Cathode Follower confusion

[Brit01]

I'm trying out various types of CF's and have started with this simple design.

I am having trouble understanding why I have such a low voltage on the grid (24V). I understand I need a 6 volt difference between cathode and grid, for a CD players 2 V input (6 volt swing). I could use 3 leds for the cathode resistor I guess.

I have tried adjusting the resistor values but have had no luck. I tried MJ's example but had only 8 volts on the grid

Your advice would be greatly appreciated.
Strangely it sounds quite good as I tried it through some test speakers.

[SY]

I understand I need a 6 volt difference between cathode and grid, for a CD players 2 V input (6 volt swing).

Nope. There's your misunderstanding. You need roughly 6V/mu. Remember, the cathode follows the grid.

[Brit01]

Hi SY,

Maybe these explanations threw me off the track:

Remembering that the quiescent voltage shown on the load line actually indicates the anode to cathode voltage, the actual quiescent voltage on the cathode is therefore equal to the HT - Vak. An easy way to read off the true cathode voltage quickly is to add a reversed voltage scale to the graph, whose '0V' point corresponds to the HT. In this case the load line indicates that the voltage across the valve is 140V. The cathode voltage must therefore be HT-Vak = 280-140 = 140V. The grid voltage must be 140+(-3.4) = 136.6V

From the Valve wizard:


http://www.freewebs.com/valvewizard/accf.html

it's showing 140 Volts on the cathode and 136 on the grid.

Am I missing something?

[Yvesm]

You cannot have 170V at the cathode and 26V at the grid simultaneously... unless your tube is dead !

But, how do you measure ?
What is the voltage at the junction of R2/R5 ?

Yves.

[Brit01]

That's what is confusing me. Tubes are fine. I tried many different types.

I will measure the junction when I get home.

[Osprey]

Your numbers, at least on the schematic do not add up.

I'll go by the schematic as I am not sure what the rest of your post means. The voltage on the cathode is 170V, divided by the cathode resistor 16kohms gives a cathode current of 10mA. That all sounds correct. Now multiply the 130 ohm bias resistor by the cathode current to give you the voltage drop across it which is about 1.4V. That is your bias voltage. R2 is your bias resistor and you will need to adjust it to get your proper current through the tube.

[Brit01]

Thks. Yep 10mA is good. Maybe a little more for the 6N1P.

Still confused about how much bias voltage I need for a CF.

I thought it was about 6V but not sure after SY's comments.

6V would mean having 600 ohm bias resistor if current remains at 10ma.

[SY]

How much cathode-to-grid voltage you need can be figured out from the tube's curves. Then you can back into what value bias resistor you need, just as you said.

FWIW, the Heretical preamp (a cathode follower circuit) has about 1.7V grid to cathode and happily swings 10V.

[oldmanStrat]

Originally posted by Brit01

6V would mean having 600 ohm bias resistor if current remains at 10ma.

if you look at your measurements you have 170 volts at the cathode. That is the voltage you need to use to calculate the current through R2 & R5. The 6V number is the delta between your cathode and grid (and is your bias voltage) although yours is much lower in this circuit.

hope that helps

[Osprey]

Still confused about how much bias voltage I need for a CF.

I thought it was about 6V but not sure after SY's comments.

6V would mean having 600 ohm bias resistor if current remains at 10ma.

If you change the bias resistor to a different value the current will change too. That's the bias resistor's job: it adjusts the current going through the tube.

If you increase the bias resistor to increase head room, the tube will draw less current. If you need more current, then increase the B+, but you will have to adjust the bias resistor again.

I don't really understand if you have built this or not, but my suggestion is to use a 500ohm pot (make it at least 1W) as a bias resistor and put a 200 ohm resistor between the pot and the load resistor so that there is always some V on the grid. Vary the pot and measure the current.