would there be any advantage in having a pp amp wihtout a traditional phase inverter

[black_labb]

hi guys, i was wondering if there was any advantage of making a pp amp without a phase inverter. i was thinking of using a stereo cable from a guitar, with each side connected to a coil in a humbucker. the effect would be 2 virtually identical signals with opposite phases. (one of the signals would have a very slight more fundamental, and the other slightly more harmonic composition, but otherwise they would be virtually identical.

would feeding these into an amp and having the 2 channels running paralell until the output transformer have any advantages? would there be any advantages in doing this other than virtually no hum.

the issues arise with it when you consider that you would have to rewire any guitar before using it (not that big a deal if its just for you, but deffinately not ideal). the insutrument would have to have a humbucker and you would have to find a suitable switch to get a on-on-on dpdt switch to do the changing of pups (assuming a dual hb guitar). you wouldnt have any chance with pedals either.

is there any disadvantage with using a phase inverter, that would warrant using this design? or is this just something that would create an annoying hassle rewiring guitars so that you dont have to buy a 12ax7, some resistors and do 5-12 solder connections in the amp.

[stokes]

There are amp designs with a transformer coupled output,instead of a PI.They are more expensive to make.I havent made one,although I do have a transformer for just this purpose,maybe one day.I have heard they dont sound as warm with the transformer in place of the PI.I dont see any advantage to doing what you describe.Splitting the signal at the input would then require you to have two seperate preamps feeding each output tube.

[Old Tele man]

...although it would STILL use two separate driving tubes (dual-triode), you could similarly just use a input-driver transformer to drive two separate cathode-followers (see old AMPEG and Ted Webers powerful bass amp) which separately drive each "side" of the push-pull power tubes.

[Steve Conner]

The Fender 300PS and 400PS had transformer drive to the power tube grids. A single triode-connected 6L6 (or 6V6 in the 300PS) drove the interstage transformer (about the size of a Deluxe O.T.) that dished out current to the grids. Fender claimed that this gave considerably more power than ordinary Class-AB1 drive: for instance the 300PS was rated at 300W RMS with only four 6550 tubes.

I have no idea what they sound like, they're rarer than rocking horse sh*t, and I've never even seen one There is a guy selling 300PS output and driver transformers on Ebay, though: I bought a set last year but haven't had a chance to use them...

Black_labb, you could easily test your theory with two Valve Jr's or something, with an extension speaker wired between them. I bet it would sound really weird!

[MWJB]

"would feeding these into an amp and having the 2 channels running paralell until the output transformer have any advantages? would there be any advantages in doing this other than virtually no hum." I guess that in reality that you would have to do more than make 5-12 solder connections. Assuming that you have achieved phase inversion at the instrument, balancing pots accross the 2 channels would be a headache, especially if tapers vary slightly. Replacing the single pots on 1 channel with dual ganged pots and running both halves through these would make things easier in this respect.

"is there any disadvantage with using a phase inverter, that would warrant using this design?" typically guitar amps use SE preamp stages followed by a PI, these preamp stages are where a lot of your SE harmonics are generated in a P-P amp. I guess that eliminating the PI and effectively running a "P-P preamp" may reduce the overall SE character in the resulting tone, compared to a similar design using a conventional PI. This may or may not be desirable...very much "not" if you like heavy metal, maybe preferable for less fuzzed out tones?

[black_labb]

i was thinking dualganged pots, what would it sound with the 2 sides at different volumes? would it give a tremolo soound (increasing and decerasing volume)?

im not sure i understand how the fender 300ps and 400ps work, in terms of how the power is so much greater than other methods. is it because there is only a certain amount of voltage that can go through a regular phase inverter, and using the transformer inverts the phase as well as being able to handle higher signal levels from the preamp without distorting. or is it the transformer turning the high voltage low current to a higher current signal? or is it a different way of having the tubes act (which makes sense as they are differerentiating it from class ab1 amplification).

i thought that it might be a cleaner sound, which makes sense for a bass amp, and even more so being able to get higher wattages, im not sure if it is the transformer or the lack of a phase inverter that is making the difference.

does anyone know where i could find a schematic?

[MWJB]

With the 2 sides at different volumes it would probably just sound like a mismatched P-P amp/bad OT - drop in output & fidelity.

[Steve Conner]

300PS schematic: http://mhuss.com/PS300/Schematic.pdf
300PS info: http://mhuss.com/PS300/index.html

The transformer driver can push the output tube grids more positive than the usual PI can. To understand this better, read up on the difference between Class AB1 and Class AB2. Ordinary guitar amps are AB1, the PS300 was AB2.

Of course the*main* reason why the 300/400PS were so much more powerful than ordinary amps was that they had a bank of huge power tubes running off a 700V B+, and about 50lbs of transformer iron on board! Even with a regular PI, the 300PS would still have made 200W, and the 400PS would have made 300W.

