more gain needed -> how?

[Danko]

Hi!

I have made the attached amplifier. Yet only one channel.
I get only a few watts from this amp. I think this is becouse the amp doasn't have enough gain.
Maybe I should use another one 6H8C, for more voltage gain?

The output xformer: Raa=2.5k, secondary is 4ohm.
The quiescent current through the output tubes is about 30mA.
I read somewhere on the net, that I can get about 30W from a push-pull triode-connected PL509.
Can I expect this?

[jane]

The grid voltage on your cathode followers is 0 volt. (Connected to GND with a 470k resistor) That is not a good working point for a CF. I think you should elevate the grid voltage to about 80V.

[Danko]

Thanks for the fast reply, jane!

Tomorrow I will try soldering an 1.2M resistor between the grin and the plate.

[ilimzn]

You might want to think about carefully chosing the operating point of the input stage (maybe using a +B drop resistor and cap for it) which may make it possible to direct couple the followers.

[Danko]

Can the 6H8C drive properly a PL509, if the PL509's grid is connected to the 6H8C's anode? (of course, through the appropiate R-C network...)
If it can drive poperly, then I can exchange the cathode-follower with a "plate-follower" which can amplify voltage.

What do you think of it?

[SY]

What's the maximum screen voltage rating of the PL509? What voltage do you have on it?

[Danko]

SY: The datasheet "says", that the maximum voltage is 275V on the G2. I have on it just a few volts lower than B+, so it's about 295-298.

Oops, it's beyond the tube's maximum limit

I gonna try a bigger resistor.

[Danko]

Now there's an 1K resistor between the G2 grid and the plate. And on the G2 there's still too much voltage. Shall I try about 10k?
I haven't seen any tube-amp schematics, where such big resistor was in that position...

[Giaime]

Danko, you aren't going to drop significant voltage with the configuration you have in mind. Use a 100ohm resistor and be happy, a PL509 is a very robust tube and don't mind a bit more voltage on the screen grid.

The problem on screen grids isn't voltage, is power dissipation. You have a class A amp (for what I see), so max. dissipation is at quiescent state. Put a DMM in series with the screen resistor and measure screen current on quiescent state. Now measure screen voltage, multiply those values by each other and see what's your current screen quiescent dissipation, and then compare it with the maximum Pg2 ratings.

Usually sweep and RF tubes like this (and the 6146 comes to mind) are specified at very low screen voltages, but they can handle serious voltage when trioded, at least that's my experience.

If you are under 300V, I wouldn't be too worried, that's pretty low

[Danko]

The screen grid dissipacion is about 1W.
The datasheet says, that the max. screen grid dissipacion is 9W.

So it's OK.

Thanks! :-)