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About as simple as it gets, for a three stage amplifier.
6AU6 voltage amplifier, gain of 25 drives a 6AQ5 miniature power valve, gain of 8, which drives the DHT output stage.
6AU6 input stage is biased with a white LED, which doubles as an "on" indicator, when placed behind a front panel, red Fender jewel lamp lens.
The smoothing filters in the power supply (bottom) are duplicated on the other channel, so what we effectively have is a split power supply, once it divides in two after the rectification and smoothing choke.
Now here's an interesting discovery: on a whim, I decided to see how my NVA P20 passive would sound, feeding the SET power amplifier. All I can say is WOW!
The P20 just "goes" with the valve power amp. The sound was good with my existing switched attenuator, but with the P20, the whole passive/valve setup just hangs together superbly well. All sources have benefitted and the overall presentation and dare I say "musicality" is much better, the P20 digging out the music like a JCB.
Sony cdp xb930,Alessandro ms1
Here is the power supply schematic:
This illustrates nicely, the "split" nature of the supply once the juice leaves the 10H choke.
As it seems - looking at your schematics - you run the JJ 2A3-40 closer to their max. design parameters for the anode voltage to gain more power from the tube. Have you tried this JJ DHT tube also with the normal 2A3 parameters according to the RCA specification (250V anode, -45V grid, 60mA)? And if yes, what is the difference in sound to the parameters used in your design?
I love the sound of a SE 2A3 amp. For me the 2A3 is a wonderful tube with a magical sound!
I'm currently using a pair of Emission Labs 2A3-S in my DHT monos, but with mainly the classic 2A3 parameters (260V anode, -45Vgrid, 60ma). The EML tubes sound great with standard specs and are really well made. It's not easy these days to get a really nice pair of original 2A3s (talking about double plates as single plates are anyway crazy expensive and therefore no option for me).
Wonderful project and very nice fit and finish! Congratulations!
Tubes and Vinyl rule, Music rocks!
Thanks for the positive comments about the amp.
I haven't tried the JJ at the normal 2A3 operating points. I can't compare the two without changing the HV transformer secondary voltage taps. This would also alter the operating points of the input and driver stages, so the comparison would not work.
However the 2A3 40 is a great sounding valve all the same.
I would love to try a pair of EML solid plate 2A3s and I intend doing so next year.
The JJ 2A3 is a great sounding tube and really priced fair for sure. The EML are unforunately more expensive but really made well. A lot of people only regard the original NOS RCAs, Sylvanias, etc. as the real thing but I have to say after trying some NOs pairs and the EML solid plate pair in my amps that the EMLs are at least on par with NOS tubes. In the bass definition department they are better in my opinion.
Worth a try for sure...
Tubes and Vinyl rule, Music rocks!
Once I've got hold of some EMLs, in the new year, I'll report how they sound.
I've always liked the 2A3/6B4G family of tubes. As you say, they seem to sound "just right"
NOS prices are stupid, so I'm glad I found the JJ 2A3-40.
Final version of power supply uses a hybrid bridge rectifier.
This config gets the most music out of the amplifier. The difference is not subtle.
This is the current version of the amplifier power supply:
The smoothing choke is now in the ground rail and the hybrid rectifier bridge has been modified with backup diodes
in order to protect the power supply caps and mains transformer, should the valve rectifier develop a fault.
The ground scheme is now a star earth, which has significantly lowered the noise floor, resultng in even more music escaping from the loudspeakers.
This amp is now sh*t hot, though I say so myself.
The high voltage value has been lowered, so that "normal" 2A3 valves can be used in the amp, which is now around 3.5W output.
With my speakers at 96dB sensitivity, the low power is not a problem.
It's a great sounding piece of kit, but needs efficient speakers to show its true capabilities.
The voltages on the schematic are around 10% lower than shown, due to the smoothing choke being placed in the negative side of the hybrid bridge rectifier, but I can't be arsed to alter them on the diagram. With a valve amp, it doesn't matter, within reason if the voltages are not quite right, and nobody on here is likely to build it anyway.