kirk57 said:The reason I was seeing 120 as opposed to 320v is because the batteries in my DMM are running low and some of the LCD segments aren't working right...so for the EL84 it's about 320v
How to increase the current then? Change the cathode resistors?
Yes, you can change the cathode resistors within reason. Remeasure the cathode voltage now that your dvm has a fresh battery - if still 12V head for about 330 ohms, if higher start with something near 390 ohms - including screen current this should still be well inside the maximum dissipation rating of the output tubes.. Don't go to much below the 330 I mentioned earlier.
I've replaced the cathode resistors with 330 ohm ones, and replaced the coupling caps with Orange Drops. Seems a bit
louder now, and definitely more detailed. I need to run this for
a while to let all the new parts break in, but it's promising.
I'm still getting some hum, regardless of the volume control position. I know on the Knight 724 page,(http://wb9k.tripod.com/) the author mentions he eventually moved the entire power supply to another chassis and only then did he get rid of the hum....
Do you guys hear this on your (working) amps? maybe it's something I just need to live with...
louder now, and definitely more detailed. I need to run this for
a while to let all the new parts break in, but it's promising.
I'm still getting some hum, regardless of the volume control position. I know on the Knight 724 page,(http://wb9k.tripod.com/) the author mentions he eventually moved the entire power supply to another chassis and only then did he get rid of the hum....
Do you guys hear this on your (working) amps? maybe it's something I just need to live with...
kirk57 said:
Hum is a tough problem. It can be caused by all manner of things. If the power transformer isn't mounted in the "correct" orientation, the field leaking from it can actually induce a hum in the output transformers. I suppose moving the power supply to a separate chassis can eliminate this problem, but it seems like a drastic solution. I'm not too keen on the idea of a 350 volt DC umbilical cord dangling across my stereo rack, either.
It is more likely that you've ended up with a ground loop somewhere. It could be something simple like a ground loop between the amp and whatever you're using as your input source. You could have a ground loop within the amp itself. It's popular to build a "ground bus", where a heavy gauge copper wire (or even a copper bar) is grounded to the chassis at only one point. Then all the power grounds are connected to the ground bar near one end, and the audio signal grounds are connected near the other end.
If you just tack solder each ground point in the circuit to whatever piece of chassis happens to be closest, you inevitably wind up with small (but audible) currents flowing every which way through the chassis itself. Ground loops. They end up causing hum. It seems like the chassis ought to be a good place to ground stuff, what with it being made out of so much metal and all that. As it turns out, the chassis is usually the worst place to solder a ground, unless you like hum.
The input jacks and the volume/balance/tone control pots can also be particularly sensitive to grounding. Oh, almost forgot - make sure all the AC wiring is twisted together, or it'll leak hum too. Stuff like the AC wiring for the heaters, or the 120 VAC going into the power transformer, or even the high voltage AC from the power transformer's secondary leading up to the rectifier tube.
Do a search for phrases such as "bus ground", "star ground", or "ground technique for beginners". Hopefully you'll turn up some useful reading.
Ty-
Thanks once again for the thorough reply. The hum level is low enough that I can live with it for now, but this would be a good project for a bit later.
I'm listening to this amp now and it's really pretty good.
Will be interesting to see how it sounds after everything is
broken in.
Now what part do I need to replace to make this sound like Audio Research stuff?
Kirk
Thanks once again for the thorough reply. The hum level is low enough that I can live with it for now, but this would be a good project for a bit later.
I'm listening to this amp now and it's really pretty good.
Will be interesting to see how it sounds after everything is
broken in.
Now what part do I need to replace to make this sound like Audio Research stuff?
Kirk
Now that this amp is up and running again, I am wondering what I could do to tweak it a bit. While the sound is good, it is not that clean i.e. seems somewhat distorted Here are some things I've seen on the net for possible improvements:
better volume contol (ALPs, etc.)
bypass tone controls
replace electolytics in PS with poly caps (does this matter enough to bother with?)
metal film resistors (a waste of time?)
better wire, etc.
These are things I could do myself.
More advanced items include PS enhancements which may be more difficult for me.
Also are the OP transformers on this unit a limiting factor that I can't get past?
I know Knight was not high end even in it's day (not EICO, Marantz, Pilot, etc) so may not be possible to get there from here...
Could this unit be as good as say this Music Angel:
http://www.e-audio.net/servlet/Detail?no=13
Have any of you guys compared vintage amps such as the Knight to the new stuff coming out of China?
It would be interesting to compare schematics.
better volume contol (ALPs, etc.)
bypass tone controls
replace electolytics in PS with poly caps (does this matter enough to bother with?)
metal film resistors (a waste of time?)
better wire, etc.
These are things I could do myself.
More advanced items include PS enhancements which may be more difficult for me.
Also are the OP transformers on this unit a limiting factor that I can't get past?
I know Knight was not high end even in it's day (not EICO, Marantz, Pilot, etc) so may not be possible to get there from here...
Could this unit be as good as say this Music Angel:
http://www.e-audio.net/servlet/Detail?no=13
Have any of you guys compared vintage amps such as the Knight to the new stuff coming out of China?
It would be interesting to compare schematics.
kirk57 said:
This is probably the single biggest improvement you could make, and it'll cost you essentially nothing. It would be the first thing I'd do.
After that you'd probably be best off completely gutting the poor thing and rebuilding a proven good design on the chassis, using better output transformers.
I wouldn't bother with boutique caps and resistors or fancy silver wire.
Here's what I did to my HK A230 to snip out the tone circuit. I also took out the phono preamp section. The second photo thumbnail doesn't show up well in this post, but it'll look fine if you click on it and view it full sized.
After I took out those tubes, I noticed the front end voltages were a little higher than specified on the schematic. I'm betting this wouldn't have been a problem, but I stuck a ~22k ohm, 5 watt cemented resistor in place of the tubes I removed.
After I took out those tubes, I noticed the front end voltages were a little higher than specified on the schematic. I'm betting this wouldn't have been a problem, but I stuck a ~22k ohm, 5 watt cemented resistor in place of the tubes I removed.
Ty-
I am using the Knight in the office system for now. Every speaker I've tried it with, I need to turn it up to about 2 o'clock to get
any decent output (and I do not mean *loud*)
I have a couple of very low power SS recievers (15-20 wpc) and with them I get the same kind of volume at about 9 o'clock.
Using your EL-84 based amp, what setting on the volume control do you usually find yourself using?
Kirk
I am using the Knight in the office system for now. Every speaker I've tried it with, I need to turn it up to about 2 o'clock to get
any decent output (and I do not mean *loud*)
I have a couple of very low power SS recievers (15-20 wpc) and with them I get the same kind of volume at about 9 o'clock.
Using your EL-84 based amp, what setting on the volume control do you usually find yourself using?
Kirk
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?
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?
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
Thanks
GAry
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
Thanks
GAry
brocks said:
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.
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?
Thanks
GAry
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?
Thanks
GAry
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.
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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