"Mirage" - 6H3N-EB preamp

There are many strange and wonderful tubes you can build preamps and power amps with. When repairing an Asian made 300B I thought the driver tubes (6H3B) may have been faulty. Accidently I ordered two triplets of 6H3 tubes one triplet 6H3N-EB Russian tubes. The 6H3Ns ended up in a headphone amp and the "EB" tubes in this two-stage preamp. Thankfully both builds perform beautifully and are as pleasing to the eye as to the ear.

"Mirage" is another example where a solid plastic enclosure looks like a metal one. This trick I have performed a few times in the past with good results. Also to add to the look this one is sprayed two-tone in Epoxy Enamel. Inside there is a mix of carbon film resistors, metal film and wire wound. Caps are a mix of polies, X2, and low ESR electrolytic. Further HT filtering is carried out at the front-end. This preamp is dead quiet. But the interesting component is the output cap. This is a vintage Australian PMG made NP 1uF cap. The PMG is where I started work in 1970 and was Australia's fledgling telecommunication government owned company. Evidently, they made a lot of their own components and that's where I picked-up the rare cap fifty years ago.

The PS is external and there is another external box with the stepdown tranni and power filter. The PS is choked in a double Pi configuration (300uf of filter caps) and there is DC on the heaters. The AC PS plugs into the PS and the PS plugs into the preamp. Two inputs are provided and hookup wire is fine silver plated copper wire. The knob is custom-made of American Oak on a black Al base. 

The topology is simple plate follower front-end followed by an SRPP backend. This combines good gain with very low output impedance. The sound is very clear almost SS like. Bass extends to 30hZ and mids upfront and very clearly and very cleanly presented. I've heard greater transparence in my other tube preamps but not the clarity this preamp presents. From cheap tubes and a plastic enclosure comes a true treasure of a beautiful simple preamp. 

"Player" - MM phono stage

 

When Jaycar discontinued this MM phono kit, some years back, based on the LM833 chip, I bought the last six in Oz. This is the last from 2006. They come complete with standard parts but you provide your own enclosure and AC wall wart. To that end I had an extruded Hammond anodised Al enclosure and a 16VAC 1.5A wall wart. Perfect. 

Wiring is via silver plated fine copper wire (heavier for the power cct.) and WBT silver solder. Two regs and basic filtering on board ensures a dead quiet preamp, no hum, buzz or his. The chips are almost vintage LM833 which are slightly warmer than the OPA2134 (which can drop in) but provide excellent detail and good bass articulation. Most caps are MKTs known for the accuracy and stability. Resistors metal film. 

Because the phono stage has a gain control (second chip acts as a buffer with variable gain) "Player" can be connected direct to your power amp. And this is how I am listing to it. When I think my DIY EAR834p is the best tube MM phono stage I have heard this inexpensive preamp betters it! Transparence and a rich tone just locks you in to the vinyl and commands your attention. Very impressive, sad the kit is no longer available. 

"Vintage" - Radiotron 6SN7GTA preamp

 

I love the modern Chinese tubes especially the 6SN7 equivs. But this vintage Radiotron 6SN7GTA  is making me think again about vintage tubes. Very smooth and balanced. 

270V HT, 6.3V filament and 3.3V bias. The Alps blue velvet pot is on the output. PIO caps on the output and 100V NP for Ck. A mix of 1W carbon film and 1/2W metal Rs used throughout. Silver plated fine copper mil spec wire as hookup and WBT silver solder used throughout. Enclosure is cast Al with epoxy enamel paint. 

Two tone to match external PS which has triple Pi filtering for the HT and DC on the heaters. Hammond power tranni. Untra Fast soft recovery diodes and the filtration is fully snubbed with small polies. Glorious sounding and appears to allow each instrument and voice to have their own space. From such a simple design and simple parts a truly exceptional preamp. 

"Zoi" - EAR834p tube phono stage

 

After the sale of my latest SET amp to a local he came back to order a tube MM phono stage. The EAR834p kit was a top of the range selection with very high-end parts on a extremely well produced PCB. It comes partially assembled but to complete it I added the taller parts on the back of the PCB to allow the tubes to poke through the top. 

