OK, another dumb question: Is it possible to have a ground loop in a tube amp (ST-70) that is not connected to any source?
did you tried what's explained/replied previously?
as far as I remember, st70 was sold in kit form too, and there is great assembly manual
if done by manual, with inclusion of ground breaker block, and properly connected case to safety GND, no reason for ground loops
as far as I remember, st70 was sold in kit form too, and there is great assembly manual
if done by manual, with inclusion of ground breaker block, and properly connected case to safety GND, no reason for ground loops
I'm working through it all, @Zen Mod! It's not really an ST-70 but a modern upgrade with different circuity but using same chassis, power tubes, and transformers. I've cleaned up the wiring (better twisting of AC wires in particular, and making sure wires aren't running parallel to each other) but the hum is still there, though quieter than before. (My old ears have to be near the speaker to hear it but my teenage kid can hear it just sitting a couple of feet away.) It's basically a peak every 60 Hz (though there's also something of a peak at 90 Hz). The US mail has been insanely slow with delivering the ground-loop breaker board I ordered...I hope to have it tomorrow and will install. I'm still conceptionally unclear how to get a ground loop in a simple device like one of these amps if it's not connected to any other piece of equipment.
Right now the boards aren't attached to their mounts, so I could have easy access to check solder joints, etc.
Ignore the white wire that's extending out of the chassis. That was an extra ground for testing some things but it has since been removed. All of the stuff at the front of the amp (the top of the picture) is decorative only, not connected to anything. (Original bias-testing sockets, original RCAs).
Currently the RCAs for the amp are not connected and the inputs to the circuit are shorted with two pieces of short wire tied together for each channel.
Larger PCB in top pic is auto-bias module. Small boards are negative feedback selectors (that I really don't need because I only have one speaker output terminal but thought I'd leave in just in case I or a future owner, if I ever sell this, wanted to go to 4 ohm or 16 ohm output.,)
The voltages that I've been told to test are all within normal operating ranges.
Just can't get rid of the hum.
Next step is to install ground-loop breaker which I hope to do over the weekend.
I do not have and cannot get the schematic.
Ignore the white wire that's extending out of the chassis. That was an extra ground for testing some things but it has since been removed. All of the stuff at the front of the amp (the top of the picture) is decorative only, not connected to anything. (Original bias-testing sockets, original RCAs).
Currently the RCAs for the amp are not connected and the inputs to the circuit are shorted with two pieces of short wire tied together for each channel.
Larger PCB in top pic is auto-bias module. Small boards are negative feedback selectors (that I really don't need because I only have one speaker output terminal but thought I'd leave in just in case I or a future owner, if I ever sell this, wanted to go to 4 ohm or 16 ohm output.,)
The voltages that I've been told to test are all within normal operating ranges.
Just can't get rid of the hum.
Next step is to install ground-loop breaker which I hope to do over the weekend.
I do not have and cannot get the schematic.
one thing I can tell from pic - if NFB route is long (and it is) be sure it's made with coax; best with screen connected at signal pcb side, open at speaker terminal end
inform us what's happening after ground breaker install
inform us what's happening after ground breaker install
In this case, could I use shielded single-conductor wire rather than coax (or are those synonyms in this situation)?
Also, is it reasonable to run a wire (in this case from the pcb end of the wire) to the central grounding point for all the circuit grounds?
Also, is it reasonable to run a wire (in this case from the pcb end of the wire) to the central grounding point for all the circuit grounds?
yeah synonyms
in tech slang, coax is shielded, be it single (usually RCA duty) or dual (usually XLR duty); technically, it is coax - only 1 central and 1 outer
if you have GND point at signal pcb handy, ground shield there
It's unclear to me if a solder pad that tests as zero resistance to ground is the same as a GND point. But basically the only obvious GND point that isn't where I have ground wires going to the auto-bias board is so far from the negative feedback solder pads that I should probably just connect the shield to the star ground. The one place that is close to the NFB solder pad that tests as zero resistance to ground is a capacitor (one on each channel) that seems to go from Pin 1 of each channel's 6SN7 to the central 6SN7 and that doesn't feel like a place I'd want to start attaching shields...
