Can I use a LM317 bias on lower cathode of SRPP?

[Brit01]

I'm running a SRPP and trying out with different CCS on top and bottom cathodes.

Set up a DN2540 on the cathode of the top tube which works nicely with 18mA.

But I thought I understood how to use a LM317 on the cathode of the bottom tube. Obviously not.
I had a Voltage of 1.21 V on the output of the LM317 with a resistor of 68 ohms.
So about 18 mA no?

So why does it not work? Just get a total distorted mess.

What am I doing wrong?


LED bias: If I have a voltage of 2.2V on the cathode with an LED with 130V on the anode of the bottom tube, how can I work out the current? I realise it sets up a constant voltage source but still confused.

I'm using 6N6P's in a headphone configuration.

Thanks

 

[Globulator]

The SRPP is already a constant(ish) current device that is designed to work with a constant voltage.

By adding a CCS you are causing the CCS to fight the SRPP, the voltage will be all over the place and you will just hear a distorted mess.

The only place for a CCS is with a single tube in place of the anode resistor.

 

[Brit01]

😀😀😀

Now that makes sense. Now I feel a little silly.

The DN2540 seems to work well setting the current on the upper cathode though.

Back to resistor bias on the lower.

Is there any method of biasing the lower? LED bias?
Or better to stick with the old school method.

 

[kevinkr]

The LM317 does not work properly if the voltage across it under any circumstances falls much below 3V or so and this is NOT including the voltage drop across the current setting resistor. This will result in lots of distortion as you found out.

The SRPP is only operating quasi-constant current if you use as a voltage amplifier and take the output from the plate of the lower tube.

Adding a ccs between the cathode and grid of the upper tube turns it into a beta-follower which is an improved variant of the mu-follower and is not PP in any sense. Gain very closely approaches the mu of the lower tube and the effective plate load resistance is pretty high, linearity is good..

LED bias makes sense for the beta-follower..

 

[Wavebourn]

However you can, but what for?

 

[Brit01]

The LM317 does not work properly if the voltage across it under any circumstances falls much below 3V or so and this is NOT including the voltage drop across the current setting resistor. This will result in lots of distortion as you found out.

useful to know

The SRPP is only operating quasi-constant current if you use as a voltage amplifier and take the output from the plate of the lower tube.

taking the output from the plate below the cathode resistor of the top tube? I was thinking of trying that. Currently have it taken from above the cathode resistor.

Adding a ccs between the cathode and grid of the upper tube turns it into a beta-follower which is an improved variant of the mu-follower and is not PP in any sense. Gain very closely approaches the mu of the lower tube and the effective plate load resistance is pretty high, linearity is good..

CCS like a cascode DN2540? I have seen some designs with a resistor between the grid and cathode of the upper tube. I have just a direct connection.

LED bias makes sense for the beta-follower..

Still not too sure how to calculate what constant voltage I need to set the current I need for a particular tube.
Lets say I need 18mA and I have 130V on the plate for a 6N6P.
Are we around the 2V benchmark?

 

[Brit01]

Interesting article about the mu follower.
The upper triode, CF, a high gm tube is preferred like the 12AU7 (which I have plenty).

Think this might be my next project😀

The Valve Wizard -Mu Follower