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Old 22nd January 2011, 05:13 AM   #1
hvguy1 is offline hvguy1  United States
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Default knight KG-240? low output help

I pulled this from somebody's trash on the side of the road, it was mounted in what looks like an early 50's radio frame. Im not 100% if this is the right model, I found a similar looking knight here: Knight KG-240 stereo amplifier Radio Allied Radio Corp.;

to my surprise it worked when I plugged it in and applied a signal to the phono input.

it worked for about... 3 hrs, later the next day I was playing some music through it when the audio slowly faded away... I turned up the volume pot hoping it was a rough section on the pot but the audio stayed very low and scratchy. When I switch the inputs and flip the input switch it only outputs a little bit of scratchy audio on the phono input.

My buddy suggested I replace the filter cap, but after an hr of googling the internet, this is probably not the problem since there is no 60hz hum.

Here are the tubes that it has in it:
the 2 tubes with metal shields over them are: ECC83/12AX7
2 tubes side by side: EL84/6B05
one tube all by itself over by the filter capacitor: all i can read is EX80

any assistance is greatly appreciated since I have no idea how this amp even works lol.

as for my workshop, I have access to pretty much anything you can think of: DMM, scopes, leakage/capacitance measurement tools etc

thanks in advance!

here are some pics I took:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

and here is a video of the actual amp and its symptoms:
YouTube - knight amplifier broken
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Old 22nd January 2011, 02:46 PM   #2
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Nice find! The 'EX80' is probably an EZ80 by the looks of it, which is a rectifier.

Well, a good place to start is by drawing a schematic (or finding one online). Then perform some DC measurements as well as AC ones (scope). At first glance, the underside of the chassis looks really good; all I can see is that there are these two components (they look like high voltage, small value caps) that are obviously fried. Those will have to be replaced in any case, but before doing so, try to analyze this amp a bit further.
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Old 22nd January 2011, 04:37 PM   #3
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Look here:

MANUAL+SCHEMATIC KNIGHT 83Y784 Tube Amplifier 83 Y 784 - eBay (item 120593078200 end time Feb-02-11 07:16:07 PST)

It appears to be the original manual for your amp!

I bet you need some tubes, a couple caps (at minimum) and some switch/pot cleaning.
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Old 22nd January 2011, 05:00 PM   #4
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Default A Sweetie

I got one for Christmas in 1967 when I was 12. Mine was a 12 Watt Knight Kit KM-15. I sold mine in 1972. I missed it and was delighted to find these pictures of one online. My gripe about that unit was it's small output transformer, as the amp is power bandwidth rated 30-15,000 hz, +/- 3db and will only do 6 watts at 30 hz. A pair of those with heavy output transformers would be really sweet!


Yours looks like an older version of that model, but the circuit is probably identical to the Knight Kit KM-15 and you might find such a manual on ebay or somewhere else online.
Attached Images
File Type: jpg Knight KM-15 01.jpg (109.3 KB, 149 views)
File Type: jpg Knight KM-15 03.jpg (103.3 KB, 139 views)
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Last edited by imfree707; 22nd January 2011 at 05:05 PM. Reason: To add info.
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Old 22nd January 2011, 06:17 PM   #5
LAJ is offline LAJ  United States
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Default Slowly faded away?

My guess is you lost one of your filaments in a tube. Either the filament opened up inside the tube, or a loose connection on the socket.
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Old 22nd January 2011, 06:17 PM   #6
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Hi hvguy1,

Welcome to the forum.

The phono socket isn't going to sound very good if your playing something else other than vinyl through it, stick to the tuner socket, operate your switch a few times just in case there is a build up of corrosion.

Those EL84's usually have about 300v dc on their anodes so be careful, recommend you read the sticky thread on safety if you havn't already done so!



ps try swapping the ECC83/12AX7's around.

Last edited by Soonerorlater; 22nd January 2011 at 06:21 PM.
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Old 22nd January 2011, 06:42 PM   #7
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Quote:
My buddy suggested I replace the filter cap, but after an hr of googling the internet, this is probably not the problem since there is no 60hz hum.
This sounds like good advice to me - at least in the long run. If it doesn't leak now it will in the future. The absence of a hum through the speakers doesn't necessarily mean (imho) that your filter cap isn't leaking. If it is then maybe not enough B+ voltage will reach the valves. This would be my first check with the meter if nothing more obvious showed up on inspection.

Also on the safety issue, I can't see an earth cable - is there one?


Brgds Bill
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Old 22nd January 2011, 07:40 PM   #8
hvguy1 is offline hvguy1  United States
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its like Christmas with all these valuable responses!

im going to replace the obvious wax caps with .02uf 600v caps. then run it.. if no good, then ill switch tubes around.

thanks again for all the responses!
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Old 22nd January 2011, 08:03 PM   #9
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Quote:
Originally Posted by hvguy1 View Post
its like Christmas with all these valuable responses!

im going to replace the obvious wax caps with .02uf 600v caps. then run it.. if no good, then ill switch tubes around.

thanks again for all the responses!
Keep in mind that if a filter cap is bad it could take out the rectifier - and you can't diagnose the rectifier tube by switching it around.
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Old 22nd January 2011, 09:30 PM   #10
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It is possible that the filter cap (metal can on the top in the back) is leaking. After it got warm the leakage increased causing an increased drain on the rectifier tube leading to a slowly decreasisng B+ voltage and the sound fading away. As stated this can (and might have) kill the rectifier tube. Continued operation can lead to a fried power transformer and even an exploding cap.

If the can cap got warm during the initial testing this is surely the case. It the plates (the large black metal parts) inside the rectifier tube glowed red the cap is probably dead.
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