[AMRadio] Loudenboomer MK2 hv supply tubes

D. Chester k4kyv at charter.net
Sat Mar 29 12:52:57 EST 2008

If the power supply is designed to use 866A's, don't worry about using 
3B28's; they will work just as well.  They are exact replacements, and tend 
to give less trouble than the MV tubes, even though they don't have the 
pretty blue glow.

If you look closely in dim light, you can see a violet glow deep inside the 
plate structure.

Likewise, the 4B32 is a direct replacement for the 872A.  The 3B28 and 4B32 
use xenon gas instead of mercury as the ionising medium.

Of course, solid state replacements will also give identical performance. 
Mercury, xenon and solid state rectifiers all have a fixed voltage drop that 
is insignificant in HV applications.  High vacuum tubes have a substantial 
voltage drop at normal operating current.  There is a high-vacuum version of 
the 866A, the 836.  I'm not sure if it has as high a p.i.v. rating as the 
866A, or if it is more like the old 866 that had only 7.5 kv piv instead of 
the 10kv rating of 866A's and 872A's, 3B28's and 4B32's.

I have found the best on-line source for tube info in Frank's Electron Tube 
Data sheets at http://tubes.mkdw.net/index.html.  For some reason, the USA 
link is dead, but any of the other links will work, and they all have 
identical data.  I usually click on the Australian one.  You can find data 
on obscure tubes such as subminiatures, European types and special military 
types.  On most tube types there are multiple data sheets of data published 
by different manufacturers, which sometimes gives you more information than 
books like the RCA, GE or Eimac manuals.

Don k4kyv 

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