Bias range.. which resistor ?

[worth]

In a Fender with a 10K bias pot , does it matter which resistor , ( the 470 , or the 27K soldered to the bias pot ) I change to bring the bias into range? I wondered about this because , on this site, Doug says to adjust the range by changing the 470, in Gerald Webers' video, he says to change the resistor on the pot itself ..... any difference ?

[JayB]

In a Fender with a 10K bias pot , does it matter which resistor , ( the 470 , or the 27K soldered to the bias pot ) I change to bring the bias into range? I wondered about this because , on this site, Doug says to adjust the range by changing the 470, in Gerald Webers' video, he says to change the resistor on the pot itself ..... any difference ?

That 470ohm will limit the max voltage/current you got. The pot and the resistor on it will vary the range you have from 0 - max. I think that 470 ohm is just there to protect against a dead short to ground anyways.

[worth]

I think the 27K is there to prevent a short to ground , not the 470 ?

[tubeswell]

What worth said.

There are various ways of making a bias supply. The Fender example given here
http://www.el34world.com/charts/Biascircuits.htm is one of the typical way's Leo did it.

The 27k-to-ground provides a tail that ensures a minimum negative bias voltage is maintained. You can change the 27k (to 22k, or 33k or 39k, for example) and you will get 'leaps' of change in the bias adjustment range provided by your 10k pot. As to what you need, it depends on what your rectified and filtered bias voltage is, and what kind of tubes you are running and how you are running them.

[sluckey]

The 470Ω will protect the PT bias winding from a dead short in the bias supply circuit, ie, shorted filter cap, etc.

The 27K will prevent you from dialing the bias voltage down to zero which would probably melt your fuse if you're lucky or melt your output tubes and/or OT if unlucky. So it protects against operator error. But it's main function is to form part of a voltage divider.

Since the 470, 10K pot, and 27k are all in series, changing the value of any one of those will change the bias voltage. If there's too much bias voltage, you can increase the 470 to decrease the bias voltage. I would not go above 1K. I would not decrease that resistor though, for the reason JayB mentioned. You will have much more range by adjusting the 27k up/down, just don't take it down too low.

[Jack1962]

I thought that the pot was there to adjust the bias 

[sluckey]

I thought that the pot was there to adjust the bias  

We're talking about setting the range (limits) that the pot can swing the voltage.

[J Rindt]

I thought that the pot was there to adjust the bias 

Sometimes, depending on the power tubes, you can not get the bias voltage negative enough (or positive enough) to set the bias correctly. Changing the resistor will allow the Bias Pot to get you where you need to go......

[JayB]

With the typical fender bias circuit, you're limited to around -45 Vdc if I remember correctly. If you look at a Marshall, a 22k pot is functioning like the 27k on the fender 10k bias pot except it's adjustable. The marshall also has a higher bias voltage on it's bias winding though. Plus the 100 watters usually derived the bias from the B+ winding.

I used a 10k pot and a 15k( forgot exactly) resistor for my bias voltage and that gets me a range from -32 to -58.  Now I Can use EL34's or 6L6's.

[Jack1962]

I thought that the pot was there to adjust the bias  

We're talking about setting the range (limits) that the pot can swing the voltage.

Oh ok my mistake