Newbie question - how can I test output tranny polarity?

[moonshot]

Can it be tested with a multi-meter?

I have a vintage harmony 18 watter that I have recapped.

When I first bought it, I could immediately hear that the 2x12's were out of relative phase with each other. After putting the speakers in relative phase, I realized all of output wiring had been futzed with so I have no way of knowing if the two leads from the tranny are the proper polarity.

It sounds good but I am afraid to crank it up fearing that I might cause the speakers to suck in rather than extrude.

The leads come out of the transformer and attach to a terminal tree. One leg of that tree connects to the chasis so I assume when it's properly wired that the negative output wire should attach to that leg.

Forgive my ignorance, I don't know circuits so any insight would be appreciated.

[rfgordon]

The secondary leg tied to chassis ground is the ground. As for the speakers, they are just motors, and they don't care which way you connect the leads, so you can't damage them. Some famous amps, like the tweed Bassman, play backwards, relative to the guitar. Simply connect things the way they sound best to you.

[austinb]

What is important is that both speakers polarity be the same. You don't one sucking while the other it pushing. If the speakers aren't marked with a red dot or something , you can use a 1.5 volt battery to test them to find what polarity pushes. That will be the positive of the speaker hooked to positive of battery when speaker is pushing.

[benoit]

The only potential for a serious phase problem (provided your combo speakers are in phases with each other) would happen if you ran another amp along with it that it was out of phase with (with respect to the dry guitar signal) this amp.

[rmb550]

Is there a feedback circuit in that amp? I would think the relative phase of the feedback signal, if used, would be of more importance.

[wscrane]

I just re-labeled my speaker phase switch "suck" and "extrude." Much better than plus and minus.

It would be cool if there was a way of mis-wiring an amp where it would actually suck all the sound out of a room.

[moonshot]

Thanks for your responses!
wscrane's reply cracked me up! It was worth posting just for that.

I definitely have both speakers in relative phase because hearing out of phase speakers sounds like an icepick driven into my ear.

I don't know if the circuit features feedback or not. It is a push/pull A/B amp with a phase splitter 12ax7 so I assume it does based on my limited knowledge.

I have the schematic. In a related issue, I find the tremolo to be a little wimply on my amp. I am wondering if somebody can help me tweak it to have more depth? I am going make a post about this.

In response to rfgordon: Last night I found some color codes for output trannys and most list black being the secondary leg which is tied to the chassis so that's what I have gone with. The other wire is green which I found out is usually the code for 4ohms which would be correct for my amp. That being said, I suppose it's wired properly and I won't worry about it.

[rfgordon]

moonshot,

Yes, plain black is usually the common/ground leg.

However, if you work with any vintage output transformers that were in something other than a guitar amp, you might run into other arrangements. A lot of older amps/receivers had OTs designed for both direct speakers (4, 8, 16 ohms) and 70 volt distributed lines, like were used to run long strings of speakers in a store ceiling, for instance. What you have in those cases is actually two secondaries on the OT. Some makers had that ground as black/white striped, but not all.

I bring that up because a lot of those OTs are out there, and many of them were absolutely top quality, and sound amazing in a guitar amp.