Can I obtain a suitable B+ voltage with spare parts?

[santonel]

So I've gotten into electronics and tube audio lately and Ive decided to play around with tube circuits. As a first project I'm converting my old tiny solid state practice guitar amp into a really basic SE tube amp.

The thing is I'm kinda strapped for cash atm but I have a bunch of spare components lying around including a few transformers and I'd like to find a way to get a high voltage out of them which I need for the tubes. They are all step-down transformers though.

I can't find datasheets on any of them and all I really know about them is the voltage I measured on the secondary (unloaded). I don't have them in front of me but I'll write everything down later on and post it. The largest one gives ~24v on the secondary if I remember correctly.

Right now I'm using one with dual secondaries. One is 12v which I'm using to test my filament supply with. The other is center tapped and IIRC gave me a reading of about 22v or 11v from each end to the center tap. I have no idea what current it's rated for though.

Right now I have a couple 12AX7s and I plan to use just one in the design. I don't have a power tube yet. I was planning on copying the AX87 design and using an EL84. The prices were reasonable but every one was charging more for shipping than for the tubes. I did some research and found the 6CH6 has similar characteristics but just a different pinout which is fine with me. I found a good deal and I have two of those on the way and I plan to use a single one in the amp.

So for the 12AX7 heaters I'm using 6.3v in parallel which draws 350mA. The 6CH6 draws another 750mA. Anode currents are another 1.2 and 45mA respectively so I need to supply about 1.14A, right? It shouldn't take a big, heavy transformer to supply that current at the proper voltages should it?

To increase the voltage I was thinking about chaining a couple of them together "backwards" with the secondary acting as the primary. But the secondary has more turns using smaller wire which will significantly limit the amount of current I can draw right?

Basically what am asking is, can you rig together a power supply out of commonly found transformers that can supply the 250v or so that is needed for the tubes? Has anyone attempted this or is it a totally stupid idea? Are there other tricks you can use like a "boost" configuration or voltage double circuits to increase the voltage? Obviously I'm not going for quality here, this is all strictly just for fun. Plus it's kind of a fun challenge to do it with scrap parts instead of the real thing. At least for me.

[Svein_B]

The popular solution is to:
wire two 12V transformers back-back.
use 12V for your filament.



You will have 110 (or 220V) AC for your HT, which should give about 150 (or 300V) DC.
If this is not enough, use a voltage doubler.



If you need 6V for heaters, use another trafo.

SveinB.

[Mosquito]

OBTAINING 250vDC & 6.3vAC FOR VALVES by Harry Lythall

[santonel]

Wow that was quick! Thanks! I appreciate it.

[Eli Duttman]

So for the 12AX7 heaters I'm using 6.3v in parallel which draws 350mA. The 6CH6 draws another 750mA. Anode currents are another 1.2 and 45mA respectively so I need to supply about 1.14A, right? It shouldn't take a big, heavy transformer to supply that current at the proper voltages should it?

Santonel,

It's not that simple. As another poster indicated, the heaters should not be attached to the same winding the B+ rail is derived from. Also, when cap. I/P filters are employed, allow twice the DC draw times any voltage multiplying going on of winding rating. It's a matter of I²R heating in the trafo. BTW, a 12AX7 draws 300 mA. of heater current, at "6" V.

The Greinacher ("full wave") voltage doubler circuit Svein_B posted is the most cost effective method for obtaining the 300 or so V. of B+ your project needs. Unless you are truly penniless, $11.41 plus taxes and shipping buys Allied Electronics stock # 967-9502, which is a 50 VA Triad isolation trafo. When Greinacher doubled, that "iron" will comfortably provide 100 mA. of B+, which is plenty for your conversion project. You will need a pair of UF4007 diodes and a pair of 250 WVDC/220 μF. 'lytics, to complete the doubler. Leverage your shipping costs by ordering those parts from Allied too.

[santonel]

Originally Posted by Eli Duttman

Santonel,

It's not that simple. As another poster indicated, the heaters should not be attached to the same winding the B+ rail is derived from. Also, when cap. I/P filters are employed, allow twice the DC draw times any voltage multiplying going on of winding rating. It's a matter of I²R heating in the trafo. BTW, a 12AX7 draws 300 mA. of heater current, at "6" V.

