|
Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
|
|
|
Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
|
In Reply to: Low cathode resistor bias posted by Doug Flynn on July 16, 2005 at 21:47:01:
Using a cathode bias resistor offers the advantage of DC feedback that stabilizes the operating current as the tube warms up, ages and as the PS voltages drift. But the value may be higher than you wish for AC signal gain since the resistor provides the same negative feedback for AC as for DC, lowering gain and increasing the equivalent plate resistance to ((mu+1)*Rk)+rp. When this is the concern, we usually add a bypass cap across either all, or part of, Rk. But now we’ve introduced the “sound†of a cap, plus a low frequency roll-off.Another option is shown in the link cited above. If Rk is the cathode resistor, R1 is the resistor to the bias supply (maybe the heater supply), and Vdc is the voltage of this bias supply, then the equivalent circuit, at least for DC bias purposes, can be redrawn with a new single cathode resistor Rkeq which is equal to Rk in parallel with R1. This new resistor is connected to a new DC supply, Vdceq, whose voltage is Vdc times the voltage divider ratio Rk/(R1+Rk).
So what? Well now it may be more clear that this technique offers a combination of both fixed bias and cathode resistor bias. It doesn’t matter that the fixed bias is in the cathode circuit instead of the grid side for DC bias setting purposes. This is a compromise between 100% fixed bias and 100% cathode bias. The advantage is that the new Rkeq is smaller than what would normally be used in 100% cathode bias, allowing more stage gain and a lower equivalent rp. Disadvantages include less DC stability due to less feedback, and the possibility of added noise and distortion due to imperfections in the Vdc supply. Still this technique, if used with care, may be very useful.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Another way to look at this - BBeck 07/17/0506:31:16 07/17/05 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
