Cheap power Q

[Chuck H]

Thinking about a new micro amp build. Two 12ax7's and a single 12au7 push/pull. I have most of the parts and some time

I have a suitable PT, that is, not too large and low HV, but I think it's better saved for a different project since it'll handle a pair of small bottles and a rectifier tube. So I want to get a PT just for this project. I was shocked to see how expensive little, simple PT's suitable for tube projects are. The most affordable I found was $45 and barely suitable. Most were more like $60! Jeeze!?! I can get a PT for a Deluxe at that price! So I got to thinking and I cooked this up. Why not use two cheap filament transformers back to back and a voltage doubler? You can get these little filament trannies for $10 all day. So two for the job is still a bargain. My only quandary is the current available for the HV duty. Do I need to assume the current will be limited to an inverse of the ratio with the 6.3V secondary?

Any info on why this won't work, tips to optimize or suggestions for a PT for this project are appreciated.

[olddawg]

Thinking about a new micro amp build. Two 12ax7's and a single 12au7 push/pull. I have most of the parts and some time

I have a suitable PT, that is, not too large and low HV, but I think it's better saved for a different project since it'll handle a pair of small bottles and a rectifier tube. So I want to get a PT just for this project. I was shocked to see how expensive little, simple PT's suitable for tube projects are. The most affordable I found was $45 and barely suitable. Most were more like $60! Jeeze!?! I can get a PT for a Deluxe at that price! So I got to thinking and I cooked this up. Why not use two cheap filament transformers back to back and a voltage doubler? You can get these little filament trannies for $10 all day. So two for the job is still a bargain. My only quandary is the current available for the HV duty. Do I need to assume the current will be limited to an inverse of the ratio with the 6.3V secondary?

Any info on why this won't work, tips to optimize or suggestions for a PT for this project are appreciated.

Been awhile since I thought of something like this and I'm half awake, lol. The output of the second transformer would be less than 100ma (you can't get 100% efficiency) and the doubler will drop that to less than 50ma? I wouldn't count on more than 40ma. There was I time I knew the math. Now I will wait for the real engineers to make me look stupid!

[hylaphone]

What about a 6G15 PT? This one is $25.

https://taweber.powweb.com/store/wrvbptsch.jpg

[potatofarmer]

Fred Nachbaur did something similar on the Real McTube: http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm

You also might want to look at the approach EHX took with the English Muff'n (and I'm guessing their other tube-based pedals as well).

You're going to draw, what, 15-20mA through the second transformer?

[defaced]

Allied has some small tube transformers that might be what you're looking for. Check the attached PDF from their product catalog. They're made for Allied by Hammond. If I remember right, I used the 6K88VG on a preamp project. A little big for the job, but for the cost I wasn't going to complain. 990.pdf

[R.G.]

The back-to-back filament transformers connection is an old trick that works well. I've used it before, as have some others. I think it's in the musty archives of MEF or its precessor.

There are some things to be aware of in doing this, and they have to do with the current ratings (as you've guessed) and the resistance of the windings. Transformer ratings are expressed in volts AC rms and current AC rms, and they are stated as measured at full rated current. A transformer rated at 120V to 6.3V @ 2A will have an output of 6.3V (+/- the tolerance %) only when it's fed 120Vac on the primary AND loaded at 2.00A on the secondary. If you remove some load, the output voltage goes up because there is less voltage lost inside the transformer due to the resistance of the primary and secondary windings.

In order to give the paying customer that 6.3Vac (or whatever) at the full load of 2A (ditto) the transformer has to be wound so that the no-load output voltage is higher than 6.3Vac. How much higher, expressed as a % is what they will sometimes tell you is the "transformer regulation". Ordinary, non-ferroresonant transformers do not regulate, they only sag from their no-load voltage. "Regulation" sounds so much better than "sag" in advertising, though.

For 120Vac (or 240Vac, etc) input trannies, the smaller the transformer, the more turns of primary wire are needed to get the smaller core's inductance up to enough to not burn up from core saturation. So the smaller the transformer, the more turns of finer wire needed, and you quickly run into dramatic increases in the resistance of the primary wire. The secondary gets more resistance in finer wire, too, but for low-voltage outputs this isn't too bad.

Transformers in the 1.5-3 cubic inch volume range have big "regulations". 15-20% isn't uncommon.

In the back-to-back arrangement, you get 120V to 6.3V (or whatever) at whatever voltage loss the loading makes happen. This is the sum of the heater loading and the loading from driving the second transformer. But in the second transformer, the extra voltage ratio on the secondary works against you in transforming UP to the second "primary". You lose the extra voltage ratio of the maker put in there for "regulation", and you also lose the current-times-resistance in the output winding for HV. You get a lot lower voltage than you thought you'd get. It's not uncommon to lose 1/4 to 1/3 of the rated voltage. This also sags badly with loading.

