applying fixed bias to cathode followers

[angsuman]

Hi, I was wondering how I would apply fixed bias through cathode followers that then connect to the power stage. I've been looking around and I think I found something that would work from the Radiotron Handbook but I'm not sure.

here's the schematic

http://www.flickr.com/photos/14031769@N06/2719918663/sizes/o/

Basically it says that R7 equals the bias voltage/plate current. So I'm guessing if I have -24 volts bias and 88ma on the plate of the following power tubes then I would make R7 272 ohms?

So I came up with this schematic

http://www.flickr.com/photos/14031769@N06/2720742956/sizes/l/

Will this properly bias my output tubes or do I have everything completely wrong?

 

[Miles Prower]

^^^^

Here's how I did that: Renard Main Schemo.

Adjusting the grid bias for the followers (6FQ7 sections) determines how much current they pull, and that sets the grid bias for the finals. Works just fine, and once the VTs settle down, the no-signal Q-Point plate current remains quite steady.

 

[angsuman]

Okay so let me get this straight. You apply the bias to VKK, which flows through R21 and 22, the load resistors and then gets stabilized by the zener diodes to 30V? Also I'm guessing that R16 and R17 set the bias point for the cathode followers? And that C7 and C8 are simply cathode bypass caps?

Could you please let me know what the component values are in that circuit?

Thank you.

 

[analog_sa]

Okay so let me get this straight. You apply the bias to VKK, which flows through R21 and 22, the load resistors and then gets stabilized by the zener diodes to 30V? Also I'm guessing that R16 and R17 set the bias point for the cathode followers? And that C7 and C8 are simply cathode bypass caps?

Hmmm. Not even one correct guess. What are the chances?

 

[ErikdeBest]

It is really not that hard: see the second picture on the attached link.

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

It uses a mosfet, but that is essentially the same thing as a valve in this position (drain = plate, gate = grid, source = cathode). The left picture shows the 'standard' connection, the right picture the addition of the source follower.

Erik

 

[Miles Prower]

Okay so let me get this straight. You apply the bias to VKK, which flows through R21 and 22, the load resistors and then gets stabilized by the zener diodes to 30V?

The Zeners, CR5 & CR6, may not be necessary. The negative rail is solid state, and comes up very fast while the cathodes are still cold. That will exceed the Vhk rating of the 6FQ7 by a wide margin. These Zeners are 90V units that limit the voltage to a safe level. Once the 6FQ7 heats up, they drop out. When testing the prototype, I didn't have any, and so just floated the 6.3V heater winding. I expected lots of hum and buzz, but didn't get hardly any (6.0mVp-p of 60Hz sine wave at the output which you can hear only if you stick your ear right into the speeks) so I never bothered to add them. Better if you don't need them as that removes a nonlinear capacitance from the signal path. I get very quiet operation with floating heaters, and so the Vhk limitation isn't a problem.

Also I'm guessing that R16 and R17 set the bias point for the cathode followers? And that C7 and C8 are simply cathode bypass caps?

The two pots set the grid bias for the 6FQ7 sections, and therefore the Q-Point current of the 6BQ6GTBs. C7 & C8 simply bypass the grid bias network so that the signal sees the same impedance to AC ground regardless of the settings of the bias pots. The only problem with that is that they slow down the response to the bias pots, so you simply have to make any adjustments slowly so that you're not chasing the Q-Point current.