So for this batch of kits I had a hell of a time sourcing all the parts. Both the transistors and the capacitors appear not to be as plentiful as they once were - or as inexpensive. For this reason I have raised the price by $10 to $89. Hopefully still a great deal.
See links at top of page for more information on this kit.
2/22/12
BHL Buffer
I have had a couple requests for a preamp circuit to turn the phono stage into a full preamp. I have also wanted to design an active crossover, both as a kit and for my own projects. Both of those things would incorporate a few (or lots of) buffer stages. A buffer is basically a circuit that presents a high impedance load to the source feeding it, and a low impedance load to whatever it is driving, with no gain.
Here is what I have in mind. It would use the same JFETS and Russian caps as the phono stage. As usual, it uses the minimum number of parts required to implement the circuit. The power supply requirements would be the same as the phono stage - something around 12-24VDC.
One of these for each channel, plus an input selector and a volume pot would make a great preamp. One of these feeding a set of caps and resistors, followed by a couple output buffers would make a great active crossover. You could add one to a phono stage to help drive long cables. Lots of options.
If this sounds like something you would be interested in, please leave a comment on this post. Price would be around $30-$40 each (that's for one channel, use 2 for stereo).
Here is what I have in mind. It would use the same JFETS and Russian caps as the phono stage. As usual, it uses the minimum number of parts required to implement the circuit. The power supply requirements would be the same as the phono stage - something around 12-24VDC.
One of these for each channel, plus an input selector and a volume pot would make a great preamp. One of these feeding a set of caps and resistors, followed by a couple output buffers would make a great active crossover. You could add one to a phono stage to help drive long cables. Lots of options.
If this sounds like something you would be interested in, please leave a comment on this post. Price would be around $30-$40 each (that's for one channel, use 2 for stereo).
2/9/12
DIY Unipivot Tonearm
My first shot at a DIY tonearm. Well, actually the second shot - I built a Ladegaard air bearing tonearm, but I made too many changes to the design and as a result, it didn't work at all. So I thought I would get my feet back under me by building a super-simple unipivot arm.
Yes, that is a penny. A penny seemed like an easier and more ironic solution than cutting a very small piece of aluminum for the headshell mount. I didn't have much luck with making the headshell angle adjustable, so I picked an angle that looked right and glued it. I figure I can make the holes into slots with a file if I need to adjust it.
I really like this Rek-O-Kut. It is an idler wheel design, and some of the nicer idler turntables are attaining cult status. I hope that with a nice massive plinth, this one will sound good. That big cast aluminum washing machine knob is just so awesome. The motor and bearing seem really solid, but there is a bit of mechanical noise that can be heard by putting your ear to the base while the platter is turning. I can't imagine an idler drive being dead quiet.
There is no anti-skating yet, so the sound is quite skewed. The wiring is that Radio Shack Kynar insulated wirewrap stuff.
Patented Boozhound Laboratories liquid damping system.
The pivot is made by drilling a hole through the arm from the bottom, and then using a small bit to make a little dimple in which the metal point sits. Drilling a hole through the arm and locating the pivot on the upper inside wall of the tube allows the center of gravity to be further below the pivot point, and also makes the arm slightly more difficult to knock off the pivot.
Yes, that is a penny. A penny seemed like an easier and more ironic solution than cutting a very small piece of aluminum for the headshell mount. I didn't have much luck with making the headshell angle adjustable, so I picked an angle that looked right and glued it. I figure I can make the holes into slots with a file if I need to adjust it.
I really like this Rek-O-Kut. It is an idler wheel design, and some of the nicer idler turntables are attaining cult status. I hope that with a nice massive plinth, this one will sound good. That big cast aluminum washing machine knob is just so awesome. The motor and bearing seem really solid, but there is a bit of mechanical noise that can be heard by putting your ear to the base while the platter is turning. I can't imagine an idler drive being dead quiet.
There is no anti-skating yet, so the sound is quite skewed. The wiring is that Radio Shack Kynar insulated wirewrap stuff.
