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Old 24th September 2008, 09:00 PM   #1
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Default Knight Amplifier Repair Question

I have a Knight KN940 stereo amplifier that I have been using for a while now and I have decided that it has come to the time where it needs a little work done on it. I don't know why but unless I am missing something or skipping over something I am having a bit of trouble digging up a schematic for the KN940.

The main thing I am wanting to do is switch out the rectifier bridge. It is original and by the looks they are Selenium or very very old Silicon. What would the experts recommend to replace these? I was thinking some 6A10's, UF5408 (or UF4007 since 3A isn't really needed) or the old stand by 1N4007 since I have hundreds of them. Any others I should look at instead?

I assume I will need to possibly add in dropping resistors since again I am assuming the V drop will be less with the newer devices. Any other issues I will need to watch out for?

There is a small bit of hum at the output, I am yet to scope it though to see what kind of hum. My guess is that it is power related since the power caps are all original too. I am wondering if it wouldn't be a bad idea to consider switching these out along with some of the coupling capacitors. Most of these (coupling caps) are orange-drop style or cheaper...some are ceramic. My guess is that switching these, even with Solen's...though Mudorf's M-Caps would be my first choice staying cheap...should bring about a bit of a sonic change for the better.

So far I haven't had any major grips with this amp, except that the bass can sound a bit loose and boomy at times. The valves are all original; all the 12AX7's are RCA's and they all test very strong, and the 6BQ5's are a mix unfortunately. Two of them are Raytheon's, though unfortunately one is US made and one is from Japan, the other stereo half has a beautiful and strong pair of Mullard's. I am getting worried about replacing tubes in the future though since the price of both these tube types are shooting skyward. I have 8 strong testing mixed brand 12AX7's in store, the majority RCA's...but only a single 6BQ5

Anything else you think I should attend to please let me know.

Cheers

James

P.S.: For those that where following the 2A3 build, it is still under way. I have all the parts except iron and 2A3's due to the lose of one half of the RCA pair. Sorry my building is slow, I have college to attend to, I am poor, and iron ain't cheap. Thanks though too all for the input on it and at my old 300B thread too.
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Old 24th September 2008, 09:14 PM   #2
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Schematics for old Knight amplifiers are ridiculously hard to find. There was a long thread about a KN928 here:
Very low output from Knight KN928 amp

Perhaps you can find bits and pieces of other Knight schematics which might have something in common with your KN940.
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Old 24th September 2008, 09:32 PM   #3
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Hi James. I think you are very wise changing out the rectifier set if they are that old. I would recommend using SKA3/13 which is a fast diode with a high PIV. You might consider fitting a surge resistor in with them, 30 ohms or so should do. If the smoothing caps are original they should definitely be changed and even if they are only a few years old they might have suffered from leakage from those old diodes. Any original electrolytics still in place ought to be changed too and it's wise to replace the output tube coupling capacitors with polyprop caps. You can't be too careful with leaking caps so if in doubt change them. Check the value of any high value resistors and replace any discoloured ones too, not that, that is necessarily an indication of them being out of spec but it's worth doing for the sake of a few pennies. It's worth taking half and hour just looking at the insides to see what's going on.

Geoff.
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Old 24th September 2008, 09:35 PM   #4
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The good news - the KN940 schematic can be found in Sams folder 1035-SED. The bad news... it's not in my Sams collection. See if your library can get you a copy, or try Bob at findatube.com (maybe $5)

Good Russian 6BQ5s are not expensive - and I've heard good things about JJs (unlike many OTHER JJ tubes...)
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Old 25th September 2008, 12:03 AM   #5
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I haven't heard anything bad about the JJ's either, maybe in the future I will try them. I can very close to pulling all the 6BQ5's, especially the Mullard's and replacing them; use the Mullard's later on a better project or just to have in my collection. Since then though I have kind of taken a different stance on my NOS tubes...Some tubes are rare to the point where they are nice to have and look at. I feel though that even increasingly rare audio tubes should be listened to, not just sat on a shelf and gazed at. They are meant to be heard and enjoyed...so for now, the Mullard's stay. Maybe JJ's in the future though, or dig in deep and get some more NOS.

Thanks for all the info, especially on the schematic, I will look into that more!

Cheers

James
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