I've started the build to replace the SET I currently use. There were several things I wanted to accomplish by doing this, but one of the most important was that if I was going to bother doing this at all, I wanted the result to look nothing like other SETs. Exhibit "A": I took inspiration from roughly the 1930s when the 300B was developed and construction like this was more common. This picture shows the amp in it's first dry-fitting of the parts. The electronics have not been installed yet. The black screws are attaching turret boards for the components. The circuit has a 6SL7 (or 6SN7) input tube with the two halves paralled. This feeds a 6SN7 cascode driver stage which in turn feeds the 300B output tube. Two vintage NOS Triplett milliamp meters will monitor bias current. Fixed bias is used and DC is used for all of the filaments, with dual regulated DC supplies for the output tubes. The output transformers are Edcor units rated for 25 watts, far more iron than needed, but this amplifier is going to be heavily over built everywhere. The entire power supply is mounted outboard in a 19" rack and will occupy 10 1/2" of rack space, with a 10 conductor umbilical attaching to the SET. Power supply filtering is very extensive and dual-mono except for the common power transformer and solid state rectifier. With my 107dB/watt HF drivers, absolute quiet is very important. More to come......
Excellent! You da man That is a real piece of engineering art. I was at a shop last week that builds mining power equipment. They had shelves full of similar meters/shunts, mosr were Simpson, same style/era.
Yes! Great job. If you are going to go through the trouble of building something, may as well as go the extra mile and make it exactly as you want. In this case, it looks like you went beyond an extra mile. More like a whole marathon.
Yes, those are cool meters! I found them on eBay for $90 each and they are new-in-box. While I'm not insane enough to insist on only using cloth covered hookup wire, I will be using cable lacing instead of Ty-Wrap cable ties. The signal capacitors will come from my stash of vintage military NOS oil-paper caps filled with wonderful toxic PCB oils! The jury is still out on using carbon composition resistors - you can still buy those from Digikey.
I'm sure someone, somewhere back in the 1930s took off with their grandma's spice rack to build their radio transmitter project.
I've built tube preamps using inverted cake pans. A friend stared at one for a while before finally asking "Is that... a cake pan?! Does your wife know?"
Very cool. Did you base the circuit on an established one? And why a solid state rectifier? Do you remember that group of designers who used to do these weekends at the OMA mill building and breadboard these lunatic old amps using crazy vintage tubes? I used to enjoy reading about those weekends, but didn't know anybody in that group well enough to get an invite.
The circuit is my own, but really 300B SET circuits are so basic and ubiquitous that there is no 'original'. I'm using a solid state rectifier because there's nothing gained by using a tube one - after heavy filtering the end result is pure DC either way and a tube couldn't deal with the extreme filtering I'm using (for noise performance with sensitive horns). Also, the power supply circuitry is outboard in an out-of-sight rack - the only thing in the amplifier part in the picture is signal circuits with an 8 foot/10 conductor umbilical connecting it to the power supply.
Thanks for the reply. My phono stage is so sensitive to rectifier changes it prompted me to ask. But the "sound" of this amp is really the 300bs, no? I like the retro look- those old WE stacks always amazed me; ditto, someone here had some Klangfilm amps in racks that were very old fashioned looking though I suspect they were post-war. I expect you'll give us a write up on how these sound compared to your existing amps (which I'll have to go back and look up- I remember you had quite a bit of period equipment in your system(s).)
SET = Single Ended Triode. Just in case you're a Millennial like me and had no clue what that stands for.
Well, that’s one person’s opinion. Not mine. In any case, your amp looks really cool and I hope it brings you much enjoyment.
Gorgeous!!! My only comment/thought... Maybe alternate the polished/brushed sides of the brass plates, so that either the vertical or horizontal plates are polished while the other are brushed(?)
The sound will be dominated by the 300Bs (or 2A3s) and the input tubes. The overall sound of this amp will undoubtedly be similar to the one it replaces since the circuit, while different, isn't that radically different. I mostly wanted to beef up the power supplies and get them out of the same chassis as the audio signal path - the closest thing I could get to running the amp on a rack of remote batteries. Also, the old amp had sequenced startup of the various power supply voltages, but I wanted to take that a step farther in this amp. Builds like this were not uncommon in the 20s and 30s, though its use specifically as a 300B power amp is uncommon (although I've seen a few wild DIY amps using things like WE 205 tubes etc). I've seen some early radio transmitters built this way.