The Greinacher ("full wave") voltage doubler circuit Svein_B posted is the most cost effective method for obtaining the 300 or so V. of B+ your project needs. Unless you are truly penniless, $11.41 plus taxes and shipping buys Allied Electronics stock # 967-9502, which is a 50 VA Triad isolation trafo. When Greinacher doubled, that "iron" will comfortably provide 100 mA. of B+, which is plenty for your conversion project. You will need a pair of UF4007 diodes and a pair of 250 WVDC/220 μF. 'lytics, to complete the doubler. Leverage your shipping costs by ordering those parts from Allied too.

This is just for testing purposes so nothing is set in stone. The transformer I am fiddling around with has 2 windings. One is 12v which I use a 7806 to get 6v of regulated DC. The other winding is the center tapped one that puts out ~22v. I ripped it out of an old clock radio. I was going to just use that transformer for the filaments since I've heard people say it's less noisy if you use a separate transformer. I don't know if it's true but it makes sense I suppose.

Thanks for the tip on the transformer, I'll take a look.

[Eli Duttman]

A 78nn 3 terminal regulator is going to need some VERY serious heatsinking to provide the 1.05 A. a 12AX7 and a 6CH6/EL821 draw.

How hefty is the 11-0-11 winding? Can it deliver 150 mA./22 V.? If it can, full wave CT rectify that winding, with Schottky diodes, and feed a 7812, which will power the 'X7 heater. At a 150 mA. draw, simple good ventilation is all that's needed for the 7812.

A tweak for the "6" V. rail is to use a 7805 IC, with a 1N4001 diode connecting the common terminal to ground. Use every trick imaginable to ease the thermal strain on the "6" V. rail regulator.

BTW, DC heating is something usually reserved for HIFI preamps, especially phono stages. Simple AC heating should be quite sufficient, in a guitar amp. The paralleled secondaries of an AnTek model AN-0206, which costs $11, get the job done.

[santonel]

Originally Posted by Eli Duttman

A 78nn 3 terminal regulator is going to need some VERY serious heatsinking to provide the 1.05 A. a 12AX7 and a 6CH6/EL821 draw.

How hefty is the 11-0-11 winding? Can it deliver 150 mA./22 V.? If it can, full wave CT rectify that winding, with Schottky diodes, and feed a 7812, which will power the 'X7 heater. At a 150 mA. draw, simple good ventilation is all that's needed for the 7812.

A tweak for the "6" V. rail is to use a 7805 IC, with a 1N4001 diode connecting the common terminal to ground. Use every trick imaginable to ease the thermal strain on the "6" V. rail regulator.

BTW, DC heating is something usually reserved for HIFI preamps, especially phono stages. Simple AC heating should be quite sufficient, in a guitar amp. The paralleled secondaries of an AnTek model AN-0206, which costs $11, get the job done.

Well when I got the 7806 they said it should supply 1.5A but originally I was going to use it with a pnp pass transistor before I broke one of the leads accidentally. I actually ordered a lot of LM 317 regulators so the plan is to keep the 7506 for the 12AX7 and use an LM317 for the power tube on a separate circuit. Like I said I couldn't find any data on the transformers so I don't know the current rating on it. I'm just fiddling around at this point and if I end up needing a new one I'll probably get the one you recommended. Otherwise I kind of want to see if I can build it out of parts I have lying around. Just for fun, really. Not for any practical reason.

[hemgjord]

If you are not afraid of spending some time it is quite easy to disassemble the core and scrap the secondary winding and wind your own secondary.
If you don't touch the primary winding and secure that you have approx 0.5mm transformer paper or some fairly heat resistant plastic(not PE) between the primary and secondry you should be safe.
Then it is just to wind on a known number of turns, say 10 or 20 and measure the voltage to determine the number of primary turns.
When you know this it is just to choose a wire dimension allowing for max 3 A/mm2 and to start winding.
For each complete layer of secondary you coat with 2 component PU or similar(not solvent based, it will never dry out)
Finish it up with the same insulation material as between primary and secondary and coat a last time.
This way you can tailor what ever voltage you want.
Oh I forgot, you also need to check or assess that the VA rating of the core is sufficient.

/Olof