Patented Boozhound Laboratories liquid damping system.
The pivot is made by drilling a hole through the arm from the bottom, and then using a small bit to make a little dimple in which the metal point sits. Drilling a hole through the arm and locating the pivot on the upper inside wall of the tube allows the center of gravity to be further below the pivot point, and also makes the arm slightly more difficult to knock off the pivot.
BHL-15 guitar amp
Oh great, yet another guitar amp schematic. Just what the world needs.
While this isn't exactly earth shattering, I think it addresses one of the major problems with guitar amplifier design in an elegant way.
Most guitar amp designs are direct descendants of hi fi designs of the 40s and 50s. The earliest amps that we love so much were taken straight from the recommended operation points of the tubes in question. This is a good thing, because the circuits are straightforward, and do not require any drastically unique engineering to make them sound good with guitar. It is also a bad thing because there are a few small differences between a guitar and a record player for example. The primary difference between a hi fi amplifier and a guitar amplifier is that a guitar amp spends a great deal of time heavily distorted, and must sound good when distorted. This sounds like an oxymoron - good distortion - but for those of us who have embraced the vacuum tube as an audio device, it has become clear that it is not the amount of distorion that causes an amp to sound good or bad, but the kind of distortion.
In Hi Fi circles good distortion tends to mean low order distortion, which is distortion that is only a few harmonics away from the fundamental tone, and even order distortion, which tends to sound more "euphonic" (a classic hi fi mojo buzzword) because it is a simple doubling or halving of the fundamental tone and therefore creates less dissonance. For guitar, this does not appear to be as well defined. Guitarists like many different tones, and at least 2 distinct kinds of distortion. Many players like Fender Champ style single ended amplifiers because of thier rich blues tone. Single ended amplifiers are heavy in even order distortion, especially second order. This is the nature of a single ended design. Some guitarists find this preponderance of second order distortion somewhat muddying to the sound. A guitar sound heavy in second order distortion can lack definition of the individual notes, and have a hard time cutting through the other sounds in a performance. These players tend to like push-pull amplifier designs. In a push pull amplifier, the signal from 2 opposing tubes is summed in the output transformer. This summing cancels out any even order distortion, so a push-pull amp should have primarily odd order distortion. Many guitarists prefer this sound.
The effect that this design attempts to address is that when you drive a tube way into distortion, the grid can swing positive, and grid current will flow. Most guitar amplifiers have a low current 12ax7 driving the grid of the output tubes, and at 1mA of current, and very high plate resistance, the 12ax7 is just not equipped to deliver any current whatsoever to the output tube's grid. As the output tubes require grid current that isn't available, distortion increases dramatically. As a result, the amplifier might sound good at a particular volume level, but the range of volumes at which you get desirable distortion is very narrow.
So what we need is a beefier driver tube. Something with lower plate resistance that idles at higher current, and can therefore deliver more current to the output tube when necessary. There are 2 ways to do this that come to mind:
1) Cathode followers. A cathode follower is a common solution for situations where you need a lower impedance output. This is commonly used in preamps to drive long interconnects for example. Many Marshall/Bassman style amps use a cathode follower to drive a complex tone stack. In this case, you would insert the cathode follower between the phase inverter and the power tubes. The only problem with a cathode follow is that they sound like crap. As a general rule of thumb, no hi fi amp or preamp with a cathode follower in it will sound good. There are exceptions, and in fact many legendary guitar amps use cathode followers to drive the tone stack. I even admit to creating a few designs that use cathode followers. To drive the output tubes, we need a better approach.
