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Old 23rd May 2008, 09:21 PM   #101
kirk57 is offline kirk57  United States
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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?
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Old 23rd May 2008, 10:00 PM   #102
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by kirk57
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.
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Old 1st June 2008, 04:43 AM   #103
kirk57 is offline kirk57  United States
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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...
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Old 2nd June 2008, 02:10 AM   #104
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Quote:
Originally posted by kirk57
I'm still getting some hum, regardless of the volume control position...

Do you guys hear this on your (working) amps? maybe it's something I just need to live with...
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.
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Old 2nd June 2008, 10:50 PM   #105
kirk57 is offline kirk57  United States
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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
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Old 6th June 2008, 10:31 PM   #106
kirk57 is offline kirk57  United States
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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.
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Old 7th June 2008, 03:15 AM   #107
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Quote:
Originally posted by kirk57
bypass tone controls
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.
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Old 15th June 2008, 05:19 PM   #108
kirk57 is offline kirk57  United States
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rainy day today, so I had a chance to look at this again. How do I bypass the tone controls? Is this a matter of snipping the wires to them, or do I need to insert some equivalant resistance?
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Old 21st June 2008, 03:17 PM   #109
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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.

Click the image to open in full size. Click the image to open in full size.

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.
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Old 23rd July 2008, 10:19 PM   #110
kirk57 is offline kirk57  United States
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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
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