"Superfly" - submini tube version of Doug H's Firefly

[Taylor]

I have more inductors, but they're the same. What rating should be upped on it? There's no mention of voltage rating on the datasheet at all. I appreciate your help; I feel bad that, since nobody else has built this, the support can't be spread over many people like with most projects here. I know you've got lots of forums to hit; I see your posts at tons of forums all over the web!

[frequencycentral]

Inductors are usually rated for current as well as voltage. The current rating would appear to be pretty important with this type of circuit, though the inductor I used is a low curent type, and woks well. As you probably know, I'm not an EE, so most of my experience is practically based. I'm using a 700ma power pack with my amp, so I guess there is a set limit there to how much current can actually be drawn through the inductor, given that the tube heaters are drawing 300ma. It might be that my inductor would burn out if I used a higher curent power pack and I set the SMPS to deliver more volts. The way I understand the SMPS is that if you want more volts it wants more current, that's why mine starts to hum if I turn the volts up to 230v - the power pack can't provide the current. So what I'm saying is that maybe (just maybe) your higher rated power supply allows you to pull more current through the inductor than it can handle without burning out. Maybe.

I had the SMPS on breadboard for a good while before I built it into the amp - so I was sure that it's components would work well in the project at the voltage I selected. It may be worth your while doing the same - breadboarding the SMPS on it's own, making sure it all works, then applying the B+ to the amp part of the circuit.

If you look at gut shots of the Zvex Nano Head and the JJS Class A amplifier you'll see that the inductor used in those SMPS's is of a different type - more heavy duty looking, and able to handle more current. I just got hold of some of them for my further experiments with SMPS's to see if I can get more volts.

I'm pretty sure that the limiting factor in these SMPS's is the current capability of the inductor, that's why Im guessing that it's your inductor that smoked. I did manage to smoke an inductor on my breadboarded SMPS (found out it's limits there for sure!). I also managed to burn out a IRF740 by not heatsinking it properly. But the IRF740 just stopped working whereas teh inductor smoke and stank. But hey - this is proper electronics if you run the risk of mild electric shocks and smoke! What fun!

As for me helping out - there would be little point posting a project and then providing no support. So no problem. And your experience once you've got yours working will add to the collective knowledge.

BTW, if you get hold of some 7327's I'd be interested at some at that price. And I'll also take the 6111's off your hands if you end up with no use for them - though they do have some interesting uses if you choose to keep them.

[Taylor]

Man, I am getting nowhere with this. It's frustrating, because I have built monstrously more complex things successfully, so I feel like I'm getting whipped by a fuzz box or something. I managed to kill several BC547s by letting my multimeter probe slip and connect the B+ to the base of the transistor.

I now have a working power supply, but no sound. I'm not sure how to debug, since I can't really use an audio probe connected to an amplifier as I do with recalcitrant effects. My scope can't do more than 50 volt signals. How do you debug something like this?

I remembered that I substituted a 470UF for the 330UF cap - C3. I figured this should be fine, am I wrong here? Is that value important?

[frequencycentral]

I remembered that I substituted a 470UF for the 330UF cap - C3. I figured this should be fine, am I wrong here? Is that value important?

That value is fine - it's anti ripple stuff.

[frequencycentral]

Talk us through it. You replaced the inductor and your SMPS is working and adjustable between about 100v and 200v? You have the full B+ hitting the output tube, and a much lower voltage hitting the preamp's plates? The tubes are glowing, and their heatered powered in series from 12v, with about 6v on the 'centre tap'? Transformer connected correctly?

[Taylor]

Yep, I've got all that, and in fact I have sound now, but again the nasty digital noise.

I think I'll buy several different inductors and see if that's what's causing the problem.

What's supposed to happen by changing the B+ voltage? Higher voltage=louder, or...?

[frequencycentral]

Yes, higher B+ equals higher volume. Can you describe the digital noise? Is it present even at lower volumes? The regulated 1 amp supply that you bought - is that a switching supply by any chance? If it is a switching supply it could be that you'd get stange ring mod type intermodulations between the HF oscillator in the power supply and the HF oscillator in the SMPS.

[Taylor]

Aha! That's a very clever point I hadn't thought of. Although it doesn't say, and it's the size of a transformer-based supply rather than the "One-Spot-size" of most switching supplies I've seen, it is quite lightweight, too light to have a transformer inside. I think this must be the problem, because at 45khs I shouldn't be able to hear the SMPS of course, but if the 2 oscillators are heterodyning as you say, it would dip into the audible spectrum.

[frequencycentral]

......but if the 2 oscillators are heterodyning as you say.......

I found this out when I had a 555 based SMPS and a MAX1044 based charge pump on the same breadboard. Sum and difference for sure. Some great swoopy synth noises there though.

[Taylor]

Yeah, it actually reminded me of your tube synth you posted here awhile ago, "Crushed glass" I think it was called?

