F5 power amplifier

[Zen Mod]

TempCo, you know that stuff

 

[2 picoDumbs]

Yeah.

Just curious if Papa has something new up his sleeve.

 

[Zen Mod]

cheeky boy

 

[6L6]

He always has something up his sleeve...

 

[Nelson Pass]

I also have something old up my sleeve....

 

[2 picoDumbs]

Hahahaha

 

[Variac]

Something borrowed
Something blue?

 

[DIYHarry]

couldn´t bias high?

hello dear community,


I have a F5 since 2018 and I´m happy-- standard JFET and 2SK1058/2SJ162



Now I tried a 2nd F5 built with A-Grade MOSfets and JFETs(bought 2020).


I´m unable to bias high, set R6 up and up till 4k7+4k7 and can´t go over R7R8-320mA bias. 4.2V on Gate of MOSFETs...



on the second site, which is build according to the V3 schematic (R6 stay at 2k2) cant get over 80mV -170mA.
I do not think, that I made a mistake twice (each board) so I guess, I oversee special Resistor change cause the A-Grade IRFP and JFET???



I´m disappointed, sad and tired......


please can somebody help me.


best thanks in advance

Harry

 

[Attila I]

@DIYHarry
I have not heard of A-Grade MOSfets
but if you refer to the LSK170/LSJ74 (A-Grade) sold by the diyaudiostore (in the past)
your trouble is caused by low idss of these items.
-I did buy them once myself accidently-
A-Grade is not the best for most circuits - B-Grade is.
There might be some workaround; but better get jfets with proper idss
(Toshiba BL-Grade or LinearSystems B-Grade) not nessesarily tightly matched.

cheers
A.

 

[Zen Mod]

clarify

I can make sense of A grade JFets (LS production, A suffix?) , but I can't find any connection between "A" and mosfets

 

[DIYHarry]

hey, thanks for your comments


everything above stock items are "A-grade" )




for my fist F5 I took JFETs from Reichelt


I measured them with 9V now


2SK170 -N-FET - 24-28mA --on the JFets ---2SK170_BL-2I
2SJ74 -P-FET - 2,78-3,02mA --on the JFets ---2SJ74_GR-4K


from diyaudio-store i got---also 9V


LSK170A -N-FET - 11,7mA --on the JFets ---LSK170_F-1529C
LSJ74A -P-FET - 38,4mA --on the JFets ---LSJ74_F_1412C


what would be your choice


thanks
Harry

 

[Dennis Hui]

Harry,

I am not sure if I understood what you wrote so I tried reading between
the lines and made some guesses. Please confirm if this is correct:

0) You are using F5 v3 schematics as shown on page 2 here: https://firstwatt.com/pdf/art_f5_turbo.pdf
1) You are using LSK170A/LSJ74A jfets.
2) You are using IRFP240/IRFP9240 mosfets
3) On one board you have R5=R6 = 2.2k and you can't get voltage across source
resistors R7/R8 more than 80mV (170mA bias current)
4) On second board, you have R5=R6=4.7K and you can only get bias bias current to 320mA.

Do you know the Idss of your jfets? Can you try measuring the
voltage across the R3 and R4 (10R) resistors? This will tell you how
much current is flowing through your jfets.

My first thought is you may need to increase R5/R6 and/or decrease
R3/R4.

 

[DIYHarry]

hey, thanks for your comments


everything above stock items are "A-grade" )




for my fist F5 I took JFETs from Reichelt


I measured them with 9V now


2SK170 -N-FET - 24-28mA --on the JFets ---2SK170_BL-2I
2SJ74 -P-FET - 2,78-3,02mA --on the JFets ---2SJ74_GR-4K


from diyaudio-store i got---also 9V


LSK170A -N-FET - 11,7mA --on the JFets ---LSK170_F-1529C
LSJ74A -P-FET - 38,4mA --on the JFets ---LSJ74_F_1412C


what would be your choice


thanks
Harry

 

[DIYHarry]

thanks for your comments



I thought everything above stock items are "A-grade" )


for my first JFets I used some from Reichelt


I measured them with 9V


2SK170-N-Fet- 24 to 28mA---label--2SK170BL-2I
2SJ74 - P-Fet - 2,78 to 3,02mA -label--2SJ74GR_4K


from diyaudio-store
MOSFETS with IFRP92403,915 and IRFP240 3,932 on it

LSK170A-N-Fet- 11,7mA---label--LSK170F-1529C
LSJ74A - P-Fet - 38,4mA -label--LSJ74F_1412C


Voltage on R2/R3 ist 23,8 to 24,3mV


thnaks
Harry

 

[DIYHarry]

@ dennis
4) R5 stock - R6 9,6k

 

[indra1]

Harry, the jfets you have are not recommended for F5. Use Linear Systems Matched JFETs – diyAudio Store instead. And read description about grades of 170/74 jfets a bit lower on that page.

 

[DIYHarry]

thanks Indra1


but at my first prototype I could help me out with a resitor and with more sensitive Jfet it is not possible - and i have to buy JFet 10$ each?

 

[Zen Mod]

you're making amp worth few kiloE, going to cost you just parts, and thinking about 50E worth JFets?

make GainClone, it will cost you same, if you use same amp case

I'm not criticizing you, just trying to serve you right perspective

 

[cubicincher]

to DIYHarry #16377

Hello DIYHarry,


I fully agree with ZenMod!


I have also tried to buy JFets (Toshibas) and to match them myself.
The problem is, that there are many fakes on the market (especially at e..y)
and that you have to buy a lot of JFets to get a good matching set of complementaries (N-Ch and P-Ch)in the BL-range. I am sure it would be much more expensive than the 39$. I made that experience!

The DIYAudiostore offer with the very good matched Linear System JFets is a more than a fair offer!
Although it costs the price of the JFets (39$ is a bargain!) + transport to Europe+ customs tax and VAT.
Greets
Dirk

 

[Zen Mod]

something is wrong

how you get A grade LSK JFet , having Idss of 11.7mA ?

short conclusion - if you have (any origin) genuine JFets , in Toshiba's BL range (6 to 12mA) , your F5 should work without any change of circuit

that means - both JFets ( N channel and P channel) must be in range of 6 to 12mA