That's too coincidental of 1/2 of the B+. It would suggest you are losing 1/2 of the PT supply to or through the rectifiers, and not a high loading pulling down the B+. That would make the PT growl like a bear, too.
I agree. Redressing the wiring while OP is awaiting the arrival and installation of sturdier HV diodes seems like a useful thing to do, since that is what Mr. Miller, the kit vendor recommended.
Once the new diodes are installed thorough testing of the power supply would be prudent. I encourage OP to trace out and draw the power supply, and show DC voltage as well as ripple at each node. Once it is ascertained the power supply works correctly a systematic testing of other issues and components will hopefully yield a dead quiet amplifier.
Bonjour @Francois G !
I did a lot of aggressive twisting of AC-carrying wires and making sure as best I can that wires don't run parallel and that wire crossings are close to perpendicular.
Hum is quieter but still definitely there. My old ears need to put my ear up to the speaker to hear it but my teenager can hear it from 2 or 3 feet away.
It's still basically every 60Hz (60, 120, 180 etc) all the way up to at least 600Hz (and also a little peak at 90Hz).
I need to try replacing the EL34 power tubes...haven't done that since doing this other work. (I have the tubes...just haven't had the time to do the switch.) I did replace the 6SN7 and except for reduced microphonics it didn't make a difference....hum was the same.
Seems like the hum is very slightly quieter in one speaker than the other but it's subtle. Swapping the EL34 tubes from one channel to the other did not move the louder hum to the other channel.
I have a ground-loop breaker board on order. But since using a cheater plug to plug in the amp didn't fix the hum, I'm not very optimistic.
I bought some RF/EMI shielding film and also some shielding tape and I might try using that stuff on some of the signal wires and/or AC wires. I guess I'll have to connect the tape to ground...
At the advice of someone online, one thing I'm just starting to do now is to replace the long run of wire for the negative feedback with a shielded wire. In this picture you can see the long yellow wires for each channel.
Here's how it looks right now, except that the white wire that you see extending out of the chassis is gone. That was just for testing some stuff.
The PCBs are not attached to their mounts at this moment as I need access to the bottom of the main board for some of these modifications.
I did a lot of aggressive twisting of AC-carrying wires and making sure as best I can that wires don't run parallel and that wire crossings are close to perpendicular.
Hum is quieter but still definitely there. My old ears need to put my ear up to the speaker to hear it but my teenager can hear it from 2 or 3 feet away.
It's still basically every 60Hz (60, 120, 180 etc) all the way up to at least 600Hz (and also a little peak at 90Hz).
I need to try replacing the EL34 power tubes...haven't done that since doing this other work. (I have the tubes...just haven't had the time to do the switch.) I did replace the 6SN7 and except for reduced microphonics it didn't make a difference....hum was the same.
Seems like the hum is very slightly quieter in one speaker than the other but it's subtle. Swapping the EL34 tubes from one channel to the other did not move the louder hum to the other channel.
I have a ground-loop breaker board on order. But since using a cheater plug to plug in the amp didn't fix the hum, I'm not very optimistic.
I bought some RF/EMI shielding film and also some shielding tape and I might try using that stuff on some of the signal wires and/or AC wires. I guess I'll have to connect the tape to ground...
At the advice of someone online, one thing I'm just starting to do now is to replace the long run of wire for the negative feedback with a shielded wire. In this picture you can see the long yellow wires for each channel.
Here's how it looks right now, except that the white wire that you see extending out of the chassis is gone. That was just for testing some stuff.
The PCBs are not attached to their mounts at this moment as I need access to the bottom of the main board for some of these modifications.
Attachments
May have found a small clue...could sure use any expert hints on best way to proceed: I noticed last night that the auto-bias green LEDs (that show that the tubes are operating at intended bias voltages) are slightly dimmer on the right side than on the left side. Mr Miller said that suggests a grounding issue on the dim side which could also be a source of hum. Obviously I need to look at soldering issues, whether a cold joint or a loose connection or a hint of a trace or wire getting near something it shouldn't. If anyone has other advice on what I should check or test, I'd appreciate it. (If this is a grounding issue in that channel, it is propagating to some degree through to the other channel, though I do think the hum is slightly louder in the right speaker than in the left. Also, I suspect the problem could at least as easily be in the main PCB as on the auto-bias module.)
Any thoughts much appreciated!
Any thoughts much appreciated!
You could disconnect from AC line and measure grounding points with an ohm meter to see if you have one that is higher than others. However, with most 2 wire meters, you won't see much difference if it is a low ohm reading. A Simpson 260 meter would be great for this. Or another analog meter.
