Cathode follower driver

[croccodillo]

Hi guys,

I just wrote down this schematic.
It is a three stage integrated amplifier:
- 1st stage (12AX7) is a CCS loaded SE driver, should be very linear.
- 2nd stage (6N6P) is a cathode follower
- 3rd stage (KT88) is a parafeed output stage, loaded by a CCS (actually a KT88 pentode CCS).

I know first stage can work, already tried it in the past; I have also built the third stage (parafeed output stage, at the moment on my desk under test and development), and drove it with a 1:10 transformer, fed by my PC sound card.

The new stage is the cathode follower in the middle, included to add some juice to the KT88 driver.

The feedback is optional, at the moment there's no feedback at all in the prototype.

My questions:

1. Is the second stage correctly developed? Would it work well?

2. How can I determine the values for R4 and R5? I would like to fix the idle operating point of 6N6P tube to around 120V@20mA.

3. What happen if I substitute the R4 resistor with a CCS@20mA?

4. The alternative to this topology is to use the 6N6P tube as a plate follower, using a CCS to load it: would this topology work worse or better than cathode follower?

5. 12AX7 and 6N6P have two triode each, so I could use one of them for each channel (I'm building a stereo amplifier, of course): is there any problem using the same valve for both stereo channels?

Thanks in advance.

Ciao,
Giovanni Albergoni

[Gluca]

Best thing to do with that cathode follower is to direct couple it to the following grid (ie get rid of that coupling capacitor): this will require a complete redrawn of the amp as you will probably need a negative supply.

Ciao
Gianluca

[danFrank]

I second Gluca's opinion. Direct coupling to the power tube by driver as a cathode follower. The Italians have used this topology a lot; ie the Geloso amps. You will need to have a split supply for the driver, so the driver's cathode will feed the proper negative bias voltage to the output's grid. This makes for a powerful driver that is extremely fast; this will sound good.
Using a cathode follower direct coupled to the power tube has many advantages but is harder to implement.
Capacitors are generally bad but a necessary evil and if you can avoid using any, then that is the way to go.
Daniel

[Wavebourn]

You don't need R4.
Value of R5 depends on a maximum current you need to drive your output monster from one side, from the other side it is limited by max current 6N6 can sustain (power dissipation) without significant distortions and plate melting.
And I would use some high voltage power PNP instead of pentode for CCS...

Gentlemen who suggest you direct cupling forgot about the 1'st triode that needs some positive voltage on the plate to work properly.

[ray_moth]

Gentlemen who suggest you direct cupling forgot about the 1'st triode that needs some positive voltage on the plate to work properly.

Well spotted! Two possible solutions? (a) move the coupling cap so it lies between the first and second stages; or (b) use the negative line instead of ground at the first stage.

[Miles Prower]

Originally posted by croccodillo

1. Is the second stage correctly developed? Would it work well?

2. How can I determine the values for R4 and R5? I would like to fix the idle operating point of 6N6P tube to around 120V@20mA.

3. What happen if I substitute the R4 resistor with a CCS@20mA?

4. The alternative to this topology is to use the 6N6P tube as a plate follower, using a CCS to load it: would this topology work worse or better than cathode follower?

5. 12AX7 and 6N6P have two triode each, so I could use one of them for each channel (I'm building a stereo amplifier, of course): is there any problem using the same valve for both stereo channels?

Thanks in advance.

Ciao,
Giovanni Albergoni

1) Looks OK, however, I would add a negative rail, and DC couple the cathode follower to the final. That way, you get the advantages of fixed bias, with less power wasteage, better THD, enough current to charge up Ci + Cmiller + Cstray without overdrive blocking. Move the AC coupling to the first preamp.

2) Loadline. Also, R4 either isn't necessary, or pick one with 1 -- 5V across it and bypass with an electrolytic if stability and/or PSRR is gonna be a problem. With a cathode follower, you want the plate to be at AC ground.

3) You waste a CCS, and invite instability. If you're gonna do that, then the CCS belongs in the cathode lead, not the plate lead.

4) Much, much worse. Do that and your feedback becomes positive instead of negative. All you'll get is a fixed frequency square wave generator.

5) Yes. No.

Furthermore, I would ditch C4, and connect that gNFB at the load on the OPT secondary, That way, the xfmr is included in the error correction. This will help to linearize it, and reduce the Zo for better woofer damping and better bass performance.

[Wavebourn]

Originally posted by ray_moth

Well spotted! Two possible solutions? (a) move the coupling cap so it lies between the first and second stages; or (b) use the negative line instead of ground at the first stage.

And what's the reason?


"There were rocks in the pool. We took racks and removed rocks. Now there are racks in the pool" (C)

[ErikdeBest]

Hi Wavebourn

I can only use other people arguments, but actually that is not that bad at all! I will use the argument's by Morgan Jones, in his Valve amplifiers book. Hmm...before spitting rubbish here (2:33 in the morning) I will read Morgan Jones again, and understand exactly why. If noone explained why ray moth's approach is better, I will try to do it, maybe tomorrow.

But it has to do with overloading of stages and class of operation. THe cathode follower can't be overloaded, but the output stage can...

I have to go to bed... ray moth, jump in (I know you have experience with the Crystal Palace).

Erik

[Wavebourn]

Originally posted by ErikdeBest
Hi Wavebourn

I can only use other people arguments, but actually that is not that bad at all! I will use the argument's by Morgan Jones, in his Valve amplifiers book. Hmm...before spitting rubbish here (2:33 in the morning) I will read Morgan Jones again, and understand exactly why. If noone explained why ray moth's approach is better, I will try to do it, maybe tomorrow.

But it has to do with overloading of stages and class of operation. THe cathode follower can't be overloaded, but the output stage can...

I have to go to bed... ray moth, jump in (I know you have experience with the Crystal Palace).

Erik

I mean, what reason in moving the coupling capacitor one stage ahead?

Possible answer is that a grid current of the output toob will charge the cap causing dynamic bias.

[SY]

That's very much it. "Blocking" is the word you're looking for. In most designs, the output stage will clip first, and direct-coupling the cathode follower to the output tube grid minimizes the pain caused by grid current piling up charge on the coupling cap, overbiasing the stage for a time constant or three.