ARC PH5 help

[cygnus x1]

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.

[rayma]

It's necessary to get these parts from ARC, they are all hand selected for the exact circuit purpose.
They always keep parts in stock.

[baudouin0]

The problem with FET is they have variable pinch off voltages. So the 6525 will have to be matched. By the time you got 20 or so j112/j113 and matched them you might as well have got them from the supplier.

[denny9167]

Originally Posted by cygnus x1

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.

[analog_sa]

Originally Posted by denny9167

It’s like playing Russian Roulette

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.

[analog_sa]

Depletion mosfets such as the DN2540 will work fine and can be adjusted in situ.

[denny9167]

Originally Posted by analog_sa

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.

[cygnus x1]

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.

[analog_sa]

Originally Posted by cygnus x1

Will 2SK170 handle the current needed for 4 fets in the difamp?

According to the indicated voltages this current should be around 14mA. So, yes, a selected violet device will do.

[baudouin0]

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.