[Vintage-Audio] 12AX7 and other Tube questions

[Philip Atchley]

Hi Chuck et al
Cleaning the pins didn't help.  Today I borrowed a tube tester and checked
all the tubes in it (Peavey Triumph 60).  What I found was interesting.
Both sides of tube V1 (12AX7) only measured '5' which is barely off the
lower peg of the meter.  One section is used in the first pre-amp stage for
all channels, the other section is used along with two more 12AX7 stages in
the distortion channels.   All other 12AX7's read approximately 68-70, well
into the green.

When I replaced that bad tube (as would be expected) gain came up
tremendously on all three channels (Clean, Crunch & Ultra Gain).  What I
thought was an amplifier that needed to have the volume controls turned up
unusually high now has more output than I can use with the gains well under
halfway!  Though if I use it 'Live' in the church band it'll need to be
'cranked' to fill the auditorium as my amplifier isn't "miked" through the
house sound.

Gone is the occasional light "crackle" in the sound, especially at B+  turn
on (standby switch).

What I found especially interesting is that the tube that failed is a Sovtek
tube with a '98 date code while all the original Peavey tubes tested good.
Guess that just proves that the newest tube in a set isn't necessarily the
best one 8^)

73 de Phil  KO6BB

REAL Radio is heard on Boatanchor Receivers.
REAL Music is played through  'Valve' Guitar amps!
k06bb@elite.net
Merced, Central California


> While 12AX7 / AT7 types often go microphonic, they rarely wear out, and
are
> usually NOT the source of crackles and pops. While the amp is on, and with
> the volume SLIGHTLY  turned up, rock these small tubes in their sockets.
If
> you hear explosions etc, put a small quantity of oil ( thats right, OIL )
on
> a "Q-Tip", and apply sparingly to each tube pin. Re-insert in the socket
>  amp off ) and rock it around a few times. Do this until the noises stop.
> With regard to 6L6 tubes-- For new, the best bet is probably the ones made
> by Sovtek. As you are aware, the Chinese ones are awful.
> Groove Tubes are nothing but commonly available tubes ( Mostly Sovtek /
> Tesla etc )  that are "screened" / graded / and matched. For this
"service"
> you pay a premium price, but the convenience is often worth it.
> The absolute best 6L6 types available are the 6L6-WGB / 5881 tubes made by
> Tung-Sol. The average amp tech will not have these in stock, as they are
no
> longer being made. Antique Electronic Supply has them in stock, and the
> price of these is quite reasonable. This type has higher design ratings
than
> commercial 6L6 types, even better than the 6L6-GC. The 5881 can be used to
> sub any 6L6 type, as well as KT-66, and the hard to find 7027-A used in
some
> Ampeg amps.
> I have been using / repairing tube amps for over 40 years, this is what I
> use in my own amps.
> Chuck N1LNH