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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

increasing the bias will cause the amp to.....
increasing the bias will cause the amp to.....
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Old 31st May 2005, 12:49 PM   #1
frank2395 is offline frank2395  Australia
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Join Date: Apr 2005
Location: AUD
Default 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 ........
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Old 31st May 2005, 06:00 PM   #2
Radames is offline Radames  United States
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Join Date: Apr 2005
Location: New York
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).

Cheers,

Rada
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Old 1st June 2005, 01:12 AM   #3
frank2395 is offline frank2395  Australia
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Join Date: Apr 2005
Location: AUD
thanks Radames, I have emailed back.
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