ValveCaster Advice

[hyped]

Hi,
I'm doing my first build based on the valvecaster and I had a few questions (which Im sure are pretty simple) I was hoping that some members might be able to provide me some clarification/confirmation. I've put together the circuit using 12AU7 with a 12v DC power supply and it is seemingly working ok (outside quite abit of general hum and noise) but I had a few areas Im not sure of?

Firstly, the schematic shows that the 9VDC+ is connected to pin 5 heater (in my case I've used 12VDC) whilst pin 4 is grounded. In all the other heater wiring diagrams I've seen it has both pin 4/5 connected directly or pin 4 and 5 bridge together with solder/wire across the pins.

So my first question is: Is It correct to only connect VDC+ to pin 5 and ground 4 without running power supply to it? If so, just curious why the second heater is grounded?

Secondly, is it ok to run 12v to the heater or should I look to regulate this down to 6.3V?

and lastly in my case I'm running 12v, but whether your running 12, 15, 18 or 24v should any of the other components resistor/caps change as a result or is the overall schematic ok to run unchanged at different rail voltages.
Sorry if the questions seem basic just trying to understand/learn more during the build process any feedback or comments would be greatly appreciated.
Thanks heaps

[Marcos - Munky]

The 12AX7/12AU7 have two heaters and two ways to wire them. One goes from pin 4 to 9, the other goes to pin 5 to 9. Each one requires 6.3V and 150mA to work. So we can:

1- connect them in parallel. Since both heaters are connected together on pin 9, we need to joint the other end of them, creating a single connection point. Since the other end of them is on pin 4 for one heater and pin 5 for the other one, we joint those pins together. So the power is fed on pins 4+5 and 9. The voltage stays the same but we need double the current, so the heaters will need 6.3V 300mA to work.

2- connect them in series. We already have them connected together on pin 9, so current needs to get in by the other end of one heater and get out from the other end of the other heater. We leave pin 9 alone, disconnected, and feed power to pins 4 and 5. It doesn't matter which one is connected to ground. So we have the same current as one heater but double the voltage. 12.6V 150mA.

If you're powering the Valvecaster from 12V, you have to connect the heaters in series, the same way you're already doing. Well, you don't have to, you can wire them in parallel and power them with 6.3V, but that doesn't makes any sense.

For the higher voltage question, resistors are ok. You have to watch for caps voltages. It's a good practice to have some safety margin on the voltages. For example, if you're powering the circuit with 9V, use electrolytics rated for 16V or 25V. Ceramics are usually rated for 50V and polyester caps are rated for 250V or more, so don't need to worry on them.

But you do need to worry on the heater voltage. A voltage a little bit higher will short the tube lifespan. A voltage too higher will kill your tube. No matter what's the plate voltage (well, it have a limit, but let's put that aside for now), but the heater voltage should be as close as possible. That's why you see tubes powered by let's say 300V, but the heaters are powered by 6.3V.

[PRR]

> the schematic shows that the 9VDC+ is connected .....

They are running the nominal 12V heater on 9V.

If you are working the tube far below its maximum cathode current, this can be fine.

I'm sure these tubes intended for 100V-300V of B+ will be FAR below maximum cathode-plate current with 9V of B+.

So it's "not right", but it works.

[PRR]

> 12V, ...connect the heaters in series, .... Well, you don't have to, you can wire them in parallel and power them with 6.3V, but that doesn't makes any sense.

To clarify: 12V on the 6V connection, heater life is reduced from many thousands of hours to a couple of hours (about 1/8000). Yes it will "work" but maybe not all the way through your second set.

[hyped]

The 12AX7/12AU7 have two heaters and two ways to wire them. One goes from pin 4 to 9, the other goes to pin 5 to 9. Each one requires 6.3V and 150mA to work. So we can:

1- connect them in parallel. Since both heaters are connected together on pin 9, we need to joint the other end of them, creating a single connection point. Since the other end of them is on pin 4 for one heater and pin 5 for the other one, we joint those pins together. So the power is fed on pins 4+5 and 9. The voltage stays the same but we need double the current, so the heaters will need 6.3V 300mA to work.