2) An additional driver stage between the phase inverter and the output pair. Keep the phase inverter the same, but use it to drive a higher current tube, which in turn drives the output tube. This is like seperating the tasks of phase inversion and driver into two tubes, each optimized for the specific funtion. This is a good strategy, and has been used by my guitar amp guru friend Woody with much success. There are 2 things that I don't totally like about this design, and neither of them are true criticisms, becuase they may not actually affect the sound in the end. I don't like the idea of having to add an additional tube just to drive the outputs. The fewer stages the better. The fewer tubes the better. This is my minimalist side. The second thing I don't totally like is that the signal passes through 2 stages before being summed. I think it is a good idea to sum the signal as soon or often as possible after splitting it in the phase inverter. Every gain stage where distortion is added to the signal whle it is split causes differences between the signals that will either be amplified or cancelled when it is eventually summed, and that leads not only to distortion, but the loss of detail.
So not totally buying into either of these solutions, I went in search of a way to cheat. I wanted a high current driver, but I wanted that tube to be the phase inverter too. So I played with building a long tail pair phase inverter (the most balanced phase inverter during heavy distortion, and therefore most liked by Woody, and I trust him) that used a higher current tube than a 12ax7. I like octals, so I chose a 6sn7 (which is quite a legendary hi fi tube and sounds great in guitar amps). The problem with implementing a high current long tail pair is that the balance of the pair relies on a large shared cathode resistor. Additionally, to get a good amount of gain and linearity, you need a fairly large plate resistor. Ohms law begins to cause problems for you when you start trying to use large resistors with lots of current flowing through them. You begin to drop large amounts of voltage across those resistors, to the point that your B+ supply isn't enough. So you are forced to compromise by using smaller than desired plate and cathode resistors and hurting the linearity of the phase inverter.
Short rant: Isn't engineering great - a solution always raises other problems. I get so pissed when I see ads that promote "uncompromising" amplifier designs. In my experience there is no variable for cost in Ohm's law. These companies would have you believe that there is a little known correlary that states that R = V/I$. As if when you use very very expensive resistors, you can bend the rules. There is always a compromise. The whole point of engineering is getting the job done with a solution that makes compromises you can live with. Tubes designed in the 30s and 40s are always going to sound better than later tubes because those tubes were designed to sound good. Period. They were also a tremdous pain in the ass to use. Check out old Western Electric theater amps to see what I mean. Later tubes were designed to sound good and be smaller. Or to sound good enough, once you add a bunch of feedback. Good with feedback will never sound as good as just plain good. End rant.
Back to the design - what we need is a device that has relatively low DC resistance so that it won't drop a lot of voltage, but high AC impedance so that it will provide a substantial load for the cathodes of the phase inverter, and give us the linearity we want. One such device is the poor misunderstood choke (inductor). Chokes are the first component eliminated from a design to reduce cost because a resistor is much cheaper than 50 feet of wire wrapped around a hunk of steel. Another Hi Fi truism is that anytime you replace a resistor with a choke, the sound gets better. Ideally, we would replace the cathode resistor and both plate resistors with chokes. Maybe even on all stages of the amp. But that would be a very expensive solution, and have other problems, like picking up hum in the hundreds of feet of wire wrapped around the hundreds of pounds of steel :)
So this amp simply replaces the cathode resistor with a choke, to allow the use of a high current 6sn7 as a phase inverter/driver for the output pair of 6v6s. This high(er) current driver allows more current to be deliverded to the grids of the output tubes, extending the sweet sopt well into the region where grid current flows. Phase inverters tend to have neat names: cathodyne, long tail pair, concertina. I nominate this phase inverter be the "choke tail" phase inverter.
The phase inverter no longer looks like a long tail pair because the increased AC impedance of the choke makes the "tail" unnnecessary. The bias voltage is generated by the DC drop across the choke, but the large AC impedance typically generated across the cathode bias resistor plus the large tail resistor is generated across the choke as well. A side benefit is that we get to lose the capacitor that grounds the undriven grid in a long tail pair. Removing capacitors is a good thing in Hi Fi, so why not in guitar amps too.
