Harman Kardon HK250 Corrected Schematic and Restoration Photos

pbnelson

pbnelson

New Member
These photographs of my Harman Kardon HK250 restoration were on the web in 2006, but my free hosting service recently died and so I have moved them to Flickr.

I have just finished restoring another HK250, and now run them both in "parallel" mode, as a dual mono-block, giving me 50 watts per channel of pure tube power.

Something new I've added to the image collection is the corrected HK250 schematic. The corrections were made based on the wiring and components I found during restoration. Both amplifiers matched my corrections, although they were purchased years apart from different sources, so I believe the deviations are due to mistakes in the schematics and not an individual discrepancy. Either that, or Harman Kardon had multiple versions of the design at different times.

Full Album/Set, here:

http://www.flickr.com/photos/14879633@N07/sets/72157622937961057/

Corrected schematic, here:

4211395616_e8fe5a8523_b_d.jpg
 
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Interesting - the Sams schematic has those changes... the H-K schematic must be an EARLY one... here it is for your collection:
 

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Sam's has errors, too

I found some problems with the Sam's schematic, too, while I was restoring. Can't remember what they were off the top of my head.
 
The "mod" has less open-loop (before feedback) gain than the original "bootstrap" configuration. I wonder if that's a problem in any systems? Or, if the feedback loop works as well, with less overall open-loop gain?
 
Great! I'm in the process of restoring an HK250. What caps did you use for coupling in place of the .05uF 600V wax/paper caps? I noticed a handful of ceramic caps by the power supply. What are they for? Any tweeks you encountered in the rebuild process?
 
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I used 716Ps for coupling, and silver Micah for the little bitty ones. It was Tim's (Nirky) amp if IIRC and man oh man it sounded really good. We used the best Sovtek 6L6s and they sounded great in that amp behind black plate 6CG7s with the shield between the sections. Easily one of the top ten best sounding amps that have been through this shak.:music:
 
jaymanaa said:
I used 716Ps for coupling, and silver Micah for the little bitty ones. It was Tim's (Nirky) amp if IIRC and man oh man it sounded really good. We used the best Sovtek 6L6s and they sounded great in that amp behind black plate 6CG7s with the shield between the sections. Easily one of the top ten best sounding amps that have been through this shak.:music:
Click to expand...

Interesting. Did you keep the coupling caps at the original .05uF values? I'm also going with silver mica caps in place of the 100pF ceramics and the 4700pF's ceramics. I might also use silver micas or polystyrene's in place of the two 0.02uF ceramics that is if the silver mica deal comes through. All the valves are the originals (outputs are RCA black plates branded with Harman Kardon name, so I am told). The 6CG7's are branded HK but I am unsure of their maker.
 
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It's busy around here this Christmas day, but when I get a chance I'll post photos of the wiring on my second restoration job (it's much neater). The disc caps are bridging each diode, on the recommendation of Mr. Yorck Neudenberger, "to reduce impulse peaks in the DC". I'll get you their values, and also the coupling caps information.
 
dBfreak said:
Interesting. Did you keep the coupling caps at the original .05uF values? I'm also going with silver mica caps in place of the 100pF ceramics and the 4700pF's ceramics. I might also use silver micas or polystyrene's in place of the two 0.02uF ceramics that is if the silver mica deal comes through. All the valves are the originals (outputs are RCA black plates branded with Harman Kardon name, so I am told). The 6CG7's are branded HK but I am unsure of their maker.
Click to expand...

.047 for the coupling caps

I can't wait to hear your thoughts on the sound of this guy.:music:
 
Modifications (thanks to Mr. Yorck Neudenberger)

In a few places this amplifier has been modified with better capacitors.

1. The coupling caps (C2/C5/C7/C10) were one place I used a component with different specs than the original. Instead of the 0.05uF 600V I used 0.22uF 1500V non-polarized caps from www.fjz-ajf.de.

