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In Reply to: Re: Lessons from my 800B experience posted by PeterI on March 26, 2007 at 20:20:32:
I'm working on an 800C now. First, you can find a schematic on Tom Bavis's website. You'll see the 4 links to the gif files most of the way down the page. Also there's a number of posts that discuss the rebuilding of these, and similar, receivers if you search this forum.After reading through a number of these posts, I ordered the parts I needed from Jim McShane -- and he made many very helpful suggestions in the process. This just saved me tons of head-scratching and made the experience quite fun.
Follow Ups:
Thanks, I'm starting to put together the shopping list. Sounds like adding the resistors from cathode to ground on the 7591's is a priority, but I am hearing values from 1 ohm to 10 ohm @ 5W+. What is preferred?Pete
Adding the resistors lets you pick the best matched tubes if you have any extras. I used 10 Ohm, 1/2W. At 50 mA there's 1/4W of heat, and they usually are biased about 30-40 mA. A 5W part won't protect the output transformer... If you're gong to use modern tubes (only JJs will fit if you have a case...), you might want to reduce the grid resistors to 220K or 150K and increase the coupling caps by the same proportion. This may keep a tube with marginal leakage or grid emission from going "runaway".
Thank you, Tom. By coincidence, I got this 800C just after completing the restoration of my Fisher 680-A power amp with 7591's, and used your advice on that one. So you might say one was practice and 'learning curve' for the other. Obviously, they are different.
10ohm, 1/2W resistors are now on the shopping list.I grew up in Western NY, and my brother worked for Kodak, though in the now long-gone pharmaceuticals division.
Your website is helpful.
Any and all advice welcome.Pete
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