The rest of the design is pretty conventional. Gain stage -> tone/volume -> gain stage -> phase inverter -> output pair. I am using a "moonlight" tone stack, which is a signle control low loss tone control that I think sounds nice. I am building this using all octal tubes, so we end up with a 6sl7, a 6sn7, a pair of 6v6s, and a 5y3 rectifier. You could replace the 6sl7 with a 12ax7 and the 6sn7 with a 12au7 if you like. You would get a brighter tone, a bit more gain, and smaller tubes :)
I implemented this circuit in a Fender Pro Junior. The chassis, cabinet, and transformer make a nice compact package, and once you rip out the PCB, there is room to build some good stuff in there. The stock 10" speaker even sounds pretty good.
Bob, the official Boozhound Labs guitar playing amplifier evaluator and co-conspirator says this design has a very wide sweet spot, sounding good from 3 to almost cranked. Sounds like the higher current driver is doing it's job.
Bob's official report:
"It has an awesome bluesy sound that starts to break up early (volume = 3), and just gets better as you crank it up. It doesn't turn dirty until some point above 9 (these amps go to 12, that's 2 more than 10)... so the sweet spot is really big. I tried this with a few of my 10" speakers that I've collected lately but it actually sounds really good with the O.E.M. ceramic magnet Eminence..."
While this isn't exactly earth shattering, I think it addresses one of the major problems with guitar amplifier design in an elegant way.
Most guitar amp designs are direct descendants of hi fi designs of the 40s and 50s. The earliest amps that we love so much were taken straight from the recommended operation points of the tubes in question. This is a good thing, because the circuits are straightforward, and do not require any drastically unique engineering to make them sound good with guitar. It is also a bad thing because there are a few small differences between a guitar and a record player for example. The primary difference between a hi fi amplifier and a guitar amplifier is that a guitar amp spends a great deal of time heavily distorted, and must sound good when distorted. This sounds like an oxymoron - good distortion - but for those of us who have embraced the vacuum tube as an audio device, it has become clear that it is not the amount of distorion that causes an amp to sound good or bad, but the kind of distortion.
In Hi Fi circles good distortion tends to mean low order distortion, which is distortion that is only a few harmonics away from the fundamental tone, and even order distortion, which tends to sound more "euphonic" (a classic hi fi mojo buzzword) because it is a simple doubling or halving of the fundamental tone and therefore creates less dissonance. For guitar, this does not appear to be as well defined. Guitarists like many different tones, and at least 2 distinct kinds of distortion. Many players like Fender Champ style single ended amplifiers because of thier rich blues tone. Single ended amplifiers are heavy in even order distortion, especially second order. This is the nature of a single ended design. Some guitarists find this preponderance of second order distortion somewhat muddying to the sound. A guitar sound heavy in second order distortion can lack definition of the individual notes, and have a hard time cutting through the other sounds in a performance. These players tend to like push-pull amplifier designs. In a push pull amplifier, the signal from 2 opposing tubes is summed in the output transformer. This summing cancels out any even order distortion, so a push-pull amp should have primarily odd order distortion. Many guitarists prefer this sound.
The effect that this design attempts to address is that when you drive a tube way into distortion, the grid can swing positive, and grid current will flow. Most guitar amplifiers have a low current 12ax7 driving the grid of the output tubes, and at 1mA of current, and very high plate resistance, the 12ax7 is just not equipped to deliver any current whatsoever to the output tube's grid. As the output tubes require grid current that isn't available, distortion increases dramatically. As a result, the amplifier might sound good at a particular volume level, but the range of volumes at which you get desirable distortion is very narrow.
So what we need is a beefier driver tube. Something with lower plate resistance that idles at higher current, and can therefore deliver more current to the output tube when necessary. There are 2 ways to do this that come to mind:
1) Cathode followers. A cathode follower is a common solution for situations where you need a lower impedance output. This is commonly used in preamps to drive long interconnects for example. Many Marshall/Bassman style amps use a cathode follower to drive a complex tone stack. In this case, you would insert the cathode follower between the phase inverter and the power tubes. The only problem with a cathode follow is that they sound like crap. As a general rule of thumb, no hi fi amp or preamp with a cathode follower in it will sound good. There are exceptions, and in fact many legendary guitar amps use cathode followers to drive the tone stack. I even admit to creating a few designs that use cathode followers. To drive the output tubes, we need a better approach.
