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Adjusting screen voltages on 7591As
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Author:  charnich74 [ 20 Nov 2019, 17:06 ]
Post subject:  Adjusting screen voltages on 7591As

I purchased an original Sherwood S-5000 ii, 7189 based 80 watt variant in original condition on ebay. I believe the amp is either untouched or minimally so. I've decided to go through it completely, recap the electrolytics, check resistors, update the coupling caps etc and see if I can't get it fired up. I haven't done so yet because one output tube was missing and 2 more were cooked. I've just purchased a new set of the re-issue tung sol 7189As as a matched quad. The issue I keep reading about though is that this amp was designed hot, and exceeded the max dissipation of the original 7189's but about 15%. Not to help is a line voltage above 120VAC, and the quality of the reissue tubes are no where near as robust as the originals. I'm going to run it on a variac and shoot for 110 V, and I'm going to adjust the bias by replacing resistors in the bias supply ( based on cathode current measured with 10 Ohm resistors from cathode to ground). But even with those steps I still feel that the screen voltage will be too high. Based on the spec sheets for the tubes, I can drop the screen voltage from 400 to 350 watts, and output only drops 2 W (from 45 to 45) and bias voltage can be reduced from -21 to -16.5. I think it is worthwhile to cool the tubes down a bit and get them running in a safer dissipation (considering the cost of the things). For only a 2 watt trade it seems like a no brainer. I've read a few suggestions from people on the forums of how to go about that, but I can't find anyone who can help me develop a plan to do so, and I don't know enough about electronics to do so myself. One suggestion was that I could simply put a zener diode in the resistor chain from the 400V source but I wouldn't know how to pick the right one etc. I was wondering if dropping the screen voltage an be relatively easily done and if so how can I go about accomplishing it. I'm attaching the schematics and the tube spec sheet. Thanks

Author:  charnich74 [ 21 Nov 2019, 23:09 ]
Post subject:  Re: Adjusting screen voltages on 7591As

*edit* the tubes are 7591's. I mis-typed in the body and said 7189 but they are in fact 7591 as the title states. Cheers

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