Rectification also makes the current worse. Rectifiers effectively cause high RMS currents, and this causes more sag in the output DC.

An now back to your original question - how much current can you pull for what transformer rating?

The full wave doubler will produce a heating/rms current in the winding driving it of 1.6-1.8 times the DC out of the filter caps. So that's a good estimate of your output "120V" winding current. For power rating purposes, this winding can still carry its full rated current as though it was being used as a normal, not-backwards transformer. In the case of a 120:6.3V @2A transformer, this is about 2A* (6.3/120)* (1/0.85) = 123ma, roughly. You'll never get there, because most of the 6.3V from the first transformer is going into the heaters, not the second transformer. Still, it's a good question to ask.

Your problem with this setup will be getting enough current and voltage out at the HV output. The DC output will sag *a lot* as well as not being as high as you thought it should be.

[Chuck H]

Thanks R.G. And all who responded. It looks then like it would "work" with some compromise since I only need HV current in the 25mA range. The consideration of voltage sag due to resistance, current and other imperfect losses would likely leave me a lower HV than I'd like. Not to mention that two transformers back to back lacks aesthetic appeal. Probably better to just use the cheapest (uh, most affordable) single transformer I can get that will work. The one offered by Weber looks like a good option (thanks hylaphone) even though it's a lot more transformer than I need. I wanted to keep size and weight proportionate to the amp but I'm not paying three times the price for the luxury. And, as R.G. was good to educate us about the nature of smaller transformers it does seem like a better option electronically anyhow. I could still see doing this trick if I happened to already have the filament transformers in hand, but I won't order them only to compromise in the end anyway.

[loudthud]

There is one option not mentioned so far. Find a transformer with dual primaries and dual secondaries. It has to be rated for twice the VA that you need, or more. Use only one of the primaries for the 120V input and the other primary for the HV secondary. Connect the LV secondaries as needed to get heater voltage.

[Chuck H]

There is one option not mentioned so far. Find a transformer with dual primaries and dual secondaries. It has to be rated for twice the VA that you need, or more. Use only one of the primaries for the 120V input and the other primary for the HV secondary. Connect the LV secondaries as needed to get heater voltage.

Thanks. Yep. I was looking at other creative options like that too. I just found the simplest filament transformers to be in the right price range. Short of buying someone else's greasy old salvage from Fleabay. My biggest problem is that I've just used up so much of my salvage that I have to buy parts for my personal projects now .

[g1]

Also, instead of a doubler, you could try a tripler or quadrupler.

[tedmich]

if you want small you can't beat a charge pump, heres a 12AX7 and 2x 12AU7s (Firefly type) powered off a 7660 based charge pump and a 12v wall wart, he says it works low gain with 3 x 12AU7s

DIYstompboxes.com has explored the charge pumps up to about 180-200v see also Rick Holts "Murder One" with minitubes

[Zipslack]

You could try a SMPS with a wall-wart. Plenty of micro-build threads at diystompboxes.com dealing with pre-built and diy SMPS modules. I have a DIY 555 nixie supply I built that puts out 250V with enough current to drive 2x12a_7 tubes. This module seems to be popular - http://www.tayloredge.com/storefront..._1363-1364.pdf.

Here's a good thread that discusses charge pumps and nixie tube SMPS supplies (with links and schematics) - http://www.diystompboxes.com/smfforu...?topic=74088.0


Use a 12VDC 1A wall-wart, parallel off for the heaters (LM7812 regular with a diode in the COM path gives 12.6VDC) and use the HV from the 1363. Pretty cheap (especially if you DIY) and you can pick up wall-warts from thrift shops for $0.50-1.00 each.

[hylaphone]

There is one option not mentioned so far. Find a transformer with dual primaries and dual secondaries. It has to be rated for twice the VA that you need, or more. Use only one of the primaries for the 120V input and the other primary for the HV secondary. Connect the LV secondaries as needed to get heater voltage.

Do you happen to have a schem of this configuration?
May be a senior moment, but I just can't wrap my head around this

[tedmich]

or if you pickup a lot of smps you do what this guy did, ostensibly before his house burned down..

https://greenvalve.wordpress.com/2011/07/11/smps-for-b/
yikes! too many leads and clips and 700VDC...I can almost smell the smoke!

[Leo_Gnardo]

or if you pickup a lot of smps you do what this guy did, ostensibly before his house burned down..
yikes! too many leads and clips and 700VDC...I can almost smell the smoke!

Crikeys! What's he call that rig? The Dog's Breakfast? What's he got, big 10-turn bias pots, all clipped together with RadioShack's finest clip leads. Maybe he should learn how to build amps first with Banana Jack kits, "safe for kids of all ages" so we've been told. Bet you there's no fuse.