HV power supply for tube pedal

aot · Post by **aot** » 16 Feb 2011, 14:06

Hi,

I'm planning to build a HV (300V B+) overdrive pedal using 12ax7 tubes. However I'm not sure what the best way to proceed with the HV power supply (12V for the heaters can be easily derived). My options are:

1. Find a suitable transformer (as would be done in an amplifier) that provides galvanic isolation and a secondary voltage of around 240V, then rectify and smooth with capacitors (and resistors for providing lower voltages).
2. Use a low voltage AC power supply (something between 12 and 24V) and then use a voltage quadrupler (or centupler or sextupler) to get the 300V.
3. Provide either low voltage AC or DC and a commercially available switched mode HV DC-DC converter.
4. Other options I'm not aware of...

So my remarks on them are:

1. This is likely to be heavy and bulky, sort of going against the point of a pedal (easy to carry around etc). Also most power transformers are rated for much higher powers than I need in a preamplifier pedal.
2. I've heard they are inefficient, but I guess I'm not too bothered as it's running from the main. Might also be quite bulky due to HV capacitors etc.
3. I've only done a very quick search but the ones available seem to be for 1000V+ and are expensive and low power. Perhaps there will be switching noise?

Can anyone recommend a good solution?

Cheers,

Aaron

DrNomis · Post by **DrNomis** » 16 Feb 2011, 14:44

aot wrote:Hi,

I'm planning to build a HV (300V B+) overdrive pedal using 12ax7 tubes. However I'm not sure what the best way to proceed with the HV power supply (12V for the heaters can be easily derived). My options are:

1. Find a suitable transformer (as would be done in an amplifier) that provides galvanic isolation and a secondary voltage of around 240V, then rectify and smooth with capacitors (and resistors for providing lower voltages).
2. Use a low voltage AC power supply (something between 12 and 24V) and then use a voltage quadrupler (or centupler or sextupler) to get the 300V.
3. Provide either low voltage AC or DC and a commercially available switched mode HV DC-DC converter.
4. Other options I'm not aware of...

So my remarks on them are:

1. This is likely to be heavy and bulky, sort of going against the point of a pedal (easy to carry around etc). Also most power transformers are rated for much higher powers than I need in a preamplifier pedal.
2. I've heard they are inefficient, but I guess I'm not too bothered as it's running from the main. Might also be quite bulky due to HV capacitors etc.
3. I've only done a very quick search but the ones available seem to be for 1000V+ and are expensive and low power. Perhaps there will be switching noise?

Can anyone recommend a good solution?

Cheers,

Aaron

You can use a standard transformer that transforms the mains voltage down to 12.6V @ 150mA,and a 12V AC @ 1.5A plugpack adaptor to generate the 250 to 300V DC, you simply wire the 12.6V/150mA backwards so that it acts as a step-up transformer,and use 4X 1N4007 rectifier diodes....

aot · Post by **aot** » 16 Feb 2011, 14:51

ah yes, I had sort of forgotten about that, thanks!

DeanM · Post by **DeanM** » 21 Feb 2011, 22:58

look up nixie power supply. its an SMPS. should come up on the search above. i think Frequencycentral also posted an SMPS somewhere as well but not certain.

DarkRain · Post by **DarkRain** » 22 Feb 2011, 08:23

DeanM wrote:look up nixie power supply. its an SMPS. should come up on the search above. i think Frequencycentral also posted an SMPS somewhere as well but not certain.

This is my version of SMPS (it's oscillate @ higher freq.): https://www.freestompboxes.org/viewtopic ... 28&t=12195

Best regards,
Gabriel Tudoran
Analog Sound

PokeyPete · Post by **PokeyPete** » 22 Feb 2011, 09:30

I've made a few nixie clocks in my time. A common way to to get high voltage is simply to:

DANGER...DANGER.....DANGER....WARNING....WARNING....WARNING....DANGER.....DANGER
I have made these, and use these, but I advise against it if you are not accustomed to working with high voltage.

If you have 220V or 240V, simply add a 1k resistor to the hot wire then full wave bridge w/ filter cap; there you are...+320-340VDC.

If you have 120v, add a 220R resistor to the hot wire, followed by a 1M resistor parallel with a 2.2u/250V cap, followed by
two 1N4005 diodes (one reversed biased to ground (along with the white wire), the other forward biased, followed by another
2.2u/250V filter capacitor to ground. Instant +250V (or so), if that's enough for your purpose.

Those first resistors should be safety fusible types.

I'm scared to even submit this! Please do not use this information! For information Only! Just common HV tecnique for nixies!

aot · Post by **aot** » 22 Feb 2011, 10:21

thanks for the info about the nixie SMPS and your version Gabriel, that looks great. I'd prefer some level of galvanic isolation, so I don't think I'll just be using the mains voltage directly, although that might be useful for testing purposes.

I was just wondering what the specs are on the SMPS - how much current can it supply? What effect would bumping the frequency even higher do? What value is pot 1 on the schematic?

Thanks,

Aaron

aot · Post by **aot** » 22 Feb 2011, 10:24

oh and do you have to be careful with placement in relation to the audio circuit board to ensure stray fields from the SMPS are not coupled? I'm guessing the inductor will be the biggest source, perhaps some ferrite shielding will do fine?

DarkRain · Post by **DarkRain** » 22 Feb 2011, 10:42

aot wrote:oh and do you have to be careful with placement in relation to the audio circuit board to ensure stray fields from the SMPS are not coupled? I'm guessing the inductor will be the biggest source, perhaps some ferrite shielding will do fine?

Hey aot,

The trimmer value is 1k in my case, but anything between 1k - 5k will be excellent. I've test it with 4 double triodes @ full blast for about 6-7 hours... the voltage droped from 300v @ about 280-275v but that i find more then acceptable. I I used ferrite shilded inductor (100uH @ 2.1A), the voltage ripple is 150mV @ full blast (more the enogh for any tube applications). If you need anymore help please do not hesitate to write me.

Best regards,
Gabriel Tudoran
Analog Sound

tictac · Post by **tictac** » 23 Feb 2011, 02:42

I've built a few tube pedals and they are bulkier than solid-state pedals but still compact enough to fit on a pedal board.
Hammond makes some small PT's for tubes that fit inside a larger die-cast box with room to spare.

Another thing, it's not really necessary to get fancy with regulated power supplies and such. 12AX7's draw such little current that regulation will have a minimal if any sonic benefits. Standard PI filter PS with or without a choke will perform well and are easy to build. What would really be cool would be to use a small tube rectifier with some resistance in series to create a bit of sag and compression which is part of the vacuum tube vibe and why folks like tube circuits in the first place...

Just sayin...

TT

DarkRain · Post by **DarkRain** » 23 Feb 2011, 09:14

That is correct, that you can use a reversed tranny, but again i like to have some room inside and i would like to use it in a 1590B type box, but that is a more of a personal prefference .... i don't use a big cap for filtering, i don't use a rectifier bridge, the noise floor is a little bit smaller and i do enjoy building miniature things.

Best regards,
Gabriel Tudoran
Analog Sound

Ohhh and maybe the most intresting thing: i don't need a separate AC tranny to power up the tube pedal: i can use any 12DC power supply that you can find it in any pedal board brick.