Super Reverb with 212 Speakers.

[Tonefree]

Hi, this is my first post, I am considering putting a pair of 8 ohm 12" speakers in my Super Reverb and wanted to know if there is a resistor or something I need to do to match the ohms for the amp? The 410's parallel series wired create a 2 ohm load so I assume the two 8 ohm wired in parallel are at 4 ohms, so I think I need to create a 4 ohm dummy load to end up with 2 ohms. Am I making sense? Cheers.

[DonMoose]

If it's not too late, get a pair of 4-ohm 12"s - wired in parallel, that's 2.

The next-best option is an output transformer with more load options.

The resistor works, sort-of, but it's not the best option.

[Tonefree]

I was hoping to put some EVM 12L's in. I think they are only 8 ohms. Thanks I haven't purchased anything yet, just doing some research to see if it is feasible. Cheers.

[Cliff Schecht]

You can buy some dead EV speakers cheap and have the voice coils set to whatever impedance you want.

[Firestorm]

And then, there are more than a few folks who think the Super sounds better into 4 ohms anyway.

[Tonefree]

Would this stress the output transformer?

[Reeltarded]

No, not really. 100% mismatch in either direction works ok if everything is in spec. A higher load is better than a lesser. Your mismatch goes up 100%. Cool. (er)

I think at some point we decided (me and my other monkey) that the only thing you wouldn't want to do is 100% down mismatch from 4 to 2. It smells funny, and may stop working a few times out of 10.

I wouldn't mismatch downward with any tweed amp, but Marshalls and Blackfaces? Done it a billion times.

[Cliff Schecht]

When mismatching low (e.g. 8:4) you have to watch out for plate dissipation and with smaller OT's they will get hotter than normal. I wouldn't push one of the dinky 6L6 transformers Fender used in some of their tweed stuff. Both of these things are easy to monitor though and like stated above, any beefy OT should be able to take this.

When mismatching high you can get flyback voltages that are MUCH higher than usual. They can easily exceed 1000V if you suddenly go from not playing to full output power with a lot of distortion (large transients) or pretty much any scenario where you kick out the speaker cones quickly. The impedance mismatch makes any reflected voltages go from a 1:1 ratio to a higher ratio (any increase here is bad really). If you feel like getting mathy (I don't at this hour ) you can derive these ratios using voltage ratio = square root of the impedance ratio. You can look at these kickbacks really only using a scope and even then, if they are high enough they will quickly blow the front end of your scope out! Also, since most OT's are only HiPot tested at anywhere from 1500V-3000V (depends on the quality) you will also quickly develop a short in your OT. FWIW this is I think why KF added the three 1000V diodes coming off of the plates in the Express. These squash any dangerously high transients and prevent damage. Perhaps this is why in his literature he mentions that the Express can be mismatched no problems for use in the studio, small gigs or at home at night.

[Reeltarded]

That is an interesting point. I have been lucky.

Flyback kills. I have some of the most prone to be killed amps ever made and 25% of them are in fact dead, even though there was never a mismatch.

Now you have me paranoid.

[Tonefree]

So if I have an extension cabinet and want to put two EVM 12L's in it, what are my options?