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Post subject: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 11:13 am
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I have a '76 Twin Reverb that was previously owned by a world-famous professional musician that he used as his main concert and recording amp (I am only the second owner). He specifically modified by replacing the stock 8 ohm speakers with two older 4 Ohm Oxford speakers (in parallel, giving a 2 Ohm impedence). The amp sounds absolutely amazing, but what effect exactly would this have on the amp in terms of tone, headroom, tube wear, etc? I know that the output transformers in the older TR could handle the 100% mismatch in impedence, but why would someone do this (unfortunately this musician is no longer alive, so I can't ask him personally)? Should I change them out and replace them with 8 Ohm speakers for any reason or leave in the Oxford 4 ohm speakers?

Any insight that anyone might provide would be appreciated! Thanks.


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 4:50 pm
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Yes, better to put 8 ohms speakers. 100% mismatch for a 8 ohms is 4 ohms, 2 ohms is too low.

You can damage the output transformer and output tubes tubes .

For now put those two speakers in series and play at low volume.

You are sure that your Oxford are 4 ohms ? How do you know that ?


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 5:40 pm
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Thanks for the reply. The Oxfords in my TR are stamped "4 ohm", so I assume that they're 4 ohm impedence. A Twin Reverb's two 8 ohm speakers are typically wired in parallel to give the 4 ohm impedence standard for the stock output transformer, right? The 4 ohm Oxfords in my twin reverb are wired in parallel to give a 2 ohm impedence. I've generally heard that the 4 ohm output transformer of these amps can handle a 100% mismatch (either 2 ohm or 8 ohm impediance would be ok), so I thought the two 4 ohm speakers wired in parallel would be ok, but I'm not sure and wanted to get some more expert opinions. And even if its ok to run the amp with two 4 ohm speakerss, I don't know what the effect would be (and why someone might purposefully modify their amp to this configuration for a particluar reason).

Thanks for any help!


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 5:44 pm
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Check the number on the output transformer and let us know what it is....it could have been changed.


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 7:10 pm
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H ijman72,

Congrats on the amp, one of the great ones!

I believe that the TR was made for a 4 ohm speaker load with the internal speakers (two 8 ohm in parallel), plus it has an ext jack which would allow even lower impedance with an external speaker cab. A 2 to 1 mismatch for any tube amp and speaker is fine, and you can go as low on the impedance as you want, all the way to a short circuited output without any damage. The matching is only about amp efficiency, and when impedance is mismatched the amp won't develop full power. In a TR, you might not need all of the power anyway. The damage to a tube amp can occur with too high an impedance, and the most that I've gone is 4 to 1, using a 16 ohm speaker with 4 ohm amp (the limit IMO). There is not a set number for what is too high, but at some point, up to and including an open circuit such as when no speaker is connected, the impedance is too high and can cause damage. If your amp has a functioning, shorting jack, some do some don't, then the amp is protected in case the speaker is not plugged in. It looks like the TRs have the shorting main speaker jack.

Short version - even if the speakers are 4 ohm each in parallel, giving 2 ohms total, this is a 2 to 1 impedance mismatch and is fine for a tube amp. Even if the amp were an 8 ohm, a 2 ohm speaker would work fine, no damage, but at this point the tone might suffer. So, in this case if it sounds good, let it be, don't worry and enjoy. 8)

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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 8:10 pm
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If said world famous dead guitarist used the amp in this configuration for so many years, who are we to suggest changing it. :lol:

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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 9:38 pm
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Excellent point bluesky636!

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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Mon Jan 02, 2012 9:40 pm
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OK... I'll bite. Plus the curiosity if killing me. Was it Danny Gatton?


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Tue Jan 03, 2012 2:39 am
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jman72 I do a mistake when I write your amp impedance is 8 ohms and a mismacht to 2 ohms is too low. 4 ohms is safe.

Follow what Shimmilou write , he said the right thing .

I believe if you have a good tone leave the amp like that. You could have a great amp, a great musical amp.


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Tue Jan 03, 2012 2:40 am
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+ 1000 shimmilou


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Post subject: Re: 4 Ohm speakers in a Twin Reverb
Posted: Fri Jan 06, 2012 8:47 am
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It shouldn't be a problem at all. My 76 Twin was rated at 4 ohms as well. Great sounding amps as long as you don't pull the boost knob on the master volume. I kept the master volume on 10 and just used it like the MV never existed. It gave me over 30 years of trouble free service. Congrats on the new amp!!

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