Single Ended Triode Hifi Amplifier Kit
The full, illustrated, story of the build.
Updated Feb 2013
I'm relatively new to this type of electronics building. I have made a Charlize T-Amp, and a modified SI – but these were simply a few low-voltage solder points and a case. SETs have dangerous internal high voltage, and the kit involves stuffing a circuit board with components and rather more scope for errors. The first power-on is not called the “Smoke Test” for nothing. However – nothing ventured, nothing gained. I can read a schematic (from a period in my teens), and own a soldering iron and a multimeter. I already (in the Charlize) have a very good amp, however SETs by all accounts offer a musical transparency and presence that no other amp type can (with limitations of course), and can set you back truly ridiculous sums of money. Decware's kit falls into my price range, carries reasonable reviews, uses cheap and widely available tubes - time to satisfy my curiosity.
I am in a good position here - I already have suitable speakers. These are huge, efficient, double-mouthed horns (Scotmoose Sachikos) with EnABL'd Fostex FE206E drivers and are listed as suitable on the Decware website. My sources are also good enough, and I have solid silver in Teflon wiring so no problems there. The amp will have a suitable home.
Reservations? 1.8 watts per channel into 8ohms. Even though I know that power is exponentially related to volume (ie 10 watts is only twice as loud as 1 watt), that doesn't sound much. Speaker efficiency is the key factor … fingers crossed. Another worry is simply that it is tube gear. My big push-pull valve amp takes a full half hour to sound its best, generates kilowatts of heat, and then does not sound as good as the Charlize which is hugely efficient, has a completely silent background, and is instant-on. Could I live with this little Decware as my everyday amp? We'll see.
Enough preamble – on with the build. I had already planned the case. Got a sheet of 3mm aluminium on Ebay for about £3. Was going to drill this (somehow) for the valves to stick out the top, and as a mounting for the board. The power transformer looks good (and also generates heat) so was going on top, the output transformers (which do not look so living-room-friendly) were going inside. I took some time over the external layout and included an on-off switch (not part of the kit – I got this from Maplins). I wanted the final amp to look good, but also to have a tidy internal wiring layout.
The aluminium sheet (protective covering) with initial markup (later all the components were moved forward).
The kit arrives
The kit arrives in a box containing a bag of components and parts, a single circuit board, a power transformer and two output transformers. There is no parts list or documentation – you download the schematic and instructions yourself from the website. This was slightly disconcerting, but the circuit board is very well marked out and – well – if you need too much hand-holding then perhaps this game is not for you. The downloaded instructions are OK, but contain a lot of how-to-solder stuff, and disappointingly little about actually assembling the kit. However - the Decware support forum is invaluable, and Steve Deckert is in constant attendance there. A forum link took me to another website with an account of the build and a high-definition photograph of the completed circuit board. This was very reassuring to have during the build, and I have included similar photographs on this web page. I don't want to make a meal of this – building this amp is not a particularly difficult task!
Components in the kit
The Power Transformer
The well-made printed circuit board.
The IEC connector and the speaker terminals
Putting the board in a case involves some changes to the basic wiring plan - the RCA connectors, volume potentiometer and speaker terminals will be mounted on the case, not on the PCB. Fortunately the attachments are fairly obvious, and in the case of the speaker terminals this actually shortens the signal path. You do need different RCA connectors to the ones supplied though, and I scavenged these from an earlier T-Amp build of mine.
It does not take long to put the parts into the board. I started with the tube holders (make sure they go on the correct side). Easy, their design means no danger of solder running into the pin holes (a danger with some designs). Poke them through, don't try to bend the legs, then simply put a book on top, turn the whole sandwich over, and solder each leg. The solder will hold them firmly enough. I added the “Christmas” mod (a capacitor connected between pins 3 and 6 on the two power tubes) later, but this would be OK at this stage. This capacitor is not included, but cheap and I found this on eBay.
