How to change an amp's output impedance

[jondanger]

Thanks to anyone who can help with this problem! I have an old Yorkville guitar mate combo that I want to disembody and use with a 4 ohm bandmaster cabinet. The only problem is that the amp is 8 ohms. Is there any way to change the amp's impedance to 4 ohms so that it will match this cabinet? Thanks again for any help anyone can offer.
-Jonathan

[jondanger]

How can I change my amp's impedance? (repost from mods and tweaks)

Thanks to anyone who can help with this problem! I have an old Yorkville guitar mate combo that I want to disembody and use with a 4 ohm bandmaster cabinet. The only problem is that the amp is 8 ohms. Is there any way to change the amp's impedance to 4 ohms so that it will match this cabinet? Thanks again for any help anyone can offer.
-Jonathan

[chocopower]

Hi!

Maybe is an easyer job change the cab inpedance instead mod the amp.
If you put a 4 Ohm resistor (apropiate wattage) in series with the speaker/s, you´ll up the final impedance to 8ohm....

[jondanger]

Thanks a lot! Will this significantly affect the performance of the cabinet? I would still like to change the amp, since I use the same cabinet with another head. Is there an easy way to do this?

[J Martin]

I think the impedance of the resistor matters little at that power and frequency range. It will still have four ohms resistance that will turn some of the amps output power into heat. Better to just swap output transformers. Get something with multiple taps, save the old one. They are not expensive at that power level. Probably needs 8K to 4, 8 ohms or thereabout.

jm

[jondanger]

Thank you! I was wondering if changing the OT would do the trick. Are there any other components that would need changed out to make this work? Thanks so much for your help. - Jonathan

[J Martin]

You shoudn't need to change anything else.

[mjmiller]

Hmm... if the output transformer doesn't have taps for 4 and 16 ohms... some ways to do it would be to rig the bandmaster cab in series with another 4 ohm load, or, if the speakers are rated for 25 watts (the guitar mate is 25 watts into 8 ohms, right?) you could run the amp into just one of the two speakers in the cab. Or you could get a new output transformer, but I'm guessing you don't want to do that.

[Steve Conner]

What mjmiller said. Also, Ted Weber sells an impedance converting transformer quite cheaply. He would probably have a low cost replacement OT too, if you'd rather go down that route.

Finally, you could just hook it up and hope for the best. It will strain the tubes a little more, and you'll not get as much output power as you would with the proper impedance match. Also, the tone may change somewhat. But I say it's very unlikely to actually damage anything. If it were me, I'd try that first, and if you don't like it, then try the options that actually mean spending money on stuff

I have a 50 watt amp head with a 16 ohm only OT. I keep meaning to get a replacement OT or a converting transformer for it, but at gigs and jam sessions I tend to just cross my fingers and plug it in to whatever cabinet is handy. I like the tone best with the correct 16 ohm load, but it still seems to work all right on 8. At 4 it sounds pretty weak and crappy, and the tubes start to red plate.

[mjmiller]

I had a buddy with one of those Pignose 40watt tube amps. He decided the thing sounded better plugged into the wrong jack - I forget exactly HOW he mismatched the impedance, but he fried either the OT or his Tubes - I forget which... this was 10 years ago. Just thought I'd throw out a little warning...
Steve's ballsy

[Steve Conner]

Putting on a heavier load than the amp was designed for is relatively safe. OTOH, mismatching in the other direction is known to do awful things, like melting the screen grids in your tubes, or arcing your OT with excessive voltage. The extreme example of this is forgetting to connect a speaker at all, so you're driving an open circuit.

[R.G.]

Putting on a heavier load than the amp was designed for is relatively safe. OTOH, mismatching in the other direction is known to do awful things, like melting the screen grids in your tubes, or arcing your OT with excessive voltage. The extreme example of this is forgetting to connect a speaker at all, so you're driving an open circuit.

Usually a 2:1 mismatch upwards is benign. I don't know of any case where a 2:1 upwards impedance mismatch caused failures.

Even a 10:1 larger resistor soldered across the output jacks is good protection against completely unloaded by unplugging the speaker. At least, it's better than nothing.

[Fred G.]

Just rewire the cabinet for 16 ohms.

Voila! Problem solved. Won't hurt anything, RE-wire the two 8 ohm speakers that are currently wired in parallel (for 4 ohms)now, in SERIES for 16 ohms.

This is EASY and WORKS GREAT. This is all you should need to do. I've been doing this for years. Like prior posters said, it won't hurt a thing.

Swapping out transformers is an utterly needless and expensive pursuit. Not a good idea (unless you just want to do it).

[hasserl]

Why do that? Why not just leave it at 4 ohms? If you're going to run a mismatch, it's better to go with a lower load than a higher load.

[Fred G.]

Why do that? Why not just leave it at 4 ohms? If you're going to run a mismatch, it's better to go with a lower load than a higher load.

That's just not true. You should ALWAYS run into an equal or higher speaker load.