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On this thread I intend to document the build for a 60 watt per channel KT120-based amplifier. I want to try the KT120 tube because I've never used it before, and everything I read about it indicates its a really great tube.
This is a continuation of http://www.audiokarma.org/forums/showthread.php?t=492619.
Attached are the schematics. I’d like some feedback on the circuit design. Is there some dumb thing I’ve done that if changed will improve the design? Is there anything missing that would greatly improve the sound or performance? Fire away with your comments.
After I get feedback on the circuit design, I plan to document the build on this same thread with a parts list and pictures as I progress through it. It’ll take a while (2 or 3 months) but I think it will be a good thing to share in the public forum. I hope tubes never die, and I’m doing my part to ensure that they don’t.
I’d love someone else to try this build also. It’s not exactly a beginner build, but you’ll have a great sounding high performance amp when you’re finished.
===========
Here’s the circuit description and design notes for those who are curious: (warning, the rest of this is kinda long)
One thing you should know is that I’m only showing one channel for the audio frequency amplifier (schematic page 1), so you’ll need to build page 1 of the schematic twice for stereo. The rest of the circuitry (schematic pages 2 and 3) are sized and shown as-is for two channels.
My design principles for this amp were to have no preconceptions of what works or doesn’t work and to make it as absolutely simple as possible while still maintaining very high quality sound reproduction. Input sensitivity is 5V RMS to drive the output stage to full power. (Edit--this has now been addressed. Input sensitivity is now about 1.15V RMS). This amplifier will produce about 60 watts per channel.
Schematic Page 1: (Audio amp)
The first stage is a common cathode voltage amplifier. I’ve tried other configurations for the first stage, such as a mu follower, SRPP, constant current sources, even a pentode, and they don’t make it sound better and just add complexity. I’ve also tried other tubes for this stage such as a 6DJ8/6922, and I don’t like them as much in this position. The good ole’ 12AX7 is a perfect tube for this stage and there are lots of choices for tube rolling, including many good NOS choices. It might be interesting to try a pentode strapped as a triode. If I did that, I’d consider the EF86.
Capacitor C23 is used to reduce HF instability. This is one part of the circuit I haven’t tried before so I’m guessing at the capacitor size. Somewhere between 330 pF and 470 pF seems about right and actually anything in that range will probably work just fine.
Circuit ground (labeled G in the schematics) is expected to be a ground bus. The circuit ground should be connected to the chassis at one point only, near the input tube. When I build it, the ground bus will be a 12 gauge bare copper wire pulled from a length of Romex.
The second stage is a cathode coupled differential pair phase splitter. I’ve tried the cathodyne configuration also, and while it’s a very simple phase splitter, I’d need a lot of headroom to run the output tubes and I didn’t have that kind of voltage available. I’ve also tried other tubes for this stage, including the 6H30p and 12AU7. The 6SN7 is the best tube I know of for this stage. It has inherently low distortion, allows very large signal swings, and has an octal base so it’s easy to work on when wiring it up.
The first stage is direct coupled to the second stage. I’ve tried capacitor (AC) coupling these stages and it doesn’t improve performance and adds complexity when instrumenting the feedback circuit.
The output tubes run at ~485 volts on the plates and are biased at about 60 mA per side. This is a medium idle for these KT120’s (!!) so these tubes should last a good long time. There is a balance adjustment circuit that is intended to be accessible from the top side using a screwdriver and a volt meter to keep each side of the push-pull pair at equal idle current. Since I didn’t make the bias of each output tube separately adjustable (seemed like added complexity that was unnecessary), you’ll need to use a matched quad for best left/right channel balance. You can direct substitute 6550, KT88 or KT90 and the amp should work just as well.
If you don’t change the fixed bias setting, you can even directly substitute a KT66 (but not a 6L6!!) and it will work pretty well, although it will idle the KT66’s at about 60% of capacity and output power will drop to about 45 watts per channel. Maybe try this if you’re really bored and want to tube roll, but ideally it’s not designed directly for a KT66. Just don’t try a 6L6. It’ll likely red plate them or fry the screen grids, or both.
The two output transformers that I know will work well with this amplifier are the Hammond 1650R and the Triode Electronics A431 Dynaco clone. Both should give really good performance, but the Hammond will reduce the output power to about 70 watts per channel since it's primary winding is a little higher impedance.
Schematic page 2 (Power supply)
Here’s where it gets a bit more interesting. The power transformer is a toroid. I’ve never had good luck with stacked EI core power transformers — they are more prone to mechanical buzz and can create hum problems in the audio circuit. Toroids are designed to keep most of the electromagnetic field inside the confines of the core. The toroid I’m using is from toroid.com. I chose it because it’s an “off the shelf” part and is a multi-wind, so I can fit everything on one chassis. R26 is a thermistor that limits current surge so the fuse doesn’t blow when you flip on the power switch.
