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Old 10th November 2012, 12:09 PM   #1
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Default Harmon Kardon A500

Morning All,

I have a set of HK A500 transformers. The amp used 7355 tubes which are rare and expensive. I'm looking for a design that will work with these transformers. B+ of 360. Output impedance of 4k and 25 - 30 watts of power. That low power and low impedance seems to require a rare output. The best candidate I've found is a 6550 strapped as a triode. Could I just use any PP 6550 schematic and then strap the outputs. Or do I need to start the design from scratch?

Kevin
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Old 10th November 2012, 12:15 PM   #2
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Anybody ever run a Dynaco Mark III in triode?
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Old 10th November 2012, 12:20 PM   #3
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Just found this:
1. Disconnect the GREEN or GREEN/WHITE wire from pin 4 of each output tube and insulate the bare end so it doesn't short out against the chassis.
2. Solder in a 100 ohm 1 watt resistor between pins 3 and 4 of each output tube.

Nevermind...
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Old 10th November 2012, 02:56 PM   #4
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4k output is perfect for a 300B in pushpull.
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Old 10th November 2012, 05:43 PM   #5
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300B = $$$$$$$
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Old 10th November 2012, 06:42 PM   #6
rmyauck is offline rmyauck  Canada
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But they will last for decades if run right! Sound better also!

Randy
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Old 10th November 2012, 10:56 PM   #7
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
4k p-to-p would be good for push-pull-parallel EL84 or 6V6, triode or pentode.

You can get the Chinese 2A3's for not too much money. It's hard to beat a good pair of push-pull, class A 2A3's. 4k primary would be fine for that. +360V B+ would be good if you cathode bias them (2A3 max plate voltage = 300V, grid bias will be about -55V or thereabouts). Actually, I really like that idea. 2A3's will last as long as 300B's, but don't cost nearly as much. 2A3's are a lot easier to drive, too. You'll only get about 6 or 7 watts per channel, though.

I'm running triode-strapped 6L6GC into Dyna ST70 OPT's (4300 ohm primary). I use the 4 ohm secondary taps for 8 ohm speakers. The idea is to use the turns ratio to raise the apparent impedance of the primary plate-to-plate load. It means the OPT's won't be capable of as high power output, but I'm only asking them to deliver a max of 11 watts rms each.

You can do a lot of fun things with those trannies.

--

Last edited by rongon; 10th November 2012 at 10:58 PM.
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Old 11th November 2012, 03:59 PM   #8
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Thanks Everyone,
This amp will be a "junkyard dog" I.E assembled out of parts I already have. I have five winged "C' 6550, two 6AN8's, two Dynaco Mark III boards and the HK transformers. Plus I have some big power transformers. And that 1.5% THD spec on the 6550 in triode ain't too shabby either.
Kevin
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Old 5th January 2013, 06:09 AM   #9
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Default Help with HKL A500 amp build

Greetings and Happy New Year to you all in the Asylum. I have been drulling over the info i have seen on the H-K A500 amp. Can anyone help me with schematics, and if possible if anyone has done a DIY on this amp could I ask for directions on how or where I can go to get what I may need-meaning schematics, parts list any directions. I would be grateful beyond what words can express. Please don;t make me beg, k.
Thanks in advance for anything that will steer me down right path. Humbly Yours Paul in Portland, Oregon.
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Old 5th January 2013, 06:17 AM   #10
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Do a search in google and you will find more than one site with schematics and info.
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