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

1966 Ampeg Reverbrocket 2

This was a nice project. Problem was it cut in and out. The screen grid resistor was fried, filter caps were going bad.

                        Burnt 470 ohm 2 watt resistor:

I always make it a point to replace these with 2 1k ohm 5 watters in parallel. A lot of current can be drawn on the screens of these tubes and I've seen this resistor burned up just due to that.

In this case it was the filter caps failing so this one got a full re-cap:

Grounded plug too. Always sand where you intend to ground to insure a good solder joint. I've seen many amateur grounded plug jobs gone wrong and the results can be disastrous!

This amp was re-covered at one point but the circuit had never been touched and the alnico CTS speaker is original. All dates are super close: 

 In combo amps I always pull the speaker to check for any tears and to remove any dust hanging around the edge of the cone. Good for avoiding that "it has a buzz" call after the customer has taken their amp home!

Unfortunately the reverb tank was dead so I replaced that as well. Amp sounds lovely now even with it's old Ampeg labeled 7591 tubes (test good but I recommend replacing them in this amp as they are showing their age! The Sovtek or Tung Sol version is quite nice. JJ? Risky. Amp is cathode or "self" biasing so you can just buy a matched pair and drop them in.) and gives up a nice even tone with my trusty Esquire. These amps tend to be nice jazzers for those of you who dig a hollow body guitar.

Saturday, January 10, 2015

1/10/15 Kalamazoo Reverb 12

It's been a minute since I've written here. I ran into some folks in Maryland at a wedding that read this blog and wanted more so I figure, well, give the people what they want!

A lot has happened in my world since the last blog post, lotta projects. I've been getting some good little jobs that require modifications. Amp behaves this way, can you make it behave another way. Here is one of them:

The Kalamazoo Reverb 12 or as I like to call it: the poor mans Princeton Reverb. The last cool amp I had on the bench was a Vox Cambridge 12 (I may write about that one soon) which, well, they aren't cheap any longer. They aren't as good as a Princeton but with some mods, those amps really open up.

This Kalamazoo was produced by Gibson in 1966. It's go a pair of EL-84 tubes producing about 15 watts going into a 10" CTS alnico speaker. I like these amps. They aren't built nearly as well as a Fender but who cares, they can be had for less than half a G and they make great studio amps or they are perfect for your singer songwriter gig at Pete's Candy.

The complaint with this one is the reverb kicks on way too strong and way too early. It also cuts in and out. Gibson reverb is like no other reverb. It's the wettest and wildest to begin with and this amp has a new MOD tank which makes it even wetter, very long decay. I dig it but I can see how it is hard to control. Quite literally, the mix is terrible, like 2 different amplifiers in one box competing for space. What I do like about these, and in a perfect world where I have my own studio, is you can turn the volume all the way down and turn the reverb up for pure reverb, so in that perfect world I would use it just for reverb and mic my other amp for the dry signal. Amazing.

So the objective is to get it balanced without losing its Gibsonee qualitees.

First thing: The reverb pot is 2 Megs! Get that out of there! I replaced it with a 250K audio pot for a nice smooth taper. I also added a .001 cap in series with the 150K resistor that feeds the reverb circuit after the first gain stage. Took some experimenting but this did the trick. Reverb is "wet enough", still nicer than a Fender reverb, but way more in control.

These amps with a Fender guitar can be very harsh too. I so want to open it up but the customer wants to keep it as is. Some simple tone stack mods would work coupled with a bit larger coupling caps in the phase inverter. I've done this before. He has a humbucker so it will be full and fine.

I also added a line out off the speaker. Very simple mod that requires a jack and 2 resistors: 220 ohm and 2.2k.

This amp runs exceptionally quiet. The Kalamzoos can be quite hummy. The filter caps were already replaced but even with that one still needs to address the filament. I've had success by lifting one side of the filament from ground and adding a hum balance. In this case another tech simply made the filaments DC. The job was sloppy so I cleaned it up but, the objective was more than achieved. This amp is quiet as a dead fish. I'll consider doing the same on the next one I get. it only requires a bridge rectifier and a 1000-4700 capacitor.

Anyway, that's it! Happy soldering in 2015. Feel free to ask any questions about this job.