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04-11-2025, 02:54 AM
My understanding is that there is an optimum impedance that will match the power tubes, whatever type they are, for maximum power. If you deviate up or down from there, it will give less power and more overall distortion with different mixtures of harmonic content. If you go too far out, then damage can result. It's possible to simulate the impedance mismatch with the primary winding by using a different speaker load to an extent, and I've read that going higher than the optimum match makes the tubes work harder and there is a possibility of arcing inside the transformer with high voltage and wide swings.
This is quoted from R.G. Keen's Geofex page;
'generally a mismatch by a factor of 2 either way is ok, with lower than expected load impedance being preferred.
Extended A: 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.'
And here's a quote from the '57 Custom Classic Owner's manual apparently;
'There are multiple things that can happen when you mismatch the impedance, and what is actually safe really depends on the particular design. If the impedance is too low, the anode dissipation of the tubes and the power lost in the output transformer (due to winding resistance) will increase, causing both to heat up more than during normal operation. If the impedance is too high, anode dissipation will decrease, but if you overdrive the amp the screen grid dissipation will increase. There is also the possibility of excessive flyback voltages, which can destroy the output transformer or the output tubes (or both).'
My brother and I used to run his Silvertone 1484 stock 4 ohm tap into an 8 ohm speaker and it had less power, more distortion, and kind of a squashed sound. It sounded fantastic for single note leads, but chords were a bit muddled sometimes. Surprisingly the output transformer is still working fine 25 years later with constant use, though now it is properly matched up to a 4 ohm speaker load, and is louder and cleaner with more definition than it used to be with the mismatch.
I remember way back on Ampage/Music Electronics Forum, 18 watt, and some other forums guys were talking about JTM45's and different impedances with those and the sound differences. You may search out those threads if you're curious.
Greg
This is quoted from R.G. Keen's Geofex page;
'generally a mismatch by a factor of 2 either way is ok, with lower than expected load impedance being preferred.
Extended A: 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.'
And here's a quote from the '57 Custom Classic Owner's manual apparently;
'There are multiple things that can happen when you mismatch the impedance, and what is actually safe really depends on the particular design. If the impedance is too low, the anode dissipation of the tubes and the power lost in the output transformer (due to winding resistance) will increase, causing both to heat up more than during normal operation. If the impedance is too high, anode dissipation will decrease, but if you overdrive the amp the screen grid dissipation will increase. There is also the possibility of excessive flyback voltages, which can destroy the output transformer or the output tubes (or both).'
My brother and I used to run his Silvertone 1484 stock 4 ohm tap into an 8 ohm speaker and it had less power, more distortion, and kind of a squashed sound. It sounded fantastic for single note leads, but chords were a bit muddled sometimes. Surprisingly the output transformer is still working fine 25 years later with constant use, though now it is properly matched up to a 4 ohm speaker load, and is louder and cleaner with more definition than it used to be with the mismatch.
I remember way back on Ampage/Music Electronics Forum, 18 watt, and some other forums guys were talking about JTM45's and different impedances with those and the sound differences. You may search out those threads if you're curious.
Greg
