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Old 20th February 2005, 04:15 PM   #1
sweet is offline sweet  United States
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Default help with modifing a H/K A500 amp

I finally was able to pick up an amp to work with. This is what I've been wanting to do...Modify, Improve, and Learn as I go. I was hoping to get some help and opinions here and also share my experience with all.

The amp I have is a Harman Kardon A500, discribition ans Schematics can be found at this link.

http://intra.engr.uark.edu/~lar/fireamps.html

Please I am in the learning stage any help or suggestions would be greatly appreciated. No matter how small or trivial you may think it to be, I am in the learning stage.

Thanks Steve
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Old 21st February 2005, 02:59 AM   #2
amperex is offline amperex  United States
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Default Mods

Congrats on the vinatge amp. If I owned the amp, I would replace the electrolytics in the power supply. Next, I would change the coupling caps to AuriCap.

Next, I would build a socket adapter to change the 12AU7 to a Sylvania 6SN7GT with the 'T' type plates. Sylvania makes a short glass version that will fit under most cabinet covers. I have not ever found a 12AU7 I like sonically. Finally, I would try some various manufacture 12AX7s to tune the sonics.
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Old 21st February 2005, 03:20 AM   #3
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Default Before changing 12AU7 to 6SN7

Any restoration of a vintage tube amp should start with replacement of the electrolytic capacitors as suggested. That means Power Supply Caps AND any Cathode bypass etc. electrolytics.

If you can find someone in your area with a tube tester its a good idea to run all the tubes through the tester. Any tube which tests below 70% of published data sheet gm gets replaced.

I'll probably run into howls of indignation here BUT I do not like Auricaps. I've tried them in 3 different amps and in each case I thought they severly limited the high frequency response. Please, any other DIY'ers with experience of Auricaps vs other good Polypropylene Caps don't let me lead this fellow astray if you disagree.

Before trying the 12AU7 to 6SN7 change which requires some rewiring I would try a modern production JJ Tesla ECC99 in the 12AU7 socket. No rewiring needed and they seriously give 6SN7s "a run for their money".
Cheers,
Ian
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Old 21st February 2005, 03:42 AM   #4
HDTVman is offline HDTVman  United States
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I have the A300. Very nice little amp that uses 6V6s for outputs, otherwise very similar. I agree that the electrolitics should be changed, tubes tested and weak or shorted ones replaced, but then give it a listen. If you want to start making changes then at least you know where you started from.

New coupling caps, new resistors, different circuit configurations can all be tried. I'm just saying give the stock amp a chance before you start modifing. Get to know it before you make changes so you can better judge what the changes do to the sound.

Later
BZ
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Old 21st February 2005, 04:19 AM   #5
amperex is offline amperex  United States
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Default AuriCaps- severly high freq limited

Out of five different amps, I never had that AuriCap issue. On the Scan Speaks & Seas Excel speakers, the high frequencies are bright and ultra clear. A stick hit on a rim or brush on a cimbal is accurate and easy to hear. No siblance on female voices either.

I also find AuriCap without midrange smearing unlike other caps I tested in its price range. My present amps are DIY PPP 6B4G & DIY PPP triode connected 6V6GT monoblocks with an Amperex 7308 phase splitter (no input tube required). Both are very accurate amps.
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Old 21st February 2005, 04:27 AM   #6
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I've got an A300 and the wiring is an unholy, point to point mess inside. It'd probably be easier and less frustrating for a novice to DIY something simple from scratch than to change stuff in there!
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Old 21st February 2005, 04:57 AM   #7
HDTVman is offline HDTVman  United States
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Now ya see I like he old point to point wiring stuff. Printed circuit boards were invented for manufactureabilty (is that a word?) and lower cost (read cheaper labor), it took some skill to do point to point. Those who try for the shortest signal path use point to point.

In the tube era a correctly done point ot point wiring job was more reliable than a circuit board. The heat from the tubes could damage the circuit board and cause intermittent problems and there are more solder connections with a PC board.

It's all in what you are used to.

Later
BZ
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Old 21st February 2005, 05:07 AM   #8
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Quote:
Originally posted by HDTVman
Now ya see I like he old point to point wiring stuff.
I certainly wasn't lamenting the lack of a PCB. I wire all my amps p2p.

There's a lot of complexity inside this integrated amp with the phono stage and the tone circuits, etc. The wiring is all over the place. It looks more like a bird's nest than an amp. A novice poking around in there with fingers or a soldering iron might break something and have a hard time fixing it.

When I first got the A300, I was a novice and I thought of modifying it. I opened it up, took one look, put the cover back on and went on to the next idea. Maybe I shouldn't have been daunted, but building a simple SET kit looked far easier to me at the time.
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Old 21st February 2005, 06:08 AM   #9
HDTVman is offline HDTVman  United States
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Well when I started out that amp wasn't that old. Point ot point was the norm and circuit boards were the "new thing".

I just learned on that old stuff so its easy for me to follow because functions are grouped in areas.

I agree that the volume control, balance control, tone control and filters can be tough to follow, but I don't think the rest is so bad.

Later
BZ
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Old 21st February 2005, 01:27 PM   #10
sweet is offline sweet  United States
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Thanks this is all great info that I plan to follow. My intentions are hands on experience learning. My goal is not so much to restore this amp as to learn as I go. So if you follow my link to the Schematics can anyone tell me the Power and OPT are large enough to switch the tubes, redesign....I'm open to anything. This is my first so I don't want to be boxed in to just restoring but I want to learn what I have and what I do with it and eventially knowing enough to design my own or knowing how and why to tweak.

http://intra.engr.uark.edu/~lar/fireamps.html

Thanks again for all the input
Steve
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