Tweed Bassman + AC15, polarity/phase question

[bassmanboy]

Hello everyone,

I joined the forum to ask a question about polarity when running two amps at once.
I have a replica of a 1959 5F6A Tweed Bassman, (similar to say a Victoria 45410, or a Clark Piedmont etc...) and a modern Vox AC15c1.
Sometimes I run them together by daisy-chaining/jumpering the 4-input Bassman to the AC15, via one of the low-inputs.

What I want to know is whether anyone can tell, purely by circuit design details, are these two amplifiers in phase with each other not?

I have done a fair amount of research on google, which leads to many forums, and so I believe I understand the theory with signal path, gain stages, speaker wiring. And i've read about ground loop problems, buffers, a/b/y pedals etc.. and also how to check speaker cabinets for phase with 9v batteries, and many magazine articles too.

For myself though, I don't have any of those pedals, and because the amps are combos, the 9v battery trick isn't convenient. So i've kept it simple, just daisy-chaining/jumpering the two amps together from the Bassman to the AC15 and i've had no trouble with humming luckily.

So can anyone hazard an educated guess as to whether they are in or out of phase with respect to each other?

For reference they are both unmodified amps, no added gain stages or anything like that, the Vox has a Celestion Greenback and the Bassman has Jensen P10R's.

[g1]

For a simple test, reverse the wires on the speaker of the Vox. How does it affect the sound?

[Justin Thomas]

What g1 said. Usually if the two amps are sonically out of phase, the sound will seem thinner with one of the amps' speaker leads reversed. You want both speakers to move either forward or backward at the same time when hit with the same signal.

This doesn't really have anything to do with hum/noise, but more about node/antinode in sound waves - if in phase, certain frequencies will be reinforced and seem thicker/fatter/louder. If antiphase, it'll sound thinner because one speaker pushes a certain frequency in one direction and the other in the opposite direction. So certain frequencies will seem to drop out/sound tinny... I don't know it the effect is as dramatic as a phase switch on your pickups, but same concept. Though, I've never really tried swapping speaker leads on combinations of amps, so... FWIW.

Justin

[g1]

Depending on the position of the two amps relative to each other, I doubt you will even notice a difference, maybe at the real low frequencies. It's something more of a concern with subwoofers, especially between 2 woofers in the same cab.

[bassmanboy]

Thanks G1 and Justin Thomas.

Yeah I realised the humming was nothing to do with the wavforms and being in phase etc.., just thought it was worth mentioning that no grounding issues were going on either.

As for the nodes/antinodes of the frequencies coming out of the speakers, yes, that's what i'm talking about.

When I run my Bassman on its own, I tend to run it facing backwards so that I can turn it up more without any ice-pick for the front row. I tend to like the sound too, it's surprisingly not dull sounding and still carries well into the room, it's quite an open back.

So the two times i've ran the AC15 in tandem, I have had the Bassman turned backwards.

I actually haven't actually noticed any thinness or dropping out and I have a pretty decent ear I like to think.
(I often get compliments for my tonality in terms of balanced sound that's not harsh.)

So it could be the shows were a bit noisy and it's just lost in the heat of the moment, but I always forget at soundcheck to test with the Bassman facing forwards and then backwards, to see if one of them is a clear winner.

So at home, I have just tried at lower volumes of course, both running together, in a quiet room, and with the Bassman facing backwards and with it facing forwards, without touching the controls.

My results are inconclusive: The Bassman is punchier sounding facing forwards of course.
So I turned off the AC15 for a moment, and repeated the above with just the Bassman and it was actually just the amp itself.
When I faded in the AC15 again repeated, I can't seem to find any weird artifacts/hollowness/thinness in either configuration.

As to G1, your suggestion about speaker leads, the back of an AC15 is a mess of screws...an annoying back panel design.
I will try this when I have more time thanks.
That might give me a clearer answer.