2) An additional driver stage between the phase inverter and the output pair. Keep the phase inverter the same, but use it to drive a higher current tube, which in turn drives the output tube. This is like seperating the tasks of phase inversion and driver into two tubes, each optimized for the specific funtion. This is a good strategy, and has been used by my guitar amp guru friend Woody with much success. There are 2 things that I don't totally like about this design, and neither of them are true criticisms, becuase they may not actually affect the sound in the end. I don't like the idea of having to add an additional tube just to drive the outputs. The fewer stages the better. The fewer tubes the better. This is my minimalist side. The second thing I don't totally like is that the signal passes through 2 stages before being summed. I think it is a good idea to sum the signal as soon or often as possible after splitting it in the phase inverter. Every gain stage where distortion is added to the signal whle it is split causes differences between the signals that will either be amplified or cancelled when it is eventually summed, and that leads not only to distortion, but the loss of detail.
So not totally buying into either of these solutions, I went in search of a way to cheat. I wanted a high current driver, but I wanted that tube to be the phase inverter too. So I played with building a long tail pair phase inverter (the most balanced phase inverter during heavy distortion, and therefore most liked by Woody, and I trust him) that used a higher current tube than a 12ax7. I like octals, so I chose a 6sn7 (which is quite a legendary hi fi tube and sounds great in guitar amps). The problem with implementing a high current long tail pair is that the balance of the pair relies on a large shared cathode resistor. Additionally, to get a good amount of gain and linearity, you need a fairly large plate resistor. Ohms law begins to cause problems for you when you start trying to use large resistors with lots of current flowing through them. You begin to drop large amounts of voltage across those resistors, to the point that your B+ supply isn't enough. So you are forced to compromise by using smaller than desired plate and cathode resistors and hurting the linearity of the phase inverter.
Short rant: Isn't engineering great - a solution always raises other problems. I get so pissed when I see ads that promote "uncompromising" amplifier designs. In my experience there is no variable for cost in Ohm's law. These companies would have you believe that there is a little known correlary that states that R = V/I$. As if when you use very very expensive resistors, you can bend the rules. There is always a compromise. The whole point of engineering is getting the job done with a solution that makes compromises you can live with. Tubes designed in the 30s and 40s are always going to sound better than later tubes because those tubes were designed to sound good. Period. They were also a tremdous pain in the ass to use. Check out old Western Electric theater amps to see what I mean. Later tubes were designed to sound good and be smaller. Or to sound good enough, once you add a bunch of feedback. Good with feedback will never sound as good as just plain good. End rant.
Back to the design - what we need is a device that has relatively low DC resistance so that it won't drop a lot of voltage, but high AC impedance so that it will provide a substantial load for the cathodes of the phase inverter, and give us the linearity we want. One such device is the poor misunderstood choke (inductor). Chokes are the first component eliminated from a design to reduce cost because a resistor is much cheaper than 50 feet of wire wrapped around a hunk of steel. Another Hi Fi truism is that anytime you replace a resistor with a choke, the sound gets better. Ideally, we would replace the cathode resistor and both plate resistors with chokes. Maybe even on all stages of the amp. But that would be a very expensive solution, and have other problems, like picking up hum in the hundreds of feet of wire wrapped around the hundreds of pounds of steel :)
So this amp simply replaces the cathode resistor with a choke, to allow the use of a high current 6sn7 as a phase inverter/driver for the output pair of 6v6s. This high(er) current driver allows more current to be deliverded to the grids of the output tubes, extending the sweet sopt well into the region where grid current flows. Phase inverters tend to have neat names: cathodyne, long tail pair, concertina. I nominate this phase inverter be the "choke tail" phase inverter.
