Some ideas on Heater Wiring

[yakkmeister]

Hello Fellow Nutz!

I am planing on a very simple, small build of an amp with my daughter on the Easter break.
Now, since it's Australia at this time of year, I don't have ready access to reasonably-priced ... anything.
Maybe sheep?

So, I'm fabricobbling some jiggers to make a whatsit.

The chassis is a little vintage radio-box which has all the holes you'd ever want drilled in there already ... and since it's a little'un, I figured I might see if I could squeeze in a pair of EL84 power choobs in a PP power section with the noisy part pushed in via either a pair or just a single (probably a single) 12AX7. This for being what I have laying around the place.

Speaking of what I've got hanging around ... I have 4 small 12v transformers which are lovely and new.
2 are beefy while the other 2 are less so (I can furnish specs when I get home should we need 'em). I don't got no fancy-pants, proper transformer with a legit heater winding.

I realise the 12AX7 gents will be right at home with the 12v - either 12vac or 12.6vdc - but the EL84's don't like them no 12v nuthin'!

I'll be running a pair of transformers back-to-back to arrive at 250vac for further refinement ... but I expect I will have to run a third as a heater supply.
I give all the info, of course, because I am more than happy to field other ideas on doing clever things.

I had a Frankenstein's Monster of an idea ... I'll whip out a little diagram and apologise fir the dark feeling of dread you get looking at it ...




I expect I would use an NTC Thermistor or twain to ease up the inrush current on startup to help preserve the life of the tranformers out at the end there ...

So ... no idea if this is even sane ... so, please, hit me with some solutions, if'n ya like to

[yakkmeister]

www.chasingtone.com/schematics/PPl25/Supro_6420_2.pdf

I am planning to use the above schem (I can't seem to make it a link from my phone) as a basis for the project - 2x 12ax7 2x el84 (I realise the schem uses different power tubes).
Since the supro wastes the second half of a 12ax7, I'll pinch the high-gain section off an AX84 Firely's pre as an added bit of fun.

I have 2x Soanar MM-2004 and 2x Soanar MM-2013 power transformers - looking at them again, it may be too big an ask for them.
The MM-2013 has secondaries rated at 12.6vac with 6.3vac ct ... 500mA max
While the MM-2004 has a 6vac tap ... 2A max. I ran some numbers this afternoon and I can't see running these tubes on less than a 3A filament.

It could be my numbers are borked (I used max values ... the intention is to run the lot on 250vac straight into a full-wave recto) ... or I'm right and I have to spring for a proper tranny.

[reTrEaD]

You might just as well fork over the dough for a proper transformer. Running mains straight into a bridge is a serious safety hazard. Your neutral would necessarily be connected to the chassis via the diodes in the bridge. A break in the neutral circuit would mean your chassis is hot. Like 250vac mains (minus about a half volt across the diodes) hot. That could ruin your whole day...week...month...year...decade...life.

[yakkmeister]

Mar 23, 2018 6:58:52 GMT -5 reTrEaD said:

Running mains straight into a bridge is a serious safety hazard.

You are, of course, 100% correct.
Which is why I have no plans, whatsoever, to do that.
I probably haven't been clear enough on this point - I would have to run 2x transformers, back to back.

250vac mains --> 250v pri || 12v sec --> 12v sec || 250v pri

Giving a 250vac output.

To be even clearerer; I may not be able to do any of this with what I have. Ideally, I won't need extras parts - perhaps I should build a Firefly or use the firefly power amp on the supro pre. 1 Watt is better than none
And then it's just 2x 12AX7s...

[reTrEaD]

If you're looking to go cheap on a low power amp you might use some toroid transformers salvaged from scrapped computers. Or maybe you already thought of that?

[yakkmeister]

I did think of it ... I threw out a whole mess of PSU's before deciding to do this project.
Hoarder's worst nightmare

I have located a solution, however - and I think it's a really neat concept.

There's a bloke here in Oz who does amp building classes and he runs Valve Heaven. On that site is a small selection of his designs, one of which is the 'Lamington Lite' amplifier. This uses a 12vac adapter to feed on on-chassis 12v Transformer. Since we have 240/250 VAC here, the 12v transformer can take the 12v up to the 240v. The unit I have can easily drive the HT on a few small bottles while the 12vac adapter I have can easily drive the 12v heaters + HT of the tube compliment I am going to use.

On the subject of tube compliment ...

I am around 80% sure i can build this little project with a pair of 12ax7's by using the supro 6420 preamp (2x triodes) and the self-splitting power section off an AX84 firefly (another 2x triodes). It also happens that I had a speaker of the correct size just hanging around ... lol

Firefly docs

The power section on the firefly (taking this as being C7 to OT) is driven with a 12AU7 ... though I expect (and I'll check) that this can be subbed for a 12AX7.
It's cathode biassed, so I assume the biassing resistor would be the main point of difference. Rectified 240v will easily reach decent HT voltages.

[yakkmeister]

Results!
Can't really use the 12AX7 as it doesn't push enough current.
Now I just gotta trade for 12AU7 ...