Let's Turn It On | JMI Vox AC50 Part 3

Hinzugefügt: 2026-04-30

[00:00:00]Coming back to this very early 65 AC50 uh an awful lot of stuff has been done.

[00:00:07]I got to be careful with where I put my fingers. Some of this stuff stores voltage.

[00:00:10]Don't be so casual around amps as you may may think I'm being in these videos.

[00:00:15]I know where not to put my fingers and where to put my fingers. Though occasionally I do make exciting mistakes.

[00:00:21]I don't want you to be as excited if you're playing the home version.

[00:00:25]Anyways, you can tell everything on this board, this is the uh bias board and the output board has been changed except for this capacitor and the pots in the sockets. So, we've got new uh grid stoppers at the output tubes. We've got new grid leaks. These are some Vishays.

[00:00:43]Um I have all the Mullard diodes. The amp's got a buzz. I changed these Mullard diodes out for some new uh Vishay 1N5408s and I might have lessened the buzz, but there's some other causes of buzz we'll get to.

[00:01:00]Um so, these may go back. They are pretty and they measure fine out of circuit, but you know, there's how something measures and how it actually performs under current, they're not always the same thing. So, for right now these 5408s are the better thing to have. I had to really rebuild this area of the board. The previous tech basically had these two caps in parallel with these two resistors the same way that these two caps are in parallel with these two resistors. So, this amp would never have worked cuz the bias voltage was like -7. It would have just nuked tubes. Uh one of the things I'm about to do after I talk a little bit about what I've done so far is because the bias circuit was so messed up, this pair of EL34s that came with the amp may not have enjoyed that. So, I don't want to chase my tail on noise and buzz if it's just some some hummy um damaged output tubes. But I did start with the uh provided tubes. Uh anyway, looking at how this thing should be built, looking at the schematic, this capacitor actually would have been in parallel with the board on this axis, but I uh given the size of the two watts that I have here versus the size of the 10 microfarad uh caps here, I thought this layout made more sense to have the long span with the longer axials and the shorter span with these two watt resistors here.

[00:02:20]Uh I did put back the 47 ohm uh resistors from cathode to ground on each pair.

[00:02:25]Quick note, as I'm looking at things and thinking of things not in terms of the sequence of what to check in an amp. The Vox schematic says to adjust the bias on each of these two pots until you measure 2.2 volts DC across each resistor.

[00:02:40]Uh that equates to a a bias of about 90% idle dissipation.

[00:02:44]Uh they were biasing it as as if were cathode biased, which was a terrible mistake and I've got these biased on the cool side uh until I get everything healthy. Um I've got it biased at about 25 mA per tube, which turns into just under 50% idle dissipation. I can increase that once I get everything healthy, but I wanted to make sure that the tubes had a fighting chance while I sorted everything else out. Um certainly gets noisier. I did try it at the recommended 2.2 volts and the amp has a ton of noise when you do that because the output tubes want to self-immolate between the uh the stock bias setting that Vox would have put on this thing and then what some tech had done by rebuilding the bias supply wrong. I'm surprised that there's not a whole bunch of burnt charred section of this of this amp.

[00:03:36]Though this does have one replaced octal socket. Oh, real fast. I also uh lifted this uh light brown or grayish wire going to ground here. It's one of the uh secondary center taps of the power transformer. It turns out that the lighter of the two is the uh filament center tap and with that lifted out of the way, here are the filament connections from the uh transformer. I did put in these 100 ohm half watt resistors to ground. This is both ground here just to see if that would help. It It has [clears throat] a slight bit, but that's not the culprit for the remaining buzz. We'll get there. All right, you can see you've got a new volume pot for the normal channel and it has a new cap here because the old cap was not long enough, the leads.

[00:04:22]Uh so, they're Vishay here.

[00:04:24]I've got a new uh bass pot here as well.

[00:04:28]Uh the old one had a broken off shaft.

[00:04:31]All the other pots are new and I'm waiting for the new grub screws to arrive from Britain for the knobs that go on these three pots.

[00:04:41]Throughout the amp, I've got Vishay and F&T capacitors. These are the dual section 16 um microfarad uh caps that I mentioned that were a little bit difficult to find. Uh this green wire here is so that as I was setting the bias, I wanted the preamp to be silent. So, when it's the bias off, so this is one of the grounds and this is the input to the phase inverter. So, I just silenced the output while setting the bias because before I did that, even with the knobs at zero, not only was there a little bit of noise in the amp, but it was really hard to get a steady uh voltage reading across the primaries of the output transformer. Uh it's not very stable circuit in its current state.

[00:05:24]More on that. Uh so, Vishay cathode bypass caps, all new resistors throughout. All the Eries and Hunts were were junk as to be expected. This thing was taken apart. All the grounds cleaned up. I know I'm repeating a little bit of stuff I told you in the last video, but it's important in context. Grounded power cable reinstalled. You can't see that right now. I'll move the camera down just a bit and show you that. Um Right now, everything is as it was when it left the factory if they had used these components. So, carbon films in the signal path, the audio path and uh these metal films for droppers.

[00:06:03]And there's a cleaned up AC wiring down here with a new bulb installed. It's nice and pretty.

