Peter said:
>I find the 572b a very expensive, tricky and fragile tube, and recommend
against it.
I've not found them very fragile, but I have found them prone to VHF
parasitics because they have very long grid leads, and as a result, by the
time you've cured the parasitics, they're not very good performers on 12
and 10. There are much better tubes. Interestingly, the 572B didn't appear
in the ARRL handbook tube section until well into the 1950's, which
suggests that it didn't appear until about then. My suspicion is that it
was intended for Class B zero bias AF power amplifier use, being the next
step up from a pair of 811A's and down from a pair of 810s.
Like Peter, I don't recommend it.
If looking for low price, you probably can't get cheaper than using surplus
4CX250Bs, even though you have to blow them fairly hard. 4 of them will
give a kW out with reasonable linearity: it may well be possible to put NFB
on to improve linearity while using up some of the excess gain that there
would be with a 100 watt driver. (I don't believe they're any good in G-G
service)
73
Peter G3RZP
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