12ax7 preamp tube bias

[George Adzich]

In a document about tips on modifiying a Dumble preamp, it talks about adjusting the bias point of the individual preamp triodes.

It said to simply measure the voltage drop across the Plate resistor and divide by the resistance value of the Plate resistor. This would give a current value for the bias that should be around .75ma

Is there any merit to this approach??

There seems to be a lot of information on the correct bias point for the Power Tubes, but a lot of confusion about the preamp tubes.

I have had great experience simply using the values shown on the schematics, but it peaked my interest. Any feedback on the matter would be greatly appreciated.


-George

[Bob-I]

Personally I use plate and cathode voltage as a guideline. The sweet spot seems to be 200V on the plates and 1.7 to 2V on the cathodes.

[George Adzich]

Bob-I,

Thanks for the feedback.

With the values you have shown, it sounds as if the triode is biased to the "cold" side. I would have expected something like 1.3 volts for the Cathode voltage based on the tube charts and using a "middle" value.

I know it is all about trial and error, but it seemed like an interesting topic that I have not seen on this message board before.


Cheers,

George

[MarkB]

In a document about tips on modifiying a Dumble preamp, it talks about adjusting the bias point of the individual preamp triodes.

It said to simply measure the voltage drop across the Plate resistor and divide by the resistance value of the Plate resistor. This would give a current value for the bias that should be around .75ma

Is there any merit to this approach??



-George

None. All you've done is to use ohm's law to determine the current passing through the plate resistor. If you want' a Dumble preamp, you need to use Dumble's values.
If, on the other hand, you want to learn how to bias a preamp tube, you need to learn about load lines, or read Randall Aiken's article on designing common cathode stages in his Tech Info section.

http://aikenamps.com/TI_Aiken_adv.htm

[Bob-I]

In a document about tips on modifiying a Dumble preamp, it talks about adjusting the bias point of the individual preamp triodes.

It said to simply measure the voltage drop across the Plate resistor and divide by the resistance value of the Plate resistor. This would give a current value for the bias that should be around .75ma

Is there any merit to this approach??



-George

None. All you've done is to use ohm's law to determine the current passing through the plate resistor. If you want' a Dumble preamp, you need to use Dumble's values.
If, on the other hand, you want to learn how to bias a preamp tube, you need to learn about load lines, or read Randall Aiken's article on designing common cathode stages in his Tech Info section.

http://aikenamps.com/TI_Aiken_adv.htm

Av = (mu*Ra)/(Ra+ra)
= (100*100K)/(100K+62.5K)
= 61.5
= 35.8dB


My brain hurts......

I agree, just use Dumble values and listen. Dumble uses his ears, his greatist asset.

[paddy]

With the values you have shown, it sounds as if the triode is biased to the "cold" side. I would have expected something like 1.3 volts for the Cathode voltage based on the tube charts and using a "middle" value

Hi George,
If we were building Hi-Fi amps then we would bias the valves to operate in the centre of the most linear region of the load line.Guitar amps are a different story.Cathode and Plate resistor values are often chosen so that the stage operates in a non linear manner,adding distortion and harmonic content to the sound.

cheers
paddy

[llemtt]

yes but

a triode in common-cathode configuration is one of the most linear amplifier stage available, actually way more linear than semiconductors...

in order to obtain audible effects you have to really skew the bias point, put a trimmer in place of cathode resistor and try!

take a look also at the penultimate gain stage in classical hi-gain preamps (marshall, soldano, boogie,...), that very high value cathode resistor is the key...

cheers
teo