The Christmas Comes Early (CCE) Mod
nb This was a modification done after the amp was finished - it would be easiest to do this as you go, though two capacitors are required and are not supplied with the Kit. (nb there is a thread about this on the Decware forums, also the assembly instructions have not yet been updated)
BEFORE installing the tube holders, check your printed circuit. Later production boards have a trace on the non-silk-screened side connecting pins 3 and 6 on the two power tubes (the ones at the back in the picture below). This had been found to have a positive effect on the sound, and if these traces are NOT present you have two choices. You could simply jumper pins 3 and 6 on the two output tubes - which would give you the equivalent of the current production boards, or better - you could install two 0.1uf 400V polyester film capacitors, one across each pins 3 to 6 in the output tubes. If you have a current production board with traces already joining pins 3 and 6, then to perform the CCE mod you will have to cut the traces before installing the two capacitors. nb Cutting the traces will be difficult to do AFTER you have installed the tube holders! Those on the Decware forum who have experienced their amps before and after this mod claim a significant improvement across the board in sound quality and (at least subjectively) in amplifier power. You will see further down this page that I installed the capacitors during the build - turned out these are actually shorted out by the traces, and I am just now contemplating how best to cut these with the board in situ.
OK - did it. Had to shine a light behindthe board to see where the trace lies behind two of the tube holders. Then drill a hole which cut the trace running between pins 3 and 6. I used a Dremmel type drill. All went well - see photo below.
Hole drilled through the trace beneath one of the power tubes.
You can see the tube holder through the hole. OK, now back to the initial build ...
Tube holders in position
The tube holders soldered into place. Started properly now!
Now for the Resistors
Then the resistors. I put each one in position, pushed the wires through, and bent them out to hold them loosely so that after soldering they would sit above, rather than on, the board. Turned the board over, supported at the edges and let the resistors all hang. Checked they were where I wanted them, and soldered them. Carefully clipped off the excess wires. This is fun! The big black resistor generates a lot of heat, and needs plenty clearance – leave the leads long, and gently angle it away from the adjacent capacitors.
The resistors in position - their leads pushed through the correct holes
The resistor wires bent, loosely, to allow them to hang away from the board
Then the jumpers. I used snipped off bits of the component leads for these. Same process, bend, let them hang and solder.
Resistors (and the two diodes) now soldered into place
Next the capacitors
Then the capacitors and diodes. Same process, polarity important here. Photo reassuring as confirmation that these were done correctly.
Jumpers and capacitors now in position
All components and jumpers now soldered into place
The Christmas mod - two extra capacitors (not supplied) (nb the PCB RCA connectors are just placed and were not used)
Final amp construction
Next stage was a bit more of a fiddle, because it involved final wiring and case construction at the same time. Drilled the aluminium. Small holes I used a metal drill bit, with some 3:1 oil as lubricant. Easy. Big holes I used a hole drill bit with the same oil. You need to run the drill relatively slowly, with a lot of pressure. Three holes for the valves, another two for the bundles of wires from the power transformer, and a smaller hole to mount the on-off switch (get one with a circular fitting). I was glad when this was done – and I could remove the protective coating from the aluminium plate. Looking good.
The aluminium sheet, drilled, and protective covering removed
Built the wooden frame. Mitre saw for the corners, 4 small glued blocks on the edges to support the aluminium sheet. Thinner bit at the back added for the IEC connector,RCAs and speaker terminals; and space at the front for the volume pot.
The back plate
Back plate - inside view
Amp front plate
Mounted the circuit board using the supplied spacers. I had long screws to go right through, very tight fit and difficult.
Circuit board positioned (not yet attached)
Now mounted the power transformer, and fed the wires through. This becomes the centre of gravity for the whole thing, and a good solid support for the rest of the build. Wired it to the board (checked the schematic – the extra wires can be cut, they are for the tube rectifier which is not part of this amp. Being 240vAC in the UK there are also spare wires in the power-in part. These can be cut. Power in lead was led up the side to the on-off switch, all well away from the signal paths.
Power transformer now bolted to the top plate
Power transformer input wires come through beside the IEC inlet, output wires beside circuit board
Power transformer output wires soldered to circuit board
Aargh, idiot, the spacers are too long – the tube supports are way too low. So off it comes, and junior hacksaw out and cut the spacers to the length that would have the tube holders just below the level of the top plate. Refitted them – ended up using a fine mole wrench to grip the screw heads as it was too easy to strip these. Finally finished. Happy again.
Spacers cut to properly position the circuit board
Mounted the output transformers on the underside of the top plate, Angled – because my case turned out quite tight here. The RCA input wires I attached to the PCB and routed these in between the output transformers. I had some 0.5mm silver in Teflon wire left from my Speaker cable build – ideal hookup wire here.
Another view of the internal wiring
The blue & black wires from the output transformers will go directly to the speaker terminals on the case (you can work this out by following the traces on the PCB).
