bias amp with 2 tubes out

[Bash_Man]

hey everyone,
looking to retube my Splawn Nitro. I use it in half power mode with 2 outer tubes taken out. Using EL34 tubes.

The recommedation is around 35mA per tube.. that's based on Safe Plate Dissipation for Class AB: 70%

With taking 2 tubes out, do I then cut that amount in half? OR regardless whether I have 2 or 4 tubes, its 35mA per tube?

Thanks

 

[Bad Brain]

I just did this. You need to halve the impedance, IE drive a 16ohm cab from the 8ohm tap. I noted that my plate voltage increased as well so you might want to recheck bias.

 

[glpg80]

Bias a little less, 30-32mA per tube to account for an increase in B+.

When pulling two tubes you half the impedance switch to the cabs. So a 16 ohm cab needs to run in the 8 ohm slot and two 16 ohm cabs would be ran set to 4 ohms on the amp.

 

[Bash_Man]

I just did this. You need to halve the impedance, IE drive a 16ohm cab from the 8ohm tap. I noted that my plate voltage increased as well so you might want to recheck bias.

Thank you. Yes, I am using half impedence as well as using half power. Typically i use 16ohms cabinet or load box I have the switch set to 8ohms.

 

[Bash_Man]

Bias a little less, 30-32mA per tube to account for an increase in B+.

When pulling two tubes you half the impedance switch to the cabs. So a 16 ohm cab needs to run in the 8 ohm slot and two 16 ohm cabs would be ran set to 4 ohms on the amp.

thank you. already done with half impedance

thanks for the input. I'll try 32mA per tube.

 

[Bash_Man]

So one thing I noticed is the tubes start with around 31/32mA per tube, and then when the amp gets hot (say 20-25min in), i do another reading, 1 tube will show 32mA and other tube shows 39mA/40mA.

Is this normal or does this mean the tubes arent matched?

Thanks

 

[311boogieman]

They do not appear to be matched and yes, you want to let the bias settle in. Did you check your plate voltage first?

 

[Bash_Man]

They do not appear to be matched and yes, you want to let the bias settle in. Did you check your plate voltage first?

If I recall, it was around 490.