MVD]

That amp and the Ameritron AL-80, it's alter ego, push things pretty hard as
far as tube life. The plate voltage is higher than the SB-220, and the
filament voltage is even a bit high (shouldn't exceed 5% above nominal 5.0)
for many people (line voltage dependent). The higher voltage results in
slightly higher mu and thus slightly more gain and any problems that may
accompany high gain (like layout issues - despite beign grounded grid and
thus normally not needing neutralization, there is much less
isolation/shielding between the input and output on the SB-1000 than, say,
the SB-220, which has a sub-chassis.

The elevated filament voltage results in excessive emission and shorter
life. One mod suggests a wire wound (homebrew, determined in situ) resistor
in series with the filament to drop the voltage back to the in-spec range.


I seem to recall being able to get more out of it, but it's been a while. If
you run it as hard as the manual suggests you can, the plate tends to run
almost orange-yellow hot, and they are only supposed to have a dull red
glow.

I guess it depends how much money you have for replacement tubes.

Solid-state exciter, keying etc, are fine...12 VDC nominal coil voltage for
the relay.

The 'spitting' could be parasitic oscillations if you subscribe to AG6K's
assessment of amplifier misbehavior (I do - & haven't seen any good
alternative explanations). Continued use while that happens could result in
arcing damage to whatever is spitting - possibly the plate tuning capacitor.
Once they have arcing damage, they're prone to repeat the phenomenon because
the damage reduces the voltage breakdown at that point.

I recommended the AG6K plate parasitic suppressor mod to everyone while I
worked at Heath. I've seen that amp 'take off' on the bench with no drive,
just power applied. Replacing tubes costs less (well, maybe it doesn't
nowadays) than replacing fried bandswitches, meters, zener, etc.

Murray

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