You can build this amp from the kit available here.

This amp has been in my head for years. In the heyday of the old Boozhound Labs blog, when I was building a ton of audio gear and was super into tubes, an amp using this basic topology — transformer coupled push-pull 6c45s — was the last and best tube amp I built. It incorporates all of the things I strive for — it’s simple, uses capacitors super sparingly, can be built inexpensively, and sounds amazing. I’m all about bang for the buck, and I think this design strikes a great balance, while being very upgradable for folks who want to take it to the next level.

Circuit Design

The core of this design is a push-pull pair of 6c45pi tubes. These tubes have a very unusual combination of plate dissipation and gain that allows them to be used without a driver tube. This saves a ton of cost and complexity.

I love designs that use interstage transformers. Most designs use a tube based phase inverter that introduces a lot of compromise. In this case the transformer is both the input transformer and the phase inverter. This has lots of advantages — great balance, the tubes see a very low grid impedance, you can choose the amount of stepup, and distortion should be super low. With inexpensive Edcor transformers, the main disadvantage of cost is removed.

I am shooting for an operating point around 175V and 30mA per tube, biased at -2V. This exceeds the datasheet maximum plate voltage of 150V, but these tubes are extremely robust so they will be fine. The maximum plate dissipation is 7.8W and many folks have run them much hotter, but this operating point is dissipating around 5W each so they are not working too hard. 6W is a good rough estimate for power output. The transformers specified have both 4 ohm and 8 ohm taps. 8k primary impedance is ideal for this operating point.

The power supply is super simple. The version of this amp I built back in the day had an absurldly fancy power supply with a current source, a big inductor, and and lots of voltage regulator tubes. One easy upgrade to this power supply would be to replace R10 with a chassis mounted inductor.

So what is up with that 10uF cap? There is some very cool info about this topology at Lynn Olsen’s Nutshell Hi Fi site. Checkout his article on Western Electric circuits and his talk on loop distortion. These articles were the genesis for this design. Basically the 10uF cap, connected to an unbypassed cathode resistor, reduces the number of components through which the output signal passes, and provides a negative feedback effect to any imbalance between tubes due to one tube clipping before the other.

So let’s build it!

Choosing transformers

Output transformers — You want a push-pull transformer with an 8k ohm primary winding with a center tap. This presents each tube with a 4k load. You want at least 5–10W of power handling, but more is better. I recommend the Edcor CXPP10–8k (http://www.edcorusa.com/cxpp10-8k). Transformers with 10k primaries are super common and will also work, possibly with slightly less output power and slightly less distortion. There are a lot of old vintage transformers that meet this spec so feel free to scrounge something. Check the frequency response for a good indication of the quality of an old transformer. This is where you can spend as little or as much as you want, from a tiny cheap open frame guitar amp transformer, to a huge potted milspec vintage beast. The Edcors are ~$50 each.

Power Transformer — you need a high voltage winding of 250V with a center tap and at least 125mA of current capacity, and a 6.3V filament winding that can supply at least 2A. I recommend the Edcor XPWR104 (http://www.edcorusa.com/xpwr104). This is available in 120 and 240VAC primary so it can be used worldwide. The Edcor is ~$50. Feel free to build monoblocks with one transformer per channel! There is no downside to using a trasnsformer with a higher current rating, so use whatever you can find. Note that some manufacturers spec their transformers with a primary voltage of 115VAC instead of the actual 120VAC so the secondaries are all a bit higher. You can correct for this or just live with the slightly higher voltages. This is why many builders are surprised by a higher than expected output from Hammond transformers.

Input/phase inverter — Edcor PCW Series matching transformers (http://www.edcorusa.com/pcw_series). You need 2 of these. They come in many different windings for matching various impedances. To drive the 6c45 tubes to clipping you need ~4 volts of swing, so I recommend using the 600:10k version. If you are building this balanced and have a full pro level source, go with the 600:600 version. The best part about these is how cheap they are at~$12 each.

