bias pot wiring

[fopoman]

could someone double check my bias pot wiring. Its a 25k linear pot. Bottom lug goes to the 6k8 which goes to ground, middle lug goes to the 220k's and the top goes to the diode.

thanks
Nathan

[Super_Reverb]

So, you have a divider string from the anode of the grid bias diode connected to a 25K pot and 6.8K to gnd. With the wiper turned all the way CW, the wiper is connected to the tap between 6800 and 25K pot. It looks to me that the voltage:

Vbias(CW)=Vanode(DC)*6.8/(6.8+25) In other words, with pot all the way up, the bias voltage will be minimum. And with pot full CCW, wiper will be shorted to top of pot (maximum negative grid voltage = minimum plate bias current)

Alot of guys place a "pullup" resistor from the pot wiper to the more negative end of the bias pot as a failsafe, in the event that pot wiper goes open. If you add this, you may have to adjust resistor values to get the range of grid bias voltage you need.

If I were building bias circuit, I'd place a small electrolytic cap from pot wiper to gnd to further smooth the DC bias voltage - you don't need much capacitance as the ripple voltage is very low due to low current draw into power tube grids.

Mississippi State country? I grew up in Gulfport

cheers,

rob

[fopoman]

This is what the schematic looks like. Born and raised in Michigan. Came to starkville in 93'. Loved it too much too leave!

thanks,
Nathan

[Firestorm]

The schematic is different from what you have. The wiper goes to the 6K8 lug, the other lug goes to the diode, cap and 220Ks. The schematic shows the "safer" way to do it.

[tubeswell]

Yes the schematic is good. If the wiper contact fails, then you are left with 25k + 6k8 in the leg of the voltage divider, i.e.: defaulting to a maximum amount of -ve voltage going to the grid(s).

[fopoman]

like this

[selloutrr]

this how it should look

[fopoman]

will this work?