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Does every tube amp have a PI tube?

lang.murphy

Member
Messages
3,582
Was thinking about it today. I have an Orange TT... I think it has plenty of gain, but just realized today that only one of the "pre's" is actually a pre-amp tube. T'other is the PI. So I guess I understand, now, why the OR15 is thought to have more gain than the TT... it's got 3 "pre's", so one assumes one of those is the PI and t'other two are actually used for gain. I mean, it's a single channel amp, so one assumes both pre's are used for the single channel, not split to cover more than one channel. Forgive the wandering... PI in every tube amp?
 

kbgear

Member
Messages
1,129
Tube amps that are "push-pull" use one power tube (or set of tubes) to push the positive waveform, and another tube (or set of tubes) to pull the negative waveform. The PI (phase inverter) is used to invert the phase of the guitar signal to one side, while sending an in-phase signal to the other side. This is generally the most common configuration for guitar amps, especially above ~10W.

A basic "single-ended" tube amp will have a single power tube (or set of tubes) that amplifies the full waveform, so it doesn't need a phase inverter (PI).

Worth keeping in mind that some phase inverter circuits only need one triode (half) of a tube, so a tube could be part PI, and part something else.

Generally push-pull amps are listed as Class AB, while single-ended amps as Class A. However, the class is independent of push-pull vs single-ended, so it's best to check the amp specs for push-pull vs single-ended.

That being said, an amp with only one power tube will by definition be single-ended, and thus won't have a PI. The Fender Champ for example has one preamp gain tube, one power tube, and one rectifier, and is thus single-ended. An even number of power tubes could be either, but an odd number of power tubes (does that even exist?) would have to be single-ended (no pairs).

Tubes can also be used to drive things like reverb tanks, effects loop send/returns, tremolo, etc. So while more preamp tubes can often mean more gain stages, it isn't necessarily true. For example, a Fender Brownface Super has 6 preamp tubes, but it's by no means a gain monster. ;-)

Hope it helps!
 

rstites

Member
Messages
1,642
FWIW, all class AB amps have PI's. It's inherent in the class of amplification. SS state amps will also have PI.

All class A amps won't have PI's. Again, it's inherent in the class of amplification. The power tube/transistors are always on and thus no need to split the signal to them.
 

makerdp

Member
Messages
617
Kbgear is pretty spot on with that reply but you actually can have a single power tube and still be push pull. There are low power designs that use a single 12A_7 tube with one triode being push and the other pull.

Also there are some tubes that have a triode and a pentode in the same tube used in push pull amps. The triodes from the pair are used for the PI and the pentode halves for the PP power so in that case there is no dedicated PI tube per se.
 
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makerdp

Member
Messages
617
FWIW, all class AB amps have PI's. It's inherent in the class of amplification. SS state amps will also have PI.

All class A amps won't have PI's. Again, it's inherent in the class of amplification. The power tube/transistors are always on and thus no need to split the signal to them.
This is not true.

All single ended amps are class A but not all class A amps are single ended and you can have class A push pull amps. In PP amps the class totally depends on how the designer set the bias point/load lines of the power tubes.
 

kbgear

Member
Messages
1,129
Kbgear is pretty spot on with that reply but you actually can have a single power tube and still be push pull. There are low power designs that use a single 12A_7 tube with ine triode being push and the other pull.

Also there are some tubes that have a triode and a pentode in the same tube used in push pull amps. The triodes from the pair are used for the PI and the pentode halves for the PP power so in that case there is no dedicated PI tube per se.
Ah, good point, absolutely correct. I was assuming the common guitar single-tetrode/pentode power tubes as the output stage, but as you say there are lots of ways to package triodes/tetrodes/pentodes. Is there one term that covers a single "tube stage"? A "tode"? ;-)
 

kbgear

Member
Messages
1,129
This is not true.

All single ended amps are class A but not all class A amps are single ended and you can have class A push pull amps. In PP amps the class totally depends on how the designer set the bias point/load lines of the power tubes.
Though to be fair, you can have a class AB single-ended amp, it'll just sound like hell. ;)
 

Fulldrive-1

Senior Member
Messages
5,926
My old Orange OR120 had only two 12ax7 in it. The reason is that instead of having a standalone phase inverter tube like a Marshall, half of one of the tubes in the Orange was a split-load phase inverter.
 

rstites

Member
Messages
1,642
This is not true.

