The amp does not waste precious 300B tube; it uses it efficiently, in class A2. As you may see from the explanation, distortions increase slowly, so at 10W it has only 0.4% of total harmonic distortions. And it still has a headroom, up to almost 18W output. It is driven by cathode follower V3. It isolates the coupling capacitor from it's control grid, eliminating dynamic distortions that happen on peaks of class A1 amplifiers (hence the belief in the sound of coupling capacitors). Also, low driver impedance shunts Miller capacitance of the output tube improving frequency response and increasing slew rate limit.
VR2 sets the idle current of the output tube. However, I would improve this bias setup network a bit, controlling the resistor in the voltage divider that goes to the ground, instead of the one that supplies negative voltage. Why? Because if the pot fails, it would decrease the bias current to the minimum instead of increasing it to the maximum.
Since 300B needs huge voltage swing to be driven, the amp uses 2 amplification stages to drive it, linearized by 3K/100 Ohm feedback voltage divider. 20 Ohm+0.05 uF is a Zobel load of the secondary winding, it is used to damp ringing of the output transformer on high frequency.
20k/40 Uf filter in anode of the cathode follower also limits voltage swing on control grid of the output tube, to avoid overheating of it's grid on peaks of the signal.
The amp was definitely designed by a knowledgeable Japanese designer, who also built it and fine-tuned practically. Even if you need only typical 8W output from a 300B tube, A2 approach is much better, because on the same power the sound is much better. Don't listen to gurus who repeat a nonsense about "adding euphonic distortions". The less distortions the amp adds, the sweeter is the tube sound, the closer it is to the real thing.
I am attaching the schematic diagram again, for convenience.
Yes, why not?
Except probably the capacitor that rather needed for self-satisfaction, to see less ringing on o-scope when testing square waves above an audible band.
Just please let me be polite and don't answer you.
I have no idea. I did not have an intention to present a complete recipe with all ingredients. You got the idea, the difference between using 300B in class A1 and class A2 amps. There can be many other ways to skin the same cat.
The question I have about those designs is, what is the advantage of DC coupling the first two stages (the two 6SN7 triodes), but not the second stage driver to the 300B grid?
It seems to me the DC coupling would do the most good between the driver stage and output stage, not between the first (voltage amp) stage and second (driver) stage.
DC coupling the first two stages would make a difference if there was a global feedback loop, but in this case there is not. So...
??
It seems to me the DC coupling would do the most good between the driver stage and output stage, not between the first (voltage amp) stage and second (driver) stage.
DC coupling the first two stages would make a difference if there was a global feedback loop, but in this case there is not. So...
??
Actually, it looks to me like in that design from diyAudioProjects, the 6SN7 driver might not supply enough signal voltage swing to drive the 300B to full power before the driver stage itself clips.
As Wavebourn pointed out earlier, the 300B requires great gobs of voltage swing to get to full power; something like 80V peak—and then you want some more voltage swing available for headroom. So figure the driver stage has to swing 100V signal cleanly to the grid of the 300B. Not a trivial task. Komoru's design solved that problem by using a power tube (6V6-triode) with a plate choke to drive the 300B. You could also use a small power triode like 6N6P into a MOSFET source follower to drive the 300B. Figure the 6N6P with 200V plate-cathode with 10V grid-cathode and real-world amplification of about 10X will get you that 100V output signal swing.
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As Wavebourn pointed out earlier, the 300B requires great gobs of voltage swing to get to full power; something like 80V peak—and then you want some more voltage swing available for headroom. So figure the driver stage has to swing 100V signal cleanly to the grid of the 300B. Not a trivial task. Komoru's design solved that problem by using a power tube (6V6-triode) with a plate choke to drive the 300B. You could also use a small power triode like 6N6P into a MOSFET source follower to drive the 300B. Figure the 6N6P with 200V plate-cathode with 10V grid-cathode and real-world amplification of about 10X will get you that 100V output signal swing.
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I think I'll go with this schematic:
Any tipps or modifications for this circuit? If I use a single power transformer (GZ34 tube rectifier) - how high should be the HV amperage?
This is pretty similar:
Or are there any reasons for mono blocks?
Greetings,
Felix
I would go with the first schematic, it's a bit simpler. You could build a stereo amp. The power transformer should be capable of at least 250mA. However, if you are going to use a GZ34 rectifier, you should reduce the HV secondaries to 375-0-375, otherwise your HV will be much too high.
Well, that was kind of my point. ;-) The OP has output transformers, probably does not have a 'scope to fine-tune a feedback network and is looking for a simple first-time 300B build. Also, a 300B is not rated for A2 operation, so while a direct-coupled, low-impedance driver is a good idea you'd want to be sure that you don't get into an A2 situation.
Or, he could just buy a $100 solid-state amp and be done, eh? ;-)
Except that they don't sound good, even in double blind testing. Pure nonsense.
yes, indeed
No scope - good multimeter, knowledge in building tube amps - guitar tube amps.
Fine tuning guitar amps and trying ptp wiring. I can read and build things out of schematics, and I know what wires I shouldn't touch during and after running my amps.
some latest builds:
Vox AC15
JTM45
Hiwatt DR103
4S Universal Preamp
Gibson GA5
So - I have a preamp, transistor but it works pretty good, it was a kit from the German Tubeland.de side. One day I will build a aikido preamp or sth else - two weeks ago I've sold a tube preamp.
I dont want a 100$ transistor amp
- I have an old Technics SU-V650.
Why not answer him? eventually he will discover on his own that the statement he made is just plain wrong.
No offence, I use SS devices quite a bit when it makes sense. Mostly regulation...
No reason other than that I've seen your posts mentioning local/nested feedback and I wondered what you'd do if starting this circuit for yourself.
Is that the 500pF?
Thanks.
I think I'll go with this schematic:
6SN7 + 300B = very classic. But is it good?
I don't think the way the 6SN7 is used in this circuit is a competent driver for the 300B. Once the 300B starts drawing grid current, the driver will "fart out", so don't expect the best result from this.
Using a buffer to drive the 300B would work much better, as suggested in some of the previous posts (for example post 6 and 11).