FourT6and2 wrote: ↑Mon May 26, 2025 5:11 am
Now I have to figure out how to remove the cap and ground it to the correct place. Looks like I might just have to remove the turret board. Ugh.
Dremel with a cutting wheel? Try to think out of the box. There's no set obligation to configure it the way it came to you -- I'm talking about the can cap.
I don't think I understand. Can you explain how to get a Dremel UNDER the turret board to cut the ground connection of a cap that can't be accessed unless you remove the turret board? Maybe it there was a long 90-degree wire cutter with a head maybe 1-2 inches long, that might be able to get under there to cut the ground lug off the cap. But I've not been able to find such a tool.
Without a picture I'm guessing. I think you said the cap is upright on the outside of the chassis. Those are usually fixed in place one of two ways. If you are lucky, there is a clamp at the base of the can that you can release. If not it will be held by metal tabs that go through 2 or 3 slots in the chassis. After the tab is inserted, it is twisted with a pliers to lock it in place. It is possible to cut out a can cap like that by using a Dremel to cut through the twisted tabs that are holding the cap in place. Once you free the cap, any wires can be snipped. If you want to make it tidy, do your best to fish out the snipped leads and deal with them as best you can. It is a good idea to put a clip or hemostat on the wires so don't lose them because you should cover the ends with heat shrink.
Phil_S wrote: ↑Mon May 26, 2025 12:23 pm
Dremel with a cutting wheel? Try to think out of the box. There's no set obligation to configure it the way it came to you -- I'm talking about the can cap.
I don't think I understand. Can you explain how to get a Dremel UNDER the turret board to cut the ground connection of a cap that can't be accessed unless you remove the turret board? Maybe it there was a long 90-degree wire cutter with a head maybe 1-2 inches long, that might be able to get under there to cut the ground lug off the cap. But I've not been able to find such a tool.
Without a picture I'm guessing. I think you said the cap is upright on the outside of the chassis. Those are usually fixed in place one of two ways. If you are lucky, there is a clamp at the base of the can that you can release. If not it will be held by metal tabs that go through 2 or 3 slots in the chassis. After the tab is inserted, it is twisted with a pliers to lock it in place. It is possible to cut out a can cap like that by using a Dremel to cut through the twisted tabs that are holding the cap in place. Once you free the cap, any wires can be snipped. If you want to make it tidy, do your best to fish out the snipped leads and deal with them as best you can. It is a good idea to put a clip or hemostat on the wires so don't lose them because you should cover the ends with heat shrink.
The ground lug on the cap is soldered directly to the bolt that holds the cap's clamp/mount to the chassis. There is no way to free the cap. You can't pull the cap out to access its lugs. You can loosen the clamp all the way until the nut falls off, but that cap isn't going anywhere. And there's no way to access its solder lugs/tabs without removing the turret board.
Maybe this image will help? This is all under the turret board. The (+) leads go up to the board to provide B+ filtering. The ground is soldered directly to the cap's mounting clamp bolt. Even if you disconnect the (+) leads, you can't remove the cap without severing that ground connection. And the ground connection is under the turret board with no way to access.
Screenshot 2025-05-26 at 2.20.19 PM.png
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Based on how replacing that cap with one with appropriate ground lead dress resolved the issue, I’d say it’s probably time to pull the board anyway. At minimum, leaving a dud cap behind with known defective impromptu ground lug would drive me crazy. I also wouldn’t be keen on leaving stray leads loose under the board.
The cathodes of the power tubes, or the ground side of the cathode resistors in the case of cathode bias. This should also include the center tap.and the negative sides of the power tube plate and screen grid filter caps.
On some builds, if you also include the P.I. filter, you reduce noise better than grounding the P.I. with the preamp ground.
With other builds it doesn't matter.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
TUBEDUDE wrote: ↑Tue May 27, 2025 2:35 am
The cathodes of the power tubes, or the ground side of the cathode resistors in the case of cathode bias. This should also include the center tap.and the negative sides of the power tube plate and screen grid filter caps.
On some builds, if you also include the P.I. filter, you reduce noise better than grounding the P.I. with the preamp ground.
With other builds it doesn't matter.
It's easy to move most of these. But tying the power tube cathodes together at one of the filter cap (-) nodes isn't really feasible. As in every Marshall ever built, that I know of, each power tube cathode is grounded to its corresponding mounting bolt lug. Would it reduce any noise to disconnect them all and route them all to a filter cap ground? Maybe. But I don't think it's worth the effort now.
I lifted the board and moved that cap's ground to the preamp node. I don't think the cap is faulty. I think it was just grounded to the wrong spot. Amp is now hum-free
I haven't noticed a difference when moving the output cathode grounds. I just leave them where they are and focus on the preamp grounding and making sure the power supply chassis ground is far away. So basically I am following the idea of the Larry scheme but I leave the cathodes and heater CT alone.
This thread is a reasonable explanation of why random grounding schemes require a a lot of tinkering and experimentation, while star grounding techniques work, without a lot of experimentation.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Maybe I'll take the time to move the bias ground over to the screen ground. But it will most likely accomplish nothing. I already solved the noise issue. The amp is now quiet. Easy enough to do though.
