O/T Power amp pentode grid 1 negative biasing question

[frequencycentral]

Well I just breadboarded a little SE amp using a 6112 and a 5902. I used my nixie SMPS for a B+ of 165 volts and a MAX1044 for a negative grid bias of -33 volts. Of course the MAX and the SMPS didn't play nice together, as the two HF clocks interacted in a weird ring mod type way in the background. But I'm encouraged. A lot louder than my Murder One - it's working title is "Serial Killer".

My next step will be to use just the SMPS, and have a voltage divider to get a vref of 33 volts which I will call ground. I'll use true ground as my negative grid bias. I'm assuming that the OT ground would now become vref too?

I looked at Dana's submini wreck, he didn't do a new schemaic for it, as it's very close to the full size express, which biases the grid at -33 volts.

Back to the breadboard.............

[frequencycentral]

My next step will be to use just the SMPS, and have a voltage divider to get a vref of 33 volts which I will call ground. I'll use true ground as my negative grid bias. I'm assuming that the OT ground would now become vref too?

Hmmm, that was a complete failure, don't know why. Maybe a vref isn't good enough and I really need a -ve voltage rather than a vref. I wonder if it's possible to derive a -ve voltage from my nixie SMPS? That would solve some issues. Or maybe it's time for me to 'grow up' and get a PT?

Now I'm playing about with a cathode biased arrangement instead. I've tried both 6111 and 6112 as preamps. The 6112 give a lot more gain, but makes the whole think hum like a mutha. I've come to the conclusion that this hum is because the 6112 wants more ma from the SMPS than the 6111 does - it's higher gain but it needs more ma to achieve it.

Next I'll try a cascoded 6111 into the 5902, cathode biased.

I don't know why I'm typing all this, just thinking out loud really......

[Ripthorn]

So are you just using a wallwart that is already in DC?  If you use an AC wall wart, you can do a voltage doubling circuit to get both + and - voltages, then charge pump.  Or you could use a voltage divider to get you 12V, invert it using a charge pump, and pump that "up" to -33V or so using two doubling stages (a tripling stage?).

[maarten]

Hello Rick, for making a negative voltage using a 555 timer, have a look at:

http://www.circuitsonline.net/circuits/view/139

Should be good for up to 30 mA, which should be enough for negative biasing, I suppose. 1N4148 should work, according to the text: will give you a slightly higher voltage (-11,8 or so, I guess). You could add two voltage multipliers to this to get to to -36Volts (or even closer to -33 when using 1N4148's). Just observe polarity of diodes and capacitors: should be directed opposite to your positive multiplier.
Maarten

[sean k]

I had the most help with this stuff from the Norman H Crowhurst PDF's at audioxpress.
http://www.audioxpress.com/resource/audioclass/index.htm

Basically take your output impedance, of the OT, and divide it by the plate voltage to get a line accross the plate characteristics graph and then choose where you want it to swing from. Say 200V and 5k you've got 40mA so your line goes from 40mA to 200V. You want it to swing from -24 to 0 then you want -12 on the grid for class A (class AB you'd have each tube swinging from -24 to 0 so you'd bias at -24). Find where the line you've drawn crosses the -12 and that point is just over a hundred volts and about 20mA so a cathode resistor of 600 ohms will do it and if you want fixed bias then just give it -12 on the grid. The fixed bias would mean 212V but whats a few volts between friends.

At the -12 at a hundred volts the screen current is just under 2.5mA so I suppose dropping 20V at 2.5mA for 180V on the screen means an 8K resistor.

I haven't re-read the articles and it's been a long time since I did this stuff so I may be remembering incorrectly but I'd always thought the graphs are the way to go... unless you have a tube tester and can draw up your own graphs to suit the tube.

[frequencycentral]

Hello Rick, for making a negative voltage using a 555 timer, have a look at:

http://www.circuitsonline.net/circuits/view/139

Should be good for up to 30 mA, which should be enough for negative biasing, I suppose. 1N4148 should work, according to the text: will give you a slightly higher voltage (-11,8 or so, I guess). You could add two voltage multipliers to this to get to to -36Volts (or even closer to -33 when using 1N4148's). Just observe polarity of diodes and capacitors: should be directed opposite to your positive multiplier.
Maarten

Hmmm, I'm wondering if it would be possible to add a negative voltage section to the nixie SMPS I'm using? I'll have a play with it and see. I've come to the conclusion that the inductor I'm currently (!) using in that SMPS isn't supplying enough current - it's a low current inductor. I think I need to uprate it to be able to use 6112's in my design.