Because they run the tubes at 6V the current draw on filament is at an amp. I had to make a special external PS to handle the current. The PS get quite hot but the preamp runs dead quiet due to choked dual Pi filtering. A 5H choke and 400uf of high voltage caps insures that and heavily filtered filament DC.

Initially I thought it rivaled my EAR834p kit phono stage of a few years back (still in play) but after listening to both I would be happy with either. Though forced to take just one I would take the newer Zoi over my current unit. 

Looks great in Indian Dark Red and sounds big and full in sound. Excellent depth and really well presented detail with fine treble. Not the cheapest kit on the market and the sale price to the buyer was $710. I feel, based on the sound and quality of parts a fair price. I have thought about making one for myself, its that good. 

“Persuasion” - 5998/6AS7 SET

 

Finished yet another SET. Meant to be another 6AS7 SET but I setup the HT on a 5998. When I put the 6AS7 in all biases and HT was wrong. So this time its a 5998/12AU7 SET. I Prefer a 5998 anyhow.

Note: since posting this amp I have made changes to the PS section which now allows the use of either 5998 or 6AS7 tubes. Though not tested 6080 tubes can also be used. 

Hammond power Tranni, Edcor 10W premium OPTs sprayed black. Triple Pi filtration and SS rec. dead quiet PS. Last filter cap is a 4uf PIO. Ceramic sockets and all Cat5 solid core wired, no shielded cable, not required. The power tranni is isolation mounted on split rubber grommets. Mundorf Supreme interstage caps and NP Cks. 

I called the amp “Persuasion”. So far but very good sounding and excellent bass.

"Koto" - 12AU7/6AS7 SET amplifier

 

Still on a roll to use expensive parts, stored away for many years in store room, here is a 6AS7 SET amp. Its been many years since I've built one of these. I remember being impressed with my first which sold quickly. I had the Edcor power tranni, Edcor 1.7K OPTs and expensive Jensen copper, paper and oil inter-stage caps, new 5Y3GT rec. tube, base and ABS top plate plus and tube sockets. No excuse not to build a two-stage SET.

Apart from NP caps as cathode bi-pass and Cat5 wiring throughout there is nothing else exotic in parts. The PS once more is double Pi - CRCRC - with 5Y3 tube rectification and further filtering at the driver stage. 6AS7 is a well used RCA but a new Genalex Gold Lion 12AU7 was used as the driver tube. The sound is open, detailed with depth and wide sound stage. Bass extends well. Only a few watts but in my system enough. Also pictured here is my EL34/KT88 SE UL amp with KT88s installed. I could happily live with either. 

"Timbre" - UX-171A power amp

 

I was commissioned to build an amp based on 100 year old UX-171A (71A) tubes. Fortunately I had a few pairs and was able to match two. The driver tubes - 6SN7 - are Ozzy RCAs which I also had. The rectifier tube is an"80". Along with the power tranni, base and top plate, some caps, choke, ceramic sockets and OPTs (Edcor) I needed just a few minor components. 

The schematic I had only found a few days before the request to build the amp. The recipient was happy with all vintage tubes though I suggested, what I felt might be better, other driver tubes. Suited me, I had all the above parts and tubes for years and was looking for an excuse to use them.

Colour is Ocean Blue Hammertone epoxy enamel. Base is Australian Spotted Gum and heavy. Filter is double Pi - CLCRC - with further RC feeding the driver stage which is SRPP. 71As are DHT and the amp is a DHT SET producing about 1/2W. That's low power but the amp will be used with very high efficiency speakers. On my 92db speakers it played plenty loud enough for me. The "80" rec. tube, double filtering and 5H Hammond choke provide a dead quiet robust PS with 260uf of caps. Total current drain is low at 40mA. 1.5mV of noise measured in my system with all gear on. Nothing to hear at the speakers. 

The Hammons power tranni is way over spec but I had it on hand and time to put it to use. Inter-stage caps are Russian PIO copper jacket types which always sound good. The jacket was earthed. I used solder from the 60s and single strand copper wire to ensure a retro sound which the client was after. The top plate is fused and earthed. Sound is silky smooth (think low power 45 or 2A3) with exceptional bass, creamy smooth mids and clear highs. Highs are not over stated and I would say the amp is a little on the dark side providing a very relaxed presentation.