Cathode Bias Pot

[Cygnus X1]

I can't get this to work-how do I wire it up to the pot?

I tried connecting the 1 ohm to pin 8, then to terminal 1 on the 1K pot, with 2/3 tied, then to the 100 ohm resistor. It shows a 220, it's supposed to 100 ohm.
Along with a parallel wire from pin 8 to the positive end of the capacitor.
That yielded either 100 oh, or 1, with no sweep showing.

So I disconnected terminal 2 apart from three, and put the wire to the 100 ohm from terminal 2. Again, either 100 ohm or nothing-no sweep showing.

So I disconnected the wire from pin 8 to the capacitor.
No difference.

I'm screwing up somewhere.



[img:864:540]http://img300.imageshack.us/img300/7580/biaspot.png[/img]

[Wayne]

What you've drawn there, and your description, should do it. You should be able to vary your resistance from pins 1/8 to ground from 101 to 1101 ohms.

A couple of thoughts - is your pot good?

Also - your multimeter applies a voltage and measures current to determine resistance. The cathode bypass cap has to charge to this voltage before your resistance measurement is meaningful. If it is ungawdly big, this could take a few seconds. When you turn the pot, it'll take a few seconds again for everything to settle. If you're sure you've built exactly what you drew, try disconnecting the cap, and measuring again.

W

[Bruciep07]

Check the pot with your DMM, just make sure it's working properly, I always start with the obvious stuff and go from there... Are you building the SEL circuit from AX84 or something else?

EDIT: Wayne beat me to it...

[Cygnus X1]

Yes, Bruce-with some tone mods I made up from a couple of other builds.

So which is the correct setup?
1 ohm to pot terminal #1, then 2 and 3 tied together?

Or is it 2 and 3 separated?

[Structo]

I would put the 1 ohm as the tail.

Hope you are using a large wirewound for the pot.

I have a 10 watt cathode resistor on a couple 6V6's and it gets pretty warm.

[Cygnus X1]

I'm thinking the same, Structo-but that is the way it's drawn.
Still not sure how the wires attach to the pot.

[Structo]

Just so I'm clear, is this a pentode like a 6V6?

[Structo]

Yeah, I just looked up the AX84 material.

The pot should be wired when looking at the back, the left two or 1 & 2 lugs wired together.

Don't forget that the plate voltage can change with the current draw so you will have to go back and forth with the adjustment on the cathode then measure the plate voltage, do your calculations.

Now, take a reading of the anode (aka plate) voltage and use the following formula to determine the bias setting for the tube you are using: (PaMax * 0.9) / Va
PaMax is the maximum plate dissapation of the tube you are using and Va is the voltage at the anode that you measured. Once you have that number, you simply adjust the bias pot until you see that number on the
display.

[Bruciep07]

Like Structo said, from the back of the pot with the lugs facing up, the left 2 lugs tied together(which I believe are actually 2 and 3), then to the 220R/5W then to ground with the cap paralleling the 5W resistor. From pin 8 on the power tube through the 1R/1W to the remaining lug on the Pot (lug 1), your test points on either side of the 1R/1W. If it's not working wired this way then something else is going on. Did you change anything from the power amp section that's different from the schematic?
Here's a layout for the 20W SEL from AX84 for reference...

http://www.ax84.com/static/corepoweramp ... Layout.pdf