Finishing soldered the external connections. Made a nice shielded high quality RCA cable from some old Belden cable I had from my power-cable building days. Earth connected to the RCA earth pin, and a separate earth to the top plate.
The top plate
The case constructed (rubbishy temporary screws, replaced later by Japanned black dome heads)Now for the first big moment – the power-on test and that potentially lethal DC voltages check. “You'd better be here just in case,” I told Debbie, “What's the worst that could happen?” she asked looking nervous.
So crocodile clip attached to ground. Black probe from the multimeter in the crocodile clip hole, red one for testing each voltage. Switch on. Valves slowly light up. No bang, no smoke, no noise, allow to settle. Left hand in pocket, carefully touch the voltage points – all within range. And I do not die.
Downstairs, replace the Charlize with the Decware. Connect inputs, speakers, volume down. I have a TVC so planned to use the Decware as a power amp – ie at full volume, with music volume controlled by the TVC. Well music flowed, and sounded OK (a bit of burn-in is expected). But hum, increasing markedly as I turned up the volume (what's wrong - this is supposed to be a very quiet design for a tube amp).
Tried various arrangements of the earth wiring, shortened the transformer leads, put a grounded metal mesh shield on the input wires. None of this made any difference. Then noticed that if I connected the amp to the speakers (power disconnected) it was silent, but simply connecting my RCAs then gave that same hum (just quieter). The hum almost completely disappeared when I connected the power cable (with amp off ie just adding the earth) but reappeared amplified when the amp was powered up. In other words, the hum was not intrinsic to the amp, but was part of the input - presumably an earth loop, or some external contamination.
The FINAL internal wiring after earth revisions and shortening of the transformer output wires.
Anyway, running the amp with the TVC at full volume and using the amp's pot virtually eliminated the hum, and it was inaudible at listening position. Good. I can look at house earthing later.
A couple of days burn in, and the sound quality is jaw-droppingly good. The hi-fi words that spring to mind are 'transparency', 'imaging', 'relaxed', and 'speed'. The sound is nicely balanced across the frequency spectrum. To my slight surprise (given what I had read) Massive Attack and Garbage were portrayed with authority, and were very enjoyable. There was no noticeable euphony (that had plagued my big push-pull valve amp) and the sound was clean, clear, and not congested during complex music. The amp sounds great 15 seconds after power up. Only 1.8 watts? You must be joking, I can get the neighbours knocking if I want to! I am now a SET convert, and this little amp is here to stay.
Time for final touches. Colron Canadian Cedar wood dye finished the case off nicely, and some polishing to get my fingerprints off the top plate.
Cleaned up top plate
Valves, gently glowing!
So I'm very happy – but the final seasoning (yes, there's always something more!) is on its way in the form of a couple of v-caps. These are not TOO expensive at the values required for the signal caps, and I am mentally prepared for a rocky 400hrs burn in. (I had previously enjoyed Auricaps in the Charlize which took about 48h to settle. Teflon caps take much longer). I am happy to wait – this is my first serious taste of audio high-end, and it is very very enjoyable. Music – listening and playing - is one of my hobbies, and this little box has made a BIG difference.
The completed amp in it's final home - making music!
Yes I know the volume pot remains just a spindle, but I leave it set it to as high a level level as I can - where the hum/noise is not significant (about 11 o'clock with my speakers), and then use my Promitheus Transformer Volume Control to adjust the volume. This works just fine!
Two further modifications:
1) The V-Caps duly arrived, and have replaced the originals. A definite improvement out-of-the-box. A bit more tinkering: replaced the original mini-pot with an Alps Blue I had lying around.
2) As described on the Decware support forum, a further internal mod with a length of solid-core mains wire has now reduced hum to negligible levels. The circuit uses AC powered filaments which Are noisier than DC, but I am a critical listener and it doesn't bother me at all now. So I now use it as a full power-amp ie permanently at full volume. Several months on, I still have no desire to try anything new (all you hifi buffs will know how rarely a feeling of total satisfaction with your hifi lasts more than a month or so!).
Good though it is, this project turned out to be just the first dip of my toe in the water of Single Ended Triode amplifiers. There was talk of 45 tubes being "crack for audiophiles", and accounts of critical listening contests between legendary designs. Unable to resist, I plumped for George Anderson's Tubelab SE and built one for 45 tubes. Let's just say that my Decware is now gathering dust.
For me it stops with the TSE, and my now heavily tweaked Kirishima horns. Good luck to all you DIYers just starting on the trail.