1974 120W SL/A Restoration - Lets Go!

[glpg80]

Sent a PM!

I realized my dumb butt made a rookie mistake in the heater wiring for the preamp tubes. The DC filtering caps for the heaters are wired to negative instead of positive potential. Ugh.

I have to remove the wiring to replace all of the sockets anyway so it doesn't mean unnecessary work has to be done, but it's worth noting.

Is what happens when you can only work late at night.

[glpg80]

Was able to complete the bias board installation and connection, so that is now complete!

I also started the relay board layout while connectors are coming in

I also finally connected the indicator light.

Still have two sockets to replace, it’s taking longer than I wanted due to the number of shielding compromised wires needing replaced along with it.

[neikeel]

Keep them coming - watching with interest

[danman]

That's one heck of a relay board...very nice!!

[glpg80]

Thanks for the kind comments guys!

The relay functions are for the three features:

Two master volumes
True bypass control of the effects loop
Hi/Lo input

Eventually I’ll have the effects loop bypass a push/pull audio taper resonance knob, but for now it will always be in circuit and bypass-able with the footswitch.

You might be thinking, 3 features, 5 relays?

The master volumes actually take 2 relays to do properly. People normally switch the hot side of the volumes or the middle taps of the potentiometers, but neither are correct. This allows you to set one potentiometer that isn’t selected and it’s value has to be equal or greater than what you have selected, otherwise it acts as a volume limiter due to parallel connections.

To do it correctly, you must switch the hot side and middle wipers both, with the grounds kept individual so that the unused potentiometer is always ground referenced for safety.

You don’t want to switch the grounds and middle wipers because if a treble cap faults, you’ll have a potentiometer that is floating hot to B+.

The relay board is fused just in case the footswitch wire is rolled over, crushed, etc.

I have gold plated pins and brass connectors coming in to actually wire up the relay board. The relays are gold plated Panasonic’s made in Thailand and not in China. They should last quite a while.

I spent a lot of time designing the schematic for the footswitch and relay control carefully. You want the controls to act as a normal amplifier (effects loop in circuit) with the footswitch disconnected. This allows me to play it at home without the footswitch installed unless I desire the added features.

It’s coming along slowly but surely

[glpg80]

One more thing worth discussing, you have to be careful with group loops. I’ve made two large ground connections for the whole amp - the buss bar and the ground of the filter cap for the DC.

The heaters and relays all tap back into the same ground.

The bias pots have two black wires running all the way back to the bias board which are individual grounds for each side and both are ground referenced at the buss bar. This is to prevent ground loops. You don’t want to use the chassis as ground in too many places because ground loops can be horrible sources for noise.

I’ve done all that I can to make sure the grounding scheme is kept clean and proper in this amp. I’m hoping my efforts will provide a quiet high gain Marshall

It’s funny reading the old TGP topics talking about how bad of an amp this was. I’m hoping once all of this is done that it’s something that is completely opposite and speaks quality.

[glpg80]

Preamp tube sockets are completely replaced with new sockets and all compromised wiring also replaced as well. The DC heaters are officially done, as is the dual bias circuit.

Next up is to will be to assemble:

2nd master volume
global resonance control
wire the new inputs
Last will be the relay board

[glpg80]

Relay board is half way done. I added the bypass switch for the effects loop, I couldn’t find a SPST switch built into a 1M pot that I liked.

The switch will just control the relay from the back similar to the footswitch control. I go back and forth as to whether I want it to defeat the relay entirely or just control the relay.

[glpg80]

Signal inputs mounted, wired up, and ready for connection to the relay board. Relay board is mounted and wired up to the supply.

Next are the footswitch inputs, then resonance, and last is the second master.

The effects loop can’t be installed or wired up until I have it on and biased with the new tubes to get an accurate new B+ with the removed preamp heaters which will cause the B+ to raise.

Still a lot of work to do, but it’s getting there.

[danman]

Looking good so far! Your workbench looks like mine...lol

[glpg80]

It’s controlled chaos - everything is in its place just not neatly haha

I did try to keep it clean as I went but it gets crudded up again just as fast. I’ll clean it up when the project is done lol

Thanks for the words! It’s slowly getting there. I feel like I make progress but then I realize just how much more I have to do to get it done. Seems like it never ends!

I have to purchase a nice fluke meter that can do peak rms ac current readings and store them. Once I burn in the new caps under no load and check the new supply is working, I’ll need it to size the mains fuse. I think 4A would still work but if the mains voltage is ever lower than 115 I’m afraid it will pull too much current and pop it since I added the supply to it. I don’t know, I’ll see.

[glpg80]

All inputs are complete and footswitch relay control is complete. I need to modify the footswitch itself and complete the relay connections to the rest of the modifications. Slowly getting there.

Please excuse the solder quality in these, I finished late last night and plan to go back and touch them up when I have time this weekend.

[glpg80]

Question, should all original nuts and bolts be replaced? I have the hardware but have been using the originals along the way. I’ve only had one screw break in the process and it was my fault that the capcan holder wasn’t flush first.

What say you?

My gut tells me that old screws should go and not to trust them. But I don’t really know of anyone else admitting to do the same?

[glpg80]

I was saddened to find out a previous owner had broken and resoldered the original treble pot for the amplifier. I was almost done wrapping up the last wire for it and it fell apart as soon as I put heat to it.

The point of this build is reliability and gig friendliness. So I replaced the pot with a new CTS pot that I had on hand. I’m not happy about it as it’s not in the range of the original, but it is what it is.

I also only have to finish the relay connections and the effects loop wiring. Because the effects loop wiring can be done with the board mounted, I mounted the board back down and it looks great!

Getting really close to initial fire up. It will be a while before it’s complete though as I need to buy some socket bias probes, new tubes, another multimeter, and a new oscilloscope for my bench with a built in waveform generator.

I did order some new old stock ceramic capacitors to tune the resonance modification when the time comes.

[neikeel]

Looking forward to hearing it.

I have some spare 250k RS labelled CTS type pots (salvaged from Marshall PA amps), but I think shipping to your side of the pond might be silly for such a small pot. I doubt the pot in your pic is original as it is a pcb type, but you never know!

I tend to use all the original brass screws and nuts. Nothing needs to be tight enough to break any of them. I do, however always add internally serrated lockwashers on everything and nip those up.