74' Super Lead Board - What to replace & Polarity Switch?

[jgab]

This is my first super lead, so I am looking for some info on the board and what is the standard maintenance you guys do. I have a bunch of Iskra and Piher spares so I will use NOS/OEM parts to keep this looking original.

I am thinking of replacing the following:

- 27k bias resistor ahead of the diode with a 1/2 watt Piher. Current resistor is obviously not original. Terrible replacement job too. Lazy tech. This was replaced twice as seen by the carbon film and then flame proof resistors in the different photos. Don't ask, its a bit of a story.
- 10uf bias caps with a couple new Sprague 10uf/150vdc. These blue bias caps look original to me, but not 100% sure.
- Should I replace the bias pot? Is the one in the board reliable? I have some black plastic Piher 22K.
- Should I replace the V1 330uf cathode bypass cap? Does this usually need replacing?
- Weird soldering on a couple 68K grid resistors (second photo). I will clean that up a bit. Make sure the connections under the board are good. Again, lazy tech style.
- What is that huge blue cap on the left? It is 0.047uf / 1000 vdc. Is this necessary? I have seen some super lead boards with and some without this cap.





Everything else looks pretty solid to me. I measured what I could without pulling leads and everything is within 10% spec.

I know these early ST1 boards are super delicate, but I have a good iron and good technique so I will be ok.

Any suggestions are more than welcome?

Thanks as always.
J

 

[Jon Snell]

In reverse order.
The 1000v 47n capacitor is there to remove any HF interference from the mains. It may or may not do anything.
Remove components (by lifting up and hinging the board) don't do as other do and clip the component wires to solder a fresh component to the old solder joint and wire.
That is asking for trouble and bad reliability.
Replace the circled components and fit new, properly.

 

[Purgasound]

Looks original aside from the bias supply resistor. It's a balancing act for that part since it may have to be altered in the future to accommodate the different current draw of a new tube set. If you had some extra Piher's then you could swap it out. I'd leave the leads just a hair long so they can be reused if it has to come out again.

The bias caps should be replaced, a necessary evil. Same with the filter caps.

A 1M pot has been replaced. I think I have an original from the same year.

That big blue cap, I disable those with extreme prejudice. If the capacitor fails short it can put full mains voltage live to the chassis. It's part of the polarity switch wiring. I defeat the polarity switches on these whenever I'm restoring them. I keep the switch in place and use it as an anchor point for the mains wiring so there's no need to pull any wires or do any extensive rerouting. The red wire going to the polarity switch just gets anchored to one side and will no longer be tied to anything on either end. Sometimes that wire might be black, I can't see how this one is wired up. I just did this on another '74 last week.

 

[Jon Snell]

Looks original aside from the bias supply resistor. It's a balancing act for that part since it may have to be altered in the future to accommodate the different current draw of a new tube set. If you had some extra Piher's then you could swap it out. I'd leave the leads just a hair long so they can be reused if it has to come out again.

The bias caps should be replaced, a necessary evil. Same with the filter caps.

A 1M pot has been replaced. I think I have an original from the same year.

That big blue cap, I disable those with extreme prejudice. If the capacitor fails short it can put full mains voltage live to the chassis. It's part of the polarity switch wiring. I defeat the polarity switches on these whenever I'm restoring them. I keep the switch in place and use it as an anchor point for the mains wiring so there's no need to pull any wires or do any extensive rerouting. The red wire going to the polarity switch just gets anchored to one side and will no longer be tied to anything on either end. Sometimes that wire might be black, I can't see how this one is wired up. I just did this on another '74 last week.

Marshall do not use a "polarity switch". That capacitor goes from the HT +ve to chassis. Totally isolated from the mains supply.

 

[david_clough]

I find it perplexing, one long time engineer says one thing and another long time engineer contradicts it.

That big light blue cap looks to go from the HT to chassis, so should it fail it will take out the HT fuse I recall these are good quality caps, whereas I have failures with RIFA (used in mains AC) which die with a great stage show, smoke, smells etc.
It's inclusion by Marshall HQ "might" have been in response to a particular situation, for a specific market/country, so as mentioned it may or may not have much effect.

Anyway each to their own. I'm actually very familiar with all the components in the photos, they were the best of their type at the time My comment would be, if anything renew the bias trimmer. Some people might comment those capacitors have "mojo", even the resistors are good quality. My view is, "if it ain't broke don't fix it" - each to their own.

