Singularity \Sin`gu*lar"i*ty\, n.; pl. Singularities. [L. singularitas: cf. F. singularit['e].] 
1. The quality or state of being singular; some character or quality of a thing by which it is distinguished from all, or from most, others; peculiarity.
2. Anything singular, rare, or curious.
3. Possession of a particular or exclusive privilege, prerogative, or distinction.


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.

Signal section

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.

For those not able to get the 437A (or don't want to spend all that money), the 6S45P is a very good alternative. You can keep the same parts but just fit in a noval base and make the right connections. I am actually thinking of making one with the 6S45P if all goes well with the 437A.

Started physical work on the amplifier such as fitting parts etc. I placed some fotos in the section below.

I've had a lot of reactions to this project. Many people have asked me what parts I am using, so here goes:

tube 437A Western Electric 2
tube socket magnoval Yamamoto 2
tube socket P socket used 1
tube AZ1 Telefunken, mesh 1
cathode resistor 120R Kiwame, 2W 2
bypass capacitor 1000uF/25V Black Gate, NX 2
grid stopper 150R VTC tantalum, 1/2W 2
volume control 10K ALPS 1
rca connectors - generic 2
speaker terminals - Vampyre 2 pairs
OPT 5K/30mA Tribute amorphous 2
supply chokes 30H/40mA Bartolucci 2
power tranny custom Bartolucci 1
mA meters 500uA (modified) n.o.s. 2
supply capacitor 22uF/350V Black Gate, VK 1
supply capacitor 150uF/350V Black Gate, VK 2
power switch dual pole, dual contact generic 1
mains entry - generic 1


I received the power transformer and chokes today and fitted them onto the chassis.

All the wiring was done, double checked and then hooked up the amp. Everything reacted as I though it would. The 437's bias themselves nicely at 205V/25mA. I did some listening even though it is not wise to make a judgement just yet. All the parts are brand-new and take a while to play in (200 hours minimum). First impression: WOW!!!! Very direct sound, right in the middle of the music. The first minute it sounded like crap but eventualy started to turn around. Sound is a bit closed and rough around the edges, but this should change after it has been playing for a few hours.

I made some measurements on bandwidth and power output. Better than I expected really. Bandwidth is 18Hz - 225kHz at -3dB. The low end was expected to be higher as the OPT's were designed for 50Hz and up (midrange), they don't have the inductance to go lower. As the 437A was intended to power my horns, the 20Hz is better than I expected to get anyway. Power output is 0.7W before any visible rounding of the sine, 1W max with clear signs of clipping/distortion. Square wave response is excellent even at 1W output, down at 60Hz is really the limit on the square wave test, very rounded. For those building this amp and wanting more power, use a 3K or 3K5 OPT.

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...

Playing-in the parts sucks big time!!! Highs are getting smooth, low is getting tight, but mids are too woolly and agressive. I hope it will pass. I've only been playing for about 50 hours now, anybody have a time machine?! Soundstage is very good, precise placing of the voice etc. Depth is wonderfull.

Visited Bert Doppenberg and took my "amps" with me, also a nice benefit of a small amp :)  Bert is very sceptical on anything that is not his, so I knew he would complain as ussual (grapje!). I was kind of worried that he didn't say anything just after hearing the first notes coming out of the amp. After a few minutes it was obvious he liked this amp, or he would have honestly told me his thoughts. So, now I am not the only one to agree this little amp sounds good.

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.

Ai, Cold Turkey... I hooked up my system this weekend (finally). It hadn't played since the ATF2001, yes yes, I am ashamed of myself. I didn't want to hook up the Exodus amp as it was a lot of hastle with the batteries etc, so I just used the 437A amp. A friend called that evening saying he would drop by and wanted to hear my new setup (just got a new CD transport). Heck, now I really had to hook up the Exodus amps, ah well, it was a good excuse to get my ass off of the couch and get everything playing.

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.



Preliminary layout.

All the holes drilled/punched.


The modified uA meter (now 0-50mA).


Internal layout.


Fitting the plate.

All smaller components fitted and plate fixed to the frame.


Beginning of the internal wiring.


Still waiting for the power supply iron.

Close-up, not much to look at yet.


Wiring finished. It looks worse than it is.




Rear view


The amp taken into action. It replaces a preamp and two monaural power amps.


Close-up of the WE437A.