6c41s
Hi all,
The output tube is 6c41s, and I would like to drive it into class A2.
Not too keen on the DC coupled cathode follower, mainly because it requires a separate negative supply. Loftin White topology has been said to sound very good, so Im keen in that direction.
The 'normal' operating point of the 6c41s is 180v/60v/120mA. It is driven by 'improved' SRPP stage with 5965, it should provide gain of 40+, which should provide plenty of drive to the output stage..
Since this is my first attempt at this kind of design, hoping to get it checked first.
btw- I'm aware of the dangers of Loftin White, but I have been building amps for many years now so its a risk i'm keeping in mind. Thanks in advance.
Hi all,
The output tube is 6c41s, and I would like to drive it into class A2.
Not too keen on the DC coupled cathode follower, mainly because it requires a separate negative supply. Loftin White topology has been said to sound very good, so Im keen in that direction.
The 'normal' operating point of the 6c41s is 180v/60v/120mA. It is driven by 'improved' SRPP stage with 5965, it should provide gain of 40+, which should provide plenty of drive to the output stage..
Since this is my first attempt at this kind of design, hoping to get it checked first.
btw- I'm aware of the dangers of Loftin White, but I have been building amps for many years now so its a risk i'm keeping in mind. Thanks in advance.
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Could you please explain how you think your circuit will work?
Some questions:
1) Why is the upper triode of the 5965 connected to 350 V?
2) Why is the dc-coupling of the stages combined with cathode bias of the 6C41S?
3) Why are the cathode resistors of the power stage not decoupled?
4) Why would you want to drive a 6C41S in class A2?
5) Why do you think that the driver stage can provide the (hefty!) gridcurrent needed for class A2 operation?
Some questions:
1) Why is the upper triode of the 5965 connected to 350 V?
2) Why is the dc-coupling of the stages combined with cathode bias of the 6C41S?
3) Why are the cathode resistors of the power stage not decoupled?
4) Why would you want to drive a 6C41S in class A2?
5) Why do you think that the driver stage can provide the (hefty!) gridcurrent needed for class A2 operation?
(1-3), I'm not sure exactly how to explain this circuit, basically its a reverse engineering several design that has been done before;
https://hifiakademiet.dk/uf/50000_59999/52624/55ea1b50c9211e04444849ba8c258ef8.pdf
The 2A3 Tube Amplifier - A Tribute to Simplicity and Warm Sound | audioXpress
The reason Im sharing my circuit is hopefully someone can check if my reverse engineering is correct or not..
(4)- A2 is for the fun of it. Why DIY if I have to stick to standards?
(5)- I am also curious about this point. But from the example circuits that I referenced, they dont seem to have problem to drive the output tube with a wimpy driver...
Hopeful someone knowledgable can share more about this circuit.
Loftin White was never intended to work in A2.
Neither it was intended to provide quality sound, actually - it was just the cheapest possible amp in those days.
Using high gm tubes like 6S41S in A2 is not a great idea, though it might more or less work in this case because of self bias (fixed bias for 6S41S + A2 = dead 6S41S). Still it won't provide any sonic benefits.
If you want to try A2 nevertheless, you'll need the driver that is 10-20x capable in terms of current that what's shown in your schematics.
And your reverse engineering is wrong on so many levels that I'm not even sure where to start.
Neither it was intended to provide quality sound, actually - it was just the cheapest possible amp in those days.
Using high gm tubes like 6S41S in A2 is not a great idea, though it might more or less work in this case because of self bias (fixed bias for 6S41S + A2 = dead 6S41S). Still it won't provide any sonic benefits.
If you want to try A2 nevertheless, you'll need the driver that is 10-20x capable in terms of current that what's shown in your schematics.
And your reverse engineering is wrong on so many levels that I'm not even sure where to start.
Thanks.. It just struck me that I am (mistakenly) equating direct coupling with ability to provide current drive.
Anyway, forget about driving into A2..
What is the first step to correct the circuit?
Anyway, forget about driving into A2..
What is the first step to correct the circuit?
To correct the circuit...
Start with a 250V power supply and cathode bias the output tube. You need a cathode bypass cap across the cathode bias resistor. (I did not review your operating points)
Choke load 1/2 of the 5965 off the 250V supply for each channel. A 50H plate choke will do the job, and it won't be excessively difficult to find nor expensive because you could operate the tube at 5mA.
Use a coupling cap and grid leak resistor on the output tube.
Start with a 250V power supply and cathode bias the output tube. You need a cathode bypass cap across the cathode bias resistor. (I did not review your operating points)
Choke load 1/2 of the 5965 off the 250V supply for each channel. A 50H plate choke will do the job, and it won't be excessively difficult to find nor expensive because you could operate the tube at 5mA.
Use a coupling cap and grid leak resistor on the output tube.