 

[Purgasound]

Marshall do not use a "polarity switch". That capacitor goes from the HT +ve to chassis. Totally isolated from the mains supply.

With all due respect, that is incorrect sir.

U.S. import models indeed had a polarity switch that switches a cap off the hot and neutral wires. That cap is connected directly to the incoming mains. Take a second look at the photo. The blue wires for the HT come in beside the diodes. All of the other wires on turrets are terminations for the PT primaries.

If the cap ever fails short it will put full mains voltage on the chassis, hence the term "death cap".

 

[Purgasound]

I find it perplexing, one long time engineer says one thing and another long time engineer contradicts it.

That big light blue cap looks to go from the HT to chassis, so should it fail it will take out the HT fuse I recall these are good quality caps, whereas I have failures with RIFA (used in mains AC) which die with a great stage show, smoke, smells etc.

Please see above response.

That cap is not connected to HT. I know the ones you think you are referring to but this is not a snubber.

I respect Jon's knowledge and experience but they don't have the same perspective on this one. I'm in North America seeing these every day and they will likely never have a Unicord import model come across their bench.

I think it's important to correct misinformation about dangerous stuff like this though.

 

[Jon Snell]

That is a reasonable answer to what it does. I am not familiar with US built Marshalls, a more complete photo showing rhe 'polarity' switch (how weird is that to call it polarity, from the days of DC power I presume, as there is no 'polarity' with AC mains. Just live/line and neutral/cold).
You are looking at the amplifier and I have not got it on my bench. The death cap is normally fitted directly to the switch, not terminated on the PCB.
It is good to consider these things as Purgasound writes.
Thank you for that.

 

[21fret]

That is a reasonable answer to what it does. I am not familiar with US built Marshalls

No worries, neither is the rest of the amp world, no Made in U.S.A.

 

[jgab]

 

[ThreeChordWonder]

Marshall do not use a "polarity switch". That capacitor goes from the HT +ve to chassis. Totally isolated from the mains supply.

Absolutely.

On some  very early amps, Marshall copied the polariity switch over from the same era Fender amps, in an era when US electrical wiring could be "interesting".

Today, however, it is at best an anachronism, and more likely pretty freaking dangerous to include one at all.

Bypass it totally. If you want to leave the switch in place for cosmetic reasons or just to fill up the hole, but totally isolated electrically, thats fine.

You could even re-purpose it for something like a bright cap on/off.

It should not be part of a safe, modern power supply, however.

 

[Purgasound]

Your photos are from two different time periods inside the amp and neither show a complete picture of what's going on with the primary wiring. I know we've been talking through emails but I wanted to put some of the information here so others could follow along should they need to eliminate the death cap.

The easiest way for this case is to simply remove it from the board entirely.

The long way, which is how I do it, is as follows... This method can be utilized when leaving the cap in place or removing it. In restorations that have the Erie caps or these big blue ones I might leave them for posterity but I disable them from any contact in the circuit. This method involves removing no wires and simply relocating them. There is a minor caveat and I will go into detail about it. It's fairly simple if you start with the power cord...

-The HOT/LIVE wire from the power cord I will run to the mains fuse first.
(Here's the caveat... This means your mains fuse end will be hot even after the fuse is blown. Some will debate it's better to have the HOT wire run to the switch first so the fuse is "safe" when the power switch is off. I suppose there's some validity in that but on the other hand it's a boneheaded move to mess with fuses and have the equipment still plugged in. Just unplug the equipment whenever diagnosing fuse issues and it solves that problem. )

-The other wire from the MAINS FUSE will typically be yellow and this will run to the top lug of the power switch. This is left in place.

-The bottom lug of the POWER SWITCH is linked to the bottom lug of the POLARITY SWITCH. Remove this link. Snip off and desolder and remove the wire from the power switch only.

-The top lug of the POLARITY switch should have a black wire attached to it. This wire runs to one of the turrets on the PCB. This is for models with four primary voltage taps and no voltage selector switch. This turret is usually accompanied by a wire linking the power switch to the appropriate primary wire for the power transformer. Remove this wire from the top of the polarity switch and move it to the bottom lug of the power switch. The AC path should now be plug live wire - fuse - switch - PT primary voltage wire

-The wire connected to the center lug of the POLARITY SWITCH will go to the turret that the capacitor is connected to. Move this to the empty top lug of the polarity switch. Now since the middle lug is disconnected, this wire can never be energized and will always be dead. I will explain why I think it's safer this way in the end...

