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ARC PH5 help

Hi All,
I need help in identifying or recommendations for the "1039" Mosfet used in this design (Q6, Q7 and Q8). Also Fet "6606" (Q5). ARC removes all device marking.
I don't really what to order from ARC...if they have any, pretty old design.
 

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Hi All,
I need help in identifying or recommendations for the "1039" Mosfet used in this design (Q6, Q7 and Q8). Also Fet "6606" (Q5). ARC removes all device marking.
I don't really what to order from ARC...if they have any, pretty old design.

I would get a matched quad of the LSK170’s they sell out fast, load resistors may need some adjustment, you may find some authentic Toshiba 2SK170’s. These are some great JFETS, have used them a number of times!

Another problem, is when you use a large dropping resistor on a low voltage device running off a high B+, the load resistance is crucial, I would recommend using a smaller dropping resistor, and a high wattage Zener and decoupling cap to arrive at a more reasonable load voltage for those JFET,s! It’s like playing Russian Roulette, the load resistor has to be perfect and right on, for the FET’s to operate properly.
 
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And this is why those need replacement. It is truly weird they didn't even introduce at least an RC decoupling network for the input. Were this a diy project it would have received lots of criticism for this.

Arc is known for some really unorthodox practices! It’s almost like they welcome
Something to go wrong. I’ve been close to buying ARC gear in the past, but I’m glad I did the research and examined many of their schematics.
 
The PS already has several mods, including outboard Lambda tube regulator (with mods to lower noise) and LC filters for each input stage which then feed RCR filters (load R split in half with matched R and Muse/Ploypro cap). Really didn't want to add zener noise. This was a major improvement over the ARC PS.

Will 2SK170 handle the current needed for 4 fets in the difamp? I was thinking maybe 2SK369? I do have some DN2540 I can use for the tube stages.

Thanks for the recommendations.
 
Fortunately there's quite a lot of help on the schematic. So there is 200v across 50k drain so that is 4ma so 2ma in each FET. So you want a device with 2ma at VGS of approx -.19v so the 2SK170 looks OK. You will still have to match all four FETS for VGS though.

So providing RV1 is adjusted for 8ma through Q6 the VGS will find its own level.

You may only need two FETs if you can find suitable devices.

I think I would look at it in simulation to get the best fets, but there are plenty of other ways.
 
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OK its not quite that simple. The FET must not end up having a voltage on the source of less than -300mV or the gate source will conduct and the circuit goes non linear.

Tried with 2SK170 in LT spice gives +300mV on source and works fine.

Hope this helps.
 

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Yep should do - there's plenty of gain at the front so the noise figure is not so important for the P-FET.

The 2SK170 will always work as the Drain current from spec:

IDSS(Note)VDS= 10 V, VGS= 0 2.6 ⎯20mA

Min value of 2.6ma > 5.5ma tail current / 4 = 1.375ma. Which means VGS will be negative. You will need 4 accurately matched ones.