Tom said:
>>I suspect that's why some amplifiers get
>> away without a parasitic suppressor - until the tube is changed.
>Do you mean the type of tube?
- No, the tube. All tubes are not equal, even from the same manufacturer's
batch. Not using a parasitic suppressor may be OK in an HF amp, but for
reliability and repeatability, I wouldn't do it. Not even on a 'one off' for
myself.
>> I've seen it in other tubes, where a component close to the
>> tube has enough capacity to allow a VHF current at the parasitic
frequency
>> to flow from the anode to the component through a localised area of the
>> envelope: the dielectric losses are enough to heat the glass, and a
pinhole
>> appears. Seen it in 6146s and 572s.
>I've seen a melt through in may inadequately cooled tubes. How do you
>know it was a parasitic? Did you ever calculate current required to
>heat the glass with RF heating?
In the cases I saw, it was during development, the amplifiers was suffering
very badly with parasitics, and in the 572 case, the lead running near the
glass discoloured. It also happened pretty quickly - the plates hadn't got
bright red. What else was likely?
I've never been very happy with 572's.
73
Peter G3RZP
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