Building Power Supplies for 2 X 12ax7 pedals

[chaccmgr]

Re mini-tubes: I only have experiences with 6N17B and 6N16B. While the 6N17B is a very easy to handle tube which behaves very similar to a 12AX7 the 6N16B is a very low gain tube which is only useful as a cathode follower but not as a gain stage.

Re voltage pump: I built mine exactly as in thew schematic I posted above and I had no issues with noise whatsoever.

15mA for a 2x 12AX7 preamp is way too much, also for 2x 6N17B. In my 6N17B design (it was a bass amp with 200V B+ and 47k load resistor) I had 4.3mA per tube. With a normal 100k it will be half of this. Stop guesswork, draw a load line with your values and you'll know.

BTW: I found a design years ago to mimic a Fender style amp with Mini-tubes incl. load lines. The original website is offline unfortunately, but I copied the relevant parts back then. Find attached pdf

[bcmatt]

Re mini-tubes: I only have experiences with 6N17B and 6N16B. While the 6N17B is a very easy to handle tube which behaves very similar to a 12AX7 the 6N16B is a very low gain tube which is only useful as a cathode follower but not as a gain stage.

Re voltage pump: I built mine exactly as in thew schematic I posted above and I had no issues with noise whatsoever.

15mA for a 2x 12AX7 preamp is way too much, also for 2x 6N17B. In my 6N17B design (it was a bass amp with 200V B+ and 47k load resistor) I had 4.3mA per tube. With a normal 100k it will be half of this. Stop guesswork, draw a load line with your values and you'll know.

BTW: I found a design years ago to mimic a Fender style amp with Mini-tubes incl. load lines. The original website is offline unfortunately, but I copied the relevant parts back then. Find attached pdf

My apologies. I went back and read your post from before again as well. I was still trying to familiarize with these ideas back then, and realized that I now have followed in your footsteps of using the 6N17B tube through just researching several sub-miniatures and their prices on ebay. I ordered some, but they may take several weeks to arrive because they are coming from Kiev.
I see your nixie supply circuit is very close to the same as other I was looking at. The premade one I ordered looks a little more complicated, but perhaps I can draw a schematic by looking at it to know how it may be different from the ones I may try to build myself.

[Dr Tony Balls]

FWIW this is the Nixie supply schematic that I used:



I built a power supply box with that in it for breadboarding purposes and despite what the schematic says it gets 100-285VDC from a 9V source. For the tube pedal projects I just hardwired it to ~100VDC, but you could always do that with a trimmer.

[bcmatt]

I started to build the breadboard.... realized my previous schematics are wrong about the pin #s. I decided I will stick with 2 of the same type of tube: 6N17B
I also realize that the initial grid leak bias on that first gain stage is probably asking for trouble with this different tube type... I started making a new schematic using a combination of the initial Xtreme Tone Pedal, my Hot Cat's preamp, and the schematic from the newer Hot Cat 45 models that don't use the Edge and Level controls or the grid leak bias on stage 1... but a TMB tonestack.

[bcmatt]

It's my first time ever doing the "breadboarding" thing. A couple more missing capacitors values will arrive tomorrow and then I will get to fire it up and start the measuring, testing and tweaking this pretty experimental circuit.

20260720_214715.jpg

[bcmatt]

So, using this little $22 Nixie Supply from amazon, I can dial in a range of voltages. I arbitrarily decided plate voltages between 130-160V would be appropriate/safe for these Soviet mini tubes.
The supply also tries to give DC heater voltage but it's a bit hot at 6.7Volts, so I added a 5A silicon diode to bring it down to 5.9V.
In the future I'll do my own Nixie supply and try to run a couple tubes in series directly off the 12VDC wall wart. Maybe even this pedal when it goes in an enclosure.

20260723_083312.jpg

20260723_083347.jpg

Here is the current state of the circuit with the current voltages.

