Sunday, January 11, 2015

My single ended 45 stereoscopic amp!

 This winter for the first time in a long time I built myself a stereo. For a while I was building one or two a year, experimenting with different transformers and caps etc, but then I moved to the future and simply couldn't imagine taking along a record collection and a pile of heavy tube amplifiers.

But I'm in a different place now, envisioning my own homestead where I enjoy vinyl again. So I built this little amp. A single ended 45 stereo amp that wields a whopping 2 watts per channel.

  Why so little power? I get asked that. Well, for me I don't need any more than that. I live in a small room in NYC. I'm using maybe 1/4 of a watt to fill it up with beautiful sound. I also like the philosophy that it's all about that first watt. If your first watt is great.....if it's crappy and you have another 999 watts after that well, you'll have a lot of bad sound.

  I like triodes. The 45 tubes pictured above (the ones that look like balloons) were made by RCA Cunningham in the 1920's. They still make music. I like single triodes because the simpler the circuit, the more music I get out of it. I first became hip to this technology when I worked for the Melkisethians at Angela Instruments back in the early 90's. He was producing his "model 91" amp with a single 300B tube creating 8 watts of power. Very lovely sound. But I became obsessed with 'flea power', 3.5 watts and under. I like the 45 tube also because it's honest. It doesn't glorify anything. If your recording sucks it will tell you the truth. If it's good, you will know that too. Plus, the 45 tube is available. You can still find NOS bottles for less than a C-note on Ebay. You cannot find NOS 300B tubes for less than $700 a bottle and NOS 2A3 tubes are quite spendy as well. Why NOS? They are the bomb, that's why. Made during the height of American manufacturing to very high standards. Like I said, these tubes are approaching 90 years old. No noise, all music. For modern tubes I do like the Chinese Sino 2A3. You would need to audition a few to get ones that don't have bugs but they sound nice when you get a good pair. I haven't tried the many fancy boutique ones available. Too much dough for me to justify with all the good old glass still available but if someone cares to send me a set, I'll be glad to check them out and write a review!

 I chose to use the 5963/6SJ7 pentode as a driver. I like this combo. It's fast and accurate. Having a triode driving an triode is nice but a bit too lazy for me. My former favorite amp was a single 45 driven by a 24A tube. Those were sold to a nice fellow in Portland.

  When you build an amp like this it is easy to get obsessive over the passive components such as caps, resistors etc. The circuit is so simple that the slightest change makes a noticeable difference. I chose to use nothing particularly special, no fancy expensive filter caps and just a good old PS series Orange Drop cap for coupling (not the harsh sounding 715 series). I like these caps in power sections of amplifiers. Warm but detailed. You can get them at Antique Electronic Supply. For resistors I used 2 watt carbon composition throughout, available thought Mojo Musical Supply, and for the filter caps I used those Marshall LCR type caps, just the cheap ones available at Angela Instruments. The power transformer is actually made for an 18 watt Marshall, just a cheap but very nice part:
And for output transformers I used some nice potted 8 watters available through Raphelite.

Frankly, if you wish to use ultra expensive components go for it. There is a lot of jive out there in the audio world. I do like paper in oil caps and my preamp is full of them, but I'm quite satisfied with what I did use in this amp.

  The fun part of building an amp is playing with the layout.....


  There are a lot of ways to go, I like something that is pleasing to the eye. Just industrial enough but not one bit more. When I first got into this nobody I knew of was making pre-painted chassis boxes. Thank goodness Hammond makes them now! My finishing skills suck and I don't have a place to do the old spray paint without making everyone around me sick and annoyed. So this amp is my best looking yet. I will pull the power transformer and paint the bells when the weather gets nice enough.

  Initially I was going to put the filter cans on top but found I had enough real estate to put them below. I like the streamlined appearance of that. Amazing what I could fit under the hood, the box is only 12"x8"x2".

  I also always use brass screws. Hey, they did that in the 30's and I'm going for a vibe here!

 Here is the circuit. The only difference is I decided not to use the 6C6 tube, I subbed it with the 6SJ7.

Also since it's a 45 tube the cathode resistor is 1500 ohms and the output transformer has a 5k primary.

  You can build these with your choice of driver tube. 6AU6, EF86, they will work fine. I like the bigger tubes myself, could be the good old placebo effect but I find them to be warmer.

One of the best things about having built so many amps is, my skills keep getting better. When I first fire up an amp I'm not afraid of it not working or some sparks flying, I am afraid of ground loop hum though. This one from the word go is quiet as a dead fish. I'm proud of that.

Grounding is a real mystery. Proper grounding that is. Electricity is a mystery. No one can really tell us exactly how it works. So, I'll just write a bit about what works for me. I've tried star grounding, aluminum chassis, using  a ground bus etc with varying results. I've always had better results with a steel chassis. Notice how the power transformer is rotated 90 degrees away from the rest. I learned how important that is the hard way.

I once was working on an old Eico HF-87 in Portland. I was surprised at how random the grounding seemed. Everything was grounded to the chassis. And the amp was of course, silent. I was struggling with getting my amps so quiet and happened to have a valuable resource nearby named Allen Garren. The way he put it: "grounding is a relative thing from input to output".

So that is how I treat my amps. All the audio grounds are grounded to the input jack. I used 16 gauge copper wire as a ground bus, the 100k pot is the first thing to be grounded to the audio line, the negative side of the transformer secondary is the last. All of the power supply filter caps are grounded through a bus wire separately to a spot near the rectifier tube, again in order. The power transformer center tap and the ground from the power cable is grounded there as well. I've never been able to get as quiet an amp using the star ground method so perhaps I was doing something wrong? This works for me.

  So Jef, what speakers are you using? When I have the big pile of cash I asked for I will probably get a pair of Lowthers. For a flea power amp it is best to use something really efficient, 100db of sound for that first glorious watt. That being said, I'm running my trusty slightly modded old NS-10 speakers by Yamaha. You know, the ones every studio has and every studio hates. "These are harsh and have no low end but gotta have them cause they are flat!" said the engineer in just about a million recording studios across the planet. For some odd reason, I'm getting this beautiful, liquid sound with more bass than I could have imagined. For my little room, these are perfect. Whether it be Black Sabbath, Billie Holiday or Mozart, this setup is keeping me happy. And, listening to my mixes, I gotta say I'm proud of my engineering ability.

  Would I consider building one of these for you? Yup. Contact me.

Peace, JB


  1. Thank you so much for your post. This post really help me a lot and I have learnt some new things from your tube amplifier