A conventional tube amplifier has a circuit whose fundamentals were well in place around a hundred years ago, so there are few surprises to be found in building one today. Nevertheless, building one is still a challenge, as [Mike Freda shows us with a stereo amplifier in the video below the break.
The tubes in question are the 12AU7 double triode and 6L6 tetrode, in this case brand new PSVANE parts from China. The design is a very conventional single-ended class A circuit, with both side of the double triode being used for extra gain driving the tetrode. The output uses a tapped transformer with the tap going to the other grid in the tertode, something we dimly remember as being an “ultra-linear” circuit.
There’s an element of workshop entertainment in the video, but aside from that we think it’s the process of characterising the amp and getting its voltages right which is the take-away here. It’s not something many of us do these days, so despite the apparent simplicity of the circuit it’s worth a look.
These modern tubes come from a variety of different sources, we’ve attempted to track them down in the past.
I’m usually the first to be frustrated by a linked video but this one was excellent and very much worth watching. Take-aways: a modestly equipped shop and hand tools can lead to great results. I don’t know much about designing and building audio amps, but he includes a lot of practical details and whyhe did stuff a certain way. Well done.
Agreed very rare example when the video really works well.
The whole thing is amazing and love how simple he makes it seem.
I did have questions about his explanations
Like it seems the filaments are in parallel but be ous the 6l6s on the rungs closest to the power supply because they draw more current does that make sense? I’m not convinced it matters much for short runs of thicker wire (22awg)
I’d have thought itd be woeth running signal wire under the plate (in the gap between the plate and the bottom) and the rest above the plate. With the plate acting as a shield?
Wonder how vulnerable to transients the whole thing is guess the 6h choke will kill any but seemed he was running it all close to spec max?
It looks nice, but what’s so special about this amp? There is no negative feedback except for the UL output stage, so distortion must be fairly high. The circuit is severely flawed, in the 1950s you would have been kicked out of engineering school for such a silly design. I would classify this thing as an audiophool’s toy, you can do “tube rolling” all day long since every tube will sound different, even tubes from the same batch.
Ah, now I get it. It’s an advertisement for a certain tube dealer.
“There is no negative feedback”
Did you read what you wrote?
I see what you did there!
An ad for PSVane tubes, basically, but interesting.
In this world 🌎 of rapidly diminishing NOS vacuum tubes/valves, I found it interesting that somewhere, new ones are being manufactured.
From scratch? The screw holes and valve holder holes are already drilled.
Isn’t drilling a form of scratching?
I went to a high school where we had to build a tube amp from scratch starting with a piece sheet metal. You unlocked that 50 year old memory.
You mean the holes he drills at 2:54 and 3:40 or punches at 3:20? Or the ones he files at 4:49? Oh, you mean the ones that are already there at 5:00.
I mean, you can buy a pre-made chassis at Tube-Tonw, Tube Amp Doctor etc, but this guy is going the long way. Too bad he didn’t make his hookup wires and solder from scratch /s
Helllooo, anybody heard of Dynaco Stereo 70?
If you can get a kit for one, please tell me where. Otherwise, I don’t understand the point of your comment.
If you’re interested in designing audio amplifiers, then ditch the light bulbs and buy the books from Douglass Self (ISBN 0 7506 56360) and Bob Cordell (ISBN: 978-0-07-164025-1)
Although both these books are explicitly about designing audio power amplifiers, the techniques you learn are of course also valid for a much wider range of electronics design.
Yet another tube amplifier built with good craftsmanship…
Like all other modern tube amplifier design, it follows the main rule : make obvious is is built around tubes by having them fully visible, even if it sounds not that great – and without any protection around glass parts, so please get rid of your kids & cats.
My naive self was thinking that an audio amplifier should be evaluated mainly by its sound…
You seem fun at parties…