F6 Illustrated Build Guide

Jump to Latest
flg

flg

Member
Joined 2005
Maybe I missed it but, do you have a path to GND at the input? Input resister? When you say "no input" do you mean nothing plugged into the input jack? I would GND or terminate the input if making noise measurements. Are we talking about an amp with typical gain? Verify the gain.:2c::Pawprint:
 
Monk55

Monk55

ResisterClone
Joined 2011
Paid Member
Power on! Thank you DIY members for all the Wonderful enthusiasm in the electronic world.
 

Attachments

  • A8F6A235-C426-45D0-9380-A5B21269B104.jpg
    A8F6A235-C426-45D0-9380-A5B21269B104.jpg
    897.6 KB · Views: 324
SRMcGee

SRMcGee

Member
Joined 2004
Paid Member
Folks:

I am considering building an F6 and would like, if possible, to use some NOS Toshiba jfets that have been in my parts drawer for years. I have matched sets of Gr and V-grade jfets (equivalent to Linear Systems' A and C grades, respectively). My 2SJ74-Gr and 2SK170-Gr jfets have an idss of 5.02 mA and the 2SJ74-V and 2SK170-V jfets have an idss of 13.4 mA.

Are either of these sets suitable for an F6 and, if so, are there any changes to the BOM that should be made to accommodate the chosen jfets?

Thanks for the counsel.

Regards,
Scott
 
SRMcGee

SRMcGee

Member
Joined 2004
Paid Member
2picoDumbs:

Thank you! I assume "You want to keep the dissipation under 200mW to ensure they last for a nice long time" was a reference to the alternate use of the V-grade jfets and not a suggestion that I add some resistance to the Gr-grade jfets' sources.

I'm not much of a technician, so please let me know if I've just made my ignorance plain for all to see.

Regards,
Scott
 
2 picoDumbs

2 picoDumbs

Member
Joined 2013
LATB said:
Did it.

Sounds really good. Just started listening.

Could you tell us a bit more what went into your choice of MOSFETs and R1,2 values?
Click to expand...

Well if you installed the green leds then you don't need 0.47 Ohms.
With 0.22 Ohms you have less heat in the resistor. Less heat equals less thermal distortion. This was an observation I made while testing resistors, some resistors were bloody terrible. The best I measured were Vishay CPF. I am sure there are also many other good choices though.
Regarding mosfets these were carefully chosen based on transconductance values to provide a certain sonic character without needing any degeneration whatsoever.
The other method is to use degeneration but that leads to some increased higher order harmonics.

The end result should be something that approaches the sound of J2 but with only a single gain stage, and is also much better into reactive loads.

In other words, it's bloody good.
 
Last edited:
Monk55

Monk55

ResisterClone
Joined 2011
Paid Member
2 picoDumbs said:
Well if you installed the green leds then you don't need 0.47 Ohms.
With 0.22 Ohms you have less heat in the resistor. Less heat equals less thermal distortion. This was an observation I made while testing resistors, some resistors were bloody terrible.

Pico, if I go with your 3 led solution, I should change the .47’s to .22 ohms?
Just want to make sure I understand.
Thanks very much
Click to expand...
 
L

LATB

Member
Joined 2019
2 picoDumbs said:
If you are building on a fresh pcb then yes.
If you have to desolder parts, then I’ll leave it up to you to decide.
Click to expand...

But doesn't the choice of R1,2 of 0.56/0.47 vs 0.22/0.22 also influence the composition of harmonics, if I understood Nelson's talk correctly and also the discussion in some of the other threads (a while back, when I wasn't on this forum yet)?

So basically going degenerate (R1 == R2) de-emphasizes third harmonics, so it would result in relatively more second harmonics (or was it the other way round)? I hope to find some time to start playing with REW soon.

I do think I hear a difference in the sonic signature (aka sounds more better bloody good) after the changes, which in my case included going with 2picoDumbs' alternative mosfets.
 
Last edited:
You must log in or register to reply here.
Top Bottom