Bias voltage range tested at -27.5 V to -43.2V without tubes seems low

[Mike_J]

I plugged in my Sluckey inspired Plexi/800 amp without tubes.  The HT/LT and standby switches were reversed so I had to fix that.  I checked the bias supply voltage sweep as -27.5 to -43.2 volts.  I am concerned that a negative bias rating of -43.2 volts is a little bit low and may cause redplating when the power tubes are installed. 

Does anyone know what an acceptable bias supply voltage range would be for a 100 watt, four EL34 amp?  If I want to increase the bias range do I increase the 47K resistor next to the 50K bias pot?

Thanks
Mike

[HotBluePlates]

What is the supply voltage? Mostly interested in screen voltage, but in this amp it's probably similar to plate voltage. Knowing the (high, due to no current draw) number without tubes installed is fine.

[tubeswell]

EL34s generally don't need as much bias voltage as 6L6s

[Mike_J]

What is the supply voltage? Mostly interested in screen voltage, but in this amp it's probably similar to plate voltage. Knowing the (high, due to no current draw) number without tubes installed is fine.

HBP

The wall voltage measures 122.5 VAC and the screen voltage is 486 VDC.  I lowered the resistor that connects to the bias power source slightly and now have -47.8 VDC from the bias circuit.  I read on the internet that Marshall recommends -45 VDC as their bias voltage so I am a little less concerned now about redplating.

Thanks
Mike

[Mike_J]

EL34s generally don't need as much bias voltage as 6L6s

Tubeswell

I read that Marshall recommends -45 VDC bias voltage which you are correct is lower than what I am used to with 6L6s which in my experience is in the neighborhood of -52 VDC.

Thanks
Mike

[HotBluePlates]

... the screen voltage is 486 VDC. ...  I read on the internet that Marshall recommends -45 VDC as their bias voltage ...

The theory is that when screen voltage and Mu G1-G2 (the triode amplification factor from control grid to screen grid) is known, then the cutoff bias for a pentode is Screen Volts / Mu G1-G2.

The EL34 data sheet says triode Mu is 10.5, and your screen voltage is 486vdc. 486v/10.5 = -46.2v.

Reality is once you get the tube close to cutoff, it takes a lot more bias voltage to totally shut current off. So for your amp, and allowing for tube variation, you'd like available bias voltage around -25v to -50v. Your ~-48v is close enough.

[Mike_J]

... the screen voltage is 486 VDC. ...  I read on the internet that Marshall recommends -45 VDC as their bias voltage ...

The theory is that when screen voltage and Mu G1-G2 (the triode amplification factor from control grid to screen grid) is known, then the cutoff bias for a pentode is Screen Volts / Mu G1-G2.

The EL34 data sheet says triode Mu is 10.5, and your screen voltage is 486vdc. 486v/10.5 = -46.2v.

Reality is once you get the tube close to cutoff, it takes a lot more bias voltage to totally shut current off. So for your amp, and allowing for tube variation, you'd like available bias voltage around -25v to -50v. Your ~-48v is close enough.

Thanks HBP for the excellent explanation.

Mike