These transformers mount directly to the circuit board but if you need to use something else you can run wires from the board to the transformer. Not super clean, but definitely doable.

Many line transformers are described using impedances which is super confusing. I prefer to think of them in terms of voltage ratio. The voltage ratio is directly related to the ratio of the windings. The voltage ratio is the square root of the impedance ratio. So a 600:10k transformer is 1:4. For this circuit, using a 1:4 aka 600:10k transformer you will need just over 1Vrms of input to get the full output of the amp. This is a sensitivity close to most commercial amplifiers which makes integration easier.

6c45pi Tubes — You need 4. Matched is good but not required. New Sensor/Sovtek makes these so they are easy to find. These are relatively modern tubes and most of the “vintage” examples were made in the 80s and they are all Russian as far as I know so no need to shop for anything crazy. Expect to pay $30-$60 each.

Chassis

There are many ways to build this into a chassis, but I love the simplicity of a single aluminum plate and a wooden base. I don’t have a ton of patience for fine woodworking, or finishing details, so I am drawn to easy construction techniques. Having seen a million beautifully built amps, I know that there are some extremely skilled craftspeople out there, so if that is your thing go all out!

Start by laying out all the parts you will be installing. Once you have it looking good, mark the holes you will need to drill. For the circuit board, it is easiest to lay it straight on the chassis and mark the holes directly. Less error-prone than measuring and transferring the measurements.

To find the center of the tube holes, mark 2 pairs of opposing pin holes, then connect the dots to make an X centered on the hole you will need to drill for the tube socket hole.

You can use speaker terminals that mount to the metal top plate, or that mount through the wooden base. I have done both. It is nice to have everything on the top plate, but it is a bit more work sometimes. I like to use brass thumbscrews for speaker terminals if I am mounting them through the wooden base.

The amp in the pics was built on a 1/8" thick aluminum plate. This was thicker than needed and made all the hole drilling more work than it needed to be, plus the tube sockets are a little bit more recessed than I would have liked. The 12" by 12" plate from Amazon was the perfect size though. It’s nice not having to cut it to size! I have cut aluminum plate with a tablesaw or a circular saw with a standard carbide blade, but that can be dangerous and is definitely not recommended.

I think it looks really cool to have an IEC power inlet attached to the top plate that you plug into the wall via a right angle plug/cable. But that does take a lot of top plate real estate and requires making square hole, which is a pain in the ass. I wish there was a good IEC inlet that mounts in a round hole. Ideally with a fuse and filter included. Holy crap — maybe I should design one.

Once you have all the holes marked, I like to center punch everything using a spring loaded center punch. This helps keep the holes in the right place despite the drill bit wanting to “walk”. I then drill all the holes using a small 1/8" drill bit. I use a simple hand drill. I don’t find a drill press worth the extra effort. Once you have all the small holes, I like to drill the bigger holes in multiple steps using progressively larger bits. The bigger holes can be drilled using a step bit. I have also used Greenlee style hole punches on aluminum, but those work best on steel. If you are using Octals or bigger that is definitely the best option. Speaking of steel, a steel Hammond chassis would be a great option for this amp.

The standoffs that come in the kit work well with 1/8" holes. That will give you a little bit of play in case things aren’t perfect. The standoffs are 3/8" and they cause the tube sockets to be at exactly the right height.

Soldering

Side note on solder: I use traditional flux core lead/tin solder. It melts and flows the best, and is easiest to get great solder joints with. I use a big spool of the thin stuff with a medium tip soldering iron. I use a digital thermostatically controlled solder station set to 650F. For bigger solder joints I sometimes use a trigger grip Weller Jr. iron. Lead is super bad for you. I am of the controversial opinion that the smoke you inhale doesn’t contain any actual lead. But I am sure to wash my hands after soldering (with soap and cold water to not open up the pores) and I don’t eat or pick my nose while soldering :) I have used 5% silver solder and it is great but harder to work with. Use whatever you prefer.

