Fisrt post... NAD Sunn Sonaro

[Carlsoti]

Hey all. First post here. I just picked this up off a FB market post, for just $200. This is only the second "real" vintage tube amp I've ever bought. The first was a Fender 400PS I picked up in the mid 90's. All of the other's have been things I bought new-ish, then they just got old. I'm gonna go poke around and see what's inside!

[Ryan Phelps]

Congrats on your new Sonaro! Very clean example. Likely dates to 1967- 1968 given the 1st style grille cloth. A word of caution: Don't go "poking around" inside the chassis if you don't know how to dis-charge the can filter cap. Lethal voltages can be present. Some typical work needed on these old Sunn amps: changing the AC power cord from 2-prong to 3-prong; disable the polarity switch and remove the death capacitor; should probably replace the can filter cap and the discrete reservoir cap (if it has one) along with the bias caps as they are all over 50 years old. The amp may need other work to operate properly (coupling caps, tubes, etc.). The early Sonaro is a 40 watt bass amp with a single 15" speaker using a pair of EL34 / 6CA7 output tubes. Some people install 6550 / KT88 output tubes to increase the output to 60 watts, but the power and output transformers are not designed to handle them. These transformers are very expensive (around $300 each + installation) if you blow them. Recommend you take the amp to an amp tech for evaluation before you fire it up. There are many knowledgeable Sunn folks on this forum who will likely chime in to advise as well. Good Luck!

[EdBass]

What Ryan said. I'd at least bring it up on a variac at first.
I agree with the filter cap replacement, however, I have a '68 200S I got around 2000-2001, all there - all factory original but in unknow condition. I slowly brought it back up on a variac and it played fine. Still had the original Mallory can cap filter. I started using it with a JBL K140 loaded I40 cab for small venue gigs, and then when I got in a heavy gigging band in about 2014 It became my #1, because this band had pro FOH sound, and I could mic it into the mains and monitors and not worry about having enough firepower for any venue.

So, on the original 50-year-old filter caps I gigged that 200S for 6 yrs., not a hiccup ever. Gigging once or twice a month in the Fall and Winter, with a heavy (for an old man like myself) gig schedule of 2-3 gigs a week in Spring and Summer. This band pulled most of its annual revenue from large outdoor festivals/seasonal open bars. In early 2020 I had the filter caps replaced because I didn't want to tempt fate any longer with the band's heaviest Spring bookings yet coming up; and then... COVID.
The can cap still tested within spec after it was pulled, the amp was a tad louder and cleaner after the replacement.
True story, but if I had more common sense, I would have replaced them before leaning on that old amp as a #1...

You've got a great little amp there, kinda like a less syrupy, less attenuated highs and bottom, stronger Ampeg Portaflex from that same era. Congrats on your acquisition, and it looks to me like you got an excellent deal!

[Ryan Phelps]

Yep, what Ed said! It's certainly possible to resurrect old filter capacitors with a variac as Ed did. I presumed that the OP did not have the equipment or knowledge to do this. But as Ed noted, new filter caps are eventually necessary for long-term performance. I love that we have so many knowledgeable Sunn folks on this forum!

[Carlsoti]

    Thanks for the welcome, guys. My first post may have been unintentionally misleading. FWIW, I have the tools and knowledge to service this amp. Earlier this year I left the relative comfort of a menial DOD gig in favor of scraping up a living fixing music equipment in my spare room. This amp was purchased with the intent to flip it. At least, that's the story I told my girlfriend. LOL
   I saw the posting on FB marketplace listed as "make an offer." The previous owner said the tubes light up, but she didn't think the amp worked. I knew I was taking a gamble in that I might be buying a giant cabinet loaded with paperweights, but I felt it was worth the drive to check it out. The home was clean and well cared for, as was the amp, so I paid my money and carried the amp out with the side handle!
    I pulled the tubes and did some low voltage DMM capacitance tests on the electrolytics. I then stepped the voltage up through a variac and checked voltages in the amp. They were expectedly high, in accordance with today's higher line voltage. I tested all the tubes on my crappy Mighty Mite II (which I need to replace with something that does testing at "real" voltages.) I dropped the bias to it's most -, and put the tubes back in. Another quick voltage check showed everything settling in to where I was content to bias the tubes to the best of my ability. It came with an old pair of Blackburn built Mullard XF2 EL34s, certainly not a matched pair anymore, but I was able to swap them around and get one to bias just above 60% and the other just below 70%. Everything looked good on the scope and temps seemed stable, so I put it back into the cabinet for a test drive.
   I didn't get the volume up past 3. I only got to play it for a minute or a few before I heard a nasty crackly sound, followed by a rock 'n roll light-show seen through the #2 input. I immediately killed the power. With the thud of the switch, I felt my heart crash to the floor. The light show turned out to be the V1A cathode resistor going up in flames!  With the sparse layout, there was minimal collateral damage.
   I ordered new caps, plate and cathode resistors, a burned in and matched set of JJ EL34Ls, and a plug in adapter and tube to replace the 7199 with a 6GH8A. I've got good 12AX7s to swap into it, should that original GE be dead, and the MMII didn't catch it. It recently showed another 12AX7 as being good, but when driven hard, it was giving static-y/crackly noises in the Wilson 707-3 it came out of. The new parts will hopefully go in over the weekend, but I might get stuck doing yardwork instead.
   Here's a few more pics.

