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Harman Kardon A700 Red Plating

One channel is ~.330v and the other is ~.360v. Pots adjusted in both channels to make them match as close as I could.
 
I think you're good to go. If you haven't already, let it run for an hour or two, then double-check the measurements. If it's stable, you're done.
 
I played it for a good 30-45 minutes. Sounded just fine. Tomorrow I'll listen to it for at least an hour and recheck it. I'd like to be dropping it off Tuesday.

A big thanks to everyone who helped me on this.
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If you are interested, when you have the idle balanced within the channel, feed some 1KHz sine wave into the input and see how those tubes track each other (how far apart the current they will draw). I doubt they will, and that will increase distortions. Watch the output on the scope and first feed signal so that the output is about 1 watt. If your loads are 8 ohms, the scope peak reading will be around 4v for 1w of output. Then you can increase the input to unset of clipping on the scope. This will give you the max output of an unknown distortion figure, of course. Just you you want to know the basic performance of the amp
 
TriodeLuvr said:
There are only two good options in this situation. Either build a new bias circuit so each tube has its own adjustment, or replace the output tubes with a better-matched set. You might try replacing just the two RCAs with new JJ, but considering this is for a customer (and that the red plating tubes may not be at their best now), it's probably more cost effective just to replace them all with a matched quad. The reissue tubes appear to need more bias voltage, so you will still need to change the values of the resistors. It's somewhat cut and try (I don't have screen/plate curves for the JJ tubes), but the voltage can be massaged over a wide range by juggling the resistors on the other side of the pots. For example, if the 120K are replaced with 100K and the 270K replaced with 470K, bias voltage will increase to about -22.5V. That's probably sufficient to stop the red plating, and might even be a little "cold." If you have a good stock of resistors, the values can be tweaked from there to achieve a proper bias current.

Jack
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Jack, if you don't mind... I have a question regarding your advice to replace the 120K with 100k, and 270K with 470K. What is the purpose of going down in value of the 120K while going up in value of the 270K?

Also, what would happen if you skipped replacing the 120K with 100K, and just went 270K to 470K? This is what I did to increase the bias voltage in my A700. Did I mess up?
 
Its a voltage divider. Decreasing the 120k to 100k drops less voltage across it. Increasing the 270k to 470k reduces the current flow through that network which also reduces the voltage drop across the 100k. You could do just one or the other but that gets into screwing with the load the coupling cap works into. What the cap "sees" as a load in stock form is 270k in parallel with about 220k, or the 120k plus half of the 200k balance pot. It ends up about 120k total.

With a 100k and a 470k, its about 140k. Different, but not so grossly off that its likely to cause problems. If you left the 120k it would be further off, same if you didn't change the 470k. If you want it even closer to original, an 82k and a 390k would get you a 124k load but not sure if that would put the grid supply where it needs to be.
 
gadget73 said:
Its a voltage divider. Decreasing the 120k to 100k drops less voltage across it. Increasing the 270k to 470k reduces the current flow through that network which also reduces the voltage drop across the 100k. You could do just one or the other but that gets into screwing with the load the coupling cap works into. What the cap "sees" as a load in stock form is 270k in parallel with about 220k, or the 120k plus half of the 200k balance pot. It ends up about 120k total.

With a 100k and a 470k, its about 140k. Different, but not so grossly off that its likely to cause problems. If you left the 120k it would be further off, same if you didn't change the 470k. If you want it even closer to original, an 82k and a 390k would get you a 124k load but not sure if that would put the grid supply where it needs to be.
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Thanks for the explanation Gadget! So I'm going to leave things the way they are: 270K resistors changed to 470K, -20 volts bias supply, Channel A power tubes running at 30ma each, and channel B at 33 and 34ma.
 
slater529 said:
Thanks for the explanation Gadget! So I'm going to leave things the way they are: 270K resistors changed to 470K, -20 volts bias supply, Channel A power tubes running at 30ma each, and channel B at 33 and 34ma.
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My earlier suggestion regarding resistor changes was simply intended to produce a good bias voltage for the tubes. If idle current is good without both values being revised, that's great. No harm no foul. :)

Jack
 
TriodeLuvr said:
My earlier suggestion regarding resistor changes was simply intended to produce a good bias voltage for the tubes. If idle current is good without both values being revised, that's great. No harm no foul. :)

Jack
Click to expand...
Thanks for the clarification Jack! I really appreciate your contribution to this thread. Thanks to you, my A700 power tubes are much happier!
 
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