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In Reply to: Lessons from my 800B experience posted by caffeinator on March 26, 2007 at 20:07:00:
Thank you - that is helpful. I have done many of those things with my other vintage tube amps, also on a Fisher 680-A (formerly console) amp with 7591's (which sounds great). I have meters but no o-scope. I'll take my time with it and research first.
Fisher Doctor sells a restore kit for these? They include schematics, I suppose. I don't worry about original appearance, esp. underneath.Wish I had the wood case.
Follow Ups:
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.
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
Vacuum Tube Valley also sells a good kit for the 800's - and it's not too hard to figure one out on your own with a schematic.I have two 800B's - one's a nice working unit; the other's a candidate for restoration. I like the AM - FM dual display and simul-cast capability.
On the case front, good news is, the same cases that fit the 400's and 500's will accomodate an 800. And they all look great.
Also, the scope isn't so necessary for these kinds of repairs...but if you're in the Seattle area I'd be happy to help you out.
Good luck - I know these guys aren't super-audiophile quality sound, but they sure sound good, don't they? And they look great doing it. We should all be so lucky when we're 50 years plus, eh?
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