Capacitors C9 and C10 are special. They are relatively new from ClarityCap, called the TC series. These are film caps and have extremely low ESR so they deliver energy almost instantaneously. Also these caps are rated at 700 volts so there is no need to stack them to get the required voltage tolerance. Downside is they are HUGE! If you don’t want to use the Claritys, you can easily substitute two stacked 220 uF electrolytics rated at 350V in place of each Clarity, and the amp will still sound really good.
I added a high voltage (HV) regulator so that at large signal swings (high output power), the voltage supplied to the input tube and phase splitter tubes will remain more or less constant. This greatly reduces distortion for both the first and second stages. It also improves bass response at high volumes.
Here’s where I may get some questions about why I did it this way. All filaments, including the power tubes, are DC supplied. I did this out of consideration for total heat generation and hum reduction, plus the availability of the windings on the toroid—they were there. I’m running the output tube filaments in series per pair of tubes, so that only half of the current is needed to power all four filaments (about 3.85 amps). A straight up all parallel connection using AC would require the transformer to supply almost 8 amps. This DC configuration will generate significantly less transformer heat and won’t saturate the core as much. The toroid is going to get a little warm, but I intend to mount it top side, so it will dissipate heat easily. The amplifier itself should run fairly cool.
Schematic page 3: (HV regulator and power supply)
The HV regulator is a series-pass design. I’ve tried many tubes for the series-pass element, such as a 6C19p, 6AS7, EL34, and a 6L6. They all work well, but when I tried the 6V6 (a new production Tung-Sol), I about peed my pants! It sounded that good. One thing I would like to try is a cold cathode constant voltage source instead of the 150V Zener. I think that might require a two chassis build though, as it’s probably going to be tight to get everything here on a single chassis. That might be one area I will experiment with in the future. I hear cold cathode voltage sources in voltage regulators can sound really awesome though.
The filament supply for the HV regulator is DC supplied because I needed to keep the voltage between the cathode and filaments within specification, so I’ve raised the filament bias to 266 volts. The 12AX7 error amplifier receives the same raised voltage on its filament.
Well, that’s the design. This should be a fun build and I think it’ll be a cool running gorgeous sounding little amp.
Kevin
- Post #1: General discussion of design goals, circuit topology, and improvements
- Post #238: Build schematic
- Post #278: Bill of Materials
- Post #343: Chassis layout
- Post #351: Construction & Testing
- Post #454: Measurements and Tuning
- Post #499: Final schematic with 12AX7 input
- Post #521: Final schematic with 5751 input
On this thread I intend to document the build for a 60 watt per channel KT120-based amplifier. I want to try the KT120 tube because I've never used it before, and everything I read about it indicates its a really great tube.
This is a continuation of http://www.audiokarma.org/forums/showthread.php?t=492619.
Attached are the schematics. I’d like some feedback on the circuit design. Is there some dumb thing I’ve done that if changed will improve the design? Is there anything missing that would greatly improve the sound or performance? Fire away with your comments.
After I get feedback on the circuit design, I plan to document the build on this same thread with a parts list and pictures as I progress through it. It’ll take a while (2 or 3 months) but I think it will be a good thing to share in the public forum. I hope tubes never die, and I’m doing my part to ensure that they don’t.
I’d love someone else to try this build also. It’s not exactly a beginner build, but you’ll have a great sounding high performance amp when you’re finished.
===========
Here’s the circuit description and design notes for those who are curious: (warning, the rest of this is kinda long)
One thing you should know is that I’m only showing one channel for the audio frequency amplifier (schematic page 1), so you’ll need to build page 1 of the schematic twice for stereo. The rest of the circuitry (schematic pages 2 and 3) are sized and shown as-is for two channels.
My design principles for this amp were to have no preconceptions of what works or doesn’t work and to make it as absolutely simple as possible while still maintaining very high quality sound reproduction. Input sensitivity is 5V RMS to drive the output stage to full power. (Edit--this has now been addressed. Input sensitivity is now about 1.15V RMS). This amplifier will produce about 60 watts per channel.
Schematic Page 1: (Audio amp)
The first stage is a common cathode voltage amplifier. I’ve tried other configurations for the first stage, such as a mu follower, SRPP, constant current sources, even a pentode, and they don’t make it sound better and just add complexity. I’ve also tried other tubes for this stage such as a 6DJ8/6922, and I don’t like them as much in this position. The good ole’ 12AX7 is a perfect tube for this stage and there are lots of choices for tube rolling, including many good NOS choices. It might be interesting to try a pentode strapped as a triode. If I did that, I’d consider the EF86.
Capacitor C23 is used to reduce HF instability. This is one part of the circuit I haven’t tried before so I’m guessing at the capacitor size. Somewhere between 330 pF and 470 pF seems about right and actually anything in that range will probably work just fine.
