6N6P SE idea

[diezel_addict]

Hi all

I wanted to build a SE low wattage amp and I hear only good things about the 6N6P. Have a look at the attached schematic and tell me what you think. Any suggestions are welcomed.

The idea was to use a cathode follower parallel feed OP stage.

I understand that the tube load and the Zo are two separate things. Is this heading in the right direction assuming the Zo on the cathode is approx 80 ohms???

thanks!

[Wavebourn]

For the first stage you need stable voltage in cathode instead of stable current, otherwise amplification factor would be near zero.

[diezel_addict]

so you are saying just replace the first CCS with a resistor?

[diezel_addict]

If the CCS was set to 20mA with a plate voltage of 140v, wouldn't it put the cathode at a stable 4 volts (approx)..??

[TheGimp]

It woudl put it at 4 V for a bias point but it would not be stable. as soon as the input voltage changed the cathode voltage would change to compensate to insure a constant 4mA. It would be the ultimate in cathode feedback. So the anode would not change much, if at all.

[artosalo]

so you are saying just replace the first CCS with a resistor?

Yes, with 200 ohms. (4 V/20 mA) You can also by-pass it with large capacitor and put the CCS at the anode.

[Michael Koster]

I have a few thoughts to add:

The LL6404 doesn't seem to be designed for interstage use. There is no quiescent DC allowed; the 6404 is designed for zero-field input use where the actual voltage across the winding changes very little This transformer is meant to be inside a current feedback loop. The LL1660S would be more appropriate.

Given the correct interstage transformer, you could simply bypass your input stage cathode CCS with a capacitor. That would keep the DC current constant and allow signal amplification at ~ mu=20.

You don't give output idle current but looking at the max cathode current of 45mA I would expect quiescent at about 25mA. That would allow a cathode current swing of ~ +/- 20 mA. Your reflected output load is about 72 ohms. +/-20 mA into 72 ohms will swing about 1.44 volts Peak or about 1V RMS. Max output power is therefore about .013 watts.

A more appropriate load would allow the full driver plate swing. Assuming 2VRMS input, the driver plate swings about 40VRMS, 56V peak. Assuming the same +/- 20mA output cathode current swing, that would imply that you want a OPT primary reflected impedance of about 2800 ohms. Max output power is about .57 watts. OPT ratio needs to be about 19:1

Cheers,

Michael

[diezel_addict]

Hi Guys

Thanks for the input. The interstage transformer was just a place holder and hadn't been chosen properly yet.

thanks!

[diezel_addict]

Hi All

I did a redesign using a Gary Pimm active load for the output stage with both halves of the tube in parallel. It uses the low Zout of the CCS into the output transformer. Not sure what the Zout would be exactly but would be pretty low. How do you think something like this would perform?

[artosalo]

How do you think something like this would perform?

It is very quiet amplifier.
You have connected both grids of 6N6P as push-pull, but anodes parallel.
There is no output signal at all.