Re: Ground Lift Switch [thread continued from Yahoo group]

[zaphod_phil]

--- In 18watt@yahoogroups.com, "ssofty" <ssofty@y...> wrote:
> I see that some of the original 18-watters had a ground lift switch
> vs. a standby switch. (If you look at an original Marshall catalog
> for the 18-watter, you can see the ground lift in the pic.)
>
> Does anyone know how these things work technically? Were they only
> used when power cords were 2-wire? Or were 3-wire cords always the
> norm with Marshalls?
>
> In either case (2 or 3 wire), how is a ground switch wired? Is it
> even needed on a 3 wire cord?
>
> If you only had Hot and Neutral wires, how does the switch route to
> ground? Maybe I'm missing something.

In the Bad Old Days, before 3-wire mains were commonplace, ground
switches connected a cap from the chassis to either hot or neutral
(Fenders actually had a center off position as well). This lashup
usually buzzed more one way then the other, so you'd try both and
select the best for your amp and then do the same with your PA. The
problem was when "best" was hot for one and neutral for the other.
Your lips touch the microphone while you were playing and ZAP! ouch!

It's completely insane to not use a 3-wire mains connection these
days, and against electrical code everywhere.

In a modern twist, Kevin O'Connor (in TUT3) recommends a ground lift
switch between the chassis and the circuit ground. The lift switch
connect the two when closed, but when open, a 10ncap/51 ohm resistor
combination separates the two. (The chassis itself is still
permanently connected to earth ground).

--mark

[zaphod_phil]

Dang, try that again. Sometomes, when I hit Enter to insert a LF/CR into the text it posts my incomplete message! Something's not right, yet.

OK, back to the subject....

Mark, do a I take it that that the 51ohm resistor is in series with the 10n cap - and these then join the main star point to the chassis? And then the switch splits the two apart? Doesn't sound too expensive or difficult to do. I wonder how they arrived at those two magic values though... )

--- In 18watt@yahoogroups.com, "ssofty" <ssofty@y...> wrote:
> I see that some of the original 18-watters had a ground lift switch
> vs. a standby switch. (If you look at an original Marshall catalog
> for the 18-watter, you can see the ground lift in the pic.)
>
> Does anyone know how these things work technically? Were they only
> used when power cords were 2-wire? Or were 3-wire cords always the
> norm with Marshalls?
>
> In either case (2 or 3 wire), how is a ground switch wired? Is it
> even needed on a 3 wire cord?
>
> If you only had Hot and Neutral wires, how does the switch route to
> ground? Maybe I'm missing something.

In the Bad Old Days, before 3-wire mains were commonplace, ground
switches connected a cap from the chassis to either hot or neutral
(Fenders actually had a center off position as well). This lashup
usually buzzed more one way then the other, so you'd try both and
select the best for your amp and then do the same with your PA. The
problem was when "best" was hot for one and neutral for the other.
Your lips touch the microphone while you were playing and ZAP! ouch!

It's completely insane to not use a 3-wire mains connection these
days, and against electrical code everywhere.

In a modern twist, Kevin O'Connor (in TUT3) recommends a ground lift
switch between the chassis and the circuit ground. The lift switch
connect the two when closed, but when open, a 10ncap/51 ohm resistor
combination separates the two. (The chassis itself is still
permanently connected to earth ground).

--mark

[MarkB]

The resistor and cap are there to keep the signal ground connected to the chassis at radio frequencies only. Otherwise, the switch acts as you describe. It doesn't necessarily have to be connected from the main star, but you get the idea.

Mark

[markh]

As Mark B said, the cap is there to keep the circuit common grounded at RF. If you floated it altogether, you might be surprised to find out how good ao an AM receiver your amp was!

I'm thinking the resistor is strictly a current limiter, in case you happen to playing next to a transmitting tower or something, and represents idealized practice.

BTW, TUT3 is the best book from K. O. since the original, and has detailed discussions or grounding, selecting wire gauges, and mechanical layout in addition to his dissection of what he calls "Iconic" designs from the Champ to the SVT.

--mark

[zaphod_phil]

So I think I now know which TUT to start my collection with! :o)

Anyway, until I get TUT 3 I'm still trying to visualise this very simple circuit. So the cap is always connected in circuit, between star and chassis. Then the resistor is wired in parallel with the cap, but may be taken out of circuit by opening the switch. Is that right...?

As Mark B said, the cap is there to keep the circuit common grounded at RF. If you floated it altogether, you might be surprised to find out how good ao an AM receiver your amp was!

I'm thinking the resistor is strictly a current limiter, in case you happen to playing next to a transmitting tower or something, and represents idealized practice.

BTW, TUT3 is the best book from K. O. since the original, and has detailed discussions or grounding, selecting wire gauges, and mechanical layout in addition to his dissection of what he calls "Iconic" designs from the Champ to the SVT.

--mark

[zaphod_phil]

So I think I now know which TUT to start my collection with! )

Anyway, until I get TUT 3 I'm still trying to visualise this very simple circuit. So the cap is always connected in circuit, between star and chassis. Then the resistor is wired in parallel with the cap, but may be taken out of circuit by opening the switch. Is that right...?

As Mark B said, the cap is there to keep the circuit common grounded at RF. If you floated it altogether, you might be surprised to find out how good ao an AM receiver your amp was!

I'm thinking the resistor is strictly a current limiter, in case you happen to playing next to a transmitting tower or something, and represents idealized practice.

BTW, TUT3 is the best book from K. O. since the original, and has detailed discussions or grounding, selecting wire gauges, and mechanical layout in addition to his dissection of what he calls "Iconic" designs from the Champ to the SVT.

--mark

[zaphod_phil]

So I think I now know which TUT to start my collection with! :o)

Anyway, until I get TUT 3 I'm still trying to visualise this very simple circuit. So the cap is always connected in circuit, between star and chassis. Then the resistor is wired in parallel with the cap, but may be taken out of circuit by opening the switch. Is that right...?

As Mark B said, the cap is there to keep the circuit common grounded at RF. If you floated it altogether, you might be surprised to find out how good ao an AM receiver your amp was!

I'm thinking the resistor is strictly a current limiter, in case you happen to playing next to a transmitting tower or something, and represents idealized practice.

BTW, TUT3 is the best book from K. O. since the original, and has detailed discussions or grounding, selecting wire gauges, and mechanical layout in addition to his dissection of what he calls "Iconic" designs from the Champ to the SVT.

--mark

[zaphod_phil]

Whenever I tried sending my last posting, I got a very ugly SQL debug screen back. So I meesed around a bit trying to get the thing to work, little realising that each time, it had posted succesfully!