Output impedance tolerance?

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sepulchre
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Output impedance tolerance?

Post by sepulchre »

I'm wondering if I can hook up a group of speakers to one or other of my amps. The best impedance I can get is 12 ohms. My amps mostly want to see 8R but a couple have 4R, 8R and 16R outs.
So first of all, how far off can the amps tolerate? Secondly, is it better to go higher or lower in impedance if a perfect match can't be made?

Thx,
Ken
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jjman
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Re: Output impedance tolerance?

Post by jjman »

For tube amps it is safer to use a speaker with a "too low" impedance vs a "too high" impedance. For solid state is it the opposite. The risk depends on the output tranny and the amount of signal pushed. There is a consensus opinion on the internet that it is safer with a vintage Fender than a vintage Marshall to go “too high.” I doubt there is any actual quantification or proof but it could certainly be true.
If it says "Vintage" on it, -it isn't.
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sepulchre
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Re: Output impedance tolerance?

Post by sepulchre »

Thanks, though I was hoping 12R was okay for an 8R output. But maybe there's a way to get a little under 8. I have 3 8R boxes that I can rig in parallel or series.

I know it depends on the particular amp but generally speaking would 4R be to low for an 8R out? I heard somewhere once that a full step (eg: 4 into 8) is ok. But I don't recall where.

Btw, the amp I want to try this with has an Edcor GXPP 15-4-8K output tran. It's a low wattage amp, like 10 or 12 watts.
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xtian
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Re: Output impedance tolerance?

Post by xtian »

Don’t worry about it! Since I started building eight years ago, I’ve run a lot of mismatches, and intentionally stressed my amps to find out where their limits are. The only time I ever had an issue was trying to push a 16ohm cab with a 2ohm OT in a 50 watts amp—after cranking it up loud, it shorted a power tube and burned through my 100R heater balance resistors. No other damage done.

I would never sweat a one-step mismatch.
I build and repair tube amps. http://amps.monkeymatic.com
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roberto
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Re: Output impedance tolerance?

Post by roberto »

It depends.

If you have an high voltage and low primary impedance (like in most Marshalls), I would connect the 12 Ohm to the 8 Ohm tap. This way the reflected impedance would be 1.7k x 1.5 = 2.55k that is safer for the tubes. A 2.2k will become 3.3k that is ok too. Otherwise they'd become 1.2k and 1.6k respectively. A bit too low.

Also, the secondary wingind has an higher impedance than the design one, so the current flowing through it will be lower, and it's also safer for the output transformer too.
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sepulchre
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Re: Output impedance tolerance?

Post by sepulchre »

Thanks everyone! Now I'm ready check it out. If worse came to worse I have two of these amps ( my Harlow) on hand so plenty of replacement parts. But I have faith in you guys. I'm going to try eithout sweating.

Thanks again, cheers!
Ken
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