old sunn 1200s head...info needed

[bm666]

hi,
I just picked up an old 1200s head and was wondering if I could get some info and a schematic for this beast.

I see it has 4 6550's in the power section, but there is a tube socket that is empty as you look at the amp from the rear, it's right next to the 4th power tube.

the front panel has 4 inputs and from left to right are controls for:

volume/treble/bass/contour/rate/depth/reverb/midboost switch
far right has standby/polarity/power switches

I'm assuming this is a solid state preamp?...is this a 120 watt amp?

any info would be appreciated.....I have not plugged it in because the cord is sketchy and I don't know what to make of the empty tube socket i mentioned above.

thanks!

[Geoff]

RE: the empty tube socket...

A possibility is that somebody soldered diodes on the socket lugs underneath to act as rectifier in place of a GZ34.  You can pull the chassis and check for this.  The most obvious explanation though is just that the tube is missing.  If a SS rectifier replacement has been wired in correctly, you won't need this tube.

Good luck!

[bm]

ok...I opened it up (carefully).....and here's the skinny...

the empty socket tube is just that...empty and has no leads of any kind soldered to it. next to that in yet another socket is a 5R4? rectifier?...that's everything on the back side of the chassis (4-6550's,empty socket,5R4)...perhaps the empty socket is a spare tube holder?

in the middle of the chassis is a 6AN8A tube next to an enclosed cannister

on the right front side hiding behind the huge transformer, from left to right are two preamp tubes....12ax7 and a 12at7...so i guess this amp includes both tube preamp and tube power amp sections?

it'd be great to get a schematic.....the chassis is dated 11/14/70

thanks for the info!

[Geoff]

OH! Sorry, I was under the assumption that there was just one rectifier socket from your description -- sorry for not knowing my Sunn models, guys.  I know the 2000S...at least the earlier ones I've seen and the schematic posted here has 2 rectifier tubes (probably because 2000S was a bass amp).  I guess the 1200S just has one???

[bm]

ok...this is starting to make sense to me now.....perhaps the empty tube socket is used for the second rectifier (1200s and 2000s have the same chassis?)...i'll check the 2000s schematic for similarities just for the heck of it.

[Greg Z]

My "70 1200's has two gz-34's , 1st pretube is a 7025/12ax7  2nd tube 12au7
is the reverb driver, recovery is solid state along with the trem.
6an8 is the PI. Amp is rated 120w rms,in reality 130 to 135 at clip.
I dumped the gz-34's in favor of sand. this took the b+ up to a solid 600V.
Mines pushing 178w at clip.  Finding a good legible schematic is going to be a toughy. Just use a 2000's for PS and outputs and a Sentura I/Solarus for
pre-amp/reverb/trem.

F.W.I.W. One GZ ain't gonna do 4-6550's W.A.S.

[bm]

hey greg,

thanks for the info.....i wonder why my amp only has one rectifier? the other socket is there but has no components whatsoever attached to the terminals of that socket......something is up

explain the "I dumped the gz-34's in favor of sand. " I don't understand that part......just my ignorance

also....if one gz-34 isn't gonna hack it for (4) 6550's does that mean that somebody pulled the other one? i know that the 2000s has two...but it also has 6 power tubes too doesn't it?...........my ignorance again

I'll take your advice about the schematics you mentioned...thanks!

[Greg Z]

1200's and the 2000's share the same Power supply and output section.
The 2000's is a bass amp,hence no reverb or trem.
Don't know whats up with the bare socket unless it arced-over and he replaced the socket and didn't bother to wire it up. If I recall correctly( Been over a year since I've been inside mine) the connections on the 2nd socket is jumped to the first.
(Mirrored pinouts) the tubes are paralleled to double the current capacity.
As far as dumped the gz's for sand. Vacuum tube rectifiers will always exhibit a certain amount of voltage drop, as the amp is played the current demand increases and so does the drop. This is known as "sag"
As I was after a tight bottom end and did not want any "sag''. I simply made up some diode rectifiers on some old tube bases that plug in the sockets. One word of caution though, the filter caps are already near ther limits with the gz's and if you do the diode thing you will be over the ratings and need to upgrade your caps.(if their original they probably need it anyway)

[bm]

greg,

thanks so much fo the info.......i hope i didn't mislead you...the rectifier is a diode type 5R4 on the one socket that is occupied....i'm aware of how tube rectifiers sag and I'm thinking this amp was possibly transitional...the diode type 5R4 rectifier might have replaced the two tube rectifiers? that would explain the empty socket? i noticed an empty socket on the 1200s in the gallery on this site.......hmmmm

I'm gonna have the caps replaced as well....I know this is a guitar amp, but i'm wondering how it would sound with bass.....might be fun to mod it for more tone control than a 2000s and dump the reverb and tremolo circuitry....who knows....I'll see how it sounds once it's retubed and gone through.......I'm into the amp for a song, so it's just gonna be fun to get it up to snuff then see what happens from there...there are so few of these things around i don't think modding it would be helping that situation.

thanks for the info...sounds like your amp is a beast...178 watts!!! do you use it for guitar or bass?

[bm]

so i fired the beast up...and it sounded like a sunn should for the most part...super loud!!!...everything works, reverb (kinda) and tremolo....but the thing distorts very early...so i ordered some tubes and stuff....but the real news is that it sounds huge with a bass as is!!!

our bass player plugged it into his 8x10 and it sounded awesome!!! he wants a 2000s now....i'm afraid to change the tubes because this thing sounds amazing with bass !!! it's the best overdriven bass sound I've ever heard...and if you roll the volume off it cleans up.....I can't wait to hear what the bass version of this amp sounds like.

we're on the hunt for a 2000s or maybe even a 200s ...anyone got any opinions about which is a better bass amp? the 200 or the 2000?

[Greg Z]

the 200's only has two output tubes,the 2000's 4.
60w or 120w, your choice?
One thing the big sunns are noted for is clean,if yours is distorting early something is wrong.
The svet 6550c work well in this amp and are relativly cheap, uncle Neds got them for 89.95 a quad. Try real hard to find a nos 7025/12ax7 for the pre-amp hole. I've demo'd mullards,telefunkens and others and nothing
gets the sunn sound like a good old rca7025.

[bm]

thanks greg...i guess it's gonna be a 2000s

I know the big sunns are known for clean, and i'm sure something is wrong or the tubes are old or something...but the early overdrive sound this thing gets in its present condition, especially with bass is just cool...

anyway, I'll re-post after the tube change and final tune-up....I sure could use some DC Ma bias specs to work with...anybody got some? I'm using 6550 SVT (svetlana) power tubes

thanks!