Sunn Spectrum II resto

[gui_tarzan]

Sometimes people amaze me. I bought this amp many years ago to rebuild and just got the chassis done. The cabinet was covered in a nasty black vinyl, a horrible job and covered in mouse pee and rust. So today I tore off the vinyl and was stunned to see the original Tolex still intact. Scuffed up a bit but that's just character as far as I'm concerned. Apparently a former owner was in a band called "Pursuit" at one time or another.

Some days I hate people but I came out way ahead on this one.

[gui_tarzan]

Due to the amount of rust and corrosion on the original parts this is a total gut & rebuild. The original PT was junk but the OT is still there along with an original tremolo board donated to me years ago by another member. But there is one part I'm having trouble finding and also understanding why it was made this way. The trem control is a 3-gang 25k pot. Why? I looked for quite a while today and can only find what I might be able to use at a UK electronics supplier.

This one isn't mine, but it shows the 3-gang 25k pot.

[loudthud]

No tellin' what they were thinkin' when they designed that circuit. Probably fighting low beta transistors, crappy electrolytic caps and bean counters in the front office as well as trying to get some kind of linear sweep on the speed control without a reverse log pot. If you can't find that pot, you're kind of stuck re-designing the circuit including the Depth transistor and the opto-light bulb thingy.

[gui_tarzan]

I suspected that. Thanks.

[Greg Robinson]

By varying all three CR filters simultaneously rather than just the one you can get a wider, more stable range of oscillation frequencies. You could try a much more easily available dual gang pot and vary only two stages, which will still buy you a bit more range than the more common single variable resistor, but you probably won't be able to get the full LFO range of the original without the 3-gang pot.

[nevetslab]

Having restored & modified a Sunn 1200S back in 2016, one of the changes I made on it was on the input jacks. Never been a fan of leaving input jacks without a Mute function (Normal wired to Ground to short the inputs). I came up with the following arrangement using a pair of Switchcraft Type 12's and a pair of Type 13A's. Both have the Normally closed leaf contact for grounding, plus the Type 13 has an additional contact that becomes engaged when a cable is plugged in. This arrangement yielded four different responses, along with having the sensitivity change as is typical.




1200S Input Jacks wiring.pdf

Switchcraft 1_4 in. Littel Jax drawings.pdf

[nevetslab]

By varying all three CR filters simultaneously rather than just the one you can get a wider, more stable range of oscillation frequencies. You could try a much more easily available dual gang pot and vary only two stages, which will still buy you a bit more range than the more common single variable resistor, but you probably won't be able to get the full LFO range of the original without the 3-gang pot.

You might try contacting State Electronics https://www.state-elec.com/ They still show carrying the Allen Bradley Mod Pots, which are a salad-bar type pot assembly, and can be easily configured into a 3-gang pot of whatever value and taper you want. Now, it's been years since I've bought product from them, and no idea what restrictions they now have on folks like us only needing one part. But, worth a try.

I've attached the Allen Bradley Mod Pot Catalog 70.

Allen Bradley Mod Pot catalog70.pdf

[nevetslab]

You mentioned "Due to the amount of rust and corrosion on the original parts this is a total gut & rebuild" That Sunn 1200S I accepted for restoration was definitely in that category externally, though on the inside, it was at least clean.



It took a while, but I did manage to get it cleaned up.

[gui_tarzan]

Very cool stuff! Good info too. I was able to find a 3-gang pot at Mouser. It's a Bourn unit, not like the original but at least it should work the same.

What did you use for the lettering?

I had to do some cleanup on the inside and hit the top where it was bad, but the corrosion on the parts themselves told me it was in a very humid area for a long time. I did leave the factory date markings though, they're too cool to lose just to have a bright chassis.

[nevetslab]

Very cool stuff! Good info too. I was able to find a 3-gang pot at Mouser. It's a Bourn unit, not like the original but at least it should work the same.

What did you use for the lettering?

I had to do some cleanup on the inside and hit the top where it was bad, but the corrosion on the parts themselves told me it was in a very humid area for a long time. I did leave the factory date markings though, they're too cool to lose just to have a bright chassis.

For the Lettering, I have a Brother P-Touch label maker, and a variety of tapes of different character heights and background (Clear, Blk on White, Wht on Blk)