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Hello Forum:
I have a 1973 vintage HiWatt DR201 amplifier in for re-capping the power supply capacitors.
When completed, I checked the bias setting and found it to be -46 VDC.
When I installed the four Genelex KT-88 tubes and checked the cathode currents of each tube, and found to be quite high 46, 51, 46, and 63 mA respectively. The AC line voltage was set with a variac to 117 VAC, and the plate voltage is 660 VDC, and the screen voltage is 391 VDC. I have tried another quad matched Genelex KT88's with similar results, and a matched quad of Sevetlana 6550's, with cathode currents in the 50 mA region with plate voltages in the 660VDC range. I have checked the two 10K bias potentiometers, and the bias supply voltage is -49 VDC maximum.
I need to get the cathode currents down to 70% at Idle, and with plate voltages in the upper 600's, I need to keep the vintage Partridge Transformers safe.
Since I cannot go more negative that -49VDC, can I get some suggestions on solving this bias problem.
Thanks,
Jay D.
I have a 1973 vintage HiWatt DR201 amplifier in for re-capping the power supply capacitors.
When completed, I checked the bias setting and found it to be -46 VDC.
When I installed the four Genelex KT-88 tubes and checked the cathode currents of each tube, and found to be quite high 46, 51, 46, and 63 mA respectively. The AC line voltage was set with a variac to 117 VAC, and the plate voltage is 660 VDC, and the screen voltage is 391 VDC. I have tried another quad matched Genelex KT88's with similar results, and a matched quad of Sevetlana 6550's, with cathode currents in the 50 mA region with plate voltages in the 660VDC range. I have checked the two 10K bias potentiometers, and the bias supply voltage is -49 VDC maximum.
I need to get the cathode currents down to 70% at Idle, and with plate voltages in the upper 600's, I need to keep the vintage Partridge Transformers safe.
Since I cannot go more negative that -49VDC, can I get some suggestions on solving this bias problem.
Thanks,
Jay D.
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