How to determine output phase or does it really matter?

[olddawg]

I have an EL84 PP amp that seems to have the OT leads reversed at the output jack. I put a positive DC pulse into the input and it seems to be going positive across the speaker when I observe it with my scope. It sounds OK. I've read post about people switching the OT leads to reduce hum. Is my test valid? Should there really be any difference in sound with the leads reversed?

[stokes]

Would probably only present a problem if you were using two amps at the same time,and the outputs of the amps were "out of phase" with each other.

[daz]

One way i suppose you could determine it is to temporarily right up some NFB and if it's not right it will howl.

[Enzo]

I think you are confusing several issues.

Swapping the polarity of the output would indeed swap the phase of the speaker, but hum is not usually the issue. The problem is when you have NFB - negative feedback - in the amp. The phase relation of the output of the amp to the input jack is irrelevant. But the phase of the feedback signal to the stage where it is inserted does matter. If it is reversed, then your negative feedback becomes positive feedback, and the amp sits there oscillating - WOOOOOOOO. So WHEN THE AMP SELF-OSCILLATES, we reverse the OT leads.

In an amp without NFB, the polarity doesn;t matter. If you want to find out, swap the wires on your speaker - does the amp sound any different? +in to +out is the standard way, but reverse doesn;t sound different.

When it does matter is what stokes described - using more than one amp together. if you Y into two amps, and one has the speaker going positive at the same time the other has its speaker going negative, then the two amps are out of phase and they cancel.

[Chuck H]

Oh contrair monfrair... I had a project wired up with a switchable series effects loop. Switching in the loop DOES reverse the phase, but the loop is reasonably transperant otherwise.

Switching in the loop with a jumper in the jacks does not change the basic tone one bit. But when the amp is overdiving switching in the loop does change the harmonic feedback. When I had the amp wired this way I could switch the loop in on the fly while sustaining distorted harmonics at high volume. The sustained note would fade and another different harmonic would take it's place.

So IME the phase of the amp can change the OD harmonic character as it relates to the self resonant frequencies between the guitar and amp.

Chuck

[Enzo]

Well monfrair here would counter that changing the phase of the amp is not the same thing as adding a phase reversing stage in the loop. The real test -and simple to do - is the reverse the speaker leads. That doesn't alter the amp circuit in the slightest, but the phase reversal is complete. A DPDT toggle should suffice.

I am curious about your loop though. If the loop jacks are simple cutout contacts, then of course a jumper wire would sound no different. I wonder how you inverted signal phase passively in the loop? Only way I can think of offhand is a transformer, and that would certainly affect overdriven signals. If you have an active stage doing it, then that alone is enough to alter your overtones.

[Sweetfinger]

I am curious about your loop though. If the loop jacks are simple cutout contacts, then of course a jumper wire would sound no different. I wonder how you inverted signal phase passively in the loop? .

I don't think he said it was a passive loop.

It probably has a buffer that changes the phase. Even though it is "reasonably" transparent and probably close to unity gain, reversing the phase ANYWHERE in the amp will affect the way the guitar feeds back and resonates. When you're on stage and trying to find the "sweet spot" for that note by turning around towards the amp, or to the side, or contorting in painful ways, you're changing the phase relationship between the amplified sound and the guitar strings. Same thing with a harmonica player playing through a mic, trying to find the sweet spot with maximum volume WITHOUT feedback. That little phase reverse button on some acoustic/electric guitar pre-amps doesn't change the sound of the guitar one bit, but as the sound level from the amplifier increases, it WILL affect the tone and resonance of the guitar itself.

[Sweetfinger]

The real test -and simple to do - is the reverse the speaker leads. That doesn't alter the amp circuit in the slightest, but the phase reversal is complete. A DPDT toggle should suffice.

I'll add that I will be doing this very thing sometime in the future for a harp player, just to see if it will help any better than turning and contorting about. We'll see.