2. I also upgraded capacitors C4/C9 (between pins 1&7 of the 6CG7 V1/V2 tubes). Formerly they were simple orange disc caps of 20000pF, but as you can see from the picture they have been seriously upgraded with some big green NOS paper caps to 68800pF 630V. I got them on eBay.

3. Finally, as my last modification, I bridged each power rectifier diode with an orange disc cap of 47.3nF 500V, labeled C103/C104/C105/C106 in my photograph.

I would like to take credit for all these ideas, but the real credit goes to Mr. Yorck Neudenberger who first restored an old HK250 some years ago, doing all the hard work of trial and error experimentation. Back in 2006 he walked me through the restoration of one that had been sitting on my shelf for years, and even sourced many of the parts. If you're reading this, thanks again, Yorck!
 
Photo of my second restoration (clean)

The second time around I did a neater job of wiring in the rat's nest underneath the 6CG7 tubes. Also, the component labeling is much cleaner, as I was able to identify from the first job which components got too hot to label, and left the rest. Anyway, here's the photo...

4214327866_442a116c32_b_d.jpg


You'll notice that there are no labels under the 6L6GB tubes, as they got too hot and totally blackened all the adjacent thermal-tape labels. Oops!

I do have a higher resolution copy of this photo (2577x1299) but cannot figure out how to get Flickr to host it. If anyone really wants it let me know, and I'll email it you, if nothing else.
 
pbnelson said:
In a few places this amplifier has been modified with better capacitors.

1. The coupling caps (C2/C5/C7/C10) were one place I used a component with different specs than the original. Instead of the 0.05uF 600V I used 0.22uF 1500V non-polarized caps from www.fjz-ajf.de.

2. I also upgraded capacitors C4/C9 (between pins 1&7 of the 6CG7 V1/V2 tubes). Formerly they were simple orange disc caps of 20000pF, but as you can see from the picture they have been seriously upgraded with some big green NOS paper caps to 68800pF 630V. I got them on eBay.

3. Finally, as my last modification, I bridged each power rectifier diode with an orange disc cap of 47.3nF 500V, labeled C103/C104/C105/C106 in my photograph.

I would like to take credit for all these ideas, but the real credit goes to Mr. Yorck Neudenberger who first restored an old HK250 some years ago, doing all the hard work of trial and error experimentation. Back in 2006 he walked me through the restoration of one that had been sitting on my shelf for years, and even sourced many of the parts. If you're reading this, thanks again, Yorck!
Click to expand...

Interesting.

pbnelson said:
The second time around I did a neater job of wiring in the rat's nest underneath the 6CG7 tubes. Also, the component labeling is much cleaner, as I was able to identify from the first job which components got too hot to label, and left the rest. Anyway, here's the photo...

4214327866_442a116c32_b_d.jpg


You'll notice that there are no labels under the 6L6GB tubes, as they got too hot and totally blackened all the adjacent thermal-tape labels. Oops!

I do have a higher resolution copy of this photo (2577x1299) but cannot figure out how to get Flickr to host it. If anyone really wants it let me know, and I'll email it you, if nothing else.
Click to expand...

Thanks for sharing! PM sent for high-resolution photo.
 
jaymanaa said:
You guys are bad influences, I just hunted one of these down and bought it.:yes:
Click to expand...

Congrats! I predict you will not be disappointed. I liked mine so much, I got another, restored it identically to the first, and run them as a dual mono-block pair. Ohh, so nice.
 
Excellent NOS Soviet 6L6GB replacement tubes

The eBay seller jspht8338 (http://myworld.ebay.com/jspht8338/) has provided me with several sets of NOS Soviet 6L6GB replacement tubes. They sound fantastic in my HK-250. The lettering on the tube uses cyrllic and therefore I have seen it spelt in roman characters many different ways: 6Π3C, 6P3S, 6Pi3C, and 6r3s. If you go to that eBay and search for 6L6GB you will get the right product, as he lists them under both designations.

http://cgi.ebay.com/Matched-Quads-of-Russian-6Pi3C-6L6GB-amp-tubes-NEW_W0QQitemZ120343808978QQ

6234_1.JPG
 
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