24V DC Shaka Tube? Votage divider or some other trick to get +/-12V

[octafish]

Hi I've built a little SS amp, and I'm working on the Vox SS preamp that RDV introduced us to. (Just gotta figure if I want to build in Reverb or build a seperate unit, sigh). The Chassis I'm building the amp into had a tube drive effect, and so I have a nice 9 pin socket and a hole for the tube to go into. So I was dreaming about building in a Shaka Tube.

The amp is feeding off a 24V single sided PS and I figure I could just use a voltage divider to feed the op-amp, and take the centre point to ground to feed 12V DC to the heater element, the cooling fan and the preamp. I'm having a brain melt down trying to figure out how to impliment the bias control. I should  set the 24V, 12V, and 0v points for the opamp/preamp/heater/fan and then add another bias network for the bias control right? But how do I isolate that bias network, from the existing opamp/preamp/etc. bias? Because a second bias network would be in parrallel with the first, thus affecting that.

Am I pushing it up hill? (Electronics has always been a Sisyphusian effort for me anyway). Is there another way to do this I'm not seeing? (Other than to introduce a 12V AC power transformer, I can see that.)

PS to answer the question: Why am I building this instead of a AX84 P1 or similar? simple, money, voltage, and money. I'll get round to that when I can scrounge some transformers and build a new chassis.

[Paul Perry (Frostwave)]

My first thought is to use a 12v regulator (but, a sturdy or well-heatsinked one) to get a regulated +12 line.
Then build the effect using this. And use caps to let the signal connect while isolating any bias voltages.
Am I missing anything? Where is the shaka tube schem?
Note: heater + fan is goingto be quite a bit of current, at least whent he heater & fan are just starting. maybe the fan could run thru  a dropping resistor? (voltage not as critical as the heater).
To be precise, the shaka (from the viewpoint of the original circuit) would have the same ground as the amp, if you call the power rail that isn't 24V+, ground.

[Paul Marossy]

Where is the shaka tube schem?

Right here. The Shaka Tube has a +/- bias arrangement. I think it would be kind of difficult to do what you propose with what you have there, octafish.

[Paul Perry (Frostwave)]

After looking at the Shaka Tube circuit, I still think you could just run a 12v reg from the original amp's +24.
Then you have voltages 0 (original circuit ground), +12 (from the reg) and +24 (from the original amp).

Go to the Shaka diagram & replace the shaka v+ by +24, the shaka ground by +12, and the the shaka - by the original amp's 0.And put a cap on the volume out pot wiper. Same for the bias.
Now, so far as the shaka is concerened, everything in it is labelled 12v higher than before. So, it works just the same.

One possible problem, is that the (original) shaka + is really 14v (from the peak rectified voltage). But now you only have 12.
And, no accidentally connecting things that are "ground" in the original amp, to the earth connections in the original shaka artwork!!!

[Paul Marossy]

Go to the Shaka diagram & replace the shaka v+ by +24, the shaka ground by +12, and the the shaka - by the original amp's 0.And put a cap on the volume out pot wiper. Same for the bias. Now, so far as the shaka is concerened, everything in it is labelled 12v higher than before. So, it works just the same.

*slapping forehead* Of course, that's so simple! The negative bias voltage on the Shaka Tube schematic is what made me think it couldn't be done. 

One possible problem, is that the (original) shaka + is really 14v (from the peak rectified voltage). But now you only have 12

Yep, that's right on. I measured about +/- 14V in my Shaka Tube, depending on how the bias is set.

[octafish]

Hmmm I'll see how I go with the 12v vs 14V. Thanks guys.

[spudulike]

I take it that the cap on the output pot is to block the introduced 12v offset ?

[A.S.P.]

do we see a long thread coming up?