Phase inverter tube?

[Chuck H]

I just hope people don't ever get us confused.

Well... He might get confused. You'll probably see things clear enough though

[Aloha_Mark]

This is one of those questions that requires the user to look at the whole picture. Paying money for a matched pair of outputs tubes and not ensuring that matching occurs at the preamp tubes is probably a waste of money. The goal with tube amps is to have a certain level of even order harmonic distortion. You can achieve that with mismatched tubes. A little can be good, but a lot may not necessarily be so good. As the output signal level increases you will get natural even order harmonic distortion just due to the I-V transfer curve, even in an amp with perfectly matched tubes. Juicing it up with mismatched tubes may be overdoing this, as results may not happen where a player would want them to happen. There is a Peavey patent which describes a method of achieving adjustable distortion, and it is explained here:

Guitar Amp with Variable Harmonic Distortion

Knowing how a circuit will behave is better than dealing with a circuit whose performance is random (with tube choice) and irreproducible.

[Chuck H]

Knowing how a circuit will behave is better than dealing with a circuit whose performance is random (with tube choice) and irreproducible.

+1

Preamp tubes are usually 'close enough' to avoid a problematic imbalance. Current production power tubes are all over the map. I like a little mismatch on power tubes, but I always buy matched tubes just to avoid a gross mismatch. I can still get the imbalance I want by "selecting" a PI tube by ear. And there's always an imbalance in drive when I do it this way.

EDIT: Perhaps I should start selling "selected" PI tubes that are intentionally imbalanced. With enough internet campaigning about the "mojo" of imbalance and tone "More even order harmonics!" Blah, blah... Maybe it could work out!?!

[Leo_Gnardo]

And there's always an imbalance in drive when I do it this way.

EDIT: Perhaps I should start selling "selected" PI tubes that are intentionally imbalanced. With enough internet campaigning about the "mojo" of imbalance and tone "More even order harmonics!" Blah, blah... Maybe it could work out!?!

What John Lennon called (in that Liverpool accent) "a fine natural imbalance." It's somewhere in Revolution 9.

Now you're thinkin' Chuck! You could make a fortune.

I noticed long ago Fender used that combination of 100K and 82K plate resistors in the inverter to try and force a balance in the drive gains. It usually works pretty well. All ya have to do to unbalance is make both resistors the same, or even swap positions of the 100K and 82K. Then . . . I have a Telefunken ECC83 brand new in the box, that I always took as unsaleable because the gain on one triode is half the other. Hmm maybe now I can flog that one & get enough to pay half my next mortgage installment.

[Chuck H]

This may sound contrary at first, but follow along and I promise it'll make sense before I'm done

I always use a special, balanced LTP PI circuit. It uses a 91k resistor and a 9.1k resistor as a split plate load on the input grid and a 100k on the non inverted side. Now both triodes have the same plate voltage and this balances very well with a matched tube. So why do I trouble to use a slightly more complicated circuit for balance when I've already stated that I prefer a little imbalance??? Well. Because I know that I won't be getting a balanced tube unless I pay a premium. Since I'm not paying that premium I have to assume I'm getting an unbalanced tube. If I were to use an unbalanced PI circuit I run the risk of an imbalanced tube accidentally balancing the drive!?! A balanced drive circuit and an imbalanced tube guarantees I'll get an imbalanced drive for less money than if I used a balanced tube and imbalanced the circuit. Plus I can use the same circuit in all my designs. Even for customers. Then if they want a balanced drive they need only pay the extra clams for a balanced tube. Easy peezy