The boards are lead-free.

If you are storing the boards for months before building your kit, keep them in a ziplock bag, ideally with a silca gel pack. I store boards in a sealed plastic box with a huge sica gel pack. The solder pads can oxidize which makes soldering much more difficult. If you have oxidized parts, hit them with sandpaper or a scotch-brite pad, and/or dab a tiny bit or paste flux on them before soldering. I never recommend pre-tinning unless the parts are large and really oxidized.

Start by soldering the sockets — remember that they need to be soldered to the opposite side of the board from the rest of the components! Be sure the socket pins are fully seated against the board. they pins have a shoulder that should rest against eh surface of the board. This is both an electrical and a mechanical connection, so be sure to fill the holes completely with solder so things are nice and solid.

You can install the standoffs to make the board sit flat and not wobble, or use a circuit board vice. I like to use a headlamp to reduce eyestrain. I haven’t used magnification yet, but I’m getting there.

Be sure to ground the board to the chassis. Use a fuse at the power input.

Twist all wires containing AC. Try to keep signal and power wires separate and cross them at right angles.

My theory with the filaments is that you should wire them out of phase so that any noise coupled into the tube will be cancelled at the output.

The input transformer allows balanced operation. For single-ended input, connect the negative and ground terminals on the 3-pin input terminal. A simple wire jumper is all you need.

Board connections

B+ connects to the center tap of the output transformer

Plates 1 and 2 connect to the plate leads on the output transformer.

Terminals labeled “fil” are the filaments of the tubes, and should connect to the filament winding of the power transformer.

The ground symbol next to the B+ should connect to the chassis.

The High voltage winding of the power transformer connects to the header with a ground symbol and 2 AC sine wave symbols. The ground terminal connects to the center tap. The AC leads connect to the AC pins.

There you have it! Not too bad, right?

One cool thing about the way this amp came together is that you can easily get to the bottom of the circuit board if anything needs to be replaced or upgraded or whatever. I discovered this when I accidentally installed 10 ohm resistors in the power supply pi filter instead of 100 ohm. 10 ohms is not enough filtering, and hums noticeably. 100 ohms is quiet. You could try going lower than 100, but I’m going to leave it 100 ohms in the manual because it is plenty quiet.

Break-in is real. It makes me mad because it doesn’t make any sense that electronics could possibly take a little bit of playing to sound good, but there it is. When I first played this amp I was worried that something was causing a low end roll off, but the bass came in after maybe 20 minutes. It’s definitely a tube amp though. If your system is light on the low end, make sure you have nice fat speaker cables, it makes a big difference, especially in the low midrange. I have been breaking the amp in playing my AR4 bookshelf 2 ways with am Alexa Dot as the source. It sounds surprisingly great. Plenty of low end with the speakers against the wall. Way more of a 3D soundstage than I would have predicted. Plenty of power, and nice compression at clipping.

Radio Shack used to sell my favorite toggle switch, but my local Radio Shack closed, so I’m still looking for something great from Mouser or another source. If you have a favorite toggle switch, please get in touch.

Cost

My goal was to offer a kit that would allow folks to build a great amp for a reasonable amount of money. Super high value. Bang for the buck.

The kit from BHL is $149. This includes everything to build the boards for 2 channels — all the resistors, caps, etc., tube sockets, and standoffs (not the input transformers). The transformers from Edcor as shown above will run about $175. Tubes will cost about $120. This is $444 all in. As always, the transformers are the most expensive part, so scrounging from used gear, trolling eBay or hitting up your local old school ham radio nerds could get you a very inexpensive amp.

I considered selling this as a full kit with transformers and everything, but it’s so much more cost effective for folks to source their own transformers and build their own chassis it is just not worth it.

Thanks!

If you are feeling inspired and want to build one of your own, please visit boozhoundlabs.com.