Replacing the cathode resistor with a current source in SE ?

[2wo]

I did a quick breadboard with a 6V6 triode wired, 320V on the plate. I used a IXYS 1045 set to about 30ma. This gave me about 24V at the cathode.

All I got was a little distorted sound, before I ran out of time to play. So Can this work in SE? Was the signal fighting the CS or maybe I don’t have enough voltage compliance cathode to ground…John

 

[Nelson Pass]

A CCS on the cathode in a single-ended Common-Cathode
circuit will give essentially no gain. It would work OK in
a Common Plate (follower) circuit.

 

[DigitalJunkie]

If you bypass the CCS with a suitable cap,you should have sound.

 

[rdf]

Did you bypass the CCS with a largish cap? A CCS in the cathode has the nice side benefit that variations in B+ from hydro fluctuations cause a linear instead of geometric rise in static plate dissipation. Plate voltage changes, plate current doesn't.

 

[tomchr]

I had issues with the IXYS 10M45 in a similar setup. I was using it to bias the pentode section of a 6LU8 compactron. I wanted 55-60 mA but couldn't get more than 40 mA. I wonder if I was running out of headroom on the 10M45 part. The voltage across the 10M45 was supposed to be about 30 V.

You can verify the current of your CCS by measuring the voltage drop across the "programming resistor".

~Tom

 

[2wo]

Thanks everyone, the point was to eliminate the bypass cap. I will hang one in there just to try it…John

 

[revintage]

the point was to eliminate the bypass cap

All problems solved if you go fixed bias instead with the cathode grounded.

 

[mos57]

All problems solved if you go fixed bias instead with the cathode grounded.

No cap discount here.

If you use fixed bias, then the AC signal is closed on negative power supply cap.
So, you move this problem in another place, only.

Francesco.

 

[OneyedK]

No cap discount here.

If you use fixed bias, then the AC signal is closed on negative power supply cap.
So, you move this problem in another place, only.

Francesco.

True, but I thought it gets another magnitude.
Instead of Ra+Rp, Rg1 (much bigger) comes into play...
Doesn't have the negative power supply cap less impact
than the cathode resistor bypass cap?

 

[mos57]

True, but I thought it gets another magnitude.
Instead of Ra+Rp, Rg1 (much bigger) comes into play...
Doesn't have the negative power supply cap less impact
than the cathode resistor bypass cap?

Have you thinking about the other components that you must use for this?

1. the cost and the limitation band of the input transformer?
2. the cost, the complexity and the influence in sound, of another power supply with its diodes,smoothing caps, and peraphs the inductor?
3. the added power supply noise injected directly in the input circuit?

Francesco.

 

[OneyedK]

(1) sorry, but I don't see how this is relevant? No matter how you decide to bias, you still have to couple with previous stages.

(2) Negative power supply can be rather simple, since the current needed is almost nothing, unless you're talking about using an interstage transformer, wich brings us back to (1).

(3) Without the transformer (again) the LPF formed by Rg1 and the miller capacitance of the tube will get rid of most noise, not that I want to say that we have to design a negative power supply without care!

But hey, just my 2 cents, maybe I'm looking at it the wrong way
Enlighten me please...

 

[kevinkr]

In most applications the bias supply is completely divorced from the grid circuit by the grid resistor. As the previous poster pointed out most tube amplifier circuits are RC coupled. In the case of an IT application it is a bit more complex, but honestly speaking I have never found large value cathode bypass caps no matter who makes them to be particularly transparent.

I use fixed bias in just about every power stage I have ever designed, and find the inconvenience of an additional supply inconsequential.

Battery bias in some cases is convenient, in other cases one can use LEDs in the cathode circuit for what is effectively fixed bias operation. (See SY's red light district, etc. for examples.)

 

[rdf]

Battery bias in some cases is convenient, in other cases one can use LEDs in the cathode circuit for what is effectively fixed bias operation. (See SY's red light district, etc. for examples.)

Possibly a better option is to current source the LEDs and put them in the grid circuit in series with the grid resistor. Since the dynamic signal current through them is infinitesimally smaller, so is any impact from non-linearity.