1974 120W SL/A Restoration - Lets Go!

Info for maintaining and tweaking your amp to perfection.

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glpg80
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by glpg80 » Sun May 17, 2020 5:55 pm

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.

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neikeel
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by neikeel » Mon May 18, 2020 4:09 am

Keep them coming - watching with interest
Neil

danman
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by danman » Mon May 18, 2020 6:08 pm

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

glpg80
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by glpg80 » Mon May 18, 2020 7:26 pm

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
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by glpg80 » Mon May 18, 2020 7:35 pm

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
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Re: 1974 120W SL/A Restoration - Lets Go!

Post by glpg80 » Fri May 29, 2020 9:40 pm

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

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