18v to 12v

[Otsismi]

I've built the valvecaster and I like it. I've poured through the tome of correspondences on mods and such. What I haven't found is what I propose.

The boob tube valvecaster runs on high voltage that has been multiplied by a charge pump. I'm not interested in all that mess, I just want a little extra tube ness from the circuit instead of the fuzzy sound it produces at 9v. I have a brick power supply with 18v outs on it that I'd like to use for this circuit.

Can the 18v be sent the the plate and regulated down to 12v for the heaters?

If so, how? Schematics are always helpful. Simpler is better.

[psychedelicfish]

You want a voltage regulator IC, for 12V try a 7812 (NOT a 78L12, it can't handle the current your heaters need). They don't get much simpler to wire, just connect the input pin to 18V, ground the ground pin, and take your regulated 12V off the output. You might want some capacitors across the power supply on either side of the regulator too.

[Otsismi]

Can you be more specific about those caps?

[PRR]

10 Ohm 10 Watt resistor will drop 18V to 12V for 600mA worth of heaters.

If you "*must* regulate", 10uFd on In and Out of the 7812, right on the poins, observe polarity.

[Jdansti]

A schematic of the regulator circuit: 


Data sheet: http://www.fairchildsemi.com/ds/LM/LM7812.pdf

You might want to use 10μF caps that PRR recommended on the V in and out. You might also need to use a heat sink or bolt it to the metallic enclosure for heat dissipation.

[bluebunny]

...and protection diodes (1N400x) from output to input on the regulator, and from ground to output.

[Otsismi]

Protection diodes make sure the caps don't discharge into the signal right?

[bluebunny]

Stops you inadvertently frying the regulator.  Read R.G.'s explanation here.

[bluebunny]

BTW, it should look something like this with both diodes included:

[Bill Mountain]

Can the power supply or the regulators provide enough current for the heaters?

[Jdansti]

The 12AU7 heaters in series draw 150mA @ 12.6V. The 7812 can handle up to 1A. The rest depends on his power supply.

[Bill Mountain]

The 12AU7 heaters in series draw 150mA @ 12.6V. The 7812 can handle up to 1A. The rest depends on his power supply.

Are there current losses in the regulation?

[PRR]

> 12AU7 heaters in series draw 150mA @ 12.6V

Each.

If I found the right "boob toob" (links are nice), it's got two. That is 12V 300mA.

> Are there current losses in the regulation?

Read and digest the datasheet.

LM78xx chips divert a couple mA, normally negligible.

LM317 types divert less than 0.1mA... if you are within 0.1mA of failing, you are doomed already.

I read the budget as:

plates: 6mA at 18V
heaters: 600mA at 12V from 18V
reg loss: couple mA

So 606.1mA to 608mA.

Plus/minus 30% on plate suck, and +/-10% on heater suck.

So up to 668mA.

Any way you slice it, dropping 18V to 12V the simple way is 33% waste. Considering his goal, another path is to find a good 24V-25V 200+mA wart, and running heaters in series at 25V 150mA. Hey, if 18V is gooder than 12V, then 24V must be even gooderer. But if the 18V supply is in his kit already, that's fine.

[PRR]

me> dropping 18V to 12V....

A thought. What you really want are 18V heaters. Or two 9V heaters in series.

There IS a 9AU7:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/9/9AU7.pdf
And it's available very cheap:
http://www.thetubestore.com/Tubes/9-Types/9AU7  -- $4 a bottle

Wired for 9.4V each, we need 225mA(+/-15mA). So a pair in 19V string is 240mA or less. 18V 250mA wart does the job.

Leave a lot of headroom on this. The cold resistance is low so the turn-on current is higher. Maybe 1A. It is fine if the supply sags on >250mA, the tube just takes another second to heat.

[Otsismi]

I'm very new at this but I am trying really hard to keep up. I've built the dod 250 and a treble booster and the valvecaster. I'm just trying to get a better understanding. All suggestions seem well and good, but I'm not sure which one to go on. Can anyone help me out here? I recognize that higher voltages equals higher fidelity from the tube. And I recognize that 18v to 12v is wasting a lot of electricity lost as heat.

What to do? The boob tube just seems too complicated. Maybe someone could help break it down for me?

[Otsismi]

How would the 9au7 circuit work? How does wiring heaters in series differ from parallel?

[Jdansti]

Like this:




Don't ground pin 9.

As for sound, I don't know how the 9AU7 would sound relative to a 12AU7.