"Ocean" - RCA 6SN7 preamp

 

Once more drawing from vintage tubes I've had for years here is another fabulous sounding super simple preamp. On a compact cast Al enclosure and sprayed Ocean Blue Hammertone with large vernier gold knob. Two inputs and external PS which uses electronic smoothing on the HT (DC heater voltage also) with also PIO output caps and carbon film resistors. Solder is WBT silver solder.

An alps pot controls level out as it is on the output rather than the input to ensure an extremely quiet preamp. Wiring is point-2-point and componenet-2-component wiring, no PCBs and no shielded wire to dull the sound. Signal wire is all silver plated, mill-spec. fine copper wire. Gold plated RCAs. 

I have stated on the past I'm a lover of modern high-end Chinese tubes but the last few preamps with vintage Radiotron and now RCA tubes has made me re-consider tube selection in the future. There is an abundance of detail, transparency, warmth and extreme naturalism in the last two builds not sure I have felt in newer expensive tubes. Another gem built mainly from parts on hand. 

"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. 

"Apparition" - 6H3N triode headphone amp

 

Having a small Hammond power tranni and three 6H3N triodes on hand I was looking for a build. Usually my headphone tube amps are based on the 6AS7 twin triode tube as power tube and 12AU7 as driver. But I stumbled on a schematic I had had for some years and it used two triodes in a HP amp. The backend is a White SRPP know for low output impedance. This HP amp was not designed for 6H3Ns but with some tweaking I could make it work. After the build voltages and biases were way out and it took four days of head scratching and tweaking to get it all balanced. Fortunately as a preamp or HP amp it is really excellent. Not sure why there isn't more designs around this inexpensive great sounding double triode. 

The chassis is cast Al with Ocean Blue Hammertone epoxy enamel double coat. All wiring is P2P or Component-2-component and hookup wire is mostly fine copper wire with a silver plate. Not shielded wire is used. The PS is triple Pi with 300uf of caps (CRCRCRC) and a further smoothing to the driver tube with another smaller storage cap and resistor. The amp is absolutely dead quiet though AC feeds the heaters and no shielded wire is used. The chassis is earthed and mains fused. Caps are polies and low ESR electrolytics. The power tranni is mounted on split rubber grommets to isolated any vibration from tubes. 

Elna caps are used on the output and the two stages are direct linked. Soldering in the audio path is with BWT silver based solder. Resistors are mainly metal film with some 1W carbon and 5W wire wound where required. Pot is an Alps blue velvet with 4oz solid brass gold plated knob. All caps in the PS and in the audio path are snubbed with 0.1uf polies making for a very clean sonic presentation. 

As a preamp playing through my KT88 SE UL tube amp "Apparition" exhibited extreme detail with very extended reverb tails. The longest I have heard anywhere. To me this says it is bringing forth every nuance of detail even at the extreme low level of volume. Stereo image is wide with all instruments well placed and standalone. Central image is well defined and feels deep. Bass is good down to 30Hz in my system. On headphones Apparition would keep up or exceed the best I have made to date and no problems driving 250ohm phones. I'm inspired to build a dedicated preamp with this tube of which I have another quad matched set. 

"Palladom" - high performance F5

 

Palladom start life as a Turbo F5 build for another. But after some months he was not happy with the sound and offered to sell back the enclosure and PS. The PS is extreme with 2X 300VA double secondary windings trannies, 16X 15A double diode bridges all with heatsinks and 192,000uf of  capacitance. The only part I had to replace were the amp boards. I used the DIYAudio F5 boards with Dale resistors and matched FET pairs.

Easy to see the PS in a 5U enclosure is way over-speced. Fortunately it produced an extremely good sounding example of a high-performance F5. Being able to push the current up this amp will produce 32Watts of Class A and about 50W of Class B. In the image above the yellow alligator leads were used temporarily to establish the right earthing setup. Because there is so much heatsink area the amp gets just warm. Very heavy though and now sold.

