Cathode follower driver

[Wavebourn]

And your current schematics represent a multivibrator instead of an amp with NFB.

[ray_moth]

Wavebourn, you were the one who pointed out the problem, I thought you would understand my answers to it. As you said, the first triode must have a positive voltage om the plate (compared to its cathode, that is). The second triode (the cf) needs a negative voltage on its grid if it is directly coupled to the KT88.

One (obvious) way to resolve this would be to have AC coupling between the first triode plate and the second triode grid, so they can operate at their correct DC voltages. This is quite normal where directr coupled cf drivers are used.

Another (possible) way could be to have DC coupling from start to finish but use a negative rail for the cathode resistor of the first triode. I doubt if it would be practical, however, because that negative rail would also have to be the basis for supplying the bias voltage for the KT88 (via the cf) and I think it would be difficult to stabilize.

[croccodillo]

Thanks all,

You're right, DC coupled stages are much better.
My power transformer has some auxiliary windings that can be used for the purpose; it has 4 different secondary windings:

- 250V 300mA
- 250V 300mA
- 100V 100mA
- 100V 100mA

So, now I'm using only the first two windings to have a B+=300+300V=600V (two separate rectifier on thw two windings, two regulators and then the two 300V voltages are connected in serie).
In this way I can have a central voltage of 300V, useful for output capacitor (less stress) and for drivers.

I can use the last two windings to build up a 125+125V, and use those voltages for a negative supply.
With such a negative supply I can connect the 12AX7 cathode to -250V, setting its working point at about 250V, and the 6N6P cathode to the -125V supply, and thus having lot of room for the output tube drive swing.

And I would use some high voltage power PNP instead of pentode for CCS

The CCS for the driver is realised with a mosfet around a schematic I drawn; when I will be sure it works fine I will show you.
The CCS for the output tube is actually a pentode KT88 driven by a SS CCS, realised with another P mosfet; actually the pentode and the mosfet are cascoded.
This is the latest schematic, where you can see also the parafeed CCS (not the SS schematic, however).
Actually the voltage regulator for the upper pentode screen is much more complicated, involving a mosfet, diodes, zeners, in manner to keep as far as I can the pentode screen voltage stable to 300V with respect with its cathode.

My bigger doubt now is about the input: is there any problem driving the amplifier with my CD player, since the input tube (12AX7) grid is at about -250V?

Comments? Could it work? Suggestions?

Ciao,
Giovanni

[Shoog]

Croccodillo I thought you might be interested in an observation I have made comparing my choke loaded parafeed version compared to my CCS loaded version. Despite the poor efficiency of the CCS loaded version it has much tighter bass than the choke loaded version. Of course there could be any number of reasons for this, but personally I attribute this to the fact that the CCS version behaves more like a PP output with better current drive throughout the wave cycle.
This results in tighter punchier bass overall.
So poor efficiency but better sound. You pays your money and makes your choice.


Shoog