Making A Low-Power Valve HiFi Amp

Hi,

A friend of mine at work is wanting to build a small, low power valve amp to use for HiFi. It needs to be simple to build and cheap (but not a kit).

Having discussed how the most expensive parts are the transformers and valves, does anyone have any suggestions for any simple known-good designs or any good value transformers to use for a given design?

We've discussed using mains transformers as output transformers as these can be had for around £10 and supposedly can in some cases give pleasing results, but other than that, I've not listened mains transformers being put to this use.

The aim is for the amp to be around 4W or so in power, and it needn't be anally HiFi. The best sound ever is not expected for a simple design.

As for mains transformers, I can only see an OEP model available which provides heater taps plus a 240-0-240 tap. This doesn't match up with the B+ requirements for some designs, so is there any simple way around this?


Cheers,

Roddy

Roddy

OEP do a multi-tap output transformer for about 10quid, RS number is 210-6475.

3.5W, 4k - 15k primary & 3 - 15ohm secondary. Looks quite respectable

Peter

Edcor makes affordable opts though I have no experience...

http://www.triodeelectronics.com/tf65wscta48o.html
(plastic bobbin) --one OPT and a duet of these:
http://www.triodeelectronics.com/5881r.html
will get you going with most of the 'expensive bits' for around 20 pounds per channel! plus cartage I built a really rockin' williamson stereo amp with these parts and a power trans. salvaged from a really old television set.


I also built a SE stereo amp with these parts
http://www.triodeelectronics.com/sv83.html
http://www.triodeelectronics.com/tfchxfwi48oh.html
and am really impressed with the simplicity...sounds very good and plenty powerful for reasonable levels though low end is lacking when I crank it up. I have not really fooled with optimizing response

I cant find schematic i used for the SE but it looked a whole lot like half of this http://www.triodeel.com/dyna6bq5.gif

Thanks, guys. That's helpful.

The OEP looks promising as it would be only £7, plus he may get a discount with our work.

He's looking to maybe build something like this:

http://www.the-planet.org/6EM7.html

Although I suggested an SE El84 amp may be nice too since they are easy to get here.

I was told that the output transformer will need to handle some level of DC - is this the case, and if so, should care then be made when choosing an output transformer in this respect?

Sorry for my lack of knowledge in the valve world - I tend to avoid it.

Roddy

Roddy
You could take a look at the Mullard 3w amp.

There's a pretty simple amplifier design on the audioXpress page HERE. Scroll down 1 screen or so and look for "A Tiny Vintage Garage Amp" by John Stewart.

Peace,
Paul