Phase Inverter Tube Position

[charvel750]

To the experts: I know most Marshall tube amps have the last tube before the power tubes as a "phase inverter." They say you should have a "balanced" preamp tube for that position. What does that mean? And what if you place a regular preamp tube there, one that's not a phase inverter balanced one?

[Cygnus X1]

Some people like a slightly unbalanced pi.

Since it is a split tube, both sides having the same amount of input signal, then it reverses phase and feeds the power stage in a 180 out of phase.
So you would want it balanced to feed both sides of the power stage evenly. Both in sgnal and phase.
A bad pi tube can make you think either your power tubes need rebiasing, or just give a bad thin sound.
In the absence of actually buying a matched pi, it's ok to "plug and play" to see what sounds good to you.

[Infernal Death]

Tubes like the 12AX7 are dual triode tubes. That means they contain 2 independant (if you don't count that they share one heater pin) triode systems.
Balanced tubes are tubes that have both the triode systems matched for the same characteristics.
In a non-real-world perfect phase inverter, both used triodes are completely balanced. But that just isn't achieved in real world applications because all parts have tolerances.
That's the reason for example that Marshall has two different anode resistors for both the triodes in the PI to account for those differences.
Now if your amp has parts with high tolerances (for example anode resistors with 5% or even 10%) it might not make much difference to use balanced tubes for the PI. Modern amps however have "better" parts so the matching might make a difference.
You can however also use normal tubes in the PI stage. But it might decrease the performance of your amp. Only might, because it could be that the normal tube you use already has more or less the same characteristics in it's triodes. Balanced tubes just guarantee the same characteristics, it's not that they have superior characteristics.

I always use balanced tubes because as i see it, it doesn't hurt and balanced tubes are only 5 Euros more (at least where i buy my tubes).
Many people argue that it's a waste of money. I mean you never can get 100% identicial values of the triodes.
But a tube with characeristics all over the place certainly doesn't help at all.
That's basically my opinion.

Flo

[DonP]

They say you should have a "balanced" preamp tube for that position.

More smoke being blown up you ass to get you to spend money.

When you take into account all the other components that might be out of tolerance in an amp, getting a matched PI is a waste of time.

Say the circuit is off and one side has more gain than the other side by 2%. You put in a match PI, matched output tubes, and you are lop-sided.

By trial and error, you get a PI tube that is out of balance 2% the other way. Put it in and the amp sings. Case closed.

Cyg's trial and error method sounds best to me if you want to spend the time. Or, if you want to get really anal, install separate volume controls on each side of the push/pull power tube input to balance the circuit.

I say just play the damn thing.

[ken361]

Some say yes and most say no on balanced tubes. I tried a few and you can hear a little diff. in the sound, I have a nos tube in there now and it sounds better then what I had laying around here. I read that if you use a higher gain tube there then some other ones it will give your amp a higher gain sound. I can tell!

[charvel750]

Thanks, guys, I appreciate that.

[Cygnus X1]

I always look forward to DonP's and Flo's responses.
I too am an eternal student.

Don, I tend to forget that some players simply don't have dozens of of tubes to try out.
I have a habit of stocking up on them!