Late 60's Sunn Spectrum II

[Curran Tone]

Ok so I've recently acquired a late 60's? ('silverface') Sunn Spectrum II Head. I found it in the basement of the guitar shop I work at here in Westfield Mass. Looks to me like someone put a 12ax7 or something else wrong into the 6an8 tube socket, successfully lighting up all the circuitry around that tube all the way down the negative feedback loop. This thing is black and smells like a good electronics fire.

So I have a few different versions of the schematic, one from 1967 and one from 1969 for both the 100S and Spectrum II. I'm a 22year old Guitarist/Teacher/Tech., and I need to prove myself to everyone around here that I know what I'm doing so I can start designing,building,repairing,selling top quality vintage style point to point tube amps.

I'm planning on changing all the sockets for new ceramic ones, I'm assuming the PT,OT, and choke are all OK. I'm going to rebuild and reinforce the negative feedback look with a 1k5w ceramic resistor instead of the 1k1w CC resistor that literally split in half, and the rest of the burnt stuff will be replaced with high grade replacements also. Luckily none of the original Black Cat signal caps were harmed. I'm replacing some polystyrene caps with new ones of the same type, trying to keep it as original looking/sounding as possible.

One of my big concerns is the 30uf/20uf/20uf/20uf @525V can filter cap. I've heard 525V is not enough to handle the power from the PT, so I'm thinking of some individual 600V caps wired to terminal strips, too keep the original style inside the amp. I want to start with the 30uf and 3 20uf's, and listen before I change the cap values themselves.

Last thoughts, kt88s or 6550s, the old schematic says KT88s, the amp had some smashed 6550s in it but I think the KT88s should get the true sound of this thing. I do have a bunch of NOS 6an8s lying around, conveniently.

I've never actually played out of a vintage Sunn and don't really know exactly what to expect for tone. When I finish the amp I might keep it if I like the tone for my style of playing (late 60's psychedelic blues/rock), or I might sell it.

Any tips, comments, questions, or whatever would be greatly appreciated.

Thanks,
Bill Curran
Curran Tone

(P.S. I'm kinda new to forums, I hope I posted this in the right spot.)
(P.P.S. Really stupid question, is this supposed to be a guitar or bass amp? Does it matter?)

[HRobert]

Hi Bill,

The Spectrum II is a 60W guitar amp. If your amp has the diagonal silver grill cloth, I would bet it's from 1967, so I would go with the '67' schematics as your guide.  To keep the amp closer to original, I would use KT88's.  There are a couple of really great electronics guru's on this forum and they can give you just about everything information-wise that you may need for this project.  If you're not familiar with Sunn amps, Conrad Sundholm- the founder - was into high fidelity, and the amps are LOUD and clean.  

The '67' Spectrum II shared the same 2 x 15 speaker cabinet as the Sentura II.  The cab was 24"W x 36"H ( 38" on the casters) and 11.5"D.  The cab came with 2 JBL D130F speakers. Since the cab was shorter that the 100S style, the speakers were offset from the verticle center of the cab - the top speaker to the right and the bottom to the left.  I've just built a pair of these cabs for my Sentura II project.  You can see pictures at the DIY sectionof the forum.

Good luck with your project.

Rob

[noel]

I'm troubled by a few comments you've made here.  For instance, the transformers and some of the caps aren't damaged... how do you know that?  A real tech would actually verify that.  Its always good practice to be overly cautious and check every little thing with vintage equipment...especially high voltage stuff like this!  Saying "silverface" in a sunn forum is also adding insult to injury for a lot of us die hard sunn guys haha.  Silverface is fender and those bastards ended up buying sunn just to shut them down.

Regarding the caps, don't bother with the can caps.  Just get individuals and wire them in.  As you've already noted you can get higher rated caps easily now.  There again, its good practice to have "robust" equipment...never hurts to put a 600v cap where a 525v was.  It really comes down to money and ocassionally space.

For the millionth time in a sunn forum...KT88=6550 (for all intensive purposes).

My advice as always is: don't kill yourself working on this stuff.

[Curran Tone]

Cool thanks for the info guys, I was just using the term 'silverface' because the control panel is like aluminum faced or whatever, I'm not comparing it to a f*nd*r, just trying to be descriptive.

The main reason I'm assuming the transformers to be good is because the amp is fried only around the 6an8 phase inverter and I know for a fact the last guy who tried to work on it didn't have the correct tube, or didn't even know what it is. So it's been sitting in a dusty basement for who knows how long.

I'm obviously pretty new to the tube amp thing though I'm reading and learning as much as I can. What is a good way to safely test the transformers? I'm still in the dark on this one. I know it dangerous with the high voltage and all. Any help on transformers in general would be greatly apreciated. I get the general idea of how they work, but how do I go about testing them?

I've tested most of the caps