[Boatanchors] Thoughts on Reviving the Heathkit DX-100

[Rob Atkinson]

When I was first operating AM I remember being curious about tube
plate voltage ratings.  You'd have a broadcast rig with enclosed anode
tubes in the final such as 4-400s, with 3 KV on them and then at
around 100% positive modulation the voltage on them would go to at
least 6 KV.   The data sheet would say the maximum plate v. for the
tube is much less than the peak modulated positive v., maybe 4 KV.
Well, how come the tube didn't arc over or something?

I was puzzled by this for a few years and brought it up with Rodger
WQ9E at some point.  He may want to refresh my memory as I think he
subscribes to this list.  Anyway, what I recall he found out is that
those data sheet ratings aren't really hard and fast.  It isn't like
the tube explodes of you go 5 volts over the data sheet specified
plate v. maximum.  The bulb glass and size, space between the plate
and grid, degree of vacuum all matter and you could probably put
several hundred, maybe even 25% above the specified maximum on a tube,
at least a large RF power tube, but the chances of problems and
squirrely amplifier behavior go way up unless you really know what you
are doing.  Brief audio v. excursions above the specified maximum
(obviously) seem to be tolerated.

73

Rob
K5UJ

On Wed, Jan 6, 2016 at 9:49 PM,  <w5jo at brightok.net> wrote:
> It has been a very long time since I have read how tube manufacturers
> specified the plate voltages for tubes, but I do remember that altitude has
> something to do with it  along with what type of envelope, the insulating
> material that holds the elements and more.