\Sin`gu*lar"i*ty\, n.; pl. Singularities.
[L. singularitas: cf. F. singularit['e].]
I made a few earlier attempts with this idea but never got around to trying it out. I've ordered the iron for it, so now I am commited to building this small amp.
As I do not need much power, 500mW is more than enough, I can do with almost any output tube. My Exodus project combines the nicest tubes I have found into a single amp. To produce 0.6 to 0.8W it requires three tubes and three pieces of signal iron from source to speakers. Although I often prefer low-mu DHT's I can't stop wondering what an amplifier with only a single tube and transformer could accomplish. The project does not have to be as lavandish or expensive since it can be small in size and requires only a handful of components.
The setup is quite simple, what can you expect with only one tube :) It is by no means intended to be "the best amp I have ever built", but more of an experiment just like my first 71A amplifier (Puppy Power). After it is finished I can always decide which way to go.
The input is a 10K potentiometer and 150 ohm grid stopper. The grid stopper is a 'must' with high-gm tubes, I have had a few run-away tubes before and I don't want to blow the 437A's. The 10K will also help in this matter as the 437A might pull some grid current when driven to it's max swing. The output transformer has a 5K primary, along with the setting of around 180V/25mA should give me 700mW output. More than enough!!! My 71A amp has proven that I only need around 200mW to play at normal levels, add some reserve and you end up with 500mW.
The power supply uses an AZ1 rectifier which I like very much. It kind of stands out as it is quite a bit bigger than the 437A it is supplying :) All the parts will go onto a small square chassis similar to my 1H4 preamp. The filament is supplied by 6.3V AC, very simple.
The circuit will put out about 700mW with 2500mV rms input using a 5K OPT. This is the max it can handle as the grid will start to go near zero/positive and start to pull current; this will result in clipping. Please be aware of the low output power before building this amp yourself. Only sensitive speakers (98dB and up) will have enough power with this amp. It is however possible to make a preamp out of this project should you want to build it anyway. The gain would be around 40 times.
The sound is becoming ever more open and clear. Another experiment that has paid off. The compromise with DHT's is that I need three tubes and acompanying transformers/chokes to get the same level of output (0.5-1W). This project proves that an IHT with high mu might be a compromise by itself when used in an audio chain, but I think the right tube used on it's own can sound better than three great tubes strung together to perform the same task. The key factor with this amp is that I can do with 0.5W perfectly. I don't have 90dB speakers that need at least 3-5W to come to their rights. I think this amp might even stay in my system for a long time to come. I'm almost thinking of not continuing my Exodus amps :) Nah...
The amp still has a lot of hours ahead of it to burn in all the parts, but it is getting better each day. I didn't think this idea would be anything else than a nice little replacement amplifier that I could put on a shelf if I didn't need it anymore. It just may up becoming my favorite amp :) Funny how things work out.
The tubes were lit up again, the 10 sticking out ofcourse with it's bright glowing tungsten filament. Always a nice sight!!! In went the CD, click... out came the music. What?! Couldn't believe my ears. Was the 437A really that bad??? It was like the music opened up a door and let in the air. Everything wa so smooth compared to before, it was music again. I really didn't notice with the 437A as I hadn't heard the good old DHT's in a while (due to all the rebuilding). I have gotten a big slap in the face again waking me up to the difference between a directly and indirectly heated tube.
Even a single tube performing all the tasks can't come close to my three tubes per channel DHT amp system. There is no getting around the immense difference in sound. Yes, the 437A and 6S45P sound incredible and perform a unique task, but compared to a DHT it really can't hold water. I was doubtful about the comparisson before, but now I am fully convinced. Even three stages of (good) DHT's can't be beaten by a circuit using only one IHT.
© Jim de Kort