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

[neikeel]

Modern regs (U.K.) will fuse live and then switch live and neutral simultaneously on the power switch (stipulated if you have a fixed power cord - like most US amps). Not spec ccd if removable cord like IEC but I tend to do it (if not pure restoration where switch is single pole).

 

[NewReligion]

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.
View attachment 157275View attachment 157279View attachment 157280




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

Hello. There are a lot of valid points made here.

First I would like to address the yellow PCB. It was used in the 1973 Super Leads though the amp may be an early 1974. Check your letter code in the serial number.

If you must replace components, cut them out. Then heat from beneath & push the remaining lead through. (Do not pull it out back through the top as this may cause trace delaminating).

If it were me I would pull all pots & inputs to raise the PCB 45-90 degrees in order to replace the components & sold below as intended by Marshall.

Use a solder sucker to ensure the leads actually were cut on the first set of 68k resistors by removing the solder as someone may have just tried to tack a component in that location but left solder residue.

On the Power end of the PCB in a white wire coming in to a 220k resistor. These resistors are of different types between photos though the correct value. Can you explain this to us? That is your “Bias Tap” & usually reads between 97VAC & 105VAC.

Most certainly replace the 10uf 100Volt blue bias caps as you do not want these to fail.

Congratulations, great looking 73/74 Marshall.

David

 

[LoudStroud]

Learned a lot from this polarity switch discussion.

One thing noticed is the Lemco 5000pf bright cap is missing from the Ch 1 Volume pot. I know it’s preferred by many to remove, but I’ve come to appreciate it as an integral part of the Super Lead’s sound and versatility, especially in a live performance context.

 

[dtier]

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.

While it is true that the mains fuse is there primarily to protect the PT primary and all circuity down steam of that, if the mains fuse is ahead of the power switch and the power switch or wiring faults to the chassis, it will blow, instead of having to draw the 15 to 20 amps through the power cord to blow a supply breaker.

 

[Pete Farrington]

While it is true that the mains fuse is there primarily to protect the PT primary and all circuity down steam of that

My understanding is that the primary / regulatory requirement for the equipment's mains fuse is to protect the building, its electrical installation and people in the building, from an electrical fault / electrical fire in the equipment.
Any additional protection eg from collateral damage in the equipment, is a bonus.
Hence the fuse being physically and electrically as close as feasible to the cable's entry point into the chassis.

 

[jgab]

Yeah I’ve learnt a lot too. Good discussion. Appreciate everyone’s input.

 

[Mjh36]

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.

I've tried to find info on other people re-purposing the polarity switch for a bright cap setup without much luck. I was wondering if anyone ever used it like that. Maybe I can ask you about my experience...

I did that mod to switch between 5000pf/100pf, using shielded cable, and I "thought" I experienced more hum. I attributed the hum to the switch being too close in proximity to the transformers, being all the way on the other side of the chassis. So I didn't keep it and wired it back as normal and left the polarity switch empty again.

But I was kind of quick with my play testing. I've always wondered if the hum was just something else that I didn't take into account.

Should there be much extra hum in doing this you think? Others have done this too? If it's worked out for others I'll give it another try.

 

[Purgasound]

The switch is not in a good spot to repurpose for much. Running signal carrying wires that close to the power supply and other AC wires is no good. Its going to pick up unwanted noise. Wiring a relay is more trouble than it's worth.
On old Fender Bassman's it's conveniently in a spot on the back where you can toggle NFB many other different mods.

 

[ThreeChordWonder]

I've tried to find info on other people re-purposing the polarity switch for a bright cap setup without much luck. I was wondering if anyone ever used it like that. Maybe I can ask you about my experience...

I did that mod to switch between 5000pf/100pf, using shielded cable, and I "thought" I experienced more hum. I attributed the hum to the switch being too close in proximity to the transformers, being all the way on the other side of the chassis. So I didn't keep it and wired it back as normal and left the polarity switch empty again.

But I was kind of quick with my play testing. I've always wondered if the hum was just something else that I didn't take into account.

Should there be much extra hum in doing this you think? Others have done this too? If it's worked out for others I'll give it another try.

Never tried it myself, but it should be a pretty straightforward job.

I'm assuming the polarity switch would be a DPDT on-on switch with two sets of three contacts, much like a typical guitar push-pull and wired the same was as a phase (polarity) flipper on a guitar.

If that's the case you would only need one set of three contacts for a bright cap switch setup:

1. Connect the middle lug on the treble pot to the middle or common lug on one of the two banks of three lugs on the polarity switch.

2a. If you want two alternative bright caps connect one end each of both caps to the non-grounded lug on the treble pot and run wires from their other ends to the other two lugs on the same bank of three on the switch.

2b. If you only want one bright cap and a simple with bright cap / no bright cap setup, do as 2a above but leave out the second cap.

2c. If you want three different bright cap values you'll need a new three position on-on-on switch with one common and three switchable contacts.

2d. If you want two bright cap values plus a no bright cap option, replace the switch with an on-off-on three position switch with a common and two switchable contacts. The no cap position will be the "off" position.

As for positioning, I've seen setups using mini-toggles added to front panels and back panels. Personally, I would think as close to the treble pot as reasonably possible would be best.

Using the polarity switch instead is just my idea of how a redundant, and should be bypassed for safety anyway, switch could be used.

Using shielded cable seems like a good idea, provided the cable is about the same gauge as the rest of the wiring, and, of course, as long as you remember to ground the shielding.

 

[ThreeChordWonder]

Hey.

Come to think of it, replacing one of the regular pots with a matched push-pull pot could be used as a bright cap switch too.

I'm going to do that to my JTM45 kit that I'm troubleshooting right now.

 

[Purgasound]

Shielding helps against picking up stray RF interference. It's not a catch-all to prevent picking up AC hum.

 

[ThreeChordWonder]

Shielding helps against picking up stray RF interference. It's not a catch-all to prevent picking up AC hum.

Agreed, but every little helps, provided the shielding is grounded.

Running a shielded twisted pair as opposed to individual shielded " to" and "from" wires and or elevating the shielding to +25 VDC or so (DC - note that) instead of grounding it can help further I have read somewhere. FWIW miles-long underwater "umbilicals" use (or used before fiber optics were adopted) to control subsea oil wells use shielded twisted pairs bundled with high voltage high current AC cables.

 

[Pete Farrington]

Shielded cable helps to block interference from electric fields but is useless when there's magnetic fields. Mains transformers generate strong mains frequency magnetic fields.
To reduce interference from mag fields, getting sensitive stuff (eg high impedance grid wiring) away from the fields is best; failing that, ferromagnetic plates to block the mag flux can work well.

 

[Mjh36]

The switch is not in a good spot to repurpose for much. Running signal carrying wires that close to the power supply and other AC wires is no good. Its going to pick up unwanted noise. Wiring a relay is more trouble than it's worth.
On old Fender Bassman's it's conveniently in a spot on the back where you can toggle NFB many other different mods.

Thanks. Exactly what I was looking for and what I figured too, I really wanted it to work though. Probably why I couldn't find anybody talking about using that switch. Flying too close to the sun I guess...