[00:06:09]Everything clean. Uh good connections.

[00:06:12]Uh the ground's longer than the hot neutral.

[00:06:15]Blah blah blah. I've shown that a lot.

[00:06:17]Let's power it on. The nice clean shiny dress nut makes me happy.

[00:06:23]So, this is the hum and buzz the amp has with both channels not just off, but the input to the phase inverter muted.

[00:06:30]Now, I've got both channel volumes off.

[00:06:31]Let me lift that ground.

[00:06:34]And shut.

[00:06:39]There's that self noise and kind of an oscillation, which may be just picking up a lot of the um electrical interference from the room.

[00:06:46]I've got a Wi-Fi router and so and so and so and the tube shields.

[00:06:50]The amp didn't come with these tube shields.

[00:06:52]I touched the tubes.

[00:06:56]That's not making a difference.

[00:06:58]There's some kind of oscillation happening.

[00:07:00]If I turn up the channel volumes, some heater noise on the treble channel.

[00:07:10]The buzz isn't much worse. If I turn up the uh normal channel, big buzz, big big buzz.

[00:07:24]Let's see if this plays uh on the video.

[00:07:27]I might test before if I pull V1, which is a 12AU7 and yes, I've tried a different 12AU7, then I still get buzz when I turn up the volume of this channel.

[00:07:41]But notice the buzz decreases when it's at either end. If it's at zero or if it's at 10. So, if it's at 7:00 or 5:00, the buzz isn't there.

[00:07:51]Anywhere in between, the buzz gets worse. That is a sign of a ground loop.

[00:08:01]That's where we'll go next.

[00:08:03]The AC50 in this period had four chassis ground connections. One is down here and it's used only for the AC safety ground.

[00:08:10]That's fine.

[00:08:12]There's one right here that I don't entirely trust, though I have cleaned it as much as possible because the uh output transformer overlaps these two holes. So, the transformer is actually here sitting on top of these screws.

[00:08:26]So, it's really hard for me to clean this uh as thoroughly as I'd like and it's really hard to tighten this as thoroughly as I'd like. We may put a new hole somewhere where the transformer is not, which will allow me to really be sure of this.

[00:08:41]But aside from that, everything that you see here, with the exception of the AC safety cable, is grounded to here. So, you've got the preamp filter caps, all four of them, dual sections. They're grounded here, which goes to this series of ground strings, which starts over here.

[00:08:59]And it just works its way down, catches all the cathode bypass caps and goes to here.

[00:09:05]And you have the output center tap reference.

[00:09:08]Oh, sorry, the output common uh reference goes to here, which goes to here.

[00:09:13]You've got the input jacks, which go to here and then from there to there. And then everything on the pots goes in a string to the connection on the input jacks, which then goes to here. And um in theory, that should all be good. I need to go through here and see if there are any uh dup- duplicate paths.

[00:09:33]Sometimes in Voxes, there'll be more than one path to ground and it just from a builder's mistake.

[00:09:40]Um so, I need to need to make sure I haven't missed anything in here.

[00:09:44]But I think if I were to separate some of this stuff from here that's going to here and then to here and just have it all go to here independently, that might help.

[00:09:54]Um I'll play around with that. Before I move the camera angle, I've got a resistor kind of hanging in air here.

[00:10:00]It's a 47k.

[00:10:02]When testing the amp last night, I had it across the output of these two uh coupling caps in the phase inverter, just so I had a a fixed uh cross-line master volume, so I could test the amp without making a bunch of noise in the other room. It's disconnected at this end now. It won't go home like this, but that's what this is.

[00:10:19]One of the other main grounds is made here. And again, I removed this and got it really clean and tightened as much as I could, but it cannot be tightened as much as I would like. And this is one of the board standoff uh locations from the other side.

[00:10:35]And it just does not tighten much more than it is. And it's not a tooth washer.

[00:10:40]So, I may revisit this one. And there's this one here for all this stuff. Now, in theory, this one is the same as the one that's on the other side of right here is the same that's on right about here.

[00:10:54]And they measure the same, but just cuz something measures equivalent does not mean it is actually equivalent. In some boxes, I've had good results with disconnecting grounds made in one board, running the wires through, making them all at one point in the chassis.

[00:11:08]Because while this measures to a meter as ground the same as this part of the chassis does, there can always be some very slight, almost unmeasurable resistive differences with at least with the meters that most of us own.

[00:11:21]Uh whereas if you just say, "I'm just going to bypass this." And use that chassis for reference, all of a sudden everything's happy. But I don't want to go through here and just start making sweeping changes. The first thing I want to do is make sure that um when someone rebuilt this board, they didn't do something uh stupid and give it multiple paths to ground. Same for um the the grounds on the other one.

[00:11:43]You know, before I reinvent the wheel, I make sure that something simple's not going on that I just missed because there is a lot of trees here. And after a while, I don't see the forest anymore.

[00:11:53]So, it's good to take a little break.

[00:11:55]But before I end this video and dive into all the nasties lurking in the beast, I'm going to play us out. Um I'm going to say thanks for watching now.

[00:12:03]Then I'm going to let you hear it a little bit. We're going to start with just the normal channel.

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