Circuit ground (labeled G in the schematics) is expected to be a ground bus. The circuit ground should be connected to the chassis at one point only, near the input tube. When I build it, the ground bus will be a 12 gauge bare copper wire pulled from a length of Romex.
The second stage is a cathode coupled differential pair phase splitter. I’ve tried the cathodyne configuration also, and while it’s a very simple phase splitter, I’d need a lot of headroom to run the output tubes and I didn’t have that kind of voltage available. I’ve also tried other tubes for this stage, including the 6H30p and 12AU7. The 6SN7 is the best tube I know of for this stage. It has inherently low distortion, allows very large signal swings, and has an octal base so it’s easy to work on when wiring it up.
The first stage is direct coupled to the second stage. I’ve tried capacitor (AC) coupling these stages and it doesn’t improve performance and adds complexity when instrumenting the feedback circuit.
The output tubes run at ~485 volts on the plates and are biased at about 60 mA per side. This is a medium idle for these KT120’s (!!) so these tubes should last a good long time. There is a balance adjustment circuit that is intended to be accessible from the top side using a screwdriver and a volt meter to keep each side of the push-pull pair at equal idle current. Since I didn’t make the bias of each output tube separately adjustable (seemed like added complexity that was unnecessary), you’ll need to use a matched quad for best left/right channel balance. You can direct substitute 6550, KT88 or KT90 and the amp should work just as well.
If you don’t change the fixed bias setting, you can even directly substitute a KT66 (but not a 6L6!!) and it will work pretty well, although it will idle the KT66’s at about 60% of capacity and output power will drop to about 45 watts per channel. Maybe try this if you’re really bored and want to tube roll, but ideally it’s not designed directly for a KT66. Just don’t try a 6L6. It’ll likely red plate them or fry the screen grids, or both.
The two output transformers that I know will work well with this amplifier are the Hammond 1650R and the Triode Electronics A431 Dynaco clone. Both should give really good performance, but the Hammond will reduce the output power to about 70 watts per channel since it's primary winding is a little higher impedance.
Schematic page 2 (Power supply)
Here’s where it gets a bit more interesting. The power transformer is a toroid. I’ve never had good luck with stacked EI core power transformers — they are more prone to mechanical buzz and can create hum problems in the audio circuit. Toroids are designed to keep most of the electromagnetic field inside the confines of the core. The toroid I’m using is from toroid.com. I chose it because it’s an “off the shelf” part and is a multi-wind, so I can fit everything on one chassis. R26 is a thermistor that limits current surge so the fuse doesn’t blow when you flip on the power switch.
Capacitors C9 and C10 are special. They are relatively new from ClarityCap, called the TC series. These are film caps and have extremely low ESR so they deliver energy almost instantaneously. Also these caps are rated at 700 volts so there is no need to stack them to get the required voltage tolerance. Downside is they are HUGE! If you don’t want to use the Claritys, you can easily substitute two stacked 220 uF electrolytics rated at 350V in place of each Clarity, and the amp will still sound really good.
I added a high voltage (HV) regulator so that at large signal swings (high output power), the voltage supplied to the input tube and phase splitter tubes will remain more or less constant. This greatly reduces distortion for both the first and second stages. It also improves bass response at high volumes.
Here’s where I may get some questions about why I did it this way. All filaments, including the power tubes, are DC supplied. I did this out of consideration for total heat generation and hum reduction, plus the availability of the windings on the toroid—they were there. I’m running the output tube filaments in series per pair of tubes, so that only half of the current is needed to power all four filaments (about 3.85 amps). A straight up all parallel connection using AC would require the transformer to supply almost 8 amps. This DC configuration will generate significantly less transformer heat and won’t saturate the core as much. The toroid is going to get a little warm, but I intend to mount it top side, so it will dissipate heat easily. The amplifier itself should run fairly cool.
Schematic page 3: (HV regulator and power supply)
The HV regulator is a series-pass design. I’ve tried many tubes for the series-pass element, such as a 6C19p, 6AS7, EL34, and a 6L6. They all work well, but when I tried the 6V6 (a new production Tung-Sol), I about peed my pants! It sounded that good. One thing I would like to try is a cold cathode constant voltage source instead of the 150V Zener. I think that might require a two chassis build though, as it’s probably going to be tight to get everything here on a single chassis. That might be one area I will experiment with in the future. I hear cold cathode voltage sources in voltage regulators can sound really awesome though.
The filament supply for the HV regulator is DC supplied because I needed to keep the voltage between the cathode and filaments within specification, so I’ve raised the filament bias to 266 volts. The 12AX7 error amplifier receives the same raised voltage on its filament.
Well, that’s the design. This should be a fun build and I think it’ll be a cool running gorgeous sounding little amp.
Kevin
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