SV: [AMPS] Inrush Current

[Peter Chadwick]

Rich said:

>In my opinion, it is not good engineering practice to gnd one side of a
heater 
>when the cathode is driven.  To avoid the possibility of heater cathode 
>breakdown, connect the cathode to one side of the heater, ground neither 
>side of the heater, and use a RF choke.  

I presume you mean 'ground to RF'. Interestingly enough, back in the dawn 
of pre-history when the the old Command transmitters were designed, they
used 
a Hartley oscillator at only a few watts, with a hot cathode. They went to
the 
trouble of providing an extra winding on the oscillator coil so that one
side of the 
heater was connected to cathode, and through the coil tap to ground, while
the 
extra winding was in the other side of the heater. This 'floated' the heater
to RF 
as Rich is saying, even though in that case there wasn't that much in the
way
of  RF  volts that could get across the heater cathode insulation. (The 1626

isn't that big a tube!) Another reason may have been oscillator stability,
but I've 
never had a problem with that. However, I agree with Rich here that it is at
best 
undesirable to have the heater at RF ground when driving the cathode.

One point which I think that we sometimes miss is that when designing things
for the commercial market, first cost can be more important than cost of 
ownership. If you blow a $300 PA tube at 2 year intervals, that's cost of
ownership -
even if it's caused by design short cuts. And you can always blame the user!
But if 
the box was $50 dollars cheaper in first cost, that's what the marketing
dept likes.

73

Peter G3RZP


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