"Thirty-four" - EL34 SE UL amp

 

Again using up expensive parts on hand another SE UL tube amp. This time using a tweaked schematic which favors EL34s. I have a number of brands of EL34s on hand including:- Svetalan, Siemens, Black Sable, Tung-Sol and Electro Harmonix. Some quad matched sets like the Tang-Sol and Black Sables. With a mix of carbon and metal film resistors, mustard caps as snubbers on Ck and Jensen copper foil paper and oil inter-stage caps this is a real mixed build. Once more the whole idea is to use up what is on hand. KT88s can also be rolled for a different sonic presentation.

On hand I had the big Hammond 600V 200mA power tranni, open frame Edcor OPTs (under the bonnet), the wood base and top plate, inter-stage caps most resistors, ceramic and resin tube sockets, tubes and all components for the SS rectification cct. and smoothing caps. The PS is a triple Pi config but this time no choke. CRCRCRC with 100uf caps and 330ohm 5W Rs has produced an extremely quiet amp with only 0.9mV of noise. Total caps is 750uf.

The base is Australian Spotted Gum and after being sanded smooth and oiled was re-prepped with "0000" steel wool and re-oiled. The timber looks rich and is extremely smooth. The top plate is sprayed with Hammertone epoxy enamel after being sanded and etch primed. The finish looks lush and the Hammertone hides any dust particles or blemishes. The top plate is 2mm Al and the Tranni is isolation mounted to it to reduce vibration to the tubes. 

Cat5 solid core wire for the audio path and HT is delivered with either twisted pairs of Cat5 or heavier wire. There is no shielded wire and the audio is delivered to the 6N1P via Cat5 twisted pair. Where I found the sound of the KT88 amp - Toccata - very open with a huge sound stage this amp sounds a solid and grounded again with a large sound stage. Once more a very transparent earthy presentation and now with the Siemens tubes in place a cleaner open sound. Bass is full on and extends down to 30hZ.

"Toccata" - KT88 SE UL amplifier

This simple two stage power amp was built from 95% parts on hand. I had all the expensive parts, tubes, OPTs, power tranni, base etc. Inter-stage caps also on hand and are Russian PIO greens. I have used these in so many builds and always sound excellent. Only a handful of cheap components required purchase. Most of the parts on-hand I've had for nearly twenty years. Time to put them to good use and I haven't made a tube power for a while. Included in the parts I have had for a while were a pair of mustard caps (0.1uf) which I used as bypass caps (across Ck) on the 6N1P driver tube.

The power tube in the image is a JJ Blue Glass KT88 of which I have had - and never really used - for a long while. I bought them as a quad matched set for a PP amp I had years ago. The power tube can be swapped out for 6L6, EL34 and others with no bias tweak. The 6N1P is Silvania. Power tranni and choke are Hammond and OPTs Edcor. Ceramic sockets. Base is Australia Spotted Gum rubbed with a finishing oil and top plate paint Epoxy Enamel. Top plate is 2mm Al and the tranni is isolation mounted to reduce HF vibration to the tubes.

As is my usual practice there is no PCB or shielded wire. Cat5 solid core used throughout. Most P2P wiring uses a large tag brd. with components aboard then wired P2P from the brd. This results in a lot of hookup wire. A better solution is small 5 tag tag brds at each tube, two brds. each then mount components to them. Far less hookup wire, parts are right at the tube.

I'd built a SS rec. brd. with two SF 3A diodes and 150uf of 400V filtering a long way back. Using it here with a 5H choke and another 470uf in double Pi configuration (CLCRC) resulted in a dead quiet robust PS. Noise at the speaker post is 0V with the meter on the 200mV scale. That's quiet!

The amp only produces 3.5W RMS into eight ohms but sounds louder on my 90db+ sensitive speakers. Bass tests down to 20hZ and upper frequency is 70KhZ. Bass in my room is heavier and deeper than my DIY SS amps and there is a real sparkle at the top end. Best I've heard so far. Very transparent amp to listen to. Gain is 15db low for a power amp but only requires 300mV for full output.