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With considerable help from one of the guys (Jeff) who frequents what I call "Audio Sunday School", I have finally finished the first phase of a new project amplifier using that "odd man out" tube, the Russian 6C33C. I set out to design and build a prototype of a relatively basic SE stereo power amp that would provide substantially more power than the usual SE 300B design. Having had no experience with the 6C33C I had no particular expectations. However, as is often the case, I was surprised at what I heard once the "bugs" were worked out and the amplifier was freed from the constraints of my mistakes. In spite of the modest 14watt output into 4 ohms using the LL1627/200mA output transformer, the sound is big, full, and warm. What was really surprising was the degree of life-like resolution that a 2 stage D3a (IT loaded)/6C33C amplifier is capable of! I've built or modified several SE amps (2A3, 45, 300B, PX-25) in the past 12 years and none of them sounded like this!
I will be building another version in monoblock form soon, to overcome the power transformer limitation of the "proof-of-concept" amplifier. Then there is the urge to try the larger LL1693/210mA. Then there is.... you get the idea.
I'll keep you up-to-date with the progress as it grinds slowly on.
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
I'm interested in this one. Do you have design details?
Best,
Schematic as it is now playing is at https://www.dropbox.com/s/kr6fekltdymlnpl/SE%206C33C%202%20Stage%20stereo%20amp.pdf
Kevin Carter
K&K Audio
www.kandkaudio.com
Kevin,
Two different schemas are shown for the signal. One which uses an interstage and the other which uses cap coupling between the stages and mosfets in the HV. I have a few questions and I apologize if this is really basic it's just not stuff that I've seen in schematics I've studied in the past.
1. The Power tx specified for B+ is 500v. I'm guessing there is an option to connect the secondaries for greater current and 250v out? What is the current of the B+ for the output transformer? I'm sure it's in the 250-300mA range...just checking.
2. What is the function of the diode between the plate and grid? Is it some kind of protection or limiter?
3. Same question but for the diode in the B+ path right after the 270V test node.
4. In the non interstage channel, what is the function of the N channel mosfets?
5. I have a tough time understanding the power supply. I may need to print it on 11x17 and trace the paths with different colors so it's easier to see whats happening. I think D3a filament supply comes from the mains tx and 6c33c filament supply comes from a separate filament tx. Two other feeds come out of the mains tx: 1) is a 5v regulated feed that goes into a 3 minute delay relay that feeds into a DPDT relay. 2) is the main B+ feed that passes through an RC damped FWB into the shared DPDT relay. Both of these feeds leave the relay, the HV into a solid state circuit before the B+ line, the LV directly into the B+ line and then both sum into a common mode choke before the CLC filter. It may not be complicated in the grand scheme of things but it's a lot harder than what I normally look at.
6. Since 2 versions are provided...which sounds subjectively better?
Best regards,
Chris
The B+ doesn't go through the 3 minute delay though right? only a 5V component of the B+? At least that's how I read it. Wait...I get it. That 1 sec mosfet delay into the DPDT relay doesn't kick over until the 5v component hits it 3 minutes after startup. That gets rid of turn on thump that would happen otherwise and provides a full 3 minutes on the 6c33c filaments to stabilize after power on before the B+ hits. If all of the above is true, why not sum the 6.3V and the 250V at the input and put the whole summed winding through a soft start and delay like the ones you sell? Maybe it's an issue with the high current requirement of the power tube B+.
This would be a bit of a pita to wire point to point and inside a compact stereo chassis. A common supply for both channels would help some though.
Your deduction is correct. The soft start kit that I sell will not support 410mA of current. That sort of current requires a lower on resistance MOSFET and a higher power zener diode.
It makes the most sense to me to build this amp as monoblocks, so that the power transformer is not run so close to the edge, although it might be feasible to use the Antek AN-4TK360 transformer without the first 47uF cap, so that it's choke loaded to do a stereo amp and not be on the edge. Once I sort all of this out in two or three months I am considering offering PC boards for the power supply and the input stage. Sonically, it's quite worth it. Those who know me tell me that I rarely communicate excitement like this, so I guess that's a sign.
Kevin Carter
K&K Audio
www.kandkaudio.com
Sorry for the complexity, but it's all part of either the sonic plan or the user-friendliness/tube life plan.Answers numbered the same as the questions.
1. The schematic wiring reflects having the two 250V windings on the PT in parallel and then in series with the one unused 6.3V winding (no waste!) for the B+ supply.
2. The diode in between the D3a screen grid and plate is part of the triode-connecting network for the tube. The means by which it makes the sound better is a mystery to everyone I've talked to about it, but it does.
3. The diode in the B+ line between the input stage and output stage prevents the output stage from sucking current out of the PS bypass cap for the input stage when the load gets heavy.
4. Plate load current source. It is a cascode CCS.
5. The mysterious circuit with the MOSFET and zener diode is a "soft start" circuit that ramps up the voltage in about 1 sec after the 3 min time delay expires. This ramp prevents the loud "pop" that would otherwise happen. Of course you can use a tube rectifier instead of this, with the time delay relay controlling current to the rectifier filament.
6. I easily prefer the sound of the interstage transformer version, but the capacitor-coupled circuit also sounds quite nice.
Kevin Carter
K&K Audio
www.kandkaudio.com
Edits: 01/17/14
Sounds very exciting Kevin. Now I only want to know more about it!
Similar to your D3A, I have used E180F, in triode, IT coupled using 1660 with good success.
Keep feeding us the teasers.
Will this ever morph into a kit/project/product for K&K do you think? Sounds like a cool piece! I had a VK-60 for a while but never really warmed up to it, for whatever reason. Keep us posted!
-Ed
That is a distinct possibility. I am building a monoblock pair next to explore this possibility and try to keep the cost down. Historically I've seen evidence that kits costing more than $1K don't sell well irrespective of the sound quality. There's so much vintage stuff out there that can be had for less and is thought by the average guy to have magical powers and be yet improved by the usual mechanisms (cap replacement, etc.) to unbeatable value. If you think about it too much, you can get depressed, so I won't for now... ;+))
Kevin Carter
K&K Audio
www.kandkaudio.com
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