The phase inverter no longer looks like a long tail pair because the increased AC impedance of the choke makes the "tail" unnnecessary. The bias voltage is generated by the DC drop across the choke, but the large AC impedance typically generated across the cathode bias resistor plus the large tail resistor is generated across the choke as well. A side benefit is that we get to lose the capacitor that grounds the undriven grid in a long tail pair. Removing capacitors is a good thing in Hi Fi, so why not in guitar amps too.
The rest of the design is pretty conventional. Gain stage -> tone/volume -> gain stage -> phase inverter -> output pair. I am using a "moonlight" tone stack, which is a signle control low loss tone control that I think sounds nice. I am building this using all octal tubes, so we end up with a 6sl7, a 6sn7, a pair of 6v6s, and a 5y3 rectifier. You could replace the 6sl7 with a 12ax7 and the 6sn7 with a 12au7 if you like. You would get a brighter tone, a bit more gain, and smaller tubes :)
I implemented this circuit in a Fender Pro Junior. The chassis, cabinet, and transformer make a nice compact package, and once you rip out the PCB, there is room to build some good stuff in there. The stock 10" speaker even sounds pretty good.
Bob, the official Boozhound Labs guitar playing amplifier evaluator and co-conspirator says this design has a very wide sweet spot, sounding good from 3 to almost cranked. Sounds like the higher current driver is doing it's job.
Bob's official report:
"It has an awesome bluesy sound that starts to break up early (volume = 3), and just gets better as you crank it up. It doesn't turn dirty until some point above 9 (these amps go to 12, that's 2 more than 10)... so the sweet spot is really big. I tried this with a few of my 10" speakers that I've collected lately but it actually sounds really good with the O.E.M. ceramic magnet Eminence..."
Push-Pull 6c45pi Spud Amp
While I am still enjoying my 2a3 monkey amp, I really wanted to design another amplifier, using concepts I had not previously tried. My introduction to really fantastic push-pull amps was at VSAC 2003 when I heard Allen Wrights fully differential 300B amps. I am definitely a single-ended believer, but the sound coming from these things was so amazing that I had to try a differential push-pull design. I messed around with a couple versions of a dialed-in balanced design aith a current source on the cathode and a negative voltage supply for the bottom of the current source and all that stuff, but never built anything.
Then I read through Lyn Olson's ETF presentation on differential topologies and his dissertation on ancient Western electric designs. I was inspired to try some of that old-is-new-again Western Electric stuff.
The third great idea that led to this design is Poinz' Tater Totter design. Suddenly my push-pull crush could be implemented without the hassle of multiple stages and interstage transformers!
Of course no minimalist push-puller can ignore the compact amp phenomenon, and especially the super cool work done by the enigamtic Keto.
So with this enormous body of work swimming around in my head I came up with this design. It is essentially a single stage Lynn Olson amplifier, with current source fed gas regulator power supply and all. But it is a spud (A spud amp is a one-tuber. Tuber, get it. Groan.). Well actually it requires 2 tubes per channel, plus a rectifier, so maybe it isn't purely a spud amp. Of course if you define a spud amp as an amp with a single gain stage instead of a single tube... Well, things get semantically confusing :)
I originally wanted to forgo a traditional phase inverter in favor of the compact amp style of "self-split", but that would have required a somewhat large resistor between one of the 6c45 grids and ground. I have been digging the grid-choke idea, and the 6c45 is supposed to like a low DCR grid-to-ground connection. My audionerd friend and local genius Matt Wiebe turned me on to a clever trick from Paul Joppa and Mike LeFevre (I am not worthy!) that suggests using a Peerless/Altec 15335 (which is a 15k:15k single ended input transformer) as a center tapped autoformer phase inverter. Minutes later I had cannibalized a pair of Ampex input transformers from a broken Ampex PR-10 that I have been wanting to get working again. These appear to be the same thing as an Altec 15335 and should work just as well. I think the 6c45 would have liked the radio frequency rejection of a conventional input transformer rather than an autoformer, but this was just too cool a trick not to use. I'll have to see if there is an octal plugin that would give me a 1:1+1 input transformer.