I'm surprised as I just haven't noticed any real thinness or weirdness at all, but it's at low domestic volumes right now, I generally just play live and don't play a lot at home in my bedroom.
Perhaps this effect needs higher sound pressure levels to be apparent.

p.s. off topic but one note that I have found interesting is the tremolo on the AC15, running with both amps on:
With one amp fading in and out like that, there's a subtle top-end artifact thing going on, where the higher frequencies have a chewy phasiness happening almost like a chorus or phaser. Kind of vowel-like, ie: "wooaWwoaAoW" type of noise.
Very much like a brownface Fender harmonic tremolo.
I thought that this meant I had struck gold and discovered the 'out of phase' position of the amps, so I swapped the Bassman back to facing me again, engaged the Tremolo on the AC15, and it was exact same again.
So i'm stumped for now! I will get back to this thread and report if/when I find out, via G1's idea.

[pdf64]

..a question about polarity when running two amps at once. I have a replica of a 1959 5F6A Tweed Bassman, (similar to say a Victoria 45410, or a Clark Piedmont etc...) and a modern Vox AC15c1....So can anyone hazard an educated guess as to whether they are in or out of phase with respect to each other?

No, the absence of a negative feedback loop on the Vox means it's output could be either polarity.
http://bmamps.com/Schematics/vox/Vox...Schematics.pdf
Unlike many earlier such models, the normal and top boost channels of the AC15C1 look to be in the same polarity; otherwise you could simply set the channels up for similar tonal balance, and then see which matched best with the Bassman.

[Enzo]

I agree that the test is not how many inverting stages are in each amp, but really a listening test.

Set the two amps up next to each other, maybe a few feet apart. Ice picks be damned, face both at you. And connect your signal to both. Play a simple tone. If they are in phase, the sound will SEEM like it comes from the mid point between them. if they are out of phase, it will have a spacey quality that your ears cannot quite locate, it will sound like it is coming from anywhere BUT between the speakers.

And then just for the experience, reverse the wires on the speaker in just ONE of the amps. Now listen again.

If you like you amp facing the wall, you might be in the market for a "beam blocker", a little thing that hangs in front of the center of the speaker, and it deflects the beam from the center. Spreads it around a bit more.

[mhuss]

If you like you amp facing the wall, you might be in the market for a "beam blocker", a little thing that hangs in front of the center of the speaker, and it deflects the beam from the center. Spreads it around a bit more.

+1 I've both bought and made these, and they really cut down on the HF beaming, while spreading the sound around a bit more.

Although with a classic Bassman, having it turned around so the controls are "in front" might be an advantage too.

[bassmanboy]

I agree that the test is not how many inverting stages are in each amp, but really a listening test.

Set the two amps up next to each other, maybe a few feet apart. Ice picks be damned, face both at you. And connect your signal to both. Play a simple tone. If they are in phase, the sound will SEEM like it comes from the mid point between them. if they are out of phase, it will have a spacey quality that your ears cannot quite locate, it will sound like it is coming from anywhere BUT between the speakers.

Thanks Enzo, as described in above posts, I have done this (albeit at low domestic volumes) and I cannot notice a difference. Which is surprising to me, thus my query.
I have tried facing me, and not facing me, no strange qualities.
I also noticed no strange quality live, the two times I used them together, but I didn't have time to test both directions.

I will try with them further apart and at higher volume, when I next have an opportunity.

[bassmanboy]

+1 I've both bought and made these, and they really cut down on the HF beaming, while spreading the sound around a bit more.

Although with a classic Bassman, having it turned around so the controls are "in front" might be an advantage too.

Thanks mhuss, the Vox could do with a beam blocker actually, but the Bassman doesn't need it, and I can always turn the treble and presence down.
I enjoy the sound of it turned backwards at louder gigs, I play with overdriven sounds mostly and it allows for the power section to warm up a bit more, drummers can hear me better, and the cabinet still disperses the sound in a 3-dimensional way.

Comparitively the AC15 is like a laser-beam at times, very directional with the semi-closed back and non-resonant MDF cabinet, but those are off-topic thoughts!

[bassmanboy]

Thanks pdf64 - That link to the AC15 circuit will come in handy later i'm sure, and I didn't know about how the negative feedback loop affects potential polarity. Cheers.

[Richard]

Try putting them closer together. Do you have 2 microphones and recordablity? You could try inverting the signal on one mike and see the difference, assuming the mikes are similarly placed.