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Old 29th April 2012, 03:49 PM   #11
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Originally Posted by 6L6 View Post
Off the top of my head I know that you can get 2SJ74s (and K170s) from member " h_a ", and also from the AMB.org site in his store. There are probably other sources as well, I just can't think of any right now.

Your schematic link is really small... and I wonder if the schematic has different part numbers than the original schematic... Do you have a bigger image of it?
h_a don't sell only J-fets. but spencer does
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Old 29th April 2012, 11:00 PM   #12
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Well, it looks like it was a wrong value for R21/22. By putting the correct 22K resistor in there, it is working more like "normal".

I am only getting about 0.35 - 0.38V across R11 and R12, despite cranking the pots to the limit. I think I got them installed correctly, but as long as they are both in correct or reversed, that shouldn't be a problem?

Click the image to open in full size.

Again, I don't know what damage I might have done to other components, and I should probably complete the rest of the boards before spending too much time on this prototype...

Thanks all. Dave
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Old 29th April 2012, 11:12 PM   #13
6L6 is offline 6L6  United States
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Is your .35v measured with zero offset?

If so, you need to increase the value (more ohms) of R3 and R4. Start with 4K or 5K in those positions. Try to bias normally. Needing more series resistance with the pot is a very common problem.
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Old 29th April 2012, 11:47 PM   #14
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No, the offset was around 0.6v (no load attached). I didn't try to zero it, as I thought it would be best to get the R11/R12 voltage to the recommended 0.59v

I'll try adding some resistance. I see there is a thread on that...
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Old 29th April 2012, 11:49 PM   #15
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Originally Posted by AndrewT View Post
Have you measured the Idss of the jFETs?
Just did....

Out of the roll I got (from eBay/China) one measured 5.5 mA, the other 6.0 mA at 9.00v.

Looks to be in spec...
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Old 30th April 2012, 12:47 AM   #16
6L6 is offline 6L6  United States
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Maybe the Jfets are genuine! Cool!

The amount of bias with zero offset in the important number, not the bias itself.

If you can't get enough bias with zero offset, make your R3 R4 bigger. That's all there is to it... easy!
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Old 30th April 2012, 02:46 AM   #17
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by hartze11 View Post
No, the offset was around 0.6v (no load attached). I didn't try to zero it, as I thought it would be best to get the R11/R12 voltage to the recommended 0.59v

I'll try adding some resistance. I see there is a thread on that...
You need to work the pots in tandem to get 0.6v across R11 and R12 while also monitoring the DC on the outputs.
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