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In Reply to: RE: Error on Scott LK72b schematic posted by jgradio on February 15, 2015 at 16:48:35
It's kind of silly what they did..They have a shared 3.3 ohm resistor running from pin 5 to ground on each pair of 7591s..The best thing to do is install a 10 ohm resistor from pin 5 to ground on each of the 7591 tubes.This is four 10 ohm resistors total..You set the balance by putting your meter on DC volts and putting the plus and minus leads of the meter across pin 5 of the two 7591 tubes in each channel..Adjust the balance until you get 0 vdc between the two.. You then remove one of the leads to put it to ground and set the bias for .32 to .35vdc and you should be fine.Recheck the bias and touch up if necessary.A good warmup is a plus.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/15/15Follow Ups:
Mike, Do you know what purpose the slide switch labeled "bias adjust" is? Thanks.
I believe it's switch from left to right output pair. Again, it's been a long time so I'm fuzzy.
I took a look on the schematic it does not seem to show there.
That's exactly what it is. I found it on the 299d schematic.
Some used to set the balance by listening to the hum thru a sensitive pair of speakers and adjusting it for minimum hum..You could also put an AC voltmeter across the 16 ohm tap and set it for the lowest output. If you have soldering skills and a meter,the best and easiest and most accurate way of setting the DC balance and DC bias is with the 10 ohm resistors and that procedure I mentioned in a precious post..The switch looks like it may float the output ground in one position...I would ignore it.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
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