F6 Amplifier

[Nelson Pass]

I can of course put the extra LEDs in to bias higher (as Generg suggested), but since the amp did before bias up to 1.55 amps, would that just be a bandage in stead of a fix? - or would that actually fix the gate leakage difference issue?

Don't know, but it would be worth trying. It's possible that you have
damaged the Gate, but that is not indicated by anything other than the
change in bias.

 

[buzz2]

Since SemiSouth is obsolete now, what is a substitute for SJEP 120100?


Thank you

 

[OllBoll]

Since SemiSouth is obsolete now, what is a substitute for SJEP 120100?


Thank you

R125s, which you can still get from Semisouthfan.

 

[buzzforb]

Impostor!

 

[Rush]

Impostor!

No fighting now!

 

[Zen Mod]

he's just 2

 

[jugi63]

Thank you Nelson. I'll do the LED thing.
Let's hope that works - if not, I guess I'll be sending mail to Mike Mazzola...

 

[generg]

you can send it to me if you want….

I have around 10 SS lying around, one will fit for you I suppose, I can measure them.

When I found the problem i sent it back, I do the same when i did not find the problem…..

 

[jugi63]

Yes Generg, I can do that too...

I'll put the amp back together and see what the additional LEDs do. I'll get back to you after that.

Thanks!

 

[generg]

of course only the module and the SS, no case, no PSU.
I have all here....

And please I do not want to discourage you to find the fault yourself...

 

[jugi63]

Added the LEDs. Got the bias up fine - but the Gate leakage difference persists. Does strongly look like the jfet is damaged.

Generg, send me your address via private mail?

Should I send just the jfets or would you like the whole PCB?

 

[aoc]

Jugi63,
Can you clarify which resistor (or resistor values) you measured he 7.xxmV which you indicated for leakage current. Nelson once said he used 20us as limit as he had some experience with those with high leakage having higher likelihood of premature failure. I had a few that had gate leakage current of 500uA. I could rig it to bias up but this also means more DC through the Jensen, not sure how much before core gets saturated. And I wasn't going to risk my speakers.

Sent from my C6903 using Tapatalk 2

 

[Nelson Pass]

Since SemiSouth is obsolete now, what is a substitute for SJEP 120100?

The IRFP240 is a good (cheap) substitute. Other parts that will give
comparable gain, distortion and bandwidth are the UJN1208K and the
IXTH20n50d.

The IXTH20n50d although a depletion mode part, will self bias to the current
you want with a little ballast resistance. The UJN part will require substantial
negative bias voltage.

 

[Zen Mod]

so , you tried them .

 

[jugi63]

Jugi63,
Can you clarify which resistor (or resistor values) you measured he 7.xxmV which you indicated for leakage current.

R7 for 7.xx mV
R8 for 0.28mV

After two LEDs I got the bias up to 1,6A and Gate leakage was 0.3mV and 4.xxmV.

I swapped the jfets with each other to see if that changed anything. The 0.3mV value followed the jfet.

 

[generg]

yeah, you got it!

i do not understand fully why one SS needs two……

but it works, more important…..

 

[jugi63]

Generg,

I was thinking the problem persists... the other jfet Gate leakage did come down from 7mV to 4mV but the 0.3mV value persists... That can't be good? The values on the other cannel are about 1.5mV and 1.0mV.

Sent you email.

 

[Zen Mod]

how it sounds ?

 

[aoc]

Low voltage drop means less gate leakage, the higher voltage drop part has more leakage

Sent from my C6903 using Tapatalk 2

 

[Nelson Pass]

Perhaps you would refresh my memory as to what resistor values these are.