dodger916
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In a recent thread concerning an LSC output impedance question, I was surprised to read the quote below, which apparently contradicts Mesa's "safe mismatch" instructions. If I'm understanding the quote below correctly, it's better (less detrimental to the ouput transformer) to plug a lower resistance load into a higher resistance ouptut on the back of the amp. This is the opposite of the Mesa manuals (and contradicts all that I've read about it), which recommends running greater resistance loads from a lower resistance output.
Any techies care to comment?
Any techies care to comment?
The amp doesn't run any cooler with a mismatch. That's bogus and they should be ashamed of themselves for saying that.
Go here, to R.G. Keen's tube amp FAQ:
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch
The salient points:
It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.
Note that he says the OT can withstand the spikes...that does NOT mean that the tubes can.