Yes, I had it plus the 1st draft and accidentally used the 1st draft for testing. Unless Nelson says different I am going to install the Fairfields when they arrive and proceed from there. Nelson said the other day he was going to be out of town. I have been waiting for many months for this build so a few more days or weeks is not a big deal. It appears I am one of the first to try this project since it has been available. I need to read the previous articles to have a somewhat better idea what function Q5 and Q6 have. I have read all the articles at one time or another but being 67 years old have forgotten most of what I little I understood at the time. I admit to being just a solder slinger with enough electronic knowledge to be dangerous.
Judging by the data sheets, the pinout of the single turn 3386P type potentiometers should be the same as the pinout of the multi turn 3296Y type potentiometers, so either should fit on the pcb.
On page 454 of Phillip Hobbs' surprisingly entertaining book Building Electro Optical Systems it's stated that multi turn types have poorer stability than single turn, which is part of why I personally prefer single turn.
On page 454 of Phillip Hobbs' surprisingly entertaining book Building Electro Optical Systems it's stated that multi turn types have poorer stability than single turn, which is part of why I personally prefer single turn.
My consul, anatech, who's fixed more equipment than I've seen, says single turn are more reliable.
AND the second opinion is:
"What would Papa do?" (my guiding principle is to use parts that he uses in his best amps.) I'll check, but I believe he used single turn so I'll stick with that (unless he didn't)
AND the second opinion is:
"What would Papa do?" (my guiding principle is to use parts that he uses in his best amps.) I'll check, but I believe he used single turn so I'll stick with that (unless he didn't)
Just to follow up I removed the V-fets and ran the voltage test with the 2nd draft. P1 has no control for T16 to ground. I have the 28V source no matter the adjustment. Same with P2 at T17 28V . I also have 28V to T18. I am quitting and waiting for new Q5 and Q6 along with the A Tl431's before doing anything else.
It takes courage to be the tip of the spear! In the end I'm sure you'll be rewarded.
BK
Not to question the Consul's experience, but I am failing to see why they should be more reliable, all other things being equal (i.e. same brand, etc.). I would assume the turning mechanism is just an add-on to the otherwise identical conductive element and wiper construction. I do remember though that I once bought trimmers where their data sheet specified redundant wipers (2 or 3).
I'll just leave these here:
Cutting Open a Multiturn Trimpots - Robot Room
http://www.bitechnologies.com/pdfs/1appnotes.pdf
Edit: Bob Pease is in favor of single turn pots: https://books.google.at/books?id=yL... multi turn potentiometer "stability"&f=false
Btw, I've made it practice to tap a potentiometer with the setting screwdriver after setting it to make sure any jump in resistance occurs while the meter is still attached...
Cutting Open a Multiturn Trimpots - Robot Room
http://www.bitechnologies.com/pdfs/1appnotes.pdf
Edit: Bob Pease is in favor of single turn pots: https://books.google.at/books?id=yL... multi turn potentiometer "stability"&f=false
Btw, I've made it practice to tap a potentiometer with the setting screwdriver after setting it to make sure any jump in resistance occurs while the meter is still attached...
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That sounds like a good tip. Thanks.
Having just finished a pre with 12 multiturn Bourns 3296 pots I can only express frustration with the adjustments. I dont have enough experience to know whether it is brand specific or type but I sure hope there is something better out there.
There's also the multi turn Bourns 3299 series. Still 3/8", but fatter and more stout. Inspires confidence, but the specs read the same from what I can see. Would be interesting to tear into one like rodeo's link. I've used them in several builds. I bought single-turn for this particular amp.
BK
BK
I'm back. Looks like two maybe separate problems. At least you don't
have the opposite problem with the VFET bias.
Problem 1. This is possible, but depends on the idea that the VFET bias
voltages are lower and the minimal voltages of T19 and T20 are about +6v
and -6v respectively if all the resistors are correct. There were qty 6 VFETs
testing as low as 6 volts, but only the 2SK82's at that.
If you have removed the VFETs, check the range of values for T16 and T17
that you can create. Keep in mind that this value depends on the DC
offset on T18 - You can short T18 to ground for the purpose of this test,
just don't drive the input while you do it.
Problem 2. Did your Mosfet bias figures check out before the installation
of VFETs?
In spite of having a 1st draft file name, the second draft clearly says
SECOND DRAFT at the top of the first page.
Nelson, I am learning along the way. My resistor check was fine. After removing the V-fets the first Voltage adjustment is adjusting P1 for T16 equals the VGS of 2SJ28 +1V. I have 28V and adjusting P1 has no effect leading me to believe there is a problem with Q5 and Q6. The same with the P2, no effect. I have pulled the 2SJ313 and 2SK2013 and I am waiting for the FQP3N30 and FQP320 to be delivered tomorrow before doing anything else.
One question that concerns me. With these Fairchild mosfets will one have to do any changes to the resistor values. I do know I used them in a previous build when the 2SJ313 and 2SK1013 called for successfully. I also see the store is using them in their BOM.
I have a better understanding and appreciation of the directions now that I have read it a few more times and some hands on experience. I never achieved 1.5V at T6-T7 or T8-T9 when I tried the first time to bias the amp. I feel confident in my resistors being correct and the test went well with them. I am replacing the TL431's with A's not really thinking the B's are a problem but since I had to place another order at Mouser why not. I also am replacing the pots with single turns. I will see what happens when I get these new components in.
One question that concerns me. With these Fairchild mosfets will one have to do any changes to the resistor values. I do know I used them in a previous build when the 2SJ313 and 2SK1013 called for successfully. I also see the store is using them in their BOM.
I have a better understanding and appreciation of the directions now that I have read it a few more times and some hands on experience. I never achieved 1.5V at T6-T7 or T8-T9 when I tried the first time to bias the amp. I feel confident in my resistors being correct and the test went well with them. I am replacing the TL431's with A's not really thinking the B's are a problem but since I had to place another order at Mouser why not. I also am replacing the pots with single turns. I will see what happens when I get these new components in.
I don't think I would have pulled Q5 and Q6. Rather, I would have shorted
T18 to ground and checked to see if P1 and P2 are working. If T18 is
shorted to ground, then Q5 and Q6 have no influence over T11 and T16.
With Q5 and 6 out, I would concentrate on this, and when you can control
those two voltages, you can reinstall the VFETs, keeping T18 shorted to gnd,
and then see if you can adjust the bias.
When you achieve this, then report back.
T18 to ground and checked to see if P1 and P2 are working. If T18 is
shorted to ground, then Q5 and Q6 have no influence over T11 and T16.
With Q5 and 6 out, I would concentrate on this, and when you can control
those two voltages, you can reinstall the VFETs, keeping T18 shorted to gnd,
and then see if you can adjust the bias.
When you achieve this, then report back.