[Taylor]

Now on power supply number 3, a robust 1.2A transformer-based thing, I managed to smoke an even larger inductor. It's a 100uh, 1amp rated inductor, looks just like the ones in the ZVex and JJS amps. I had the 1k trimmer somewhere near the middle, not at full voltage. I also bought some 3300uh .5A inductors, because that's what JJS spec'd. I can try those, but I just don't get what's going on here. Apparently the inductance doesn't matter too much here, so the 1 amp 100uh should be fine, right? I realize my supply is 1.2A, but it's not automatically putting out that much current, right?

Edit: tried the other inductor, also melted. 

[Cliff Schecht]

The inductance DOES matter here, although the actual value isn't critical. The inductor in a switching power supply "balances" the inductor by maintaining a specific amount of voltage across that inductor for a certain amount of time for a given peak inductor current. What this looks like formulaically (new word!) is L = (V*S)/I where L is your inductor, V is the output voltage, S is in seconds and I is the peak current seen by the inductor. What this tells you is that for a certain frequency, you have to maintain a certain voltage across the inductor for a certain amount of time for a given current. If you end up losing this balance, be it increasing the current output, changing the on-time or changing the input voltage, it's the controller IC's job to sense that an imbalance has occurred and either increase or decrease the on-time of the switch accordingly. When the switch controller cannot maintain this balance, the inductor saturates and draws too much current so that either the switch will blow or the inductor will burn out (if the IC doesn't blow first).

I think that in your case, you haven't calculated or measured the peak currents seen by the inductor. For the supply I've finally finished designing (I'll post on it soon after more testing), the peak inductor current is MUCH higher than the rated output current - we're talking 2.85A peak inductor current vs. a 50mA output current. The peak currents in your circuit could be much higher than this still! The peak current is dictated by the inductor size where a smaller inductor draws more current. You're going to either need a higher current inductor or a larger value, but remember that a larger value inductor will increase switch stress.

I'm not sure which schematics you're basing your design off of either, I'm just going off of general knowledge. I will say that my new HV supply design is superior to the other designs I've seen so far in that it has pulse-by-pulse monitoring of the output voltage and would shut off (hopefully) before anything blows up. I have no grudge against 555 timer based designs or anything, I just don't know who designed the things or how good of a job they've done on their supplies. I usually don't trust something that I found on the internet until I've done at least some preliminary calculations to ensure that the design is a good/safe one.

I'm hoping to have my supply posted by this weekend, so maybe you can play around with it and give me some feedback. I have a little 1W tube amp already planned out for this supply, so I'm really stoked to get the supply finished finally and tested out in a real application. I'm also going to do a layout for the supply with a huge focus on making everything as small as possible. I'm hoping to find a FET that doesn't need to be heatsinked which would save a LOT of room, or perhaps I can use a DPAK or D2Pak part and mount it directly to the board. Either way, look for my post coming up soon.

[Taylor]

Cool, I will be watching for that.

The circuit I've got is just the amp and supply from the first post in this same thread:

http://www.diystompboxes.com/smfforum/index.php?topic=78302.msg644749#msg644749

[frequencycentral]

Strange, I've built this type of SMPS twice now, and breadboarded it a couple of time too. I've only managed to burn out a low current inductor once. I'm feeling for you though - it's a bummer when things like this hold up a build. BTW, that JJS amp posted over at FSB last week also used this SMPS. Anyway, here's my latest version of the SMPS:

[Cliff Schecht]

I'm a bit weary of placing a standing inductor so close to the switch like that, especially when you're switching such high voltages. Stick an ungrounded scope probe near the inductor and switch to see what I mean - they both throw out a lot of hash. Probably not going to hurt you or anything, just something to be aware of.

[Taylor]

Rick, Looks like you've got a 2.5A inductor there, I guess my next try should be something in that area.

[frequencycentral]

Yeah but look at the inductor in my original Superfly build on page 2 - tiny and low current. I did manage to burn one of these out on the breadboard trying for vey high voltage, but that one in my build has been working well for months now.

[Cliff Schecht]

You can estimate the max inductor current for a certain output voltage by using the formula I = (Vout*Ton)/L where I is the max current, Vout is the output voltage, Ton is the time that the switch stays on and L is the inductance. This is assuming that the inductor current falls to zero at one point (which it should for these Nixie tube 555 based supplies). The Ton parameter is just (1-D)*T where D is the max duty cycle and T is 1/Fs (switching frequency). Do some math gentlemen!

[frequencycentral]

OK oldgravity, your build is bugging me. Please list the parts you used in the SMPS. What layout did you use? Got any photos? Lets get you running.

[Taylor]

Hmm, don't know why everybody's calling me that today. Kind of bumming me out, because I don't want FSB guys digging up more of my posts to call me stupid for trying to learn something.

Anyway, don't worry about it. I just discovered that when buying a new roll of solder, I accidentally bought one with a water-soluble flux, and the flux is partially conductive.  Although it didn't show up when probing for continuity, I am thinking that the flux is acting like little capacitors connecting all my joints together. I'm going to start over with a new roll of solder. I appreciate you wanting to help me though.