Variable cathode bias?

Satamax · 10-19-2006, 11:25 PM

Hi everybody!

Well, any problem with using a pot in parallel with the cathode bias cap of a 5E3 to make a variable cathode bias? I was thinking of 470Ω and a 10K pot in parallel for example.

Thanks.

Bye.

Max.

JC@ · 10-20-2006, 01:09 AM

I can't see why not. Even a little trimer pot would be ok I believe.

carl gigun · 10-20-2006, 02:50 AM

This is for the 6V6 tubes I take it? Consider adding another resistor in series with the pot to provide a lower limit to the resistance of that leg. Without that the pot can be rotated to a very low resistance killing either the pot or the tube.

The 5E3 had a shared 250 ohm cathode resistor, if you wanted a range of adjustment from 200 ohm to 350 ohm using your fixed 470 ohm cathode resistor you could add a parallel 1k pot and 330 ohm safety resistor.

Pots and trimmers can get noisy, It'll probably be fine but keep it in mind in case you have some noise problem later and need to know where to look.

stokes · 11-11-2006, 12:19 AM

Instead of a pot,I have used adjustable wirewound 500ohm 5watt and 10 watt Ohmite resistors.You can adjust for different current draw with different tubes,no problem with noise as Carl pointed out,and less parts to deal with.

PaulC · 12-12-2006, 05:24 AM

Combination bias got a bad rep due to some ugly silver face fenders, but I actually like to do this for these types of amps. That way you can pick the amount of bias shift due to the cathode resistor while still getting the tubes to bias were you want them by adding a little negative grid voltage in.

PaulC

10-19-2006, 11:25 PM	#1
Satamax Senior Member Join Date: Jun 2006 Location: Southern french alps, right by the Italian border Posts: 752	Variable cathode bias? Hi everybody! Well, any problem with using a pot in parallel with the cathode bias cap of a 5E3 to make a variable cathode bias? I was thinking of 470Ω and a 10K pot in parallel for example. Thanks. Bye. Max.

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10-20-2006, 01:09 AM	#2
JC@ Senior Member Join Date: May 2006 Posts: 369	I can't see why not. Even a little trimer pot would be ok I believe.

10-20-2006, 02:50 AM	#3
carl gigun Junior Member Join Date: Jul 2006 Posts: 12	This is for the 6V6 tubes I take it? Consider adding another resistor in series with the pot to provide a lower limit to the resistance of that leg. Without that the pot can be rotated to a very low resistance killing either the pot or the tube. The 5E3 had a shared 250 ohm cathode resistor, if you wanted a range of adjustment from 200 ohm to 350 ohm using your fixed 470 ohm cathode resistor you could add a parallel 1k pot and 330 ohm safety resistor. Pots and trimmers can get noisy, It'll probably be fine but keep it in mind in case you have some noise problem later and need to know where to look.

11-11-2006, 12:19 AM	#4
stokes Old Timer Join Date: May 2006 Location: NYC Posts: 1,306	Instead of a pot,I have used adjustable wirewound 500ohm 5watt and 10 watt Ohmite resistors.You can adjust for different current draw with different tubes,no problem with noise as Carl pointed out,and less parts to deal with.

12-12-2006, 05:24 AM	#5
PaulC Junior Member Join Date: Dec 2006 Posts: 1	Combination bias got a bad rep due to some ugly silver face fenders, but I actually like to do this for these types of amps. That way you can pick the amount of bias shift due to the cathode resistor while still getting the tubes to bias were you want them by adding a little negative grid voltage in. PaulC

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