JD01 wrote: ↑Sun Apr 17, 2022 3:56 am
Is there a way you can plug it in and test it when it's all still in bits on the bench? So you can test it before it's fully built.
You can, sort of, kind of, not really, although there's not much to learn from doing so that you couldn't learn from just using a multimeter. At this point in my build I'm just avoiding shorts to ground. I'm looking for zero-loss continuity across that long ass heater filament twisted string. I'm looking for rock-solid connections. I'm checking resistor values. I can use a multimeter to test that stuff. Plugging in and powering on an unloaded unfinished amp isn't really something you'd do unless you are deep into tracing down a problem.
But once it's finished there is a step-by-step start-up procedure that you need to go through. That's where you can find problems before they hurt something in the amp. I'll try to detail that part of it when I get there.
Optional reading, this is gonna get a little techy:
Sort of related to testing and related to my last post, I made this to kind of generally explain filter caps and power nodes in a tube amp if anyone wants to learn something. This is my schematic. The colored circles outline the various power nodes. A power node is rectified DC voltage that supplies DC voltage to various parts of an amp. This voltage starts as AC from the wall, gets stepped up in the power transformer, gets rectified to DC voltage through the tube rectifier or diodes (Green Box) and then can get used by the amp. The rectified DC voltage supply is what is commonly called "B+ voltage", or just "B+", or sometimes "H.T." We'll call it B+. This B+ voltage goes everywhere and different parts of the amp need different amounts of it. B+ voltage travels from node to node and it's usually stepped down in stages as it travels from node to node. There are resistors in line along the B+ supply to drop voltage because not every part of the amp wants full B+ voltage.
So what happens is once our wall power is stepped up and rectified to DC, it should be a straight smooth DC signal of high voltage. But it isn't. It's still got some ripples in it. It's imperfect. And it "sags" from being used throughout the amp. That's where the first filtration stage comes in. On the schematic I've circled this in red. Those first two filter caps get charged up and they store electrons. They're kind of like batteries, in a way. These caps serve two purposes, 1) the stored electrons fill in the ripples left in the B+ after rectification, and 2) they are sort of like back up power to keep things running smooth when you hit a massive crunchy power hogging E chord. So you can see them circled in red on the schematic, and here's where I put mine in practice.
So that's the first stage filter cap node...or "reservoir caps" as they're commonly called. They are like a reservoir of electrons. On the schematic they are before the standby switch, so they are always "on". If the amp power is on, these guys are getting charged up. But what about the other caps? They are after the standby switch. On the schematic we see that after the reservoir caps there is the standby switch and then the choke - ignore the choke for now - and the B+ line splits. Part of it goes to the output transformer center tap to supply full filtered B+ voltage to the OT. This is the high voltage you see on the plates of output tubes. We're not concerned with that for this. The B+ line also splits off to the left and we come to another filter node, circled in yellow. This one is for the output tube screens. They need clean filtered high voltage for happy output tubes. So then as we move along that B+ line on the schematic we get to a zig-zag...that's a resistor. It's a dropping resistor I mentioned earlier. It's a 4700 ohm 1 watt resistor. This drops the voltage in the B+ line. And then we get to yet another filtered node, circled in dark blue. This one feeds our dropped DC voltage to the tube plates for the phase inverter. We don't want any electrical ripple on any tube plates, so pretty much anywhere plates are getting DC, there is a filter node attached to it. And again, we move on down the line and get to yet another dropping resistor (my MS Paint circles hide it, but it's 27K) and another filter node, this one circled in pink. This one has a "C" next to it on the schematic and that simply means they broke the drawing and we'll pick it up elsewhere. To the far bottom left you can see the Normal Channel preamp section. At the bottom you see a little "C" and that's where the drawing picks up again. That pink node on the "C" line filters and supplies the B+ voltage for the Normal Channel.
So now all that's left to supply power to is the bass channel. Instead of just moving down the line and powering the Bass Channel after the Normal Channel, they did something different. The Bass Channel filter node is circled in light blue. On the schematic it's broken off similar to the Normal Channel, and there is a little "B" on the drawing. If we go back and find a corresponding "B" we can see that it goes all the way back to the very first post-standby node....the yellow screen supply node. This is a really high voltage node that they're tapping off of to feed the Bass Channel. That voltage is way too high for typical 12AX7 preamp tubes, which is what this amp will use, so if you go back to the light blue circled Bass channel node you'll see it's dropping resistor is a whopping 56K. That poor resistor is gonna eat some voltage so the Bass Channel can use that B+ and not melt tubes. My assumption is Fender designed it this way for minimal sag since that is a "Bass" channel. But I'm not sure.
Any-fucking-way....that's how B+ gets used in an amp and how it's filtered for smooth DC voltage. All of those power nodes and all of those capacitors are how tubes get clean smooth power to make beautiful sounds.
And lastly, this is how it applies to my build. This little bundle of caps, resistors, and wires are my filter cap nodes, the corresponding dropping resistors, and the wires that go to feed the various parts of the amp.
If you read and understood all this, kudos to you.