[black_labb]

300PS schematic: http://mhuss.com/PS300/Schematic.pdf
300PS info: http://mhuss.com/PS300/index.html

The transformer driver can push the output tube grids more positive than the usual PI can. To understand this better, read up on the difference between Class AB1 and Class AB2. Ordinary guitar amps are AB1, the PS300 was AB2.

Of course the*main* reason why the 300/400PS were so much more powerful than ordinary amps was that they had a bank of huge power tubes running off a 700V B+, and about 50lbs of transformer iron on board! Even with a regular PI, the 300PS would still have made 200W, and the 400PS would have made 300W.

700v b+, that makes sense in upping the output, i dont know if i will make this amp, but ill see. i have other ideas before i reach that stage.

oneday ill have unlimited time and money to make every idea that pops into my head... maybe

[Steve Conner]

I found that if I work hard enough to get enough money to make everything that comes into my head, I don't have enough time and energy left over to make anything! Maybe I should have been a lawyer instead of an EE...

[Enzo]

In the typical p-p amp, the output is running AB, which means neither side amplifies the entire waveform. It relies on the two sides to provide the complete waveform to the output. In your design the two pickup signals would get to the output stage and... then what? If you ran the tubes in AB, each different signal would be an incomplete waveform - unles you ran the thing in A like two SE stages. This will wind up with the pos and neg halves of the output coming from two different signals.

Or so it seems to me.

[black_labb]

In the typical p-p amp, the output is running AB, which means neither side amplifies the entire waveform. It relies on the two sides to provide the complete waveform to the output. In your design the two pickup signals would get to the output stage and... then what? If you ran the tubes in AB, each different signal would be an incomplete waveform - unles you ran the thing in A like two SE stages. This will wind up with the pos and neg halves of the output coming from two different signals.

Or so it seems to me.

my thoughts are that the 2 waveforms would be the same waveform but out of phase, making it the same as what comes out of a phase inverter but without any of the distortion from the pi. would this not be the same thing to the power tubes and output transformer as any other pp system?

or is the pi part of governing the class of the amp?

[Enzo]

If the two pickups were the same waveform and wired together out of phase, they would cancel and there would be no signal in the guitar as wired normally.

The output tubes and transformer don't care what signal they get, they simply amplify whatever they receive. Each side has zero knowledge of what the other is doing. It is only because they both receive the same signal that a complete waveform results at the speaker terminals normally.

If you are worried about the distortion from the PI, then also worry about the distortion in all the other stages. Guitar amps are not hifi. or design the PI better. or use a transformer in place of the PI, as someone mentioned.

It would seem to me this design makes the use of NFB more difficult, and that means higher distortion from that.

The PI is electrically isolated from the power tubes - at least in DC terms - and has nothing to do with determining the class of operation.

[Chuck H]

O.K. For starters, Enzo is right. The two coils on a humbucker are IN phase and in series. To do what you propose would require that you inentionally wire them out of phase and in parallel. Even then you would not have two identical signals. There are schematics for wiring humbuckers out of phase and in parallel to achieve a specific tone. If the the two signals were identical, there would be no sound at all as Enzo pointed out. You would be surprised at just how much difference the small spacing of humbucker coils makes in the actual signal. I once had to move a pickup 1/2" closer to the bridge and the tonal difference was profound.

As far as achieving AB1 with two preamps...I don't see any reason why it wouldn't be possible, theoretically. But since most componants are +/- 5 to 20% and most tubes vary as well, it would require ALOT of balancing circuitry to get things lined up with no crossover distortion due to two dissimilar halves of the final output waveform. Also... I think it's possible that even if you were able to avoid crossover distortion, you would still have dissimilarities between the two halves such that you wouldn't get the 2nd harmonic distortion cancellation. So you may as well run class A parallel. OOO...Just thought of something else. Someone else mentioned the difficulty in implementing NFB with such a design. Well, consider that with any dissimilarity between the two preamps you would also have dissimilarities in thier distortions. Both good AND bad. And these would not be cancelled by such a circuit. Oh, you'd also have to use twice the preamp tubes. That could be very significant depending on the design.

It's a reasonable thought. Fun to consider. I had before. Not quite like your idea, but close enough. I just never found a way to make it worth trying.

JM2C

Chuck

[MWJB]

"It would seem to me this design makes the use of NFB more difficult, and that means higher distortion from that." Hi Enzo...that's a possibility, but it may also be the case that the natural distortion characteristics of the amp *may* be smoother and may not require the same degree of NFB. In any case it might be easy enough to tap NFB from a preamp cathode, like on a 6G2/SF Princeton/champ.

Out of curiosity I built a couple of amps that, whilst not exactly as black labb describes, have some similarities in principle and the NFB question came up with those...I found that so far it has not been necessary.

Chuck, why would you need twice the preamp tubes? A typical 2 channel backface amp (no effects) uses 3x12AX7, why couldn't you do the same with this?

In the long run, it's a novel idea...but the practicalities of having an amp that you could only use with 1 guitar (assuming that satisfactory phase inversion was achieved at the instrument) would seem a bind.