bassdriver wrote: Sun May 28, 2023 10:33 am Fantastic job, Nate!

Thanks!

Guess what? I have another mouse nest amp on the bench now, courtesy of FM Garth. Stay tuned!

Sunn 2000S:



FM Garth dropped this off on my porch while I was sleeping, with instructions to replace the 2-prong power cable and give it a checkup. Very early build (May 1969), well within the first year of production. Later versions apparently had a couple extra electrolytic caps that don't appear on the schematic, based on the photos I'm seeing elsewhere.

Picked it up and heard some rattling inside the chassis.



Fairly fresh mouse nest, with what looks like stuffing from a pillow or speaker cabinet in there, plus some seeds and droppings.

After not having to deal with any mouse nests inside an amp in 20+ years of fixing amps, I get two in the same week.

Not particularly gross in there, just needed some vacuuming and a douse of alcohol to clean it up.
They had gorged themselves on wire jackets though, particularly the ones on the power transformer.



Something had happened here also, near the bias circuit. Looks like there was a short or arcing between two terminals on that strip back there, which had melted and scorched some adjacent wiring. I cleaned up the carbon as best I could, and I'll relocate the connections to some of the unused terminals to prevent this from happening again.



Multi-section capacitor had dumped its guts inside as well.

I was able to get the wires cleaned up last night. Added heat shrink over the exposed sections, replaced some that were too far-gone to repair.

This thing will get a full recap job, as well as replacing a lot of those large carbon comp resistors that have drifted way out of spec. Tubes all test good.

Why do old caps have such rad names and logos. Astron... Aerovox... etc. always look cool as hell.

Kniferide wrote: Wed May 31, 2023 10:22 am Why do old caps have such rad names and logos. Astron... Aerovox... etc. always look cool as hell.

The Planet ones are great also:

A Sunn 2000s has been my “dream” bass amp for a long time. Not because I need it or have ever even played one, they’re just super cool

They're comically large!

There's so much open area inside the chassis. And the transformers are huge, at least as big as the ones on my Traynor YBA-3A, despite the output power being like a third of the Traynor's.

Nate Dort wrote: Wed May 31, 2023 11:23 am

Kniferide wrote: Wed May 31, 2023 10:22 am Why do old caps have such rad names and logos. Astron... Aerovox... etc. always look cool as hell.

The Planet ones are great also:

Capacitor Logos were universally awesome. Its weird. Bird Truline made a bunch of radio stuff and their logo also kicks ass.

Nate Dort wrote: Wed May 31, 2023 6:59 am Sunn 2000S:

Got this thing recapped and brought up slowly on the variac. Had some buzz and no signal prior to the phase inverter. Started probing around and found a couple of resistors that came apart as soon as I touched them.



Also found a couple of broken solder joints, similar to what this guy found:



Replaced the resistors and touched up the broken joints and got output, but there was a significant amount of 120 Hz hum, which means it was post-rectifier, i.e. on the B+ rails. It's probably low enough hum for rock & roll stage use, but I wouldn't record with it.

Tried different rectifier tubes first, no change. Also added some solder blobs to the terminal-strip-to-chassis ground connections as dude showed in his video, as there were a couple of suspect ones.

Just to prove my assumption, I temporarily stuck a 5 W 1 kohm resistor in series with the first B+ filter cap, which eliminated the ripple, but dropped the B+ voltage too much and was dissipating too much heat.

I think the Sunn engineers must have came to the same conclusion, that there wasn't enough filtering, so they added some 20 uF 600 V caps in parallel with the first 30 uF 525 V cap on the B+ rail on later production amps. I've seen both 1 and 2 extra caps, depending on the year of production:




A GZ34 rectifier tube can drive up to 60 uF of capacitance, but we have two GZ34s in parallel in this case, so I should be able to add some significant filtering to the rail. Need to place another digikey order.

EDIT: Looking back through that video, it appears that the big BEAVER cap IS the first filter cap, and the 30 uF cap in the can is ignored. That terminal is floating. I suspect the hum issue is due to ground loops, and relocating the first filter cap closer to the PT reduces the hum. I'm going to play around with some star grounding techniques and see what happens.

Glad you found that video Nate - it's not a hard one to find, I think the only repair video for that amp anywhere, but still found it interesting, those broken solder joints kinda weirded me out, especially since I always heard Sunn stuff was pretty well-built. I mean not like TRAYNOR well-built (throwing it off a roof) but still.

Turns out he's a local guy too! https://d-labelectronics.com/

Anyway, these updates are really cool - thank you!

Garth wrote: Tue Jun 06, 2023 8:56 am especially since I always heard Sunn stuff was pretty well-built.

There's some definite craftsmanship in this Sunn. That wire harness wrapping technique is a lost art. Yamaha did the same thing when they wired their consoles. I've spent countless hours snipping that Yamaha wire-wrap twine when I was racking PM1000 channels. Nobody takes that time anymore, not since zip ties were invented.

The build quality overall is pretty good. They used higher-quality parts and obviously had some skilled people doing the work. Overall, it's a way better build than some other amps I've seen from that era.

We're here, 50 years later, expecting it to still work perfectly? That's 40 years beyond the expected service life of one of these things.

Would I have done some things differently in the Sunn? Yeah, like not using those riveted tag strip terminals as grounds. Or routing the heater wires further away from the DC supply lines and twisting them to reduce stray magnetic fields.

Or fixing the 120 Hz hum issue like I just did by star-grounding the PT center tap and bias cap ground to the first filter cap ground point at the can cap twist-lock connection. Didn't even need to order that 47 uF 600 V cap like I did this morning. I could have saved Sunn literally hundreds of dollars in 1970-74 and got that promotion and bought that sweet Pontiac LeMans and met my wife at a mutual friend's backyard cookout and raised 2.5 children and taken them on yearly vacations on a single income and then join a bowling league and throw out my shoulder and turn into an alcoholic and alienate my teenage children and stay in a bitter marriage but still manage to put my kids through college on a single income but then Reagan comes along and fucks everything up and I die of a heart attack in 1994 because I had been eating a whopper and drinking a case of Olympia every day.

I would have done that differently.