1974 120W SL/A Restoration Project v2.0

So now that it’s completed, I feel the need to make revisions.

I just finished the schematics for the changes.
I need to add back in a bright cap for just the clean side as it doesn’t have enough brightness compared to the HI input side after the mods.
Stay tuned, I’ll have pictures of my journey :)
 
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So after much deliberation, I decided the only way to make the new features work was to do it the right way and redesign a new relay board. The downside to this is that I couldn’t get anymore of the original relays I liked and covid has complicated the process of finding substitutions. I finally found a relay that is comparable.

I had to redesign a new board itself to allow for 6 relays plus a fuse block to be mounted.

I also had to redo the schematics as certain parts will have to move around to keep some of the original features I desire. I’m still updating schematics for version 2.0 and debating where I want to drill the hole in the back.

Heres pictures of what I’ve come up with for the rough layout of a new relay board. I’m happy with how it’s turned out and I can re-use a lot the original wiring and connectors. I do have to repurpose a lot of pins to flip flop certain features either to NO or NC so that functionality when everything is unplugged is working as it should. It’s a fairly complicated build by itself and I’m taking my time adding these new features while encompassing lessons learned on the first go around.

When I have schematics, truth tables, and updated layout diagrams completed, I’ll be able to post an update on the fun side diving into the amp.


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I’ve designed a gain boost mod that I like which adds some mids. It makes the amp just a monster and still on 3 gain stages. It’s at a level you have to pull some bass out to keep it tight, but it’s equally as much a feel thing as well.

I sourced some push pull potentiometers to change out a few on the amp to make it work.

Here’s the amp without the gain boost:



And here it is with the gain boost/mid shift. In the beginning you can hear me pulling bass out of it slightly. Sorry for the bad playing, I wasn’t planning on sharing at the time of recording.



I’m still tweaking it tonally and documenting as I go. I don’t have any updates for the switching mechanism at the moment, I’m trying to get other life things in order first before I blow this thing apart a second time. I only want to do all of the changes once.
 
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Update, working on cleaning up the top end. It had a sag type of sound before and I found the culprit. I plan to make it a “smear” type of switch which can take the clarity out and smear the top end as recorded in many prior videos or add the super aggressive top end as recorded below. It’s a ton of gain and the mic has a hard time picking up what’s changed.

 
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Here’s the latest version of the amp.



4 gain stages on the hi input, 3 gain stages on the low input. The low input footswitch design was faulty and I disabled it to record this video.

The amp is currently under the knife getting an all new relay board. I had a bad relay causing high frequency oscillation. I don’t trust any of the others now so I’m trashing the original board and I’ve hand made a PTP new one.

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Once I get this board installed, I also have an amazing upgrade coming in that I just put an order in for.

Because I’m using 6550s and I added a tube, my B+ has dropped significantly. For reasons I don’t want to mention here, the dropping resistors cannot change values. The whole amp is just too low of nominal voltage and I worry about power handling of the stock PT with 6550s long term.

So, I just placed an order for a new Mercury Magnetics PT with a very high B+ (500V+ at 0.5A) on the secondaries. The model they’re using is normally lay-down style and they’re going to make it stand up style. I chose black end-bells for the color.

I’ll definitely record a before and after PT upgrade. The OT is still a stock dagnall. I’ll eventually pull the OT down the road and use both of the original dagnalls for a rainy day parts build. I’ve always wanted to build a 100W in a 50W chassis. For now though, it’s staying.

Stay tuned! Lots of stuff happening with this build :rock:
 
SpiderWars said:
Sounds killer! You could also rescue a cherry vintage piece that had a swapped OT or PT with that iron you have.
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Thanks!! She’s a bastard child if there ever was one.

I’m a bit more hesitant to pull the OT from the amp because we all know that’s a big part of the amps tone and response. I’d replace it also with a MM transformer, but really I wish I could still get a classicTone :(
 
No updates on the amp, I work 2nd shift now and I’ve had a lot of life things getting in the way lately.

I’m planning on working on this thing this weekend.

I plan to finish replacing the new relay board
I need to wire in the additional relay’s functionality
I want to solve the effects loop location. Currently it’s after the master and I want it right off the treble pot.
And last once I have all of that working, I need to tune/fix a coupling cap and also work on the depth pot

After all of that, I’ll make a video. Then I have the MM PT to install. After the install, I plan to make another video. I’ll make sure the bias points are identical for each. I will have to rebias it with the new PT but the tubes will already be burned in.

Lots of work to do to this thing to finish it, but afterwards I’ll be fully content.
 
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