[daz]

Heres a question....lets say i have a amp with NFB and i wired the OT primaries correctly so that the amp does NOT howl. Is it then possible it could cause it to howl if i changed the phase somewhere else?

[Chuck H]

Heres a question....lets say i have a amp with NFB and i wired the OT primaries correctly so that the amp does NOT howl. Is it then possible it could cause it to howl if i changed the phase somewhere else?

Only if you have a phase reversal within the loop itself. Adding a preamp stage anywhere in the preamp, for example, will not cause the nfb loop to become a pfb loop if it's the typical 'output fed to the tail of the PI' loop found in most guitar amps. Nor will reversing the speaker phase. The preamp is before the loop and the speaker is after the loop. That's why it's called a loop. Only the phase relationships in the loop between the PI output and the OT output effect it's function.

Enzo, it is an active effects loop. And yes, there must be some effect on the harmonics as a result. But the effect is more pronounced than that. You can actually hear (and feel) things flip around when you reverse the phase. In fact, other than the transformers and speaker configurations, part of the Fender/Marshall difference between the JTM45 and 5F6A Bassman amps (which are the same circuit) is because of this phase reversal. Ah, you say, but if they're the same circuit then why should the phase be different? Because old Jensen speakers were marked in reverse polarity with respect to Celestion and other brands.

Chuck

[Enzo]

Well, then there is your test. Use your loop circuit to flip between phases. Note the differences, then use a speaker phase switch to switch between phases and listen to those differences. ANy difference in differences - hows that for a sentence? - would be attributable to something other than phase.

If absolute phase through an amp was the determining factor, then it shouldn;t matter where in the signal path the reversal is accomplished.


And your explanation to daz was more concise than what I was thinking of saying, well done.

[olddawg]

Well guys, I went ahead and reversed the OT output wires to the way it was marked. The main reason I asked was because this amp was a home built Marshall clone that I picked up second hand and there was a plethera of things wrong with it. The speaker phase was the last thing that I noticed. I do think it made a difference. It sounds a little more expressive to my ears. Not a hugh difference and of course it is subtle and subjective. I didn't look at it on the scope inputing a positive going pulse. I just reversed the leads and played it. I like the way it sounds so I will leave it. I guess it is another one of those mojo things that can be discussed forever. I use pedals too, so who knows what that will do to the phasing. Probably the biggest reason I changed it was because I did not want to have phase cancellation with another amp. Hopefully it is correct now.

[Mark Hammer]

Last week I stopped in to the shop of one of our local legends in town to buy some tube sockets. This guy is in his 80s and has been doing sound, and tube stuff for well over half a century. I may well have been his only customer the whole day.

In any event, when I mentioned I wanted to make an amp with some 6GW8 tubes I had come into, he quickly suggested I ignore using any negative feedback. When I asked why, his response was that, as a novice builder, the risk was high that I would accidentally apply positive feedback, and fry the output transformer. Besides, he said, they sound better without negative feedback.

How would one be able to tell, reliably, that negative feedback was being applied, instead of positive?

[Chuck H]

This might have been a new thread but I'll answer anyway... Since it's an easy one.

NFB will create a reduction in gain and stabilize the output (reduce damping, odd distortions, etc.) and PFB will sound like your boiling a cat.

I don't think you run any real risk of damage placing PFB on ANY tube as long as your aware that things could be cattywhompus and work carefully. If you have PFB instead of NFB you'll know soon enough even with conservative amp settings so you can swap it around before anything bad happens.

Caveate... I have never built with 6gw8 tubes and am therefore unaware of any unusual PFB sensitivity they may have.

Chuck

[Enzo]

I don't know why PFB would burn out the OT anyway. The OT - and the power tubes for that matter - has no idea whether it is in the middle of some feedback or if you are simply amplifying a loud tone. If your amp and transformer can;t handle going WOOOO loudly for a few seconds, I don;t know how reliable it was going to be anyway.