cathode follower with gain

[trancy]

Originally posted by fdegrove
Hi,

Do you really need to drive a 4K7 load?

4k7 is input res of my amp....

[fscarpa58]

hi trancy

correct me if I'm wrong: gain of tube is defined by u (for ecc88=33) and plate resistor (Rp)

yes but also consider that Rl is in parallel to Rp and also the load
resistance (4.7k) will be in parallel with it.

I think you have forgot the output cap.

As depicted in your figure, the circuit will exhibit a non zero
output voltage.

give me some minutes to draw your cir

Federico

[fscarpa58]

Here is, trancy

You can try cir A or B.

the first has a bypass capacitor as in your pic.
gain is 15 (23.5 db) and Zout is about 3.5k.

Cir B has no cap (that is degeneration). Its gain
is smaller: 8 (18 db) but Zout is greater: 6.5k.

Input cap may be avoided (If source has not
DC component).

As stated by Frank (ciao Frank) your amp has
a very low Zin, and it does not seem very advisable
to drive it with a common cathode stage.

Ciao

Federico

[AKSA]

Unless you use a high gain triode and a step down output transformer in the plate.

Example: 6GK5, mu 78, rp of 5,900R. With Step down of say 12 to one, the gain will be about 6, which is 15.5dB, possibly enough, and Zout will be on the order of 41R, so driving 4K7 will be a snap. The B+ will be around 150V for best results; you should be easily able to cope with 1Vrms input, for 205Vpp at the plate and 17Vpp at the output of the trafo.

This is a great way to drive low Z loads, though the transformer is expensive. Advantage; option of balanced output, no coupling cap, wonderful drive.

Cheers,

Hugh