What does a phase inverter actually do?

[Young Angus]

Just wondering how much difference it makes if you have a good or crap tube in the phase inverter position...and i thought a good way to decide this would be to ask exactly what it does in the amp...

[Jeff Seal]

Re: What does a phase inverter actually do?

Phase Inverters (PI) have often been overlooked in the "tone chain", when in reality, it's one of the most important aspects. I'm assuming you're not looking for a detailed explanation, but rather an answer to how important a matched PI tube is.

In reality, it's probably a good idea. The amount of + voltage the PI produces between both halves will affect how "balanced" the bias voltage is on the other end of the coupling caps. Thus insuring an overall balanced output as far as the output section is concerned. Most PI's are actually an intentionally "mismatched" circuit, based upon both sides having equal performance from the tube itself.

In practicality, it's always going to be "off" spec, and a "perfectly" tuned output section rarely occurs. There are more cases than not, that "tone" is achieved via imperfection.

I've set up many an amp to perfection only to have it knocked into next week by some "tried and trued" gigging amps.

So, my best advice is..... unless you're running a Marshall Major, SVT or an amp that has quite a bit of power..... don't worry about it!

hope it helped

JS

[kmcguitars]

Re: What does a phase inverter actually do?

Just wondering how much difference it makes if you have a good or crap tube in the phase inverter position...and i thought a good way to decide this would be to ask exactly what it does in the amp...

IMO you don't want a Crap tube anywhere in your amp ( But I know what you mean ) The PI tube doesn't necessarily have to be a balanced or toneful tube, But rather a robust one.

[Curly]

Re: What does a phase inverter actually do?

exactly what it does in the amp...

the PI basically feeds the preamp to the power tubes

[marvar]

Re: What does a phase inverter actually do?

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

[Scott_F]

Re: What does a phase inverter actually do?

Okay, guys, riddle me this....

If a balanced phase inverter is so important, how come the old fender circuits (and many others) use different resistors here. (100k, 82k)? It's like they didn't want the signal balanced perfectly.

[marvar]

Re: What does a phase inverter actually do?

Scott- do you really want to open this can of worms again? LOL
Personally, a balanced PI works for me- I can hear a dramatic difference in my amps, and looking at the schematics, both resistors are the same value on my circuit,
Once again, I think it all comes down to personal preferences. I know that most amp mfg don't give it a thought when tubing up an amp- the assemblers throw in whatever is on hand-neither do automobile mfg's, hence, we have "blueprinting" of amps and engines, to get the most out of them that they are capable of. Just my .02!

[Rich_S]

Re: What does a phase inverter actually do?

Okay, guys, riddle me this....

If a balanced phase inverter is so important, how come the old fender circuits (and many others) use different resistors here. (100k, 82k)? It's like they didn't want the signal balanced perfectly.

Actually, the use of 100/82K load resistors on many PI's is an attempt to balance the circuit rather than intentionally unbalance it. The two halves of the PI tube operate at different points: the + half operates primarily above the bias point and the - half below the bias point. A tube's gain is not linear - it tends to drop off as you move higher on the operating curve. So, the 100K is there to give the + half a bit more gain to compensate for the fact that it's operating at a slightly higher point, and thus has slightly lower gain. Even if the tube's two sides are perfectly matched, the difference in operating points makes their effective gain different, and the resistors are adjusted to re-balance the circuit.

There's some disagreement about the importance of balanced PI's - seems they're not nearly as important in guitar amps as in hi-fi. Since the 100/82K thing is a compromise anyway, it's unclear how much benefit a specially-balanced PI tube can provide. Simply having a good tube is probably more important than whether or not it's matched. That was my old buddy Zaphod Phil's conclusion in this 18Watt.com thread here. Search for 'PI' at 18Watt.com for a bunch more discussion on the subject.