Sunn Sceptre

weird name, weird amp,

gut shot,  no, not a bullet wound,

has RC filter before B+ which is weird,

has 6AN8  phase splitter which is weird, triode/pentode type,

weird transistor vibrato  instead of tube which actually sounds nice, goes slower and faster than Fender.

those black cat's are gonna start leakin pretty soon, let them go for now, no positive grid mv so ok.

nice wiring job,

gleened as much info as we could without a customer teardown , 

oh yeah, another weird thing, ultra linear OPT with 6550 taps at 33%.

flat to 100K which is good on a EI 150 core (bigger core = more C, lots of leakage (700 mH))

standard Shumacher config is probably gonna go Sec - Pri - Sec - Pri - Sec  (from DCR)

problem was a Ruby 6AR4 rect, replace with GE NOS 5U4 on a new ceramic socket with tube retainer,

OPT sq wave at 1 K>

The reason for the UL transformer and 6AN8 tube is that Sunn amps started as Dynaco hi-fi amps with a custom preamp in a cabinet made by Conrad Sundholm in a garage in Oregon.  (This was circa 1965 or so.  Conrad's brother was Norm Sundholm, bass player for the Kingsmen of "Louie Louie" fame, and a bunch of other frat-rock classics, and he built the first one for him.)

They were either a Dynaco Mark III (UL 6550) or Dynaco Mark IV (UL EL34) in a head cab with their separate custom guitar/bass preamp.  Later built on their own complete chassis, supposedly still buying power and output (UL) transformers from Dynaco.  Eventually they added solid state add-on boards for tremelo & reverb to some models, and even later, went to all solid state amps, of course.

I have one of the early ones with a full Dynaco MK III amp inside.  But if you want to see something REALLY weird, I also have an amp that is some kind of early Sunn copycat, called a "Polaris," which is conceptually similar: a Dynaco Mk. IV (UL EL34) with a custom preamp in an even more strangely shaped head...

I played bass through a big Sunn head and 15" cab in one of the bands I was in high school (late 80s). It was like luggin a full grown walrus around!  But it was thunderous! Loaned to me by my next door neighbor who had played in Bob Will's Texas Playboys in the 60's (well past the glory days).  Along with a '64 Jazz Bass (lucky kid).

Not the same model, but looked very similar.

thank's for the cool info!  i wonder if hafler and keros ever found out about that,

Conrad did a two 15" folded horn thingy also,

check this out!>

https://www.namm.org/library/oral-history/conrad-sundholm

6AN8 was worked, funny voltages so we replaced that with a Sylvania,

note error in schemo on 6AN8,  (red line)

note weird feedback from UL tap>

The parallel tag strip arrangement is a nice way to go ,allows a lot of flexibillity and it can survive falling out the back of a moving van or down a flight of stairs,makes renovation and component replacement a breeze too .

Ruby tubes ,yep Ive seen em ,absolute garbage and a liabillity in any amp ,PM tubes are another I avoid like the plague.
I once ordered 1100 US worth of Sovtek from Tubedepot ,the package was light as a feather and it slipped through regular postage,so I avoided any extras. Of the whole batch only one EL34 was bad .

CJ said:

thank's for the cool info!  i wonder if hafler and keros ever found out about that,

Conrad did a two 15" folded horn thingy also,

check this out!>

https://www.namm.org/library/oral-history/conrad-sundholm

Click to expand...

You're welcome!  Cool link there, too!  I read some things he wrote about early Sunn before, but I never saw that interview clip until now. 

I have the single 15" version of the cabinet he describes that came to him in a dream!  Mine has half of the horn baffle and port along one edge (the larger, original kind he describes has the horn baffle/port in the middle, between the two 15s, but for the single cabs they just built them as if they cut it into two cabinets, along the center line of the port).

I always kind of wondered if Sunn was the first use of UL in a  bass or guitar amp until I got to look inside a Selmer and realized  the 30 watt Selmers with EL34s use UL, with an output stage that has similar specs to a 37 watt UL design in late fifties Amperex literature.  The Selmer I've seen is a croc-skin Zodiac from 1963 or 64, but I bet the earlier ones are the same basic design, since it seems like the models were pretty similar, even with different names and looks. 

So just as far as UL in a guitar amp, they might have beat Sunn by a few years!  Not that it matters, it's just kind of interesting.

> note weird feedback from UL tap

That's directly taken from Dynaco.

I forget who used the 6AN7. There were/are a ton of pentode-triodes for TV and general work. Minor value and pinout changes, they sorta interchange. You go with what is cheapest that year.

> weird transistor vibrato  instead of tube which actually sounds nice, goes slower and faster than Fender.

Both C-R-C-R-C-R phase-shift oscillators. Fender varied one R for simple/cheap. Change of F is cube-root of change of R. 20:1 change of R (2Meg-100K) is 2.7: change of F. Sunn varied 3 Rs. F is proportional to R. For the values he used, a hair over 10:1 change of F (working on matchbook; there's stray paths which will reduce that). The downside is a 3-section pot.