All single ended amps are class A but not all class A amps are single ended and you can have class A push pull amps. In PP amps the class totally depends on how the designer set the bias point/load lines of the power tubes.
I'm not entirely sure what this has to do with my post. By definition, a class B amplifier carries half the phase of an input signal in one device and the other half in another. Class AB allows just enough class A operation to remove crossover distortion. There's no way to have this separation into positive half-phase and negative half-phase without having some form of phase separation.

Yes, the definition of AB is squishy enough that you can take the same amplifier and adjust bias and call it Class A and one point and Class AB at another. It can exist along a bit of a continuum. That's not contrary to anything I've said.
 

lang.murphy

Member
Messages
3,582
When I had my TTHW in the Orange shop here in ATL, I asked for verification on which tube was the PI. I assumed it was the furthest from the input, which was correct. The diagram the Orange guy (Thank you Rory!) showed me indicated that the "second" pre was only PI, not split for other duties. Unfortunately, I believe what I saw was an internal doc, because I cannot find that layout via google. The TT online manual doesn't show tube layout.

Then... My Marshall Offset 1w. (JTM1H with new cosmetics and matching cab, referred to below as "the 60s".) I found this on AX84.com... posted by the designer, James Marchant:

"All the amps are push pull ECC82/12AU7 except the 70s which is single ended ECC81/12AT7.

The 60s and 80s are both 3 gain stages with cathodyne PI as has been correctly suspected. The 90s and 00s use a long tail pair.

The 60s has an input stage that has a cathode r/rc network designed to imitate the frequency response of a jumpered input with a little more bright than normal volume level, then gain then anode follower then the tone stack from a JTM45 - 56k slope and 220p treble. Only the treble control is fitted as a tone control. Then there is another anode follower to the PI. There is about 6dB of NFB into the grid of the PI."

Since the amp has one 12AU7 for the power tube and 2 12AX7's, I assume the middle statement above means V1 is 2 gain stages and V2 is split between a gain stage and the PI. Sound about right?
 

zzmoore

Member
Messages
7,236
Kbgear is pretty spot on with that reply but you actually can have a single power tube and still be push pull. There are low power designs that use a single 12A_7 tube with one triode being push and the other pull.

Also there are some tubes that have a triode and a pentode in the same tube used in push pull amps. The triodes from the pair are used for the PI and the pentode halves for the PP power so in that case there is no dedicated PI tube per se.
A 12_7 is not a single tube.
It is Two Separate tubes in one glass enclosure.
It is the same as two discrete tubes.
 

DrainBamage

Member
Messages
2,408
Your tiny terror in its simplest form is a hot rodded normal channel Voc AC. Hot rodded meaning 2 gain stages instead of one for the vox.
The Dark and OR expand on this by adding more gain stages among other things. All those amps have Long tail phase inverters. One full tube is being used for the PI. One half is a Driver (gain stage) and the other is the Inverter stage. Single ended amps still have the Driver stage as in the PI circuit minus the Inverter as it is not needed for this design/tone.
 

lemonpaul59

Senior Member
Messages
2,065
Tube amps that are "push-pull" use one power tube (or set of tubes) to push the positive waveform, and another tube (or set of tubes) to pull the negative waveform. The PI (phase inverter) is used to invert the phase of the guitar signal to one side, while sending an in-phase signal to the other side. This is generally the most common configuration for guitar amps, especially above ~10W.

A basic "single-ended" tube amp will have a single power tube (or set of tubes) that amplifies the full waveform, so it doesn't need a phase inverter (PI).

Worth keeping in mind that some phase inverter circuits only need one triode (half) of a tube, so a tube could be part PI, and part something else.

Generally push-pull amps are listed as Class AB, while single-ended amps as Class A. However, the class is independent of push-pull vs single-ended, so it's best to check the amp specs for push-pull vs single-ended.

That being said, an amp with only one power tube will by definition be single-ended, and thus won't have a PI. The Fender Champ for example has one preamp gain tube, one power tube, and one rectifier, and is thus single-ended. An even number of power tubes could be either, but an odd number of power tubes (does that even exist?) would have to be single-ended (no pairs).

