Two Valve + IC Regenerative RX kit

[Herald1360]

Edited circuit........

The heater side isn't ideal, but to maintain a common negative with the HT you're stuck with a half wave rectifier.

[Tony1951]

Originally Posted by Herald1360

Something a bit fundamental, too. For a quadrupler to give around 96V, the peak voltage of the ac in needs to be 24V, so you need a transformer with about 17V secondary, plus a bit to compensate for diode losses, say 18V.

Oh, and that 6.8V regulator would get a bit hot with nearly 30V across it

Originally Posted by Herald1360

If you would like a scan of the original article, I've got a pdf, but it's rather too big for the forum. PM me your email address if you're interested.

You guys are pure gold.. I'm sorry to be making such a twit of myself, but the stuff you are giving me back is great. Thanks.

[Tony1951]

What is the likely cost of output transformers suitable for a moderate loudspeaker volume from a typical out put valve, maybe running with an HT of 90 volts? Looking at new ones, they seem inordinately expensive. Where can you get them?

I read somewhere on a forum I think, that it is possible to use a small mains transformer, using the 240 volt winding as the load for the valve anode and putting the low impedance speaker across the low voltage winding. This might give a ratio of around 20:1.

However, looking on audio forums, where this is also discussed, people suggest that such transformers may get saturated, and also have inadequate frequency response. Of course a small table top, experimental valve radio does not need to be a hifi success.... I don't know.

Once again, please excuse the naivety of my problems. I am trying to discover what will work and what won't, and have learned a lot from member's input.

[kalee20]

There is a likelihood that the transformer will approach saturation, yes, and this will give distortion. I would not recommend it for a 'good' design. But it won't damage anything.

However, in practice you will probably get away with it until something better turns up.

For a typical output valve, running from an HT of 90V, you really need a load of around 10kΩ. If your loudspeaker is 3Ω this would be a turns ratio (equal to the square root of the impedance ratio) of 58:1 (so use a 240V : 4.5V). If you have an 8Ω speaker you'd need 35:1 (so 240V: 8V might be a fair match). I've chosen voltages which, from memory, I believe exist. The turns ratio of small transformers isn't quite what you'd expect from the voltage anyway, to make up for losses - but it would get you working.

There will be many scrap chassis 'out there', with a good transformer. I'm not going to suggest buy a cheap radio, salvage the transformer, and scrap the rest - that would be sacriledge - but in monetary terms, if you did this, you could get yourself a transformer for £10 or so. Best route is put up a 'wanted' post on this forum.

One warning - small output transformers are wound with very fine wire and if they've been stored in damp conditions, the primary may have corroded and be open-circuit. So don't pay more than a couple of pounds for an untested one.

Mains transformer for output stage...

This is very ballpark, finger in air, read the tealeaves stuff but.

If we assume 10mA for the anode current, a 240V primary at 10mA would be 2.4W so a 6VA transformer 'shouldnt' saturate at 10mA, and are probably the cheapest ones to get.

A low voltage wall wart should supply something suitable.

[kalee20]

Originally Posted by merlinmaxwell

If we assume 10mA for the anode current, a 240V primary at 10mA would be 2.4W so a 6VA transformer 'shouldnt' saturate at 10mA

That'd not quite right I'm afraid. Granted that at full load, a 6VA transformer will be pulling 25mA through its primary. But this will be counterbalanced by a corresponding current through the secondary, leaving an arbitrarily low magnetic flux in the core. It turns out that the flux in the core is independent of load applied.

With steady DC flowing in the primary, there is no cancelling current through the secondary, and the core sees a fairly strong magnetising field, which is highly dependent on the DC. It is quite possible that the core could saturate at far less than 10mA. The transformer has to be designed with this in mind.

Some toroidal transformers, with high-spec, low-loss cores, would saturate readily with DC at a level of a fraction of the rated AC primary current. Other transformers, with cheap-and-cheerful commercial laminated construction, can in practice cope comfortably. But it's not something that can be taken for granted!

[Tony1951]

Fascinating stuff this. I read some remarks on an audio forum that were saying the same thing Kalee20. We can have a go though and see what happens. The info about the square root of the impedance ratio being the turns ratio to go for is a key piece of info.... I had been wondering if it was just the turns ratio (ie 40:1 (roughly) being the ratio likely in a 240/6 volt mains transformer, when attaching an 8 ohm speaker, I thought I'd get 8x40 ohms load or 320ohms and far too low - BUT NO. Using the example you gave, a 240/6volt transformer with an 8 ohm speaker would present around 12800 ohms to the output valve anode...

To paraphrase Dr Doolittle, 'By George, I think I've got it!'

Saturation is another story though. Maybe suck it and see. Possibly, we might avoid that by using a somewhat larger mains transformer so it has a larger core mass.... Guesswork that though. Shoot me down??


On another tack: I contacted the maker of the kit and he said he used to produce a four valve regen set based on three df92s (IT4) and a 3q4, running with an HT at 90v. He stopped supplying it because of concern over inexperienced fingers and 90v. He has offered all the parts and a circuit diagram for £49 + £ 3p&p. Looking at the price of individual valves, reduction drive tuner, pots, output transformer, speaker, hardware and components, it isn't too bad an offer. Of course, we are still stuck there with 1.4volt filaments, as these are battery valves. There is no circuit board on offer. Construction would be of the point to point variety under a chassis with the valves plugged into chassis mounted sockets, though I think his intention was to supply the pcb/matrix board type sockets, as he mentioned matrix board. There is no power supply. Batteries were the expected power source, but that can be got around.

I'll need to think about this, and maybe price the parts for doing it with mains valves instead and getting all the bits individually. I bet it will be more than £50 though.

Kalee20, you learn something everyday! thank you. So grotty wall wart one is the best?

[kalee20]

Cheap mains transformers won't be hi-fi whan used as output transformers. But, if you try one, you'll probably find the results encouraging. And yes, using a wall wart (of the AC output variety obviously!), if to hand, would be OK.

The offer of a kit of parts for £49 isn't bad. Getting the bits individually would probably be about the same. I recognise the style of your kit, and I also built one of the other kits, which I reviewed as a diversion.

If you firm up on a output valve then other parts tend to fall into place. I can see that power supplies are going to be an issue for you, but your voltage multiplier rectifier system, while not exactly elegant, will work. (Last night while posting replies, I was running a radio from ten PP3's giving 75% of the 120V I really wanted; a couple of nicads and a diode to give me 1.9V which is near enough for the 2V I wanted; another PP3 to give me a bias supply. This wasn't exactly elegant either!)

[murphyv310]

Hi
I recently repaired an old valve Grundig tape recorder and all I had to hand was a small mains transformer which was substituted for the faulty audio output transformer.
The results were fine with plenty of volume and OK quality for an old tape recorder.
The transformer had died due to the grid coupling cap being very leaky.

[neon indicator]

Done the same regularly.