• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

increasing the bias will cause the amp to.....

thank you, some ideas/answers please

Mono block amp SET 211 with unknown OT impedance.

looking around at other circuits there sugestions for bias are around:
a)900v, -47, 70ma, 10k

valve spec leaflet says
b)1000v, -61, 53ma, 7.6k

my question is,

my voltage is 950, -45, 70ma (guess that can be adjusted)

without going into valve operating points etc and power limitations on graphs if my OT is 10k then (a) is ok but if my OT is only 7.6K what will be the effect on the sound if I use (a) settings.

ie what effect does the OT impedance have on choice of bias and the combination effects the sound how. If I reduce the -V the bias goes up and the sound ........
Hi Frank,

The lower the primary impedance (or load), the steeper the loadline. You can see the efects in the datasheet. Mark the operating point of your settings and rotate a line around that point.

You will see that the steeper the loadline, the more you go into the curved region of the Vp vs Ip plot. That is: higher distortion.

As a general rule, for triodes, the closer the load is to the plate resistance, the higher the distorion. The power output is higher too.

For a 211 the plate resistance is around 3600 Ohms, if I remember correctly. 7.6K will be just above 2*rp.

2*rp: higher power, higher distortion.
10*rp: low power, low distortion (there is little improvement in the distortion figures by going much beyond 10*rp, I think).