2- connect them in series. We already have them connected together on pin 9, so current needs to get in by the other end of one heater and get out from the other end of the other heater. We leave pin 9 alone, disconnected, and feed power to pins 4 and 5. It doesn't matter which one is connected to ground. So we have the same current as one heater but double the voltage. 12.6V 150mA.

If you're powering the Valvecaster from 12V, you have to connect the heaters in series, the same way you're already doing. Well, you don't have to, you can wire them in parallel and power them with 6.3V, but that doesn't makes any sense.

For the higher voltage question, resistors are ok. You have to watch for caps voltages. It's a good practice to have some safety margin on the voltages. For example, if you're powering the circuit with 9V, use electrolytics rated for 16V or 25V. Ceramics are usually rated for 50V and polyester caps are rated for 250V or more, so don't need to worry on them.

But you do need to worry on the heater voltage. A voltage a little bit higher will short the tube lifespan. A voltage too higher will kill your tube. No matter what's the plate voltage (well, it have a limit, but let's put that aside for now), but the heater voltage should be as close as possible. That's why you see tubes powered by let's say 300V, but the heaters are powered by 6.3V.

Hi Munky,

Thanks heaps for taking the time to reply really appreciated. The caps in circuit are rated at 100v+ so should be all good there. In relation to the wiring of the heaters I think Im getting confused as I've looked at a couple of point to point wiring diagrams that seem to either have 4 or 5 connected only and not 9.


Forgive my crude picture;D is the attached, what you are saying???

Again, thanks so much for taking the effort to reply I appreciate you taking the time!

[hyped]

> the schematic shows that the 9VDC+ is connected .....

They are running the nominal 12V heater on 9V.

If you are working the tube far below its maximum cathode current, this can be fine.

I'm sure these tubes intended for 100V-300V of B+ will be FAR below maximum cathode-plate current with 9V of B+.

So it's "not right", but it works.

Thanks Paul really appreciate your response mate - I've attached a diagram in the hope that I've interpreted responses correctly!

[FiveseveN]

Does this representation make it easier to understand how the heaters are wired?



In any case, your diagram shows the supply shorted in both options (red and black connected together).

[hyped]

I thought that was shorting, but thought I was missing something?

Not sure, looking at the attached for example it shows +9vdc going to pin 5, 4 to ground and
NC on pin 9?

Noting, my shorting 🔥    could someone point out what I need to change on diagram?

Sorry! Appreciate patience!

[andy-h-h]

I've built a few of these - great pedal, one of my favourites 

- Do not short pin 4 & 5
- Do not run above 12v, or you will cook the heaters (which are rated up to 12.5v in this configuration)
- Pin 9 is not connected to anything

I have used the point to point layout shown, and it works just fine.   

[GibsonGM]

Hyped:    http://www.valvewizard.co.uk/heater.html


They're saying (correctly) your supply is shorted because you 'jumpered' across the tube pins, when you want current to flow THRU pins 4-5, etc.  Look closely, at your 'wires' going across the tube socket.

The layout is fine as Andy says.   Do some reading on tube heaters, there is a lot of info out there and on youtbe!

[hyped]

Thanks Andy and Gibson appreciate responses a    and sorry for Qs!!

Looking at attached does this look right for series and parallel now?

[andy-h-h]

The diagram looks right - use the series option, which will work at 9 to 12v.   

I made a similar mistake once on a breadboard and cooked two valves in the process - and it took no time at all. 

[Marcos - Munky]

Yes, your diagram is correct. Just something to add: while the heater connections are correct, there's no polarity on them. In fact, they can be powered by both AC or DC. As long as you use the correct pins, you're good to go.