Originally posted by Lord Valve
I'm a tube dealer, and I say balanced PI tubes are bull{censored}.
If you *really* want your PI stage to be "balanced," the
best way to do it is witha $3.00 pot, not a $30 tube that's
been dipped in snake oil. For a typical Fender PI stage,
for instance, you use a 100K linear pot. Remove the two
plate load resistors (100K and 82K, for a blackface) and
connect the pot's wiper to the B+ eyelet (where the two
original resistors were connected together; there will be
a wire coming off this eyelet that is connected to the HV
supply) and then solder a 47K 1/2W resistor to the pot's
two outside lugs. Connect the free ends of the resistors
to the two eyelets which formerly had the plate load
resistors in them. That's it.
Now you can tweak your PI stage's "balance" to your
heart's content. Set it where it sounds best. Once
you have it where you like it, put the amp on a testload
and scope the output. I betcha fifty bucks you set your
PI stage balance pot so that the PI stage was fairly
unbalanced. I have, in fact, put these pots in a number
of amps (stuck through the back panel, where the
geetah-picker could twiddle 'em) and I can't remember
a single time where the amp came back with the PI
stage in a balanced condition.
Don't waste your money on "balanced" PI tubes.
There are situations where a tube with two "matched"
triodes is required, but very few of them are in audio
circuitry, and fewer still are in musical instrument
amplifiers.
Lord Valve
Expert
And there you have it