Phase inverters?

[nicknolte]

I've got a question that's been bugging me lately. So there is a phase inverter in the preamp, but is there a phase inverter in the power amp as well?

 

[Sixtonoize]

No - there's only one.

The phase inverter is a preamp tube (like a 12AX7) that is technically a part of the power amp.

 

[Ventura]

No - there's only one.

The phase inverter is a preamp tube (like a 12AX7) that is technically a part of the power amp.

+1

And therefore, it's critical you get a "workhorse" tube in that socket as it's a socket that's ALWAYS on!! The PI is the hardest worked tube in any and all amps. It's the last tube before the output stage so it's critical you get a piece of glass in there that's ready to shine before getting amplified.

V.

 

[nicknolte]

Gotcha.

Slightly unrelated, but are 12at7's and 12ax7s directly interchangeable without having to change anything else in your amp?

 

[Ventura]

Gotcha.

Slightly unrelated, but are 12at7's and 12ax7s directly interchangeable without having to change anything else in your amp?

For the most part, yes. However, one has to understand the two variables differentiating the two: Namely, factor/spec gain and spec current. Deciding first of all why you're needing to change the PI is somewhat important. Is it toast? Something wrong? Sounds like shit? From there - get to know the intrinsic qualities of things such as plate length and how sometimes this can attribute to sonic effects, etc.

It's a BIG study man, there's so much to learn about glass... Good luck - but if you have more questions, fire away.

V.

 

[glpg80]

phase inverters are used in class A/B push-pull power amplifier setups to flip the phase angle of the incoming signal 180* out of phase on the top and bottom halfs of a sin wave so that the duty cycle of each set of tubes is equal to one another. the OT reassembles this high voltage low current signal into a low voltage high current AC wave in which you drive your speakers with.

the phase inverter works no harder than anything else in the amplifier. all tubes are always on in any tube amplifier as long as the plate voltage B+ are being applied to their anodes. you dont need to worry about plate lengths or all of that other bullshit to get a decent tube - take $50 one night and go on a shopping spree at dougstubes.com and purchase any 9 pin 12A_7 tube you would like, and experiment to see which you like best.

it is not a requirement, but recommended you get a tube with its triodes balanced (same mu of gain in each stage, there are 2 stages or 2 triodes inside a single glass envelope, and no not the distortion type of gain)

this will provide better cleans and more solid low end reproduction - assuming your power tubes are biased correctly.

i honestly prefer Jan phillips 12AU7's - they are not cheap but are a very clean and characteristic tube. but you can use anything you would like

 

[Ventura]

So, am I supposed to take this as a kick to the beets?

http://www.guitaramplifierblueprinting. ... verter.pdf
http://www.300guitars.com/articles/arti ... -inverter/
http://www.radiomuseum.org/forum/phase_inverters.html

These were the 1st three links that came up in google. All of them actually do state the or infer that the PI does work a little harder and does have a somewhat more 'key' role in the operation of the amp and its inherent tone. So, without getting into a battle of the lexicons and I know more than you do bullshit, take it for what it's worth - the PI is a damn important tube and it sure as hell works more than other tubes especially if you're running a multichannel amp.

Have a great day.
V.

 

[Sixtonoize]

it is not a requirement, but recommended you get a tube with its triodes balanced (same mu of gain in each stage, there are 2 stages or 2 triodes inside a single glass envelope, and no not the distortion type of gain)

this will provide better cleans and more solid low end reproduction - assuming your power tubes are biased correctly.

Neglecting, of course, the fact that 90% of the amps out there use a "long-tailed pair" PI arrangement...
A circuit that in and of itself is imbalanced.

Really, your amp would be MORE balanced if you had a slightly mismatched tube in the PI, provided it was complimentary to the inherent mismatch of the LTP.

In all actuality, PI balancing in a guitar amp doesn't matter. In a Hi-Fi setup it might, but even "clean" guitar tones are dirty enough to completely outweigh any benefits of matching.

 

[glpg80]

Neglecting, of course, the fact that 90% of the amps out there use a "long-tailed pair" PI arrangement...
A circuit that in and of itself is imbalanced.

Really, your amp would be MORE balanced if you had a slightly mismatched tube in the PI, provided it was complimentary to the inherent mismatch of the LTP.

In all actuality, PI balancing in a guitar amp doesn't matter. In a Hi-Fi setup it might, but even "clean" guitar tones are dirty enough to completely outweigh any benefits of matching.

yes in a LTP's design you are correct, i did not think i would need to get theoretically heavy here in actual design though? circuit wise we can be here all day debating the neuances and importance of a PI stage and even get into natural compression explanations/equations.

if the triode in the tube itself is not balanced, and the unbalancing circuit design using 82k and 100k anodes matches with the opposite sides of this mechanical design (chances here are getting slim), you will get differentiating in signal amplitudes once the signals are amplified and reassembled using the OT which can distort minutely itself the original signal that was intended to be amplified - which can be a bad or good thing in guitar amplifiers. thats the magic about tubes and tone - mathmatically incorrect does not mean its a bad thing - alot of new designs have been accidental designs that mathmatically make absolutely no sense at all.

you are right in the fact it does not matter in a tube amplifier - but it will definately do nothing more but help to get balanced triodes. a tube used in a LTP arragement have differing anode currents of nothing more than about 878 uA between each other in a balanced setup using a perfect world tube.

depending on the quality of your OT will depend on the quality of the PI tube and balancing to be more realistic in a guitar tubed amplifier.

So, am I supposed to take this as a kick to the beets?

http://www.guitaramplifierblueprinting. ... verter.pdf
http://www.300guitars.com/articles/arti ... -inverter/
http://www.radiomuseum.org/forum/phase_inverters.html

These were the 1st three links that came up in google. All of them actually do state the or infer that the PI does work a little harder and does have a somewhat more 'key' role in the operation of the amp and its inherent tone. So, without getting into a battle of the lexicons and I know more than you do bullshit, take it for what it's worth - the PI is a damn important tube and it sure as hell works more than other tubes especially if you're running a multichannel amp.

Have a great day.
V.

thanks, i plan to

the fact you said its the only tube that stays "on" is incorrect if you want to get technical with corrections. the ordeal you mentioned with mulitchannel amplifiers has absolutely nothing to do with the importance of a PI tube at all. thats out of context in importance with anything dealing with a PI's purpose in a tubed amplifier to be honest - you dont even need a PI tube in class A amplifiers, and i know multichannel amplifiers that use class A biasing.

i disagree that a PI has a more important key role than any other tube in the amplifier - a PI serves equally in a tonal standpoint as any other tube - not more, not less. in theory though its purpose is nothing more than to allow for proper class A/B operation and also a place to insert global negative feedback loops.