Soldano on impedance, loads, etc.

[jcj]

This was a discussion here yesterday, I believe. Stumbled on this tonight. This is the exact opposite of what I had always heard was correct.

Hey Amp Man!
In your column, "Ask Mikey", in the most recent issue of Musicians Hotline magazine, you said the following in answering a question about impedance mismatching. "In a mismatched impedance situation, it's always safer to have the load be less than what the amp is set at". That's contrary to everything I've ever heard or read in the past. It's always been my understanding that in a mismatched impedance situation, that it's always safer if the load is GREATER that what the amp is set at. That's the setup I'm currently using, please set me straight so I don't blow my transformer.
Thanks, Dave

Well, Dave, I don't know who you've been listening to or what you've been reading, because they're just plain wrong. Here's how it works: If the load is lower than what the amp is set for, like using a 4 ohm cabinet with the amp set at 16 ohms, the power tubes will be worked harder and will run hotter. This, of course, will shorten the life of the power tubes. However, if the load is higher than the amp's setting, like using a 16 ohm cabinet with the amp set at 4 ohms, the voltages in the output transformer will be higher than normal. These excessively higher voltages increase the risk of arcing, which can destroy the output transformer and/or tube sockets. That's why running an amp with no load at all invariably ends up blowing the output transformer. I'd say tubes are easier to replace, wouldn't you?

That's all for this month, folks. I hope you're all having a rockin' summer!!


Check out Michael Soldano's website at http://www.soldano.com.
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[JamesPeters]

http://aga.rru.com/FAQs/technical.html#imp-1

Read that and the point below it.

Saying that "using a higher impedance is always safer for tube amps" or something has always been wrong. That's a generalization which was taken from the world of solid state amps, as far as I can tell.

Mismatching either way is potentially bad in different ways. So, don't.

BTW, the AGA FAQ point doesn't even mention what happens if you're running a mismatch already, such as using 6L6 in an EL34 amp or vice versa (which happens a lot when the company selling the amp doesn't even say to change the impedance to compensate for the specific tubes used...aka Mesa, etc.) Or, if you have dirty contacts on your speaker jacks, output tube sockets, and so on. So even in "the best case scenario" there's a bit of risk involved mismatching either way. But in "the real world scenario" there's more risk involved than people realize.

 

[jcj]

Great info, thank you. There would seem to be a LOT of misinformation out there. I match 'em, because I don't know enough to feel safe not doing so, but there was quite a bit of back and forth on this recently, with differing points of view on what was the "safe way" to handle a mismatch. Seems it's very dependent on a number of factors. Thanks again.