[Amps] ceramic vs glass

[Roger (K8RI)]

It has been my experience, hence anecdotal, that if properly cooled and 
treated, there is little difference in the useful life of Ceramic Vs 
glass tubes, but when we get into older tubes, like the  PL172/8295A. 
which are the same tube, but the 172 had glass to metal seals while the 
8295A has ceramic to metal seals.When in use, I believe there is little 
difference in life but when setting on the shelf the glass to metal seal 
PL172s have strong tendency to go gassy, while the ceramic 8295As do 
not, which is probably why the 8395A sells for a premium.

I ran a pair of 813s (Tetrodes) at the old 1 KW (which is pushing them) 
average input convection cooled for a few years and they were still 
going strong when replaced with a Hallicrafters HT32B.  That PL172/8295A 
(Pentode) has a coarse and heavy fin structure that let you easily run a 
KW using only an open fan for cooling with no shroud.

It's not the Ceramic Vs glass that requires the protective circuits, it 
is the internal structure of grid and screens with very little 
dissipation that require the protective circuits.  When 8877s were $300 
you didn't see much in the way of protection.  Now the Eimac versions 
are over a kilobuck, over drive protection has become popular, so too 
has the awareness of IM products.

It's the lack of awareness that has given Tetrodes a bad name.  The 
4CX1500B and Chinese FU728F have very good IM products and are quite 
linear when operated correctly and not over driven.  (Tuned and loaded 
as a Tetrode and not as a triode)

73

Roger (K8RI)



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