i just picked up a sunn sceptre head and 4x12 cab on craigslist. the previous owner had it for 15 years and retubed it when get got it and never since. he says he used it every 6 months or so at bedroom volume for that 15 years he owned it.

so, i crank it up and it's brittle, lifeless and with no bottom at all. diming the bass and zeroing the treble does nothing. i tried running it into a soldano 2x12 and it was the same story so it's not the cab (although, the cab is pretty lifeless on it's own but that's a different thread). i know sunns are bright and clean but something is definitely wrong with this guy.

i called my amp guy and he thought it was less likely to be a cap or tube issue and possibly something like an output transformer. i told him i was thinking of making this my first amp tinkering project. he said he's fallen under the spell of sunn's also but now thinks that they're poorly designed circuits prone to problems and that it wouldn't make a good first amp to work on.

so, does it sound like an OT? i'm a proficient solderer and i want to flex my schematic reading skills, am i asking for trouble if i tinker with a sunn rather than something more standard like a SF fender for my first project?

I was under the impression that they were well made. I believe the first ones were based on / built by Dynaco.

I would say your first bet would be the things that actually fail: tubes and capacitors. Since it's obviously an old amp, I'd suggest looking at the caps and see if they look like they are leaking or bursting. Regardless, you will probably want to recap it anyway.

yep, tubes and/or caps would be my guess. might wanna make sure and check your interstage caps, too, the ones that are blocking DC from passing along from each tube's anode (or cathode if there's any cathode follower stages). and for sure check all the electrolytics for signs of leaking, bulging, whatever. if you've got a voltmeter, check how much AC you've got on each filter cap's non-ground side... should be very little AC if the caps are doing their job.

the basis for his saying sunns were poorly designed is that he's tons of them at his shop, he said he saw 6 last week. he said they're prone to motorboating and that "if you move one wire a couple inches, you'll have a buzz you can't get rid of." he was basically suggesting they're a pain to work on and that i'd be better off learning on something like a SF fender which he called a "beautifully designed circuit" and that "everything's right there to see." i don't think he was just yanking my chain to get my business 'cause he didn't sound that excited to work on it himself.

scott's the one to listen to here when it comes to amps.

is the sceptre the 60 watt one?

Hey, I never worked on a Sunn before, and am not even familiar with their circuits, so don't trust me too much!

As far as working on a silverface Fender, I find them to be a pain in the ass compared to amps that are actually easy to work on, which include 60's and 70's amps from other non-Fender folks like Hiwatt, Sound City, Traynor, and even Marshall up to 1974 i.e. before they started using circuit boards.

The Sunn amps may be easy to work on, or may be hard, I have no idea. I'll bet somebody here has worked on one or 10 of them.

As far as moving a wire around having that drastic of an effect, I guess it's possible if it's that unstable of a circuit. If that's the case, I would expect you should be able to find numerous websites that describe what changes to make (capacitor or resistor values) that will result in a more stable amp. It's a well-enough-known brand that there's a band named after them for cryin out loud. Fender's using the name again nowadays, right? There's gotta be resources that'll tell you how to fix it, if it's truly a "broken" circuit design.

Does your tech have a tube tester that he'll use to check out your tubes? You might be up against something as simple as a bad preamp tube, which takes zero skill to fix. Pull out bad one, plug in good one, done. I would try and find out the nature of the problem before deciding whether or not to forget about working on it.

But if anything, I'd wait a day or two and see what kinda responses you get from other folks here. Like I say, I'm sure that *somebody* here has worked on a Sunn amp before, right?

madlee wrote:scott's the one to listen to here when it comes to amps.

is the sceptre the 60 watt one?

ya, it's the 60 watt, 2 kt88, reverb and trem amp. also, i guess it's the older non-mid-switch version.

thanks for the info scott, next thing i was gonna do was playing with the tubes, unfortunately, the lot of them are reasonably non-standard and not stuff i have kicking around. it might just have to make a trip to my amp guy after all....

I don't know if Sunn was a regionally-popular amp or something, but I've seen comparatively few around Mpls.

I don't even know anyone that uses or used to use a Sunn except for some odd solid-state Betas and such. I do have a Sunn Concert Bass, it's solid-state and put together very nicely.

So, I've never worked on one, and I've never looked inside either.

Scott's posts are too long, so I don't read them. Sorry if this is repeating in 50 words what he said in 500.

As I recall, the tube Sunn amps used a variation of the Williamson power amp output stage, with a feedback winding off the output transformer approximating the "ultralinear" design. If something goes wrong with that loop, or if the output transformer isn't wired correctly, you'll fuck your sound right up.

steve wrote:Scott's posts are too long, so I don't read them. Sorry if this is repeating in 50 words what he said in 500.

That's the problem with your generation, Steve: all that pong and black&white teevee growing up, and you don't have the attention span enough to read for hey look, what's that shiny thing over there, oooh neat!!

steve wrote:As I recall, the tube Sunn amps used a variation of the Williamson power amp output stage, with a feedback winding off the output transformer approximating the "ultralinear" design. If something goes wrong with that loop, or if the output transformer isn't wired correctly, you'll fuck your sound right up.

If they're derived from Dynaco hi-fi amps, that seems reasonable. Ultralinear is generally not used in musical instrument amps cause its whole purpose is to decrease distortion. Right?

That might be a fun project, to take this amp and set it up for a conventional screen supply instead of an U-L one. Or, just tie the screen to the plate supply so it operates in triode mode.