
JakeAC5253
New member
SpiderWars":3qjsuoa4 said:You can decrease the 27k to get more bias voltage (not current, * - see below) as long as it doesn't go over the bias cap voltage rating (the bias filter caps are probably rated in 150v-160v range). If you can get there that way then leaving the 47k is fine.
As you increase your idle current the plate voltage will decrease a little bit. So you have to keep re-measuring your plate voltage, re-doing the calculation, and adjusting again to get it dialed in. Also, consider that 70% max dissipation as a max and don't just blindly set it there. Listen to the amp at 70% then 50%, see if you like one better and go from there. Run it as cool as you can and still get the tone you like. You may like 55% or 60% better than 70%.
* - just to clarify: you are setting a bias voltage (at the grid) such that it controls the idle current through the tube (from plate to cathode). Under normal conditions, no real current flows in the bias circuit itself, the current is flowing from plate to cathode and the grid (bias) voltage just controls that current.
The 10uF bias caps in my amp are only rated at 100V. I'm wondering if I should replace those with higher rated ones during the process of fixing this bias issue.
So, since you are setting the voltage to control the current, then higher not lower resistance between the 47k + 22k will cause a higher negative voltage on the Grid which should help to increase the idle current of the stages? It makes sense since when talking about voltage, more parallel resistance means higher voltage on the rail.