Just fired up an Aleph J circuit with some ebay jfets

[audionut]

I just fired up an Aleph J channel...built on Brian's excellent boards...

The matched Jfets are off of EBay...from a guy in isreal. Are there any in- circuit measurements that I can make to determine if the jfets are genuine?
My correspondence with the supplier makes me feel confident that the 2sj74 bl are genuine.

The circuit biases up...is drawing the correct amount of current. offset is .3 volts (my aleph 30's are about the same offset)

anything else I can check??? I have an O-scope, a signal generator and a good fluke VOM.

Thanx

 

[qusp]

in circuit is fine, you'll load the cct a bit with the meter, so the parts after it may not get the exact current, but that doesnt matter so much for this. sounds to me like they are ok, as generally the fakes are showing considerably higher idss.

 

[audionut]

QUSP...thanx for your quick response....

How do I check idss???

 

[qusp]

in circuit you can only check the current that it is putting out, or its vgs for your gate current, but to check idss you really need to pull it out. for p-channel jfets check idss by connecting gate and source together and through a 9v battery, or a 10v supply and connect the drain to ground through a 100r resistor and measure the voltage drop across the resistor.

 

[qusp]

if you are asking for matching, its more useful for most applications to match vgs at the same gate current you will use

an excellent article by the jfet king Erno Borbely on jfets and matching them

 

[Nelson Pass]

The circuit biases up...is drawing the correct amount of current. offset is .3 volts

This should be measured after an hour or so, as the circuit needs to warm up
to its final offset figure. If you need to adjust it, best place is the resistor
off the Drain of the diff input pair. You can put a 500 ohm pot in place of
the 392 ohm resistor.

 

[audionut]

Thank you Master...

I measured the Idss of some one of the other pairs. Came out to 8.7 ma...so I think that means these are probebly genuine...

 

[anilva]

This should be measured after an hour or so, as the circuit needs to warm up
to its final offset figure. If you need to adjust it, best place is the resistor
off the Drain of the diff input pair. You can put a 500 ohm pot in place of
the 392 ohm resistor.

Papa, aren't we putting a pot in place of R8 (1K) to adjust the offset? That's what I read in the Aleph J thread and implemented....

 

[Nelson Pass]

You can do either. Adjusting R8 has the slight advantage of not having
any effect on the open loop gain, but I was too lazy to look up the part.
I remembered the 392 ohm resistor instead, which is where I put the
pot on the J2.

 

[Thago]

Hi

My Aleph 2 functioned flawlessly for over 3 years, but now I have a problem with one of them: the 392 ohm resistor (drain of the input pair) just disappeared into smoke. Does this mean that (one of) the IRF9610's is broke? Or do I have to search elsewhere?

 

[Zen Mod]

yes - you need to check everything

sorry - that's the nature of the beast

 

[audionut]

The other day a mosfet failed in my Aleph J. I checked everything...turned out to be a little piece of wire had gotten underneath the mosfet as I was tightening it down to the heat sink. I didn't notice the problem. the mosfet not being in good contact to the sink, was operating at higher temperatures and finally blew. It took about 6 mos...