Wow, did that setup complicate such a simple amplifier.
Wow, did that setup complicate such a simple amplifier.
I have a bench meter with 4-wire functionality but it's been so long since I've tried to use that function that I don't remember how. Alternatively, might there be a point that isn't supposed to connect to ground that is doing so, or would that more likely cause a more significant failure?
It looks like the ground wire from the power cord connects to the common ground point in the center of the chassis. It should connect to the chassis at its own grounding point (no other ground connections) close to the inlet of the power cable.
@hifiamps, I just installed the ground-loop breaker board from Hoppe's Brain so I have the power cord ground wire attached to its own lug and all other ground connections separated from the chassis by 47 ohms. No change in the hum. (I did not move the location of the power cord ground wire, so it's not right by the inlet. Hard to imagine that that's the source of hum...do you think it could be somehow?)
So, here's what I've done:
- Installed ground loop breaker board (from Hoppe's Brain) and verified that all circuit grounds test at 47 ohms to chassis ground (which is connected to power cable ground wire.)
- Twisted all AC wires much tighter
- Rerouted wires so that fewer are crossing, and crossings are perpendicular
- Swapped all tubes for brand new tubes (and then put back old tubes when that didn't make a difference.)
- Changed diodes but it turned out the original diodes were good ones...better than what was on the parts list and they didn't need to be changed.
- Remounted power transformer, including tightening screws and putting rubber washers between chassis and transformer
- Reheated all solder pads (in case of cold joint)
- Changed B+ resistor to raise B+ voltage. (Now it's a little too high instead of a little too low and I have in-between resistors coming, but the change didn't impact the hum.)
- Checked for differences in resistance to ground between similar points in the circuit on left and right but did not find any. (In other words, at every spot where there is the same point in the circuit on each channel, the left and right versions of that point tested the same resistance, or lack of resistance, to ground.)
- Replaced long wires for feedback with shielded wire (and shield going to ground at PCB end of cable)
- Note: RCA inputs are completely disconnected (no wires to them at all.) The inputs on the main board are shorted. (Was just two wires tied together, now it's two wires connected by 47 ohm resistor, but no difference.)
At this point, I can only think of two things to try: I bought some RF/EMI shielding material (some interesting film mad by Kemet, and also some tape), so I could try putting that on transformer or around wires. \
Or, swapping out the transformer.
When I run the transformer noise through a spectrum analyzer it seems, oddly, to have a spike at 90Hz (and a much bigger one at 120Hz). There seem to be harmonics every 30Hz starting at 90Hz (not much I can tell at 60 Hz) but bigger peaks at multiples of 120 Hz. (See screen shot below.)
When I do the same with recording (through my phone) the hum in the speaker, the results are similar. This is somewhat different than a week ago when I had clearer 60Hz noise in the mix. (Not worth posting separate screen shot since it looks so similar.)
It's possible that the results at low-ish frequencies, maybe as high as 60Hz are inaccurate due to limitations of my recording device or my cheap test speakers, but in any case the big spikes at multiples of 120Hz are clear and I'm not sure the lower peaks at other intervals are even audible.
I'm sooooooo frustrated. Even though he's very busy, Mr Miller is working through stuff with me for which I'm quite grateful. But so far we haven't solved it.
Any thoughts much appreciated.
Power supply ground (ground of the power supply filter caps) should be separate (grounded to the chassis) from the signal ground. Only the signal ground should be connected to the ground loop breaker board.
OK, this is where I start getting a little lost, @hifiamps.
I'm assuming those are the electrolytic caps rather than the film caps. But if the main PCB has one connection to ground (and the auto-bias board gets its ground through the main PCB) then how would I separate the ground for the power supply from everything else?
I mean, I don't think it's possible to separate the circuit ground from the power supply ground within the board itself, right?
I'm assuming those are the electrolytic caps rather than the film caps. But if the main PCB has one connection to ground (and the auto-bias board gets its ground through the main PCB) then how would I separate the ground for the power supply from everything else?
I mean, I don't think it's possible to separate the circuit ground from the power supply ground within the board itself, right?
It looks like the row of eight electrolytic caps along the bottom of the picture are the main filter caps. The choke connects there, and there are diodes. The ground of those caps should connect to a chassis ground point. There is a pad marked GND near the middle along the bottom edge of the PCB. That is probably the ground wire. Be sure that ground wire connects to a different point on the chassis from the other grounds. Not to the lifted ground.
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@hifiamps, that ground wire connects to the ground loop breaker board along with everything else.
But this board is power supply and signal processing in one so I don't know about moving the ground to the chassis. That would move almost everything to the chassis except the common/negative from the speakers and the center tap from the transformer and the shields from the negative feedback wires.
