Not enough range in bias pot

[bluesguitar]

I just swapped my gz34 rectifier tube with a 5r4gy. I knew this would lower my plate voltage, which it did. 470 before, 438 after. I calculated my bias point for 70% plate dissipation (6L6) at 47.9 mA. I use an Amp-Head bias probe for my readings. The tubes read 20.5 at the start. When I turned my bias pot to increase current it reached 26 mA and ran out of travel. I double checked it after making sure everything was hooked up correctly, and got the same readings. Has anyone else run into this problem? Any help would be much appriciated.

[Gibsonman63]

Normally, there is a fixed resistor in series with the pot in the bias circuit. You will have to change the fixed resistor to something a little smaller.

[bluesguitar]

Normally, there is a fixed resistor in series with the pot in the bias circuit. You will have to change the fixed resistor to something a little smaller.

Thanks. I'll research your tip, and replace the appropriate resistor.

[skyboltone]

It can be a pain in the ass to do experimentally. Sometimes you can just string another pot in there and set it high, then work down until you get the range you want. I'd take the tubes out though and just look at voltage on the grid. Negative around the -32 thru -45 range. Then plug the tubes back in and, again, starting high, adjust things until you get the range you need.