Hot Mini Sub Schematic with Voltages.jpg

I'll keep playing with it in the breadboard state to make sure I can get the controls all as user-friendly as possible... but I'm quite pleased with the sound so far. I'd say it is (or is going to be) a complete success. It is definitely worth it... a pedal that sounds like a great gain channel on an amp.

I was actually surprised by the heat that sub-miniature tubes produce. Not like a power tube, but for sure warmer than 12ax7s. I didn't assume I would need ventilation, but now I am thinking it could be smart.

I'm already planning my sub-mini-Lator...

[didit]

hello Matt --

The datasheet spec for the heater current is 400mA and there's less glass surface so unsurprisingly hot too touch. To air-cool simply, ventilate your final packaging well. A 6.7V heater is within spec tho a tad high.

..

[bcmatt]

hello Matt --

The datasheet spec for the heater current is 400mA and there's less glass surface so unsurprisingly hot too touch. To air-cool simply, ventilate your final packaging well. A 6.7V heater is within spec tho a tad high.

..

Ya, totally. I'd prefer a tad low to a tad high if they are both within spec... Then we slightly reduce heat damage and wear and tear.

While it's still in the breadboarding stage, I'll probably try making my own Power supply and do the series heaters direct off the 12V input... Then I can also start to imagine actual footprint for in a housing as well.

[Dr Tony Balls]

FWIW on my pedal using two 6112s I let it run for hours with a thermocouple inside the closed box and, while I dont remember what the temp reached, I know that it plateaued at a level where I wasnt worried about the enclosure needing ventilation

[bcmatt]

FWIW on my pedal using two 6112s I let it run for hours with a thermocouple inside the closed box and, while I dont remember what the temp reached, I know that it plateaued at a level where I wasnt worried about the enclosure needing ventilation

Thanks Tony! That's definitely handy to know.

So, I'm also realizing that I have some impedance issues with the last stage driving the tonestack (these sub-mini tubes have a much higher internal impedance) and then going to a pedal output that should be able to drive a guitar cable without losing clarity... So I'm thinking maybe it ought to be a CF tonestack stage... maybe something like this:

Hot Mini Sub Schematic CF TS.jpg

But then I would also want to elevate my heaters...

[bcmatt]

Actually, I have a couple 6N21B tubes. I think I can use one for V2 and overcome the potential internal high impedance issues that would mess with the tonestack by optimizing that stage for that tube instead...
So, I'm thinking this would mess less with the current configuration and the plate driven tonestack:

Hot Missile.png

Sadly, I'm now travelling on vacation and I don't get to try this for a couple weeks now.

[bcmatt]

I'm also planning my SMPS Dumble-ator with another 6N21B:

6n21B Dumblator.jpg

[didit]

FWIW on my pedal using two 6112s I let it run for hours with a thermocouple inside the closed box and, while I dont remember what the temp reached, I know that it plateaued at a level where I wasnt worried about the enclosure needing ventilation

Total, there's going to be < 6W to dissipate. A 6112 has somewhat lower heater 300mA, so < just 4.5W. Those Soviet tubes are spec'ed for maximum of 220C. Thermal density with a peak at the cathodes would be our concern.

How large is the box and were those tubes floating "free" or somehow thermally coupled to the box?

..

[Dr Tony Balls]

FWIW on my pedal using two 6112s I let it run for hours with a thermocouple inside the closed box and, while I dont remember what the temp reached, I know that it plateaued at a level where I wasnt worried about the enclosure needing ventilation

Total, there's going to be < 6W to dissipate. A 6112 has somewhat lower heater 300mA, so < just 4.5W. Those Soviet tubes are spec'ed for maximum of 220C. Thermal density with a peak at the cathodes would be our concern.

How large is the box and were those tubes floating "free" or somehow thermally coupled to the box?

..

Floating free but not tightly constrained....plenty of free space:

[didit]

That does really look fine though supporting the tubes off the wire feels a bit fragile long term (shrug) ..