I suspect that what you're suggesting might not make sense when the power and signal stuff all shares a ground through the board. But I'm just learning here...
But this board is power supply and signal processing in one so I don't know about moving the ground to the chassis. That would move almost everything to the chassis except the common/negative from the speakers and the center tap from the transformer and the shields from the negative feedback wires.
I suspect that what you're suggesting might not make sense when the power and signal stuff all shares a ground through the board. But I'm just learning here...
I tried to follow the wiring in your picture but a couple things aren't clear to me. The red/yel HV CT, where does it land? Also the 16ohm output tap, yellow wire, runs to the driver board for feedback it appears but the speaker ground lands on the chassis star ground. That's a pretty big ground loop for the feedback. The common for the feedback should land on the common side of the input stage cathode resistor. Feedback is measured at the input tube relative to signal common. If the common reference for the feedback is on the chassis star ground and then the star is connected to the board via another wire, any noise in the ground system will be modulated into the feedback. I also notice that the right side in the picture has the yellow feedback wire and the speaker common wires surrounding the choke. That's a sure way to pick up noise. The two wires should run on the same side of the choke and twisted is better yet. I've done several ST-70's for friends with horn speakers and to get them quiet you have to do everything correctly. They were dead quiet on 107dB sensitivity horns so it is possible. Since we don't know how the pc board is arranged it's hard to be certain about some things.
Hey @nerdorama and @hifiamps,
The red/yellow center tap connects to ground...at this point it connects to the ground of the ground loop breaker, not to chassis.
That yellow wire that looks like the 16-ohm wire is actually the feedback wire from the main PCB back up to the OPT. I have since replaced it with a (white) shielded wire. The 4-ohm and 16-ohm OPT outputs just have heat shrink over the ends and don't connect to anything. I only have an 8-ohm speaker tap. There are a couple of short wires (yellow and brown) attached to the feedback selector boards in case anyone wants to switch to 4 or 16 ohms later but since the jumper is on the 8-ohm selector, I'd be surprised if they could be impacting anything. (Tell me if that's wrong!)
The only wire running next to the choke is the shielded feedback wire.
I don't understand what you're saying regarding what should be going on with the common (black) wires from the "ground"/"negative" speaker terminal. You're right that they go to the "star ground" which is now connected to the ground loop breaker, not to chassis.
The white wire is the ground connection of what I think are filter caps. (Two caps with the outer leg of each going to green and brown transformer wires, respectively, and inner leg of each tied together and to ground (to ground loop breaker.)
Where you see screw of the former star ground, that is the lug that has only the power cord ground attached. Then the other lugs are actually connected to a terminal strip that is connected to the ground loop breaker board. (You can see it next to the circular opening in the chassis.)
I may try using some EMI/RF-blocking film around some wires or transformer or choke to see if anything changes. At this point I'm wondering if it's possible that the transformer has a problem?
And, again, everything on the front of the chassis (top of the picture) is for decoration only. None is connected to anything.
The only
The red/yellow center tap connects to ground...at this point it connects to the ground of the ground loop breaker, not to chassis.
That yellow wire that looks like the 16-ohm wire is actually the feedback wire from the main PCB back up to the OPT. I have since replaced it with a (white) shielded wire. The 4-ohm and 16-ohm OPT outputs just have heat shrink over the ends and don't connect to anything. I only have an 8-ohm speaker tap. There are a couple of short wires (yellow and brown) attached to the feedback selector boards in case anyone wants to switch to 4 or 16 ohms later but since the jumper is on the 8-ohm selector, I'd be surprised if they could be impacting anything. (Tell me if that's wrong!)
The only wire running next to the choke is the shielded feedback wire.
I don't understand what you're saying regarding what should be going on with the common (black) wires from the "ground"/"negative" speaker terminal. You're right that they go to the "star ground" which is now connected to the ground loop breaker, not to chassis.
The white wire is the ground connection of what I think are filter caps. (Two caps with the outer leg of each going to green and brown transformer wires, respectively, and inner leg of each tied together and to ground (to ground loop breaker.)
Where you see screw of the former star ground, that is the lug that has only the power cord ground attached. Then the other lugs are actually connected to a terminal strip that is connected to the ground loop breaker board. (You can see it next to the circular opening in the chassis.)
I may try using some EMI/RF-blocking film around some wires or transformer or choke to see if anything changes. At this point I'm wondering if it's possible that the transformer has a problem?
And, again, everything on the front of the chassis (top of the picture) is for decoration only. None is connected to anything.
The only