Matchless Hot Box powered by GTFO SMPS?

[george]

Hi All

I am planning to build a Matchless Hot Box as per the circuit on GGG website http://www.generalguitargadgets.com/diagrams/hotboxsc.gif.   Instead of using the conventional power tranny/diode bridge PS shown on that schematic, I am planning to build an SMPS similar to the one used by Gabriel Tudoran for his GTFO project.   I figure that as he is pumping 300v into 2 12AX7s then this should be fine to use in the Hot Box that has an A+ of 250v and a B+ voltage of 242V.   The only question I've got, is it possible to tweak Gabriel's SMPS to get the 2 A and B voltages the Hot Box circuit requires, and if so, how?

Any advice is much appreciated.   NB all safety precautions for working with high voltages will be observed! :-)

Oh and the SMPS schematic is http://dl.dropboxusercontent.com/u/15275178/blast%20from%20the%20past/BFTP_SMPS.svg from Gabriel's original thread http://www.diystompboxes.com/smfforum/index.php?topic=93936.0   I understand the output is the junction of R22 and C11

BTW thanks Gabriel for your work!


Thanks and Regards
George Watkins

[defaced]

That SMPS in the GTFO is a pretty common design and has been around for a while, the GTFO was not the first use of it.  Most of us in the stompbox probably took the idea from the Nixe guys like this one (and I can almost guarantee they weren't the first to use it): http://www.desmith.net/NMdS/Electronics/NixiePSU.html (Myself included, where I copied the design but sourced components with higher voltage ratings so it could run safely above 400v).

Yes, you can make the A and B supplies, just copy the resistor network after the rectifier diodes.  

[J0K3RX]

what Mike said should work perfectly...

[george]

fantastic guys thanks so much for your replies.   Now to build ...

[george]

oh and just a couple more questions,

- can I get the required output voltage by tweaking the pot
- is it OK to build this on perfboard or vero?

Thanks again for your prompt replies.

George

[defaced]

Yes. The trimmer does adjust the output voltage. 

Because of the type of circuit this is, I'd be real hesitant to advocate putting this on perf without some careful forethought regarding the function of the circuit and its layout.  Yes, people have done it, yes it can work, but I'm not convinced it's a good idea.  My biggest area of concern would be the junction between the inductor/diode/MOSFET/filter cap because that's where the flyback voltage and filtered exists.  Increasing the distance between tehse nodes and anything else is a good idea.  The problem is with perf/vero, the pads are spaced on a 0.1 pitch, so you have to remove the pads to meet this.  It can be done, but again, it's another detail that's not intuitively obvious to the casual bystander. 

[george]

Thanks defaced!

[J0K3RX]

This Nixie PSU can produce upwards of 400v. A 5k trim pot recommended and if you change the 1.5nF cap going to the NE555 to a higher value you can get into the 400v range. Not that you really need that for what you are doing but I just thought I would throw that out there... In any case, one big factor is the inductor coil, make sure you use the type and value recommended or you will most likely have thermal issues...  As it is the coil already gets smokin hot and will easily burn the skin off your finger if you touch it. If you built it in a small enclosure it really heats up the insides and I think a very small 12v DC exhaust fan would be a good idea as long as you get one that doesn't generate and additional noise. it needs something because my GTFO get really hot to the touch even on the outside of the enclosure! I haven't used it much and never really more than 30 minutes at a time so I am not sure what extended and or everyday use will expose, possible failure? Who knows but it makes me uncomfortable... 

[george]

thanks j0k3rx,  I'm already on the alert because of the voltages this thing can put out.    From the amount of heat you describe, it is drawing a lot of current which surprises me because I thought 2 x 12AX7s would only draw about 600mA?

Maybe I need to go to something like this 2.6A inductor, I may need to cut a hole in top of my enclosure:



 

[J0K3RX]

Nah, there's already too much stuff sticking out of the gtfo the way it is now... I like Gabriel's layout but personally I would rather use a little bit bigger enclosure and put the tubes on a daughter board connected to the main board at a 90 degree angle so they lay down inside the enclosure... The inductor coil can be found that will work, I have a couple I found somewhere...

Something like these










I really like this little layout!

[defaced]

600ma is the heater current. The HV part of a 12AX7 triode will draw 1 to 2 ma. The inductor is getting hot because of the high current pulses going through it. The switcher goes from a nearly dead short on the on pulse to full off on the off pulse. The on pulse allows lots of current to flow. If the industor is getting hot, you want a physically larger inductor with a higher current rating. This will give you an inductor with lower resistance, which because P=(I^2)*R, means lower power dissipation and cooler temps. MuRata has some large industors for good prices and are available through Mouser. I used one on my Cut Throat project and was happy with it. Part number 1410478C. Or try one with lower inductance and lower resistance, 1410313C.