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Old 25th February 2013, 09:34 PM   #251
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Originally Posted by grimberg View Post
I'll install the trimpots then. This is what I am thinking:

R7 - Replace 1K resistor with 2K trimpot
R8 - Replace 1K resistor with 2K trimpot
R27 - Replace 68K1 resistor with 51K1 resistor + 20K trimpot
r7 is more than likely all that will be needed and it, only for a while.
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Old 25th February 2013, 09:36 PM   #252
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Since there are less available trimpot values than resistor values, would the 56K2 + 20K combination be adequate?
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Old 25th February 2013, 09:45 PM   #253
PKI is offline PKI  United States
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It should be ok. I have a little bit bigger sinks, so I bias my Aleph a bit higher and have 100k pot instead of 56k resistor in schematics.
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Old 25th February 2013, 09:48 PM   #254
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r7 is more than likely all that will be needed and it, only for a while.
I'll try that on the other channel since it isn't mounted on the heat sink yet. But it may have to wait until the weekend as I have very little free time during the week.
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Old 25th February 2013, 10:24 PM   #255
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please - make difference between using perfectly matched and perfectly graded (and matched) parts ;

who sez that Pa is putting always exactly same resistor values ?
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Old 25th February 2013, 10:28 PM   #256
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please - make difference between using perfectly matched and perfectly graded (and matched) parts ;

who sez that Pa is putting always exactly same resistor values ?
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Old 25th February 2013, 11:45 PM   #257
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please - make difference between using perfectly matched and perfectly graded (and matched) parts ;

who sez that Pa is putting always exactly same resistor values ?
Can you tell me the difference between "tightly" and "perfectly"?


JFETs are graded by IDSS and grouped in 3 ranges identified as Green, Blue and Violet. For the 2SJ74 they are GR: 2.6~6.5 mA, BL: 6.0~12 mA, V: 10~20 mA

Using JFETs from the same manufacturing lot and same IDSS grade, with the help of one of the meters in the photo and an HP power supply, I measured IDSS on the self-biased devices at 10V. The output of the power supply, after about 45 minutes, is stable to the fifth digit. I always wait at least 1 hour before starting. In the channel on the photo the JFETs have IDSS 9.31mV and 9.36mV. In my view, "tightly" matched.

MOSFETS I matched for similar Vgs. They also come from the same manufacturing lot as that increases the chances of other parameters being also similar, and each pair is "tightly" matched to within 10mV. I am not familiar with grading for those.

As for adjustments in production I am fully aware of them. I even remember seeing piggybacked resistors in a photo of a First Watt amplifier and Nelson explained what it was.
I made a commitment to build the boards quickly and had the parts in hand for a "standard" build, as the boards I intended to use do not have room for trimpots. I made good on my commitment to 6L6. Now, with the help of Mighty ZM and other members, I'll try to make the amplifier as close to "perfect" as possible.


Feel free to add, as much as you want, to my little knowledge at any time. I am here to learn and have fun!

Last edited by grimberg; 25th February 2013 at 11:55 PM. Reason: Added comma
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Old 26th February 2013, 12:07 AM   #258
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Taking it further would mean matching for Yfs, Ciss, and leakage. He was simply trying to reiterate the practical use of pot at R7.
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Old 26th February 2013, 12:14 AM   #259
Zen Mod is offline Zen Mod  Serbia
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point wasn't in tightly or perfectly , but in difference between matched and graded

you can make an amp without any pot if you have parts stashed in small drawers , each of them having label by grade

then all you need is coffee , cigarette and calculator , to be able to choose from which drawer you are going to pick next part

so - be clever - put pots where they need to be , set amp after temp equilibrium , then pull pots out and replace with fixed resistors

even if my cleverness ends right after setting da amp ; I don't mind having good cermet here and there , as long I'm understanding role of the part

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Old 26th February 2013, 12:16 AM   #260
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My bad.
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