New Marshall project - 1970 50w Lead Restoration

[Roe]

Thanks for the advice Roe! I was just reading up on the switching on these amps and came across a method to eliminate the “hot switching” that Marshall built into these. Let’s see if I can type it out correctly…

1. Connect the PT Center Tap directly to main chassis ground point.
2. Attach HT Leg 1 (Red) from the transformer secondary to standby “in” pole 1. Run bias feed directly off this AC leg.
3. Run HT Leg 2 (Red) to standby “in” pole 2.
4. Route Pole 1, Output of the Standby switch to one of the AC (~) terminals of the KBPC5012FP block.
5. Route Pole 2, Output of the Standby switch to the other AC (~) terminal of the KBPC5012FP block.
6. Wire the Positive (+) terminal of the rectifier directly to the input side of your HT Fuse Holder (0.5A or 1A slow-blow).
7. Wire the output side of the HT Fuse Holder directly to the positive lug of your Mains Reservoir Filter.
8. Connect the OT/CT and choke “in” to the HT fuse output

1. CT should go to mains cap ground, which -in turn - goes to ground (buss/chassis)
6. 1A slow-blow is ok on 100w amps, not on 50w.
8. often the corner of the preamp board is used for this, not the fuse holder

consider two resistors across the standby switch, one for each leg

[Raoul Duke]

Got it Roe, thanks again.
Upon re-reading my post; I think I misunderstood the switching. What I typed is still hot switching, isn’t it?

Re: 1 - understood, RGKs “one true ground”.
Re: 6 - understood, so 2 amp mains/.5 HT then?
Re: 8 - also understood, so leave the DC routing from rectifier into the HT fuse, HT fuse out to reservoir cap, cap to the empty turret in the lower left hand of the board along with the OT/CT and choke “in”?

Also, for the resistors: across = from each “in” to “out” side to soften the voltage surge or to trickle charge the filter cap? I’ve got some 5w wire wound 82k sitting here that seem like a good choice to me, but I’m still a bit unsure as I’ve never implemented this standby mod before.

Really appreciate the help! I was looking at the empty turret and thinking it looked like the same tie-in point as my JTM45 build but was thrown off because a lot of Marshall gut shots show the OT/CT and choke on the HT fuse out. But as I’ve seen - model year context and best practices are sometimes not always the same.

[martin manning]

I was just reading up on the switching on these amps and came across a method to eliminate the “hot switching” that Marshall built into these. Let’s see if I can type it out correctly…

The main thing for the hot start issue is to take the AC for the bias supply direct from the PT's HT winding, before the standby switch.

[Raoul Duke]

Ah, OK - thanks Martin. I wasn’t sure if it also included switching the DC instead of AC.

To be honest, I’ve read so much about this in the past few days it’s all kind of blurring together.

[Roe]

Got it Roe, thanks again.
Upon re-reading my post; I think I misunderstood the switching. What I typed is still hot switching, isn’t it?

Re: 1 - understood, RGKs “one true ground”.
Re: 6 - understood, so 2 amp mains/.5 HT then?
Re: 8 - also understood, so leave the DC routing from rectifier into the HT fuse, HT fuse out to reservoir cap, cap to the empty turret in the lower left hand of the board along with the OT/CT and choke “in”?

Also, for the resistors: across = from each “in” to “out” side to soften the voltage surge or to trickle charge the filter cap? I’ve got some 5w wire wound 82k sitting here that seem like a good choice to me, but I’m still a bit unsure as I’ve never implemented this standby mod before.

Really appreciate the help! I was looking at the empty turret and thinking it looked like the same tie-in point as my JTM45 build but was thrown off because a lot of Marshall gut shots show the OT/CT and choke on the HT fuse out. But as I’ve seen - model year context and best practices are sometimes not always the same.

1 yes
6 yes. 500ma slow blow
8 yes corner is traditional solution

yes resistors go from in to out and soften the surge and charge the filter caps slowly. Normally 2-3w are used, typically 47k-220k. With low values you risk getting some sound signal when the amp is on standby

[Raoul Duke]

Fantastic, thanks again Roe!
Your Marshall expertise is always appreciated!

[Raoul Duke]

Got some more done today and starting to figure out the switching IRT routing the wires and such. Probably do that tomorrow. Getting to the point where I’m probably going to stop until the power transformer gets here.

IMG_0559.jpeg

Choke got here today and I got that in. Was a bit challenging but it all worked out. Blends in pretty well except for looking new I guess.

IMG_0560.jpeg

So I've been staring at switches for so long and getting turned around based on the switch orientation and chassis in the stand upside down, I decided to make a drawing to explain what I think is the correct way to wire them so that they operate as they should. Any critique is appreciated!

1987 Switch Wiring.PNG

Thanks!

[Raoul Duke]

Actually, I think this is the right way:

Switches Corrected.PNG

[martin manning]

If I understand correctly you are showing the lug side of the switches as viewed from the rear through the open side of the chassis. To close the contacts the bats would be thrown toward the open side of the chassis, “up” in your drawing. Looks good to me.

[Raoul Duke]

You understand correctly Martin and thank you!
I don’t know why these seemingly simple things get me so turned around. It’s almost like a “who’s on first” loop going on in my brain when I start trying to “see” the way it should work.

Second drawing, right?

[sluckey]

Second drawing, right?

Both drawings do the same thing.

[Raoul Duke]

But I think they orient the “on” position of the bat differently, don’t they?

I always thought that switches like this are either open or closed - but trying to orient the bat correctly has me second guessing. I’ve been round and round with this so much I’m a bit lost at this point - so please put me on track if I’ve got it wrong Steve.

Thanks!

[sluckey]

But I think they orient the “on” position of the bat differently, don’t they?

No. You would need to move the wires on the bottom lugs to the top lugs to change bat orientation. Or, physically rotate the body of the switch 180°.

I always thought that switches like this are either open or closed - but trying to orient the bat correctly has me second guessing. I’ve been round and round with this so much I’m a bit lost at this point - so please put me on track if I’ve got it wrong Steve.

When dealing with simple two position SPDT or DPDT toggle switches, the bat is oriented opposite from the selected contacts. IOW, bat down selects the upper two contacts or switch up selects the lower contacts. Maybe this simple pic will explain it better. I'm showing a 3-lug SPDT switch but the same principle applies to a 6-lug DPDT switch. Hope this helps.

If the toggle switch has center position the orientation may be different.

[martin manning]

Maybe a cut-away showing the mechanism will clear it up.

[Raoul Duke]

Thank you both for the diagrams - they do help. Does it make a difference if the switches are configured with one set of end lugs and one set of center lugs? Mine are DPST and it appears that’s how they are configured.

Also, to your point Steve: the difference between my two drawings is where the wires are in the switch. If it’s one end and center - then it’s the same either way, correct?

IMG_0385.jpeg

And if that’s the case - there’s truly no best practice for which end is “in” and which is “out” then?

Thanks!