DN2540

[SY]

Don't know that circuit. Does the DN have a gate stopper resistor? Nice and close to the gate lead?

[tmblack]

Yes the DN has a 330 ohm gate stopper resistor but I breadboaded it on a solderless board.
So I have 4 versions of CS:
1 two DN2540 cascode
2 one DN2540
3 one 2SK170
4 one 2sk170 inside a DN2540 cascode

the pure DN2540 versions 1 and 2 allow hum to pass through.
I'm using a cheap wall power transformer power supply.

[TheGimp]

How much voltage do you have across the current source?

[Poindexter]

Depends on what you mean by 'better'. Rdyn is about 20-30% higher. Otherwise, it's a matter of availability. There were times when the IXYS part was unobtanium, but the DN2540 has always been scarce. I would use whichever was available; I don't think there will be any difference in normal (not approaching design limits) service.

Poinz

[tmblack]

I recon the DN2540 has less than 2V Vds in the inner cascode, total of 8V across the cascode.

[Alain Poitras]

Originally Posted by tmblack

Yes the DN has a 330 ohm gate stopper resistor but I breadboaded it on a solderless board.
So I have 4 versions of CS:
1 two DN2540 cascode
2 one DN2540
3 one 2SK170
4 one 2sk170 inside a DN2540 cascode

the pure DN2540 versions 1 and 2 allow hum to pass through.
I'm using a cheap wall power transformer power supply.

Don't forget the 2SK170 come in three IDSS ranges (Vgs = 0V) :

2SK170GR : 2,6 to 6,5 ma
2SK170BL : 6 to 12 ma
2SK170V : 10 to 20 ma

The 2SK170 is very good as current source and source follower buffer but personally, I will never use them as commun source amplifier ...

Alain.

[TheGimp]

tmblack,

You may need more voltage across the current source to get good cmrr out of it.

[tmblack]

Originally Posted by Alain Poitras

Don't forget the 2SK170 come in three IDSS ranges (Vgs = 0V) :

2SK170GR : 2,6 to 6,5 ma
2SK170BL : 6 to 12 ma
2SK170V : 10 to 20 ma

The 2SK170 is very good as current source and source follower buffer but personally, I will never use them as commun source amplifier ...

Alain.

I'm just testing the 2SK170 because that was what Nelson Pass has in his JFET BOZ.
I plan on using the current source devices on a tube preamp next, modding the 12ax7a or 7247 in the EICO ST70.

[tmblack]

Originally Posted by TheGimp

tmblack,

You may need more voltage across the current source to get good cmrr out of it.

The DN2540 has been used on cathode end of power tubes. What is a typical voltage to get good CMRR?

[Alain Poitras]

Originally Posted by tmblack

Are they good as current sources in the range of 0.5mA to 2mA?
Some say a gate resistor should be used, or can I just ground the gate to the source resistor?

No problem using the DN2540 for 0.5mA to 2mA and a 2K resistor between the gate and the source is safe.

The DN2540 is very good for high voltage use in tubes circuits, VDS up to 400V but a minimum VDS over 10V is the best. A good alternative to replace the DC load resistor or choke in plate circuits to make some kind of SRPP circuit.

The 2SK170 is a low voltage device, a maximum VDS of 40V but you better use it in a range from 2 to 20V, then no good for plate circuit but OK for cathode circuit like in a "long tail pair" phase splitter.

You just have to take care not exceeding the maximun power dissipation, you may need a heatsink in many cases, sometime a very big one for the TO220 DN2540 version with a high current ...

For both 2SK170 or DN2540, you should use a multiturns trimpot to adjust the bias because there can be a big current difference between different specimens and datasheets curves, IDSS and Yfs (Forward transfer admittance). Also keep in mind JFET current drop with temperature raise ...

Alain.