"single" version of the double valvemaster at 18v/12v -- some help...

[mordechai]

I came across this very appealing variant on the Valecaster:



For my purposes I just want to build the first half of it, so it would look very similar to a normal Valvecaster, but I like that it runs at 18V and runs the heaters at 12V -- from what I can gather from the huge thread on the Valvecaster, this would yield more headroom and volume and improve the overall tone.

What I can't figure out, however, is how to feed this circuit two different voltage sources.  I would be running it off of a 9V power supply, so I know I'd run that into a charge pump...but how do I build up a charge pump that can put out BOTH an 18V and 12V supply? 

I'm keen to find a veroboard layout that could do this, but I'm also interested in actually seeing a schematic of this so I can understand better what's actually happening.

(I realize that this sort of question may have come up in the bazillion threads about the Valvecaster, and I did do a search, but I couldn't find the information I need to answer my question here.)

I'd definitely appreciate any help here!

[slacker]

What you could do is use a charge pump to convert 9 volts to 18. Then run a 12 volt regulator off the 18 volts to get 12, or just use a resistor between 18 volts and the heaters to drop the voltage. I don't think a charge pump will be able to provide the 150ma that the heaters need though, it might be easier to use 18 volt supply or just run the heaters off 9 volts.

[mordechai]

Thanks Slacker.  This is helpful but I do have a few follow up questions:

1.  I was following a similar discussion on another forum and the consensus was that it was not feasible to power a circuit like this with a charge pump.  Are there exceptions to this?

2.  Would running the heaters off of 9V (as opposed to 12V) adversely affect their functionality? 

3.  If I wanted to use a resistor to create a 12V source for the heaters, does it have to be regulated?  That is, would something like a trimpot provide a stable enough current for the circuit to function with relatively low noise and consistent functionality?

4.  Finally, I was thinking about putting a Fetzer Valve at the front end of the circuit to flavor the tone a bit before it hits the tube.  If I'm running the plates at 18V, this means that I should just bias the JFET to be 9V, correct?  Or would I run the Fetzer stage at 12V like the heaters (and then bias the JFET for 6V)?

[M.A.P]

Hi mordechai,

you could connect the heater of the valve parallel and not in row. In this way it only needs 6,3V at 300mA. You could take this voltage from your 9V in (battery or wall wart) over an 9Ohm 1Watt resistor. According to the datasheet of the 12AX7 you have to connect the Pins 4 and 5 together and use pin 9. So 4+5 will be connected over the resistor to 9V and 9 will be ground (will also work the other way round).

http://www.drtube.com/datasheets/12ax7-rca1962.pdf

I've build a normal valve caster and used a PCC88 (7DJ8) instead of an 12AX7 and it works fine with the heater voltage regulated by an resistor. I power the whole circuit with 9V so I can't help you with your charge pump question. You could breadboard the circuit and test it with 9V and 18V from a charge pump on the plates.

Greetings from Germany
Marcus

[frequencycentral]

1.  I was following a similar discussion on another forum and the consensus was that it was not feasible to power a circuit like this with a charge pump.  Are there exceptions to this?

Don't consider using a charge pump to power the heater, they don't throw out enough ma.

2.  Would running the heaters off of 9V (as opposed to 12V) adversely affect their functionality? 

Some might say that 'starved heater' is part of the Valvecaster/Master sound. 9V won't do any harm to the heater.

3.  If I wanted to use a resistor to create a 12V source for the heaters, does it have to be regulated?  That is, would something like a trimpot provide a stable enough current for the circuit to function with relatively low noise and consistent functionality?

You could use a resistor for voltage drop, though it would have to be a couple of watts to dissipate the heat without burning up. My fading memory says 18R 2W might do the trick, but you'd have to check my maths. Don't use a trimmer, unless you like the smell of acrid smoke.

In your position, I would run the heater directly from the 9V supply and use a charge pump for the higher voltage for the anodes. But, why stop at 18V? It's easy to add more diodes and capacitors to get up to say 44V, when the Valvecaster/Master begins to sound like a real HV tube circuit.

[amptramp]

There is an alternative:  the 20EZ7 which is a 12AX7 with a 20 volt 100 mA heater:

https://frank.pocnet.net/sheets/049/2/20EZ7.pdf

Not easy to find, but some people may have stock.

There is no reason to stick with the usual 9 volt DC wall wart power supply.  You can get 16, 18, 20 or 24 volt wall warts and there is no need to stick with centre pin negative either.  I think the industry has done itself a disservice by keeping standards from 50 years ago.

[Nick C.]

I'm working on a 32v 12ax7 valvecasterish design myself. The 32v comes from a printer power supply I got for free and I drop it to 12v with a voltage regulator for the heaters. Got it on the breadboard and works well. Got a little more tweeking to do and I'll post a schematic soon.

[Kevin Mitchell]

Doesn't putting charge pumps in parallel double the current? Like two lt1054 chips in parallel could put out about 200ma?

I have to do this myself for a project. I'm sure it'll fit your bill as well.

So to answer this threads name I suggest the parallel lt1054 chips used as a voltage double and a 12v regulator. There's your 2 supplies. Though be weary of the regulator's max current output as well as the main power supply's rating.

[amptramp]

Doesn't putting charge pumps in parallel double the current? Like two lt1054 chips in parallel could put out about 200ma?

I have to do this myself for a project. I'm sure it'll fit your bill as well.

So to answer this threads name I suggest the parallel lt1054 chips used as a voltage double and a 12v regulator. There's your 2 supplies. Though be weary of the regulator's max current output as well as the main power supply's rating.

If you are operating two charge pumps, you have to make sure the frequencies they operate at are synchronized or sufficiently different that the difference does not fall in the audio range since you will get sum and difference frequencies with the difference being a beat note that will probably fall within the audio range.  You should use an external oscillator connected to pin 7, the OSC pin on the LT1054.  But this is getting far more complicated than a boost converter using an inductance.