31-10-2016, 08:05 PM
(This post was last modified: 31-10-2016, 08:07 PM by Hartley118.)
Well, Andy this is puzzling. You can get 2W out of the EL84, but it's horribly distorted above 0.5W. The triode input stage looks OK, with plenty of voltage drive available for the EL84 so, as you deduce, there's something very wrong in that EL84 output stage.
NFB, by the way, never cures 'horrible' distortion because the 'horror' generally implies that the open loop gain is practically disappearing at some point on the waveform. That's why a NFB amp generally runs into clipping more suddenly than one without NFB - the open loop gain suddenly disappears at the clipping point.
My first thought was to suspect the EL84 valve itself, but you report that it seems to be drawing a sensible anode current, and a few volts either way on the screen will be neither here or there. I wonder, could your test rig be going unstable as you increase the gain to raise the output above 0.5W. Does the horrible distortion include any squeals?
I also wonder whether Mark may be on to something in looking in the direction of the output transformer. If the core were to be saturating, that would kill the primary inductance so that it would distort badly at low frequencies. Even if that were the case, I'd still expect it to perform reasonably at, say, above 1kHz. However, you say it's rated at 40mA, so should be OK - it should have gapped core and they don't suddenly saturate above rated current. I wonder whether it has a shorted turn somewhere. Might be worth testing it on an oscillator or just by feeding the secondary with, say 6V AC and see what comes out of the primary.
Martin
NFB, by the way, never cures 'horrible' distortion because the 'horror' generally implies that the open loop gain is practically disappearing at some point on the waveform. That's why a NFB amp generally runs into clipping more suddenly than one without NFB - the open loop gain suddenly disappears at the clipping point.
My first thought was to suspect the EL84 valve itself, but you report that it seems to be drawing a sensible anode current, and a few volts either way on the screen will be neither here or there. I wonder, could your test rig be going unstable as you increase the gain to raise the output above 0.5W. Does the horrible distortion include any squeals?
I also wonder whether Mark may be on to something in looking in the direction of the output transformer. If the core were to be saturating, that would kill the primary inductance so that it would distort badly at low frequencies. Even if that were the case, I'd still expect it to perform reasonably at, say, above 1kHz. However, you say it's rated at 40mA, so should be OK - it should have gapped core and they don't suddenly saturate above rated current. I wonder whether it has a shorted turn somewhere. Might be worth testing it on an oscillator or just by feeding the secondary with, say 6V AC and see what comes out of the primary.
Martin
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