>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'.
I mean that the heater should probably not be DC grounded as long as the
possibility of a B+ to ground arc exists. . If such an arc occurs, the
internal insulation between cathode and heater can arc and possibly burn
out the heater. .
>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,
good point.
>and through the coil tap to ground, while
>theextra 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!
- or the tube manufacturer. As I recall, MFJ/Ameritron's Mr. Rauch has
pretty much blamed Eimac when an AL-1500 experiences sudden failure of an
8877.
>But if
>the box was $50 dollars cheaper in first cost, that's what the marketing
>dept likes.
>
cheers, Peter.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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