I'm planning to breadboard the old Komuro 300B circuit once I've completed my current project. There are two questions that occur to me after studying the schematic.
1. What is the purpose of the connection from the voltage divider off the 6V6 cathode network to one leg of the 6.3V heater winding (dark grey circles in the attached schematic)? Is it to raise the heater voltage and maybe provide some hum suppression?
2. For the 120V power transformer (light grey circle), if my mains voltage is 120V, can I just connect the rectifier diode bridge to the power switch? Seems like it would be a good way to reduce parts cost and space usage.
Thanks!
Jeff
1. What is the purpose of the connection from the voltage divider off the 6V6 cathode network to one leg of the 6.3V heater winding (dark grey circles in the attached schematic)? Is it to raise the heater voltage and maybe provide some hum suppression?
2. For the 120V power transformer (light grey circle), if my mains voltage is 120V, can I just connect the rectifier diode bridge to the power switch? Seems like it would be a good way to reduce parts cost and space usage.
Thanks!
Jeff
Attachments
Hi Jeff,
1.) That is the lift the voltage of the filament so that to ensure the cathode-filament voltage stays within the 100V limit.
2.) For safety reason, please use a transformer and avoid connection directly to the main. Recommend avoiding using the voltage-doubler as shown in the circuit.
1.) That is the lift the voltage of the filament so that to ensure the cathode-filament voltage stays within the 100V limit.
2.) For safety reason, please use a transformer and avoid connection directly to the main. Recommend avoiding using the voltage-doubler as shown in the circuit.
Yes, you don't want a "hot chassis" like the old radio amps. And make sure the power tranny for the 300B is insulated for at least 700vdc working voltage, like the UTC he used.
https://www.hifido.co.jp/sold/19-24184-57448-00.html?LNG=E
https://www.hifido.co.jp/sold/19-24184-57448-00.html?LNG=E
BTW, I heard that original amp at the old Fi store on Watts Street in downtown Manhattan, back around 1990. It was a long time ago, and of course the speakers were a one-off 3-way design by (I think) JC Morrison using Focal drivers, but I will never forget how spectacular that sounded to me at the time. Bill Evans "Waltz For Debby" was playing on a TD124, but I forget what the rest of the system was. The cartridge may have been a Kiseki or Koetsu or something exotic like that. The glasses clinking and the inane conversation at the table near the bass mic were scary realistic. That's a fond memory...
Agreed...Indeed it sounds amazing!
Unlike the 'typical' 300B SE amplifier I heard of. This one packed with clarity and at the same time; with punch!
Unlike the 'typical' 300B SE amplifier I heard of. This one packed with clarity and at the same time; with punch!
The Komuro 300B amp is a fascinating circuit. I learned a lot from the way it biases each stage in a 3-stage direct coupled amp. It uses two power supplies stacking together to reach over 700VDC. I contemplated making it safer by grounding the mid-point and have only 411V at the 300B plate and supply negative voltage to feed the driver circuit. The only hitch is that you need to add an input transformer, or input capacitor, to isolate from the DC at the grid. But I like having input transformer (bridging 1:1 10K:10K) for power amps and it allows for balanced input if wanted. A lot of JC Morrison amps are obsessed with DC coupled and highly influenced by Komuro like the ones he designed for Silbatone, and they all have input transformers. I also saw a similar circuit in the Japanese magazine MJ so why not. And here's a spin on it.
Hey @directdriver, many manufacturers do not recommend using such DC differential between primary and secondary of input transformers. I am not saying it will not work but I have asked this question to some of them and it looks like 300V is border limit. They didn't say no but did not recommend it. I had the same thought about the 45 PSE amp with 12AY7+49 DC-coupled stage. Some transformers might be just fine, I have not made a general inquiry. If you know some or already made it, please let me know.
I know all circuits designed by JC Morrison for Silbatone have similar topology bracketed by input and output transformers and all direct-coupled in between so there's always DC on the secondary of the input transformer. Some examples here. I do know that all his transformers are sourced either from Lundahl or Intact Audio by Dave Slagle. I have not built the circuit and I do understand the risk of insulation material not able to withstand high voltages. Having a blocking cap is probably the safest but...
I deal with vintage vari-mu compressors all the time and they all need an input transformer to isolate from the side-chain voltage on the secondary that can go up as high as -100VDC or higher and most of them are vintage UTC and Triad transformers and they all seem to be working perfectly fine. To be honest, I'm more concerned about grid current with faulty input tube. Having input isolation transformer allows for more creativity on circuit designs, especially with inverted "upside down" circuit.
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Question: how can there be -295V on either side of the input transformer? Is that a typo?The Komuro 300B amp is a fascinating circuit. I learned a lot from the way it biases each stage in a 3-stage direct coupled amp. It uses two power supplies stacking together to reach over 700VDC. I contemplated making it safer by grounding the mid-point and have only 411V at the 300B plate and supply negative voltage to feed the driver circuit. The only hitch is that you need to add an input transformer, or input capacitor, to isolate from the DC at the grid. But I like having input transformer (bridging 1:1 10K:10K) for power amps and it allows for balanced input if wanted. A lot of JC Morrison amps are obsessed with DC coupled and highly influenced by Komuro like the ones he designed for Silbatone, and they all have input transformers. I also saw a similar circuit in the Japanese magazine MJ so why not. And here's a spin on it.
Not sure if there is anything 'safer' about this version as opposed to the original. 😉 I would think any HV transformer with 770VCT secondary would be isolated to 2-3kV.
This circuit reminds me of the Tubelab circuit (probably should be the other way around) but using the 6V6 instead on the mosfet. 😊
The 6J5 is not supposed to draw grid current. If you supply -295V to one side the other side is still -297 because of ohm's law. In a conventional amp the ground is 0V and the grid is also 0V so this version is relatively the same.
It's safer for human not the power transformer because original circuit has up to +715VDC to chassis ground, most likely would be touched by your hand, whereas this version has only +420VDC to chassis ground. Either voltage can kill you of course but I rather take the chance with 420VDC than 715VDC! The less likely scenario would be touching the negative rail AND the positive rail at the same time to reach 715VDC. Ouch!
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I think my tolerance is 220VAC at best. As a kid I touched a knob in the back of an old TV, it was the summer and I was barefoot on a marble floor. My father later discovered the pot was live with mains voltage. This was in the 70s and I remember flying like after a shotgun blast in a Quentin Tarantino movie. 😎