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Old 23rd July 2008, 11:56 PM   #111
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It's been a while since I fired up the HK A230. The amp I use these days isn't really an integrated amp. It's just a power amp, and I have an Adcom preamp in front of it. I honestly don't recall where I typically kept the volume knob when I did use it. Besides, I took out the tone circuit, so the overall gain of the amp probably isn't the same as it was from the factory.

There's a couple things you might be able to do to get more output from your Knight. First check and make sure the driver tubes aren't on their way out. Swap the 12AX7 for some different tubes from one of your other amps. You could also try putting your speakers on the 8 ohm terminals instead of the 4 ohm (or the 16 instead of the 8). That will put more voltage across the speaker, which will make it louder, but it will also put a lower impedance load on the output tubes, which might make it distort sooner.

You could also experiment with a higher resistance in the negative feedback loop. That's the 47K ohm part going from the output transformer back into the 12AX7. A higher resistance here (or even disconnect it completely) will put less NFB back into the circuit. The overall gain will increase, but distortion will also go up and the amplifiers damping factor will go down. You'll end up with "flabby" bass if you don't have enough feedback.

Keep in mind that not all input sources have the same output level. A typical CD player will put out a higher level signal than an iPod. Have you tried different input sources?
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Old 27th August 2008, 07:46 PM   #112
brocks is offline brocks  United States
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Ty

I have a KN924 that need the pwr sup caps replaced. I was thinking about doing what you talked about in your poast 58. What would be the adavantage of using the larger caps. Also do you know the difference between the 924 and the 928? Feel free to email me if you like at gary@brocksflyfish.com

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GAry
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Old 27th August 2008, 08:03 PM   #113
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Quote:
Originally posted by brocks
I have a KN924 that need the pwr sup caps replaced. I was thinking about doing what you talked about in your poast 58. What would be the adavantage of using the larger caps. Also do you know the difference between the 924 and the 928?
Post #58 (see also #72) describes the output tube's cathode bypass capacitors. Bigger caps at this point should increase the circuit's bandwidth (better low frequency extension). It's probably a moot point, since the output transformers probably can't handle it. You might even worsen the system's overall performance by going too big here.

Don't confuse these capacitors with the power supply caps. They are a different part. If you intend to replace the power supply caps, my advice is to stick with the same size for the "first" capacitor (the one right after the rectifier tube). Increasing the capacitance here will shorten the life of the rectifier. You can safely increase the size of the other caps (within reason). Make sure the voltage rating of all the power supply caps are the same as or greater than the original parts.

From my notes, the KN928 used a 9-pin 6CA4 vacuum tube rectifier, four 12AX7 tubes in the driver/preamp stages, and four 6BQ5 output tubes. I don't know much about the KN924.
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Old 29th August 2008, 11:07 PM   #114
brocks is offline brocks  United States
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Ty
THe KN924 uses the same tubes as the 928. I finally got around to taking the bottom plate. I remember when I was a teenager I put a buss bar in and it made a big difference with the noise. I also did what you were said in post 58 except I did it to the pair of 6bq5's, not individualy. This also made a difference in the sound. I used a 25mfd@25V with a 100 ohm resister which was the original size. I noticed in your 6bq5 diagram you used a 250v cap. How critical is the voltage? Regarding the filter caps for the pwr sup would it be better to replace them with individual caps providing I can fit them in? Also, what manufacture of caps would you reccommend?

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GAry
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Old 29th August 2008, 11:52 PM   #115
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Good catch. I copied and edited a diagram from an earlier post. Caps rated for 250V are completely unnecessary in that position. The most voltage they should ever see is the bias voltage of the 6BQ5. In most applications, that's 10 to 15 volts. I'd double that to be conservative. A cap rated for 35 volts should be more than adequate, and 25V will certainly be fine too. I'll go back and edit my copy of the diagram.

Concerning the power supply caps, I don't think it matters much whether you use a multi-section "can" cap, or individual parts. I think the cans tend to be expensive, and I'd probably go with separates myself.

I like to make sure the power supply caps can handle the voltage even with no load on the PSU, so that means they ought to be rated greater than 1.4 times the power transformers no-load voltage. If you have a 300-0-300 volt transformer, I'd go with 450V rated caps for the power supply. Sometimes this can be hard to do, since general purpose electrolytics rated for more than 500V are difficult to find. Always make sure any replacement caps are rated for at least as many volts as the part you are replacing.

I'm not experienced enough to have a strong preference for capacitor manufacturer. I guess for power supply caps I've used Panasonics, since they're readily available through Mouser and DigiKey. Lower voltage electrolytics (like the cathode bypasses) will probably be Xicon or Nichicon. For coupling caps, Sprague "Orange Drop" 715P or 716P work for me, or maybe Wima MKP "box caps" if I'm in the mood to spend extra money.
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