Monthly Archives: July 2006

Build a DIY vacuum tube amp ( EL34 / KT88)

Posted on by
9

I decided to build a tube amp for the fun. In fact I think of that stuff since a really long time now (since the first amp, I ‘ve built ten years ago).

There is a lot schematics on the internet, but finding a good one is pretty hard. I want to build something like that:

  • Good power: 10 Watts seems to be a good starting point.
  • Something simple. I want to build an amp, I can fix by myself. So I decided to build a Single Ended one. (Not a push-pull) Push Pull provide far more power, but need more components and harder to build.
  • Low cost. The best issue for this kind on amp, is the price. You need to buy some output transformers, big power supply transformer and choke. And of course, when we talk about HIFI this cost a lot of money.
  • Look’s good. Something which is important is the way it looks. So I will have to take really care of that.

For people who have never seen this kind a stuff here a little pic I ripped out.

http://www.osbornloudspeakers.com.au/opera/m100se-Bt.jpg

Tubes

For this design, I decided to use some EL34. Check out DrTube[0] for additionnals data. They offer good output power for a decent price.
The reflected load permits to use a KT88 instead if this doesn’t provide enought power.

For the preamp, I think I gonna use a 6SN7 in SRRP. This provide a good gain, and very good linearity.

Output transformers

EL34 has a reflected load of 2KOhm, so I need to find coresponding transformers. Of course this cost a lot of money, Hammond HIFi (not guitar) output transformer for 20w cost something like 99$ each. And
even worst, I unable to find 2.0K only 2.5 are currents.

I found this one:

  • EuphoniaAudio[1] (205$ the pair)
  • Hammond[2] (99$ each)
  • Electra Sud Ouest (86Euros)
  • Elettronocanovarria[3] (75Euros)

Despite, Elett.. offer good price, I unable to buy them here, cause the vendor speack only italian.. and that sucks. Electra sud has a good price too, so I guess I will buy them here if I don’t find anything else.

Update: I found a matched pair of 2.5K / 20W transformators on eBay.

Power Supply

This part isn’t really clear for me right now. Singled Ended amps doesn’t need regulated PSU because they work in pure class A, so the current is fixed (throught the tube bias) but It need to be really stable to avoid distortion.I don’t want to buy a large transformer because they cost a lot of money and doesn’t feet with my needs. EL34 use a quite low voltage (~250v) but need quite a bit of current ~100ma. I think, a good isolating transformer can do the job. Something like 230*230 at 200Va doesn’t cost a lot of money.

Schematic

This is really a hard part. But I found 2 good example:

  • first[4]
  • and second[5]

To be continue… ;)

Links