[EdBass]

This amp was purchased with the intent to flip it. At least, that's the story I told my girlfriend. LOL

  I've gotten a whole bunch of mileage out of THAT one...
I literally have a barn stuffed full of MI gear and old cars that have showed up at our house over the years under the "flip" premise. Still works!

As you experienced, the Sunn tube stuff is pretty simple. That said, it sounds great, even compared to today's supposed cutting-edge MI gear.
The only shortfall compared to new stuff is the size/weight vs SPL challenge, which can easily be overcome with modern sound reinforcement technology, tonally the old Sunn stuff is outstanding.

Interestingly, I see the "Guitar Transducer" in the cab. I'm guessing that's a replacement; it looks like a Hartzell era driver to me. When I was in buying mode, I always wanted a nice Sonaro combo amp to add to my "flip" gear. 

[Carlsoti]

EdBass, you seem to know quite a bit more than I in regards to the history and tradition of Sunn Amps. Do you think it would be hurtful to the value of this amp to install a bias balance control in addition to the bias control it came stock with? I can see both sides to this and I'm not sure which way to go. On one hand, it would allow appropriate biasing of slightly mis-matched tubes, like the vintage Mullards it came with. On the other, I can hear someone saying "Why are you using mis-matched tubes?!?!"

[Carlsoti]

IT LIVES... and I love it. My neighbors are not so fond of my new acquisition.  I did a 3-prong cord swap and "death cap" delete, swapped out the caps, even though I'm not certain it "needed" it. New cathode resistor at V1. I also found the 1K NFB resistor to be a bit crispy, despite testing OK, so I swapped that out with a 1.5K I had on hand. Additionally, I used an known good 12AX7 in V1. Overall, voltages came down a bit with the new matched tubes, which I biased to right around 60% idle dissipation; 452V and 0.031/0.032ma.

Does anyone know what kind of amp rating the stock polarity switch has or a part number I could use to find that info? I'm considering using it as a SS/Tube rectifier switch, but I'd like to know a bit more about them before committing to that mod. 

[loudthud]

The original switches were made by Carling. Search for Carling here, look through the threads and try to cross reference that against the Carling website and various online distributors.

[Soundmasterg]

EdBass, you seem to know quite a bit more than I in regards to the history and tradition of Sunn Amps. Do you think it would be hurtful to the value of this amp to install a bias balance control in addition to the bias control it came stock with? I can see both sides to this and I'm not sure which way to go. On one hand, it would allow appropriate biasing of slightly mis-matched tubes, like the vintage Mullards it came with. On the other, I can hear someone saying "Why are you using mis-matched tubes?!?!"

If you were going to mod it for the bias circuit, adding another bias pot so you have one for each of the power tubes is a better way to go rather than adding a bias balance pot. It allows for more versatility and you can adjust both tubes so they will bias properly, which means you can use unmatched tubes, which is somewhat important with the low quality of tubes sometimes today. That said, I wouldn't do it, especially if you plan to flip the amp. It's worked this long with the single bias pot, why change it now?

Greg

[Soundmasterg]

I agree with the filter cap replacement, however, I have a '68 200S I got around 2000-2001, all there - all factory original but in unknow condition. I slowly brought it back up on a variac and it played fine. Still had the original Mallory can cap filter. I started using it with a JBL K140 loaded I40 cab for small venue gigs, and then when I got in a heavy gigging band in about 2014 It became my #1, because this band had pro FOH sound, and I could mic it into the mains and monitors and not worry about having enough firepower for any venue.

So, on the original 50-year-old filter caps I gigged that 200S for 6 yrs., not a hiccup ever. Gigging once or twice a month in the Fall and Winter, with a heavy (for an old man like myself) gig schedule of 2-3 gigs a week in Spring and Summer. This band pulled most of its annual revenue from large outdoor festivals/seasonal open bars. In early 2020 I had the filter caps replaced because I didn't want to tempt fate any longer with the band's heaviest Spring bookings yet coming up; and then... COVID.
The can cap still tested within spec after it was pulled, the amp was a tad louder and cleaner after the replacement.
True story, but if I had more common sense, I would have replaced them before leaning on that old amp as a #1...

Great story Ed!

My very early 2000S still has the original filter caps in it, and it works, though I don't gig with it. I plan to put Solen caps in it when I get the time, but I've been planning that since I got the thing for free in the late 90's. The advantage of the Solens is that I won't have to open the amp up again, and the 2000S has plenty of room for the huge size of those caps. The fact it is a bass amp and good clean power is a desired goal also benefits the use of the Solens. One of these days I'll get to it.

Greg