high voltage source for 12ax7

[carlozsulca]

hi i was working on a preamp which is seymour duncan's ktg-1 that carries around 2 12ax7 and 1 12au7, i managed to put it together with this dc-dc boost, but it only goes up to 120v on load and i was wondering if anyone has another schematic of boost that can hold up to at least 220 V DC

[ThermionicScott]

120V isn't a bad supply voltage for a tube preamp, if nothing else.  Keeping plate and grid-leak resistors the same, you'd just need to adjust the cathode resistors for desired bias.  The RCA manual gives lots of examples with a 90V supply.

Is having a 12V input to the power supply a requirement?

[GibsonGM]

I often use a customized GTFO preamp (findable on this forum) using a similar (same?) supply.  I had it up to 140 or so (they say not to crank it up too much ha ha).  It wasn't giving me the kind of distortion I'd expected - I tweaked it back to 100, 110V, and got a nice brown sound. So I left it there.   If you're trying to get the EXACT preamp, well...that's your call.   

All a higher supply will do in a preamp is give you more headroom (LESS distortion).  Of course, too low and it goes to pot, there is a lower limit to this. 

Even with my brownly distorting GTFO, I can dial that right back to 'skynyrd-like clean strat'.  I opted to keep the pre gain pot after the 1st gain stage, which gives me greater control over the nature and nastiness of the distortion...YMMV...I'd try it at the lower voltage and see if you like it.

[vigilante397]

That looks similar to the common "nixie" style SMPS, but it's missing some things I'm used to seeing in it, namely feedback. This is the one I use (some part names are going to be weird because I use SMD) and I usually run it around 235V but I've used it successfully up to 350V.



Where I have the MMBT3904 just about any BJT will work, but 2N3904 is an easy to find cheap one. Also for the diode any fast rectifier (usually things like UF4004 or UF4007) works, MOSFET needs to be big enough to handle the power, IRF740 is a common one. Inductor value isn't crucial as long as it can handle the current.

[vigilante397]

I was just reading GibsonGM's comment again, if you put a resistor (560R is a good place to start) between the trimpot and ground, then change the trimpot to a B1k you could mount it outside the box and dial in some brown sound-esque tones by tweaking the voltage down like Eddie did with his variac. I have one pedal I do this on, and with a 560R the knob sweeps from 80V to about 240V, which is perfect as I use 250V caps.

[GibsonGM]

I was just reading GibsonGM's comment again, if you put a resistor (560R is a good place to start) between the trimpot and ground, then change the trimpot to a B1k you could mount it outside the box and dial in some brown sound-esque tones by tweaking the voltage down like Eddie did with his variac. I have one pedal I do this on, and with a 560R the knob sweeps from 80V to about 240V, which is perfect as I use 250V caps.

Agree. I wish I'd done that on the one I made - I didn't have a 5k, so probably put a 10k (or more!) in there and found it very 'rubber-band' like to adjust (90v, 220V, 178V etc LOL).   That would give a person much more control.   

[merlinb]

i was wondering if anyone has another schematic of boost that can hold up to at least 220 V DC

Consult the mighty thread:
https://www.diystompboxes.com/smfforum/index.php?topic=74088.0

[Rob Strand]

That looks similar to the common "nixie" style SMPS, but it's missing some things I'm used to seeing in it, namely feedback. This is the one I use (some part names are going to be weird because I use SMD) and I usually run it around 235V but I've used it successfully up to 350V.



Where I have the MMBT3904 just about any BJT will work, but 2N3904 is an easy to find cheap one. Also for the diode any fast rectifier (usually things like UF4004 or UF4007) works, MOSFET needs to be big enough to handle the power, IRF740 is a common one. Inductor value isn't crucial as long as it can handle the current.

R2 should connect to the MOSFET.

https://www.ledsales.com.au/kits/nixie_supply.pdf

[anotherjim]

Is R2, C3 a "snubber" to bypass fast spikes around the MOSFET? If after the diode it's part-blocked from doing that?

[Rob Strand]

See "RC Snubber" 60% the way down the page.  The snubber removes ringing but if you over do it it wastes energy.
https://leap.tardate.com/electronics101/555timer/nixiepowersupply/

While the ringing is less damaging than the main spike after turn off, when there's ringing present there's always a risk
of increasing the stress on components.  Possibly made worse with less than ideal layouts.

The ringing can get into other circuits and cause follow on problems.

More technical details here, (see fig 2 in first link, which is for ringing on a very short time scale)
https://www.ti.com/lit/an/slva255/slva255.pdf
https://incompliancemag.com/article/snubbers-to-kill-parasitic-resonances/

[caspercody]

Hello

I have been using the attached power supply to power my pre-amp tube builds with no problems, or noise. I have gotten over 400 vdc, and been able to power (3) 12AX& tubes with no problem.