1974 120W SL/A Restoration - Lets Go!

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

Post by glpg80 » Thu Jul 16, 2020 3:53 pm

Most relay control functions wired with shielded wire. Only the effects loop is left, and I’m wiring it as we speak externally to be able to scope the signal levels to match it correctly.

I also modified the effects loop so that the line level or instrument level switch will become the toggle switch. I plan to wire it temporarily as mounted and will then shorten the wires slightly for proper mounting.

I also plan to mount the effects loop upside down so that send is on the left when facing the rear of the amp, and not on the right.

Once I have the effects loop mounted externally, I plan to build my own 4 tube bias probe off of an external meter. They’re so expensive and 4 tube models are made in China and with questionable hardware.

The reason for the bias probe is so that I can reinstall and bias the original tubes to get the correct plate voltage measurement to know what B+ dropping resistor is needed for the effects loop.

It’s getting there slowly. I’ll have more updates on the bias probe design next.

Image

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

Post by glpg80 » Thu Jul 23, 2020 1:34 am

I finished the footswitch controls for the amplifier. The black wires are to add some stability to the small somewhat fragile signal wires, and aren’t connected or used for any purpose. The small signal wires are double shrink wrapped - once as twisted pairs after the initial splice and again near the plugs to provide more stability and to hold them to the black dummy wires. I used clear so that I could match the wires to labels later on and also keep an eye on wear/tear down the road.

Image

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

Post by danman » Thu Jul 23, 2020 6:23 pm

Nice job! Looking forward to hearing some clips when you get it finished up :shred:

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

Post by glpg80 » Thu Jul 23, 2020 10:10 pm

Thanks Dan!

I need to build some bench items first. It will be a while before it’s completed unfortunately.

I need to build an analog current meter into my iso transformer. I want to make one plug 0-5A monitored and another plug 0-10A monitored on the secondaries. I’ve considered making a standalone plug in unit as well, not sure.

I also need to build my own 4 plug bias probe. I didn’t add 1 ohm resistors to the cathodes because I installed two external bias potentiometers so that external bias management could be done without pulling the chassis. That means I need to make it now to do initial bias after function test startup.

I don’t want to pay the price that people are charging for bias probes because I believe I could build something better for the same price IMO.

The last thing I need are more handheld meters and a lot of clip on probes.

Essentially I need to upgrade my bench and these are the bare minimums. If there’s a problem in the relay wiring, I’ll need a sig gen and digital scope as im not floating my $1000 500MHz analog scope :mrgreen: :lol:

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

Post by glpg80 » Mon Aug 24, 2020 11:37 pm

Ordered my new power tubes and balanced long plate phase inverter tube. I’ve built two bias probes, but have two more to go as I post this - these things are challenging to get to sit flush, but I can build them for way cheaper than what I can buy them for. People must think they’re made of gold or something?

I also finished building my battery powered bias segment display. Each one draws roughly 60mA and can run all the way down to 3V, so a single 9V battery will do fine.

When I have the bias probes completed, I need to flip back to the amplifier and finish the effects loop. I decided I didn’t want to mount the board upside down as it would make the install with the send power level switch very messy and unprofessional looking. It also wouldn’t allow adjustment in real time of the send level for accuracy/tuning.

That’s my update. It’s getting there, but it’s challenging. The build has slowed down significantly as I am purchasing a new home at the moment in parallel. Too many projects lol

Edit: I’ll add images later. The board is having problems accepting image hyperlinks from imagur.

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

Post by glpg80 » Sun Aug 30, 2020 1:12 am

Well as a small update, my chinese 8 segment auto-ranging displays have failed me.

They came delivered with no datasheets which I thought was purplexing, but I continued designing my quad bias probe box anyway. They did auto range and measure their own respective 9V battery voltage correctly, so I did perform a function test of them individually upon arrival.

What I did not anticipate though, was that their range was stated to work from 0V to 100V. That to me meant they should measure 0.01 to 0.05 ranges just fine when in fact they displayed 0.0V the entire time. They couldn't register anything less than 100mV minimum.

I ordered a new box, new rotary switch, and supporting plug hardware to make a 4 way bias probe measurement tool that can utilize an external meter. I'll simply cut the probes off the current box (not worth the time to unsolder what is there) and re-use them as a pluggable probe into the box. In this manner, I will have the ability to measure anywhere from 2 tubes to 4 tubes at a time. The probes themselves will be modified with banana plugs so that a single probe can work with a multimeter.

It's a large setback on the bias measurement tool, but lesson learned. Keep it simple.

I need this bias probe so that I can bias the amplifier with the tubes installed, otherwise, my B+ voltage will be incorrect for the metro-loop install.

Anyway that's my update. Still no pictures - I cannot upload anything until the board is fixed.

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

Post by glpg80 » Wed Oct 14, 2020 10:59 pm

Just wanted to state the marshall is completed and wrapped up. The chassis was signed with an official completion date of 10/10/2020.

I had a relay control issue where the DC current of the LEDs on the footswitch were causing the relay to be unable to unlatch from NO to NC. I had to install pullup resistors rated to handle idle current to both sides of the shunt diodes.

I also had a problem with a shielded cable for the front master shorting out to ground for the signal. That one took me a second to debug, but in the process I noticed the original potentiometer for the master volume only went down to 8k at 0 after cleaning, so the potentiometer had a dead spot in it at low volume. I replaced the master volume on the front panel with a new potentiometer.

So that makes only two original potentiometers needing replaced - one due to mechanical failure from previous work, and another due to a dead spot or poor function at low value.

The last problem I had to fix was DC on the resonance pot. I put 0.022uF in series with it to block DC and it works great.

Anyway - the amplifier biased up and the tubes drifted a bit needing correction, but it stabilized at 46mA at 460V loaded B+ which put it well within the safe range of 60% AM PDR.

Tonal notes - it has less gain than before the complete rework, but the resonance mod added a lot of full mids. It's much more clear sounding now - not as grainy and not as muddy as before likely due to the higher B+ and fresh set of tubes all around.

Noise wise it is the quietest marshall I have ever played - at full tilt through a small 1x8" speaker with the volume pot on 0 on the guitar, it was quiet as a mouse. Virtually no hiss.

I cannot wait to grab an SD-1 and stick it in front! It turned out great and it feels amazing to have accomplished this build.

Thank you all for your help - I have a one of a kind build that is my own and it is everything I could have wanted out of a vintage marshall tone and more.

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

Post by neikeel » Sat Oct 17, 2020 9:22 am

Some final pics would be nice and some clips in use :rock:
Neil

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

Post by glpg80 » Sat Oct 17, 2020 12:31 pm

The board is broken as I’ve mentioned from previous posts. George doesn’t seem to have the funds to fix it :(

Here’s a direct link:

https://imgur.com/a/I7RLBhw

No sound clips unfortunately. I have no cabinets or anything to play through beyond my bench setup. The amp will go into storage until the boss approves spending for some 4x12s. I sold off a lot of gear in college to afford rent and groceries, so I still have a lot to recoup.

But, there’s the finished pictures at least :)

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