[Johnson] Matching Parallel Final Amp Tubes...

[Eddy Swynar]

Unless you've never seen an 811A plate go from dark gray, to cherry red, to
bright orange, to yellow, and then observed a hole form its way out across
the centre of the plate from its centre until it's the size of your pinky
finger nail just before you pop a fuse---and all in the matter of what seems
like a nanosecond!---you just can't appreciate the importance of using
matched tubes in your linear amplifier!

In my neophyte days I built a 4 x 811A G-G linear with nary a care about
"...matched tubes"...brother, was I wrong! One tube invariably would "hog"
the load, just as Mssrs. Orr & McCoy said it would, & the poor thing would
meet its maker as described above (this should NOT be so critical an issue
in a push-pull circuit, I believe---only when the tubes are in parallel).

The secret I learned was to match & pair/"couple", as closely as possible,
the tubes by individual RESTING CURRENT...and with surplus tubes, it's
pretty amazing how much variation one encounters, tube-to-tube, in this
regard!

Now, I can't vouch for the 4-400 behaving like this, but it sure was the
case with 811A's...

Lucky for me these old jugs only cost me a couple of bucks each
way-back-then...to this day I have no idea how many I "smoked", but I still
seem to have a near endless supply of 4-pin bases for plug-in coils from
within my junque box, & they all invariably have "811A" imprinted on them,
so it must have been a LOT! Hi Hi

Go ahead & try pairing together your "...mystery tubes", but be prepared to
see your investment go up in smoke (ALMOST literally!) before you can even
THINK of hitting the "off" switch...

~73!~ Eddy VE3CUI - VE3XZ