ecc40 LTP

[Jaap]

Does anybody have a schematic for a differential stage with a CCS under it with the ecc40 tube ?

[hemgjord]

It would look like any diff stage. If you want suggestions regarding komponent values, operating points etc. I think you have to be a little more specific as to the intended use, available rail voltages etc.

/Olof

[andyjevans]

what CCS are you using? I've had good results with Morgan Jones one in his Valve Amplifiers book. He uses a pair of BC149, a pair of red LEDs and a couple of resistors. He recommended the J511 but that's not available any more and I don't know the equivalent.

Damn good tube the ECC40 - I've used in in differential pair and its a really good sound. Lovely treble.

Andy

[andyjevans]

Sorry, should read "pair of BC549" above.

If anybody knows a good substitute for a J511, let me know!!

Andy

[ErikdeBest]

Hi Andy

Maybe these diodes will work?

For sale : Constant Current Diode 1N5314, 100V, 4.7mA

I have no affiliation with the seller...

Erik

[SY]

Ugh, those diodes are the thing that turned me off to using CCS in my amps until MJ encouraged me to try the bipolar cascodes. The discrete circuit is cheaper and much higher performance.

[ErikdeBest]

hi Stuart

From my understanding Andy is seeking a replacement for the constant current diode that feeds the string of leds in version C (pg 134) of MJ's CCS with bipolars.

Erik

[SY]

A plain JFET with gate and source tied together (that's what the J511 is anyway) will work fine for that application. To be honest, I think that the performance difference between a CCS feed of the LED string and just using a large-value resistor tied to B+ will be negligible. The CCS feed is more appropriate for low voltage situations where the resistor would need to be something like 1k instead of 100k.

[andyjevans]

A plain JFET with gate and source tied together (that's what the J511 is anyway) will work fine for that application.///

That's what I'm after - do you have a model number of an appropriate one?

Erik's right - the J511 is on p134(c) of MJ's book. I use the version with two BC549, two red LEDs and a multi-turn sensor potentiometer. The bottom of the string goes to -15v and the top via the J511 to earth. It's possible to replace the J511 with a resistor, but an equivalent of the J511 would be good to try.

andy

[SY]

You can do it three ways: find a 50V FET with an Idss somewhere nearby and do some selecting, find a 50V FET with an Idss higher than you want and use a source resistor to bias it correctly, or (my choice) use any old 25 or 30V FET with a series resistor to drop most of the voltage, leaving 10V or so across the FET. The rationale is that there are hundreds of usable 25V FETS, but very few 50V ones.

Alternatively, you could use a DN2540 with a source resistor. Overkill, but they're cheap.

One more thing to consider is a cascode of DN2540 instead of the bipolar CCS. No reference string, super low parts count and expense, extremely good performance. I've seen some fancy ones, but I've posted schematics of a dead simple one, just two FETS and four resistors. I suspect that you'll see this circuit in the 4th edition of Valve Amplifiers.