On 31st December, Mike wrote...
"... I was thinking about two 813 in grounded grid configurations but now
I'm thinking why not the pair of 3-500zg from RF-parts? I newer used 3-500zg
before so I can't compare it to 813. I understand the difference in pricing
but I also know it's easy to buy matched set of 3-500zg but it's just
impossible to find matched pair of 813."
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Hi Mike,
Well, I would NOT say that it's impossible to find a matched pair of
813's --- far from it!
I have never purchased a "new" 813 here, yet whenever I need replacements
for my 2x813 amplifier (which, BTW, happened only ONCE in 17+ years of near
daily operation!), I simply reach into my junque box collection of donated
813's / Hamfest acquistions / etc., and place them into my amplifier one at
a time, taking note of each tube's resting current (i.e. full B+ voltage
applied, no excitation)...
I then mark the metal base shell of the tube with the current value in ma.,
and go on to the next one. I continue this until I find two that are very
close to one another. Once I find two such tubes, I'm off to the races.
Tube brands don't matter --- it's the resting current that I find to be
critical here, a lesson that was painfully forced upon me to learn many
years ago, when I attempted to get my very first HB kilowatt on the air, a
4x811A grounded-grid set-up (have you ever seen holes in tube plates the
size of your baby finger nail...?! Hi Hi).
~73~ Eddy VE3CUI - VE3XZ
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