Tubes can also be used to drive things like reverb tanks, effects loop send/returns, tremolo, etc. So while more preamp tubes can often mean more gain stages, it isn't necessarily true. For example, a Fender Brownface Super has 6 preamp tubes, but it's by no means a gain monster. ;-)

Hope it helps!

Gotta love single power tube Class A amps. I have 2, a Fuchs Lucky 7 Mk II and a Marshall SL5.
 

makerdp

Member
Messages
617
A 12_7 is not a single tube.
It is Two Separate tubes in one glass enclosure.
It is the same as two discrete tubes.
No it's two triodes inside of one tube. The name "tube" comes from the glass enclosure... it looks like a tube, thus the name.
 

Scott_F

Member
Messages
2,984
Tube amps that are "push-pull" use one power tube (or set of tubes) to push the positive waveform, and another tube (or set of tubes) to pull the negative waveform. The PI (phase inverter) is used to invert the phase of the guitar signal to one side, while sending an in-phase signal to the other side. This is generally the most common configuration for guitar amps, especially above ~10W.

A basic "single-ended" tube amp will have a single power tube (or set of tubes) that amplifies the full waveform, so it doesn't need a phase inverter (PI).

Worth keeping in mind that some phase inverter circuits only need one triode (half) of a tube, so a tube could be part PI, and part something else.

Generally push-pull amps are listed as Class AB, while single-ended amps as Class A. However, the class is independent of push-pull vs single-ended, so it's best to check the amp specs for push-pull vs single-ended.

That being said, an amp with only one power tube will by definition be single-ended, and thus won't have a PI. The Fender Champ for example has one preamp gain tube, one power tube, and one rectifier, and is thus single-ended. An even number of power tubes could be either, but an odd number of power tubes (does that even exist?) would have to be single-ended (no pairs).

Tubes can also be used to drive things like reverb tanks, effects loop send/returns, tremolo, etc. So while more preamp tubes can often mean more gain stages, it isn't necessarily true. For example, a Fender Brownface Super has 6 preamp tubes, but it's by no means a gain monster. ;-)

Hope it helps!
If you didn't copy and paste thy from somewhere, nice job.
 

drbob1

Gold Supporting Member
Messages
29,317
FWIW, all class AB amps have PI's. It's inherent in the class of amplification. SS state amps will also have PI.

All class A amps won't have PI's. Again, it's inherent in the class of amplification. The power tube/transistors are always on and thus no need to split the signal to them.
It's the second part of this that reads wrong: some class A amps have PIs. You can run a push-pull pair in class A, Alessandro builds a number of amps like this. It can get esoteric and frustrating when tubes go into cutoff (where they're technically no longer class A) but that happens with single ended amps, too! What DO you call a single ended amp that isn't conducting voltage for the full 360 degree cycle?

As to the OP's question, you have to look at the purpose of each stage in a tube amplifier to figure out what's going to happen. A preamp stage (which can be a single triode or pentode tube OR a tube with a pair of triodes or triode/pentode in one envelope) can supply gain, drive effects/eq, recover (makeup gain) from effect/eq losses or be a PI/output driver tube (by that I mean that some output tubes require a higher voltage than dual triodes can supply to push them to their full potential, so you can have a PI, followed by a pair of pentode driver stages, followed by the actual power tubes). As an example, a brown Super has 6!! preamp tubes, but technically only one gain stage.
 

kbgear

Member
Messages
1,129
[...]As an example, a brown Super has 6!! preamp tubes, but technically only one gain stage.
Well, if we're going down the slippery slope of too much detail, the Super (and scads of other amps) passes the signal through two triodes (a set for each of the two channels) before the PI. People have used the word "gain" to mean so many things that it's essentially a useless term. Even if I say every tri/tetr/pentode produces gain-as-in-amplification, someone will roll out a cathode follower or corner-case where the tube is set for unity gain and has a separate electrical purpose. There are very basic guidelines (single-ended vs push-pull, Class A vs AB vs B vs D, don't mix the two) but past that an amp is an electrical system that can't be defined by a single label. And that system is a little OT for the OP's question. :)
 




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