I have been collecting parts for a while, and have finally reached the critical mass required to build a parts-bin amp. I had the power transformer, power supply choke, output transformers (from an old Sansui I think), big honkin oil caps (Sprague Vitamin Q even), and gas reg tubes. All I needed to buy were the 6c45s and the current sources. I chose Kevin Carter's 2 terminal CCS assemblies for ease of use since they are at the outer reaches of the signal path, and should not affect the sound as much as if they were plate loads for example. I very much like Gary Pimm's tube based current sources, but they looked so complicated that I got scared. I probably should have stayed true to the Western Electric ideal and used a choke instead of a CCS, but I need to drop some voltage and add some power supply filtering, so a CCS just seems to fit the bill.
Construction of the amp was very straightforward. I love it when almost all of the parts mount to the chassis. I would have liked for the wiring to be a little bit neater, but this was the best I could do.
After all the worrying I did about the current sources and the heat they would be dissipating, it turned out to be a total non-issue. The power transformer gets warmer than the heatsinks I used. Chassis mounting the CCS would have been just fine without the heatsinks.
6c45 tubes can be a pain. I guess they are radar tubes or something like that, and so are very good at high frequency oscillation. To fight this oscillation, I used grid soppers on both grid pins on each tube, with the resistors forming a "tent" and connecting to the grid lead. This minor precaution was sufficient on the last amp I built, but not on this one. I think a combination of the little bit of RF generated by the gas reg tubes, and the reactive nature of the input autoformer is too much for these little radar tubes, and they just can't help but oscillate. The amp was stable when there was something connected to the input, assumably because the output impedance of the source would damp and resonances that try to get going in the input autoformer, but with nothing plugged in, it would go crazy. It would oscillate audibly, and the gas reg tubes would go out! That means that the output pair, which normally conduct 60mA, were drawing the full 80mA allowed by the current source! Nuts!
To fix this, I simply attached resistors across both windings of the input autoformer to mimic a source impedance. I started with 2.7k resistrs and that fixed it, but the low input impedance gave me noticeably less gain overall, and was likely taxing the source. So I started and 1M and worked down until the oscillation went away, ending up at 211k. Anything below 250k or so should work just fine.
I hooked this guy up to the (all tube tektronix 555) scope, and could definitely see some crazy noise in there. I think I will see what else can be done to calm these hyperactive little tubes down. Unfortunately I am not much of a scope jockey, and don;t know what a happy amp should look like, so I'm not sure how to distinguish between oscillation and plain old noise.
Suggestions I have had to help the oscillation problem are to add 0.1uF capacitors from both filament pins to ground and also between filament pins, ferrite beads on the cathodes, bigger gridstoppers, and voodoo rituals. I also want to try adding small caps across the gas regulators to hopefully shunt away some of thet RF they generate.
-jsn
Then I read through Lyn Olson's ETF presentation on differential topologies and his dissertation on ancient Western electric designs. I was inspired to try some of that old-is-new-again Western Electric stuff.
The third great idea that led to this design is Poinz' Tater Totter design. Suddenly my push-pull crush could be implemented without the hassle of multiple stages and interstage transformers!
Of course no minimalist push-puller can ignore the compact amp phenomenon, and especially the super cool work done by the enigamtic Keto.
