6L6 Ultralinear cathode Follower

[ppl]

I am playing with a (2) stage Head-amp consisting of an Ultralinear cascode Input stage driving an Ultralinear cathode Follower. I was considering as the next step in the evolution of this design is to remove the inter-stage coupling capacitor (C3) and direct couple the 1st cascode stage to the (2ed) cathode follower output stage. By adjusting the anode voltage slightly I could use the DC from the (1st) Stage anode to Bias the (2ed) stage. While I see no problem with this at first I was wondering about long-term stability since as the tubes age would not the voltages depart enough from Initial design to cause Problems?

[DF96]

That's a strange circuit. It starts with a cascode, but then throws away most of the only audio advantage of a cascode (low Miller capacitance) by putting feedback to the upper grid. Then the output stage applies positive feedback to g2 in order to boost the drive capability, and makes the 6L6 behave more like a pentode than a triode (as would be normal for a CF). In both cases it is as though the designer wanted to use a particular circuit, but didn't want the normal characteristics of that circuit so he modified it to behave like a different circuit.

[ppl]

Thanks for your comment on the Circuit. the topology is my original adaptation of the Ultra-linear Line stage as described HEAR One thing I do with this Circuit is return the Resistive voltage divider to the cathode rather than ground as depicted in the reference circuit. This indeed dose adds feedback and that was my Intention. The circuit operates in the ultra-linear mode and not Pentode.

[revintage]

I used the this cascode but variable from cascode(pentode) to triode in my guitar amps long before that one was published. Didn´t behave nice until a CF was added and the return was fed from there.

[DF96]

Feedback from anode to cathode is positive feedback. This will tend to increase any distortion generated in the lower triode. Fortunately you only have a small amount so it won't do much harm.

[ppl]

Thanks for the Input. More Info on the Ultra-linear cascode can be found HEAR. this highlights the advantages of the ultra-linear cascode topology. By me returning the Ultra-linear network to the cathode and not Ground dose indeed create positive feedback. (I like to call it feed foward).

The difference in sound between connecting the Ultra-linear network to ground or the cathode are quite different with the cathode connection Highlighting detail and sharp crisp transients, whereas grounding the Network produces a warmer less Detailed presentation far to smiler to the sound one would expect from a tube Amp. Myself I like the Hightened detail and crisp high end of the cathode connection.

OK, so now back to removing that interstage coupling capacitor? will Tube aging cause any problems if the Anode of the 1st stage is direct coupled to the grid of the 2ed stage?

[DF96]

Originally Posted by ppl

(I like to call it feed foward)

I guess you can call it whatever you like, but it is in fact positive feedback.

You might get ageing problems if you made the circuit into DC coupled, but the feedback in the cascode stage will help. Give it a try. It should be OK. The output CF should not be too fussy about exact bias.

[smoking-amp]

How about taking the cascode grid's feedback off a tap from the final follower output?

[revintage]

How about taking the cascode grid's feedback off a tap from the final follower output?

That is just what we use(mentioned it above) in our guitar amps, albeit variable and no postive feedback to the cathode.

This gives different characterics soundwise, sparkling in pentode/cascode- and tight in pseudotriode-mode and also a gain difference from 34dB to 58dB

Note that the designer at Conrad-Johnson(Bill Johnson?) has a patent for the fixed feedback circuit

[smoking-amp]

"Note that the designer at Conrad-Johnson(Bill Johnson?) has a patent for the fixed feedback circuit "

1987, US patent 4,647,872 from the Tubecad article. Would be expired now. And probably was unenforcable anyway as Broskie mentions prior art and plain obviousness from UL.

Should be a helpful scheme for controlling the DC biasing in the DC coupled case. I think Wavebourn uses something along this line for DC bias control.