Fixed-Bias Williamson EL34

[Yvesm]

Allow me to suggest a "safe" bias circuitry.

Called so, cos if a pot cursor lift off, tubes just become over biased

Yves.

 

[sorenj07]

Allow me to suggest a "safe" bias circuitry.

Called so, cos if a pot cursor lift off, tubes just become over biased

Yves.

Nice looking schematic, thanks. I'll probably use it

 

[ray_moth]

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Current capability for this PT is 280mA

According to the Hammond website the 282X can supply 200mA DC, but I don't know if that's for choke input or cap input filter

 

[sorenj07]

I'm pretty sure it's cap input. The link you posted says:

"H.V. DC current below, measured with cap. input filter, full wave (two diode) C.T., rectifier circuit"

According to a PDF elsewhere on the Hammond website, choke input should give .45 * the secondary VAC, so 450V out of 1000VCT, and 1.54 * the current rating, so 308mA. Encouraging numbers.

 

[sorenj07]

Hey Yves, a couple questions if you don't mind

My bias is -45V, and supply is -70V or so. So I have 17.2K on the top top resistors, 10K below, 100K to the grids, and 1uF for a 15Hz response. I should probably know this, but what range of adjustment will your setup give me?

Here's my not-so-foolproof math: I assumed the upper pot to be halfway across, so I could replace the top 2 resistors and the 10K pot with a single 11.1K resistor (17.2+5)/2, making a simple voltage divider. In that case, the bottom pot should adjust the total output voltage between -33 and -45V right? I would rather be able to adjust to around -10+15V of -45V (keeping signs constant), since that's the central point of my schematic... Is something wrong with my math, or should I try some different resistors to suit my goals?