Anybody heard of a Mark Sheets Amp?

[Platefire]

Got a Mark Sheets Amp in that is modified 5F6A circuit in a tweed combo with 2-10 speakers. It's point to point on an eyelet board--nice work. Customer said amp works fine but he took the backplate off and ground switch and fuse is not connected--and he is concerned about that!

Gave it a close look over when I got home. The ground switch was apparently installed for looks because no wires are connected. On the fuse, the whole body of the fuse is wrapped in blue insulated sock/blanket--but the wires come in from the top against chassis and go inside the sock. So you can't see the wires unless you look close. So everything is copasetic! That was a easy fix 

[HotBluePlates]

Never heard of it, but the guy is probably a low-volume amp builder like us.

I'd bet the chassis he used is an off-the-shelf repro chassis, which would have the ground switch hole and marking cause the original had it.

Because a hole-plug will look cheap on an amp you're selling, I'd probably do the same thing he did: install a switch not connected to anything, because the ground switch is not needed.

Otherwise, you go whole-hog and have no connection from the chassis to ground, except for the 3rd wire of the power cord. With everything isolated from the chassis, you can then add a 51Ω resistor and 0.01uF 1000v cap in series from circuit ground to chassis. The SPST ground switch would then short that resistor/cap combo.

What that does is allow a safe ground lift which connects circuit ground to chassis and 3rd wire of the power cord (for everyday operation); if you get ground loop hum when splitting the guitar signal to other amps or by using certain outboard effects, you can throw the ground switch and disconnect the circuit ground from the chassis/power cord 3rd wire. The resistor and cap still shunts RF interference to the 3rd wire, and since the 3rd wire is always solidly connected to the chassis, it doesn't present a shock hazard.

The tricky part is making sure everything is isolated from ground, which means you might have to not follow certain build practices you use now.

[Platefire]

Well thanks! I never heard of that type grounding lift but I can see where it would be handy to have if you need it. This amp had an evelope stapled to the inside and I looked inside and pulled a schematic out. It hasn't got a mid control, just bass, treble, presence, a bright volume and normal volume. Kind of different. Platefire

[kagliostro]

As further documentation you can download this file from Merlin's Web site

all text is interesting but read page 13 & 14 (fig. 13.17 Hum-loop block network)

http://valvewizard2.webs.com/Grounding.pdf

K

[HotBluePlates]

HBP, could you make a schematic of what you posted? 

Are you describing a an isolated ground? 

I'm simply suggesting not using the chassis as a wire at any point of the circuit. This might require the use of plastic Cliff-type jacks (or shoulder washer for Switchcraft jacks), and pots which you are sure do not have the contacts electrically connected to the shell. All grounds, center taps, filter cap negatives get connected along a buss isolated form the chassis.

The only wire that connects to the chassis is the 3rd wire of the power cord. That keeps the chassis at zero volts, and keep you safe if some high voltage point inside the amp ever shorts to the chassis. In that case, the circuit breaker back at the service panel trips.

The connection from "circuit ground" (what the upside-down triangle ground symbol really means) to chassis ground is like p. 14 of Merlin's document that Kagliostro linked, except I've used 51Ω in series with a 0.01uF cap, instead of a parallel arrangement. This is dead-nuts common in pro audio equipment; see for example the Line Input of this Jensen Transformer schematic.

Well thanks! I never heard of that type grounding lift but I can see where it would be handy to have if you need it.

Have you ever seen someone lop off the ground pin of their 3-wire power cord cause they got hum with all the stuff they had rigged together?

They got the hum because more than 1 piece of gear had a ground pin on the power cord, but the shields of the guitar cables between the gear were also connected to circuit/chassis ground. People cut off one of the ground prongs and the hum stops.

But that ground prong is there to keep you from being electrocuted if there is a massive fault in the amp that puts high voltage on the chassis. So cutting off ground prongs is a no-go.

If instead the interconnected gear has a ground lift like I describe, all but one of the items can have its circuit ground lifted off the chassis ground (and 3rd power cord prong). They will all still find a path to that 3rd wire through the interconnecting cables, but the ground loop is eliminated and hum stops. Further, all equipment still has a solid connection form the chassis to the 3rd prong of the power cord, so there is no risk of electrocution.

There are other ways to do it; Jensen tries to convince you to buy one of their fine transformers to isolate the equipment at the input, because it's probably easier to do that as a modification on an existing piece of PC board mass-produced equipment. They still show the ground lift switch as I described.

If you just plug a guitar into an amp, or use only a couple pedals with batteries or a 2-prong wall wart, you will spend 95% or more of your time with the ground lift switch in a position that connects the circuit ground to the chassis. I know right now in my amp, I get a slight buzz with the circuit ground lifted off the chassis. That said, if I had other gear with its own 3-prong power cord interconnected, I wouldn't get buzz in the lifted position, I'd get a silent background. If I didn't have the ground lift, then I'd get ground-loop hum in the same situation.

In practice, you might not even mark which position is which on the ground lift switch. You'd flip it to whatever setting gives the least noise, just like you did in the 50's-60's with the original Fender ground switch and 2-prong cord.

[Platefire]

Here in the deep sunny south the ground plug of a three conductor cord gets clipped off because so many old buildings here still have two conductor wiring. You show up to play at one of these places and don't have a 3 prong to 2 prong adapter, your out of luck---so you cut the gound lug off. This is getting less frequent now days that a lot of the old buildigs have gone away but just a few years ago this was more common than not. Platefire

[Platefire]

Well I personally never have cut the ground off a three condutor cord but just speaking of what I have observed on other peoples amps around here where I live. I've always made it a habit when I play out to be prepared for every situation and usually have a bag of tricks with extra extension cords, adapters, strings and tools. If you play at a lot of different places you will eventually encounter about every strange and insane guitar rig set up obsticals known to man--and you have to be prepared! 

[Willabe]

I think they do that for a designated high current circuit. So no one plugs anything else into it.



                 Brad