Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kharrisma on March 18, 2020, 02:52:22 pm
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Hey folks,
Is there any need (or benefit) to designing in a phase inverter in a low-wattage, single output tube amp? I'm toying around with a 1-watt-ish "bedroom" amp, and probably using a 12AT7 as an output... I understand phase inverters are usually used with larger tubes (pentodes and such), but a dual-triode is two tubes in one envelope (sorta), so would there be any benefit in adding this to such an amp? Or does this fall under the heading "needless complexity with no real benefit conferred?" Or is it a just plain bad idea, or totally unnecessary? Thanks for your thoughts.
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go here, read the phase invertor section AFTER the single ended one.
http://www.valvewizard.co.uk/
a PI is required to drive 2,4,8 power tubes in push-pull mode
a driver is required for single ended power amps, no matter how many PA tubes you parallel together
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Phase inverters are used in push-pull amps. Single-ended amps don’t use a phase inverter.
The number of output tubes is not relevant. You can have a single-ended amp with any number of output tubes and a push-pull with only one output tube (a dual triode).
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Look at this thread. It has a lot of excellent information to answer your question.
http://el34world.com/Forum/index.php?topic=18481.0
With respect, Tubenit
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Hey folks,
Is there any need (or benefit) to designing in a phase inverter in a low-wattage, single output tube amp? I'm toying around with a 1-watt-ish "bedroom" amp, and probably using a 12AT7 as an output... I understand phase inverters are usually used with larger tubes (pentodes and such), but a dual-triode is two tubes in one envelope (sorta), so would there be any benefit in adding this to such an amp? Or does this fall under the heading "needless complexity with no real benefit conferred?" Or is it a just plain bad idea, or totally unnecessary? Thanks for your thoughts.
You can make it a "self-split" power amp. The Firefly is a good example of a DIY project with a self-split 12AU7 power amp.
Basically, for self-split you ground the grid of the 2nd triode, and both share an unbypassed cathode resistor. So when the first triode pulls more current, the cathode resistor drops more voltage and thus the 2nd triode lets less current through, and vice versa. Really, it's not unlike how a phase inverter itself works.
It's not very efficient though, for one thing you can't bypass the cathode resistor. Of course, that can help you if you're trying to keep the output level down.
It works... but I've never been a fan of it. Or at least, I've never played a self-split amp I liked.
Another thing to think about is that a dual-triode power amp won't pull much current, so it works very well with a cathodyne PI (with big grid stoppers, like 470k big). Many 1-5W Marshall amps use a cathodyne PI. That only requires one triode.
You can also run that dual-triode as parallel SE stages and then no phase invertion required.
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"Phase inverter" mostly makes sense for Push-Pull power amp. Like that stick behind two harnessed horses which splits the load between the horses.
Yes you can do P-P with both halves of a 12AT7. To do it "good" you use a phase splitter. As said, there are cheats.
All "real power tubes" need a Driver. Something to lift signal from around 1V to around 25V, maybe into small gridleak. Twice for push-pull. 12AT7 does not need a heavy-duty driver.
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I guess ”phase inverter” is not a very good name for this circuit. All gain stages invert the phase of the signal. ”Phase splitter” is perhaps more descriptive.
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I think that we must remember that one of the advantage of the use of a Phase Inverter (or self split if we are fine with less power from same tubes) is on the OT
The OT on PP amp is smaller than OT on SE amp (same output power) and, also, allow noise cancellation
so, a self split configuration (PP without the use of a Phase Inverter) if we are comfortable with less power, will give us the advantage of PP configuration
as told, a Phase Inverter in an SE amp has not much sense
Franco