26-01-2018, 04:45 PM
Made a few more tests, but first I checked the cathode current on the 2C34 which was 38mA at rest, but dropped to 28mA at full OP. Suspecting the mains tfmr/PSU I used my big bench PSU capable of 500v 3A.
Checked quiescent I again, it was 43mA but dropped to 34mA at full OP which was 4.6v RMs which is a paltry 2.6w. This is with a Z pri of 16.2k - 8 ohm load. Swapped sec taps to get a Z pri of 8k with a 16 ohm load, OP dropped to 3.1v RMS.
Voltage on the grids of the 2C34 is 19.5v RMS or g - g of 39v RMS or 109.98v AC P-P , this should be more than enough to drive the OP valve into positive grid current/AB2; AFAIK, it is. The load line for either Z pri suggests we need a swing of up to -24v to +24v P-P which were getting. Can't work out why cathode current drops at full OP, it should go up surely.
Lastly changing the bias of the OP valve changes nothing much. The OP is not clipping but develops crossover distortion. Heads swimming a bit, so am probably missing something. Need to look at my old notes for the big amp and have a think, but I suspect the OPT isn't optimum for the job. I'll draw up a schematic later.
TFL, Andy.
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Checked quiescent I again, it was 43mA but dropped to 34mA at full OP which was 4.6v RMs which is a paltry 2.6w. This is with a Z pri of 16.2k - 8 ohm load. Swapped sec taps to get a Z pri of 8k with a 16 ohm load, OP dropped to 3.1v RMS.
Voltage on the grids of the 2C34 is 19.5v RMS or g - g of 39v RMS or 109.98v AC P-P , this should be more than enough to drive the OP valve into positive grid current/AB2; AFAIK, it is. The load line for either Z pri suggests we need a swing of up to -24v to +24v P-P which were getting. Can't work out why cathode current drops at full OP, it should go up surely.
Lastly changing the bias of the OP valve changes nothing much. The OP is not clipping but develops crossover distortion. Heads swimming a bit, so am probably missing something. Need to look at my old notes for the big amp and have a think, but I suspect the OPT isn't optimum for the job. I'll draw up a schematic later.
TFL, Andy.
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