-The bottom wire on the polarity switch is typically the COMMON wire for the POWER TRANSFORMER PRIMARY WINDING. This can be left where it is but you would attach the NEUTRAL wire from the AC POWER CORD here.

That should be it. This way, the NEUTRAL wire from the AC POWER CORD is connected directly to the POWER TRANSFORMER PRIMARY COMMON WIRE. The LIVE or HOT wire from the AC POWER CORD is properly fused and connected directly to the power switch and the wire running to the turret is now completely dead and can never be energized. The death cap can be left in place for visual or nostalgic purposes. In the simple one-step version where the cap is just removed the only real problem would be that the wire going to that turret could still be energized. Nothing really wrong with that aside from having a live wire inside an amp that has no purpose. I go through all the above steps so I don't have to remove any of the factory wires from the bundle. I try to preserve as much as I can when working on vintage amplifiers as we have to make other concessions later when it comes to broken pots and aged electrolytic capacitors.

Cheers.

 

[david_clough]

I did think to add, I have no knowledge of US spec mains circuits, I'm 100% UK. I gather the US mains is more prone to rough incoming AC.
Got me thinking... Marshall know what they are doing, so some designer, maybe influenced by North America customer feedback thought that cap as described was a fix. Dunno. Yey for UK mains.
Just to add to the debate, we do use the term "polarity" in UK AC main systems - it's more of a specialist area. When I was at college training.. it got defined, used and tested for. I can see how it could be used in US mains systems but I could not comment on it.

 

[Pete Farrington]

I'm familiar with the use of 'polarity' to describe the non inversion - inversion status of an AC waveform, eg flipping live and neutral.
The term 'phase' is often used similarly but generally incorrectly, eg the 'phase' switch on mixer channels.
Phase being a property that's tied in with time and frequency, can't affect one without affecting the others. Whereas inversion and polarity are independent of time and frequency.

And DC is just 0Hz AC

 

[jgab]

-The HOT/LIVE wire from the power cord I will run to the mains fuse first.

So this is weird, started making some changes and noticed the white neutral wire is connected to the fuse. Has anyone ever seen this before? This doesn’t make sense.

This is from factory too as the connections still have the red inspection marks on them. White neutral wire went to the fuse, then the yellow comes off the fuse goes to the switch.

Should I wire the black hot to the fuse instead?

 

[Purgasound]

A lot of Marshall's were wired that way. I don't understand the logic behind that. It works but it's not ideal which is why I change it. The fuse could technically be on either side to do it's job protecting the power transformer but it's better practice to have the fuse interrupt the hot side. This would be safer so there is no chance for the hot to continue to ground out if the primary shorts to the shell of the transformer. Not a common scenario but it's better to increase safety wherever possible, no matter how unlikely a specific fault might be.

 

[Pete Farrington]

Should I wire the black hot to the fuse instead?

Yes.

the white neutral wire is connected to the fuse. ... This is from factory too as the connections still have the red inspection marks on them

I wonder if Marshall just got confused. Back then, UK colour code was red = live, black = neutral.
It feels horribly wrong with US amps that still have the original mains cable to see the black fused.

 

[jgab]

-The HOT/LIVE wire from the power cord I will run to the mains fuse first.
(Here's the caveat... This means your mains fuse end will be hot even after the fuse is blown. Some will debate it's better to have the HOT wire run to the switch first so the fuse is "safe" when the power switch is off. I suppose there's some validity in that but on the other hand it's a boneheaded move to mess with fuses and have the equipment still plugged in. Just unplug the equipment whenever diagnosing fuse issues and it solves that problem. )

Not sure I like this idea or see a point to it.
I’m just going to wire the power cord (hot and neutral) to the top two lugs of the switch and then wire the fuse and PT primary on the other side of the switch.

I’m also switching the polarity and power switches around so I get all the poles I need on my power switch, and the polarity switch with only two lugs has nothing on it.

 

[Pete Farrington]

Not sure I like this idea or see a point to it.

The point is that in the event of an issue, the circuit is de-energised as early as feasible after live enters the chassis.

 

[jgab]

And what’s that protecting? When you flip the mains switch, the
fuse still protects what it needs to whether it’s before or immediately after.

I would rather have constant energy on just the switch rather than the switch and the fuse.