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The schematic for the lk72b that is most often used shows r44 & r144 being a 8.2k. In the amp I'm rebuilding, there's an 820 ohm in that position. Anyone know if it was a builder error, schematic error, or production change? I'm inclined toward builder error, but the original work is excellent, the resistor matches the others in the amp, and I don't see an 820 ohm any where else in the amp where he might have errantly put one value in place of the other. Also, I have only been able to find the building manual and/or user manual for the earlier lk72 version, which has significant differences. Does anyone know where I might find the lk72b versions(s)? Thanks for any help.The McShane kit I purchased to re-build the amp is fantastic. High quality parts, very good price, extremely well organized.
Edits: 02/15/15 02/15/15 02/15/15Follow Ups:
Look for the 299b/c version schematics. There were many running changes with these amps I think I have at least 7 somewhere in my archives.
If I recall correctly and it's fuzzy now. I have an early version LK72b. I used the input/front end of the 299b and the power supply/output of the 299c.
Very helpful info. The schematics of the 299 variants I'm finding on line all have the 8.2k resistor, so it must be builder error. Can you tell me anything about how the bias adjustment works. My variant has a switch, 2 rca test points, and then 4 adjustable pots... 2 (l,r) for bias adjustment, and 2 for balance adjustment. Thanks.
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: 02/15/15
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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