Traynor YBA-3 dummy load to use 8 ohm cabinet?

[beedoola]

I got an old YBA-3 recently. I assumed since there was no markings about impedance on the rear panel, that 4 ohms would be alright. After doing so research online, a lot of folks said it really needs 8 ohms, and that 4 ohms, using both the main speaker out and ext. out would be ok, since they are wired in series, not parallel.

Since my 4x12 is 4 ohms, is it possible to make a dummy load to plug into the ext. out to make the amp work/run more efficiently?

So the 4x12 is 4 ohms - plugged into the main speaker out.

The dummy load would be 4 ohms and plugged into the ext out.

The amp would then be getting an 8 ohm load, correct?

If this would work, how do I build a dummy load plug or whatever for the ext. out jack?

[Chuck H]

A 4ohm dummy load in the extension jack will work fine, but will actually reduce power by more than half. Speakers have a dynamic impedance and resistors don't. So if your cabinet has, say 30ohms impedance @ 100Hz, that would be parallel to the resistors at only 4ohms. The resistor is going to dissipate most of the power at that frequency. The same follows for the whole frequency range. Most guitar speakers, for example, only offer their rated impedance at a limited range in the mids and are much higher at either end of the spectrum. The amp won't mind though and it may even sound great!

EDIT: It occurs to me that you could always use a higher resistance in the extension jack to sort of average the impedance range. There are many power attenuators that do this. So, if you were to use something like a 10 or 15 ohm resistor this would still present an impedance @ frequency LOWER on the higher impedance dynamics than if you used an actual speaker load. The amp would still be quite happy and you won't lose quite so much power.

[g1]

The Traynor output transformers are generally more than robust enough to handle that kind of mismatch. I think they probably put that series jack arrangement in to avoid people using two 4ohm cabs and having a 2 ohm load on an 8 ohm OT.
You will get more power to the single 4 ohm cab with the mismatch than you will by adding a 4ohm dummy load in the ext. jack.
But make sure the switch part of the ext. jack is cleaned/deoxidized or put a shorting plug in it, that way you will avoid an open load scenario if the ext. jack switch contacts don't close properly.
Consider that many people would have also used two 8 ohm cabs, resulting in a 16ohm load, just as much a mismatch as 4 instead of 8.

[beedoola]

The Traynor output transformers are generally more than robust enough to handle that kind of mismatch. I think they probably put that series jack arrangement in to avoid people using two 4ohm cabs and having a 2 ohm load on an 8 ohm OT.
You will get more power to the single 4 ohm cab with the mismatch than you will by adding a 4ohm dummy load in the ext. jack.
But make sure the switch part of the ext. jack is cleaned/deoxidized or put a shorting plug in it, that way you will avoid an open load scenario if the ext. jack switch contacts don't close properly.
Consider that many people would have also used two 8 ohm cabs, resulting in a 16ohm load, just as much a mismatch as 4 instead of 8.

Cool, thanks. I'll just stick with a 4 ohm load then, and I'll examine the ext. jack to make sure the contacts are clean. When you mean a shorting plug, do you mean one that connects the tip to the sleeve?

[Chuck H]

Not surprisingly there are a couple of versions of the YBA3 that I found. One prefers a four or two ohm load with series taps on the transformer and the other uses the same series jack but for four or eight ohms but doesn't use taps to compensate. I'd say just plug in and go for it. As G said, those transformers are REALLY robust and will almost certainly be fine.

[g1]

When you mean a shorting plug, do you mean one that connects the tip to the sleeve?

Yes. It will act the same as hard-wiring that ext. jack switch closed and is more reliable than counting on the little switch contact to carry all the current to the load.