Ok great, thanks for that info, I'll have a look.
Reducing screen voltage 7591A's
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 some of the 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 could 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’m not sure if that is the best plan or not but I’m wondering if anyone can help get me going in the right direction. I’ll attach a schematic to my system page. The tube specs are found here. http://http//rtellason.com/tubedata/7591.pdf Thanks
6 responses Add your response
Um.... yeah, but not what you might think. You can’t just replace the cathode resistor with a big high wattage pot. Have a look here. And for more info in general here. I need to clarify the above posts. On my initial reading, I just assumed you were talking about the main B+ voltage. My bad. If it’s simply the screen voltage you are concerned about, you do not need high wattage resistors. a 1 watt should be fine. |
As it’s just the screen voltage, there should be no sag. You might look at reducing the main B+ though using 7 watt 100R etc. resistors in the secondary legs of the power transformer. You should also strongly consider using an adjustable negative voltage "fixed bias" instead of auto biasing with cathode resistors. |
Hi NoRo, thanks for the reply. Oops I meant to say 7591's throughout the post, I was looking at schematics for a different variant when I typed the post. Do you think using a resistor is the best way to go about it? Does that typically cause amp sag? Do you have any thoughts regarding the notion of using a zener diode instead to a resistor to reduce amp sag? Cheers |
What’s with the 7591? To get the resistance value or the dropper resistor, measure the approximate current in amps in the B+ supply and divide it into the voltage you want to drop. (V/I = R). Then to get the wattage of the resistor, multiply the voltage by the current in amps. Then increase the value for headroom.e.g. 50/0.2 = 250ohms. 50*0.2 = 10watts. So you’d need in this example, a 270 ohm 12-20 watt resistor. |