So with this enormous body of work swimming around in my head I came up with this design. It is essentially a single stage Lynn Olson amplifier, with current source fed gas regulator power supply and all. But it is a spud (A spud amp is a one-tuber. Tuber, get it. Groan.). Well actually it requires 2 tubes per channel, plus a rectifier, so maybe it isn't purely a spud amp. Of course if you define a spud amp as an amp with a single gain stage instead of a single tube... Well, things get semantically confusing :)
I originally wanted to forgo a traditional phase inverter in favor of the compact amp style of "self-split", but that would have required a somewhat large resistor between one of the 6c45 grids and ground. I have been digging the grid-choke idea, and the 6c45 is supposed to like a low DCR grid-to-ground connection. My audionerd friend and local genius Matt Wiebe turned me on to a clever trick from Paul Joppa and Mike LeFevre (I am not worthy!) that suggests using a Peerless/Altec 15335 (which is a 15k:15k single ended input transformer) as a center tapped autoformer phase inverter. Minutes later I had cannibalized a pair of Ampex input transformers from a broken Ampex PR-10 that I have been wanting to get working again. These appear to be the same thing as an Altec 15335 and should work just as well. I think the 6c45 would have liked the radio frequency rejection of a conventional input transformer rather than an autoformer, but this was just too cool a trick not to use. I'll have to see if there is an octal plugin that would give me a 1:1+1 input transformer.
I have been collecting parts for a while, and have finally reached the critical mass required to build a parts-bin amp. I had the power transformer, power supply choke, output transformers (from an old Sansui I think), big honkin oil caps (Sprague Vitamin Q even), and gas reg tubes. All I needed to buy were the 6c45s and the current sources. I chose Kevin Carter's 2 terminal CCS assemblies for ease of use since they are at the outer reaches of the signal path, and should not affect the sound as much as if they were plate loads for example. I very much like Gary Pimm's tube based current sources, but they looked so complicated that I got scared. I probably should have stayed true to the Western Electric ideal and used a choke instead of a CCS, but I need to drop some voltage and add some power supply filtering, so a CCS just seems to fit the bill.
Construction of the amp was very straightforward. I love it when almost all of the parts mount to the chassis. I would have liked for the wiring to be a little bit neater, but this was the best I could do.
After all the worrying I did about the current sources and the heat they would be dissipating, it turned out to be a total non-issue. The power transformer gets warmer than the heatsinks I used. Chassis mounting the CCS would have been just fine without the heatsinks.
6c45 tubes can be a pain. I guess they are radar tubes or something like that, and so are very good at high frequency oscillation. To fight this oscillation, I used grid soppers on both grid pins on each tube, with the resistors forming a "tent" and connecting to the grid lead. This minor precaution was sufficient on the last amp I built, but not on this one. I think a combination of the little bit of RF generated by the gas reg tubes, and the reactive nature of the input autoformer is too much for these little radar tubes, and they just can't help but oscillate. The amp was stable when there was something connected to the input, assumably because the output impedance of the source would damp and resonances that try to get going in the input autoformer, but with nothing plugged in, it would go crazy. It would oscillate audibly, and the gas reg tubes would go out! That means that the output pair, which normally conduct 60mA, were drawing the full 80mA allowed by the current source! Nuts!
To fix this, I simply attached resistors across both windings of the input autoformer to mimic a source impedance. I started with 2.7k resistrs and that fixed it, but the low input impedance gave me noticeably less gain overall, and was likely taxing the source. So I started and 1M and worked down until the oscillation went away, ending up at 211k. Anything below 250k or so should work just fine.
I hooked this guy up to the (all tube tektronix 555) scope, and could definitely see some crazy noise in there. I think I will see what else can be done to calm these hyperactive little tubes down. Unfortunately I am not much of a scope jockey, and don;t know what a happy amp should look like, so I'm not sure how to distinguish between oscillation and plain old noise.
Suggestions I have had to help the oscillation problem are to add 0.1uF capacitors from both filament pins to ground and also between filament pins, ferrite beads on the cathodes, bigger gridstoppers, and voodoo rituals. I also want to try adding small caps across the gas regulators to hopefully shunt away some of thet RF they generate.
-jsn
New Blog Format
As many of you have noticed, I have had trouble lately keeping my website running. First I lost a drive and lost all the website content, then I had to switch ISPs and lost my static IP. I also have been wanting to host things somewhere free and more reliable than my DIY attempts. So here we are. It will also be fun to do things in blog format since that is basically how the website was set up to begin with. I will be adding the old pages as blog entries and even adding some new stuff. Enjoy!
-jsn
-jsn
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