Ground lift question.

[dragonbat13]

I'm repairing a vintage, 66 bassman and am having grounding issues. For the sake of improvement, I am going to implement a slightly better than stock grounding scheme via ground buss and other methods in Merlin's grounding write up.

On thing I have wanted to do to this particular amp is to install a ground lift. However, I want to use the original ground switch, which seems rather inconveniently located.

Is there any reason I couldn't run the ground buss as is typically done on these amps behind the pots, have the isolated jacks and all, and run a ground wire back to the original ground switch? I would install the lift circuit at a location close to the switch. I know it's probably more work than it's worth but I like a challenge.

Thanks in advance.

[Phil_S]

Before you go to all of that trouble, have you considered whether the original circuit board has become conductive? The material is known to absorb moisture, causing a variety of problems. Before you decide there's a ground problem, make sure that's the problem to solve. Remember, when these amps were sold in 1966, the ground scheme worked reasonably well. It may be that the board needs to be replaced and a new one built. If the old parts are still OK, they can be reused if you think there is some mojo in them. Truthfully, though, no one remembers what that amp sounded like over 50 years ago.

Of course, if you've already considered all this, please excuse me for butting in.

[romberg]

What exactly do you mean by ground lift circuit? I'm assuming you going to install a modern 3 prong AC chord and disable the whole deathcap thing?

If you do this and then follow Merlin's advise, you have only two connections to the chassis. First the safety earth ground from the ac chord connected to chassis that is NEVER removed. It is there to keep you alive.

The second connection from the circuit to the chassis is somewhere near the input jacks. This is for reference and shielding. No current flows on either of these under normal conditions. I see no reason to lift or disable this second connection either.

The whole ground switch on those fenders is a leftover from the days of non-polarized two prong chords. Rewire it to modern standards and make the ground switch cosmetic only.

Mike

EDIT: If you mean ground lift as in lifting the ground of the tone stack to take it out of the circuit and give you a gain boost, then I'd just use a push pull pot rather than running the tone stack ground wire all the way to the back

[pdf64]

I think that a network similar to CR5, CR6 & R23 is being referred to.

https://el34world.com/charts/Schematics ... manual.pdf

I can’t see any benefit in shorting such a network out.
Just leave it permanently in circuit, no point in a switch.

I suggest that the network’s chassis connection be close to the input socket. Or failing that, a decoupling cap eg 0.1uF ceramic, between the input socket sleeve and a nearby chassis point.

[dragonbat13]

15.10: Ground Lift
To combat the problem of creating a ground loop via mains earth when
connecting two devices together, some appliances will offer a ground-lift switch.
This disconnects the audio ground from the chassis, breaking the loop. However, the
connection between chassis and mains earth is NOT broken. The chassis must
remains earthed at all times!


https://www.valvewizard.co.uk/Grounding.html



Copied from Merlins site. I suggest reading the whole article like I did.

[dragonbat13]

I think that a network similar to CR5, CR6 & R23 is being referred to.

https://el34world.com/charts/Schematics ... manual.pdf

I can’t see any benefit in shorting such a network out.
Just leave it permanently in circuit, no point in a switch.

I suggest that the network’s chassis connection be close to the input socket. Or failing that, a decoupling cap eg 0.1uF ceramic, between the input socket sleeve and a nearby chassis point.

I can see what your saying. It actually did cross my mind. I may just built it at the input jack area.

I would still like to know for sure if the wire would cause issues. I guess the only real way would be to try.

[pdf64]

Unfortunately Merlin doesn’t explain what the benefit of having the network switchable is
The network is a permanent feature in plenty of gear.

A long wire run to the ground switch would probably be ok provided there was a decoupling cap at the input socket.

[Phil_S]

If you allow the circuit ground to float, what is accomplished? Why don't we see this as typical construction? You only state there are "ground issues." Please describe the symptoms that lead you to think a ground lift will be beneficial. Doing that will help us to help you.

I see that you think there may be a ground loop. It likely wasn't present when the amp was built. That is why I suggested the board is conductive. That would be a good place to look for a ground loop. I am well aware my suggestion does not address your initial question. I keep thinking you are asking the wrong question.

[dragonbat13]

I never posted the ground issue symptoms. There is an intermittent break in one of the factory grounds in the preamp. I've already fixed one, cathode on the shared stage. Amp worked great till I put it back in the chassis. New filter caps and cathode caps, and all the other E Caps.

The amp does not have a hum. But it does have a hum when used with a pedal and a one spot, in the same house receptacle. So I want to see about a solution to multiple devices plugged into the same receptacle, which is the reason for the lifted ground.

Since the second time the amp has cut out, I've decided to upgrade the amp to a buss type setup, not guaranteed that it's going to fix the problem, but it sure seems that way after all the other troubleshooting.

I definitely know what a fender fiberboard shorting out is like down here in one of the most humid parts of the country. My twin is going to have to be rebuilt because of this.

And I'm going to try this simple circuit in the ground buss to try to make the amp more user friendly for the person it's going to.

I don't make repairs to amps from a service tech point of view,if that makes sense? I look at it from the aspect of it were someone who knows nothing about them, and would be put off by using a setup such as mentioned above. It can't hurt to try, and if it doesn't work I'll leave it out.

[martin manning]

What model Bassman is this?

[pdf64]

If you allow the circuit ground to float, what is accomplished? Why don't we see this as typical construction? …

It breaks earth loops via mains earth between different items of mains powered (and earthed) equipment.
I think it is a typical feature of more recent designs, especially those where connection to other mains powered gear is expected.
See the network around D18 and D19, bottom centre on p2 of https://www.drtube.com/schematics/marsh ... 2-iss3.pdf

However, I think that generally, the DC output of the fx power supplies are isolated from mains earth. Hence earth loop hum via the mains earth shouldn’t arise in the scenario described.
Perhaps the one spot is bad, has battery power been tried?

[dragonbat13]

What model Bassman is this?

66 AB165

And FWIW, I may have said the shared gain stage, but I can't remember which circuit uses the stage for both and which uses it only on the bass channel? I'm not even positive if it's that. I found the issue via chopstick, reheated the ground and it worked on both channels. It wasn't before. I will have to look at the schematic this evening to tell for sure.

[dragonbat13]

If you allow the circuit ground to float, what is accomplished? Why don't we see this as typical construction? …

It breaks earth loops via mains earth between different items of mains powered (and earthed) equipment.
I think it is a typical feature of more recent designs, especially those where connection to other mains powered gear is expected.
See the network around D18 and D19, bottom centre on p2 of https://www.drtube.com/schematics/marsh ... 2-iss3.pdf

However, I think that generally, the DC output of the fx power supplies are isolated from mains earth. Hence earth loop hum via the mains earth shouldn’t arise in the scenario described.
Perhaps the one spot is bad, has battery power been tried?

The one spot may very well be suspect. I don't have the ability to work on it or access it at all times. The setup was a DS-1 with the onespot. I didn't have a battery, but that will be the first test on the next go round.

The ground issue cutting the amp, or at least what I suspect, was with the guitar directly plugged in.

I'll see if I have some pics I can post if possible.

[dragonbat13]

My plan is to detach the fender grounds on the pots to the cases on the back, add a buss to each channel, tie the appropriate grounds to each, isolate the jacks, and then install either a ground lug directly to the chassis or the lift in between the lug and buss. I'm gonna forget about the switch

[pdf64]

…
The ground issue cutting the amp, or at least what I suspect, was with the guitar directly plugged in.
...

The network can’t help with that.
That issue must be within the amp (or guitar or its cable).