Is there any downside to connecting shield to star ground? It's not really inconvenient. If I do it that way, does that wire itself need to be shielded? (I wouldn't think so...)
Is there any downside to connecting shield to star ground? It's not really inconvenient. If I do it that way, does that wire itself need to be shielded? (I wouldn't think so...)
installed shielded negative feedback wire...didn't make a difference that I could tell but it's still cool!
I notice that my B+ voltage is too low (at least 10V) and much lower than it should be given the wall voltage. I am going to try replacing all the EL34 power tubes with brand new (not NOS, actually new) tubes and see if that changes anything even though I doubt it will.
And hopefully my ground loop breaker will arrive tomorrow but I'm not optimistic it will help because a cheater plug didn't help. But we'll see!
I notice that my B+ voltage is too low (at least 10V) and much lower than it should be given the wall voltage. I am going to try replacing all the EL34 power tubes with brand new (not NOS, actually new) tubes and see if that changes anything even though I doubt it will.
And hopefully my ground loop breaker will arrive tomorrow but I'm not optimistic it will help because a cheater plug didn't help. But we'll see!
The ESP version is correct. The mains Earth-to-chassis lug is used also for the GLB connection as shown in Rod's Fig. 5. I've installed that exact breaker circuit in dozens of DIY power amps and have no audible hum or buzz despite the use of single ended interconnects always. Just read the full article at ESP and you'll be fine. https://sound-au.com/earthing.htm#s9
Hi @Milsdrewbulch, I have the board from Hoppe's Brain coming. My current understanding is that it attaches to the chassis with two screws and metal mounts, that the mains ground is attached to the chassis (nearby) with its own lug, and that I run a wire from the Hoppe board to some spot (maybe a little terminal block) where I have connected all the circuit grounds (and which is not connected to the chassis.) Does that sound right?
I wonder if it's rational to think that if a cheater plug on the power outlet didn't help then probably this won't help. But we'll see.
I'm really at a loss right now.
I wonder if there's any way to tell if the transformer itself has a problem, even though that doesn't seem very likely.
I wonder if it's rational to think that if a cheater plug on the power outlet didn't help then probably this won't help. But we'll see.
I'm really at a loss right now.
I wonder if there's any way to tell if the transformer itself has a problem, even though that doesn't seem very likely.
Does it still hum with your input tubes removed ?
Either way it can help you determine the cause.
Either way it can help you determine the cause.
`Most of the hum goes away without the center signal tube and I think all of the hum goes away without all 3 signal tubes. Although I can still hear a rather significant hum from the power transformer. (Hard for me to tell if it's "buzz" or "hum", and I don't know if that behavior is normal, but the main peaks are at multiples of 120Hz, more than at 60Hz.)
At this point I suspect a bad connection to ground somewhere in the right channel because the two LEDs for the auto-bias module are slightly less bright on that side. Cold joint, other bad soldering connection, wiring issue...have to really check small details.
I have a bench meter that has 4-wire resistance capability so I can try to use that to compare connections to ground in the right channel vs left channel and see if I can find something that has some resistance in one but not the other. (I'm kinda guessing here...but not sure what else to try.)
And the ground-loop breaker board arrived so I'll put that together and install it but gonna look for the cause of the slightly dim LEDs first. I mean, it's not inconceivable that something that could cause the LEDs to be dim could also cause a bit of hum, right?
At this point I suspect a bad connection to ground somewhere in the right channel because the two LEDs for the auto-bias module are slightly less bright on that side. Cold joint, other bad soldering connection, wiring issue...have to really check small details.
I have a bench meter that has 4-wire resistance capability so I can try to use that to compare connections to ground in the right channel vs left channel and see if I can find something that has some resistance in one but not the other. (I'm kinda guessing here...but not sure what else to try.)
And the ground-loop breaker board arrived so I'll put that together and install it but gonna look for the cause of the slightly dim LEDs first. I mean, it's not inconceivable that something that could cause the LEDs to be dim could also cause a bit of hum, right?
What you need is dedicated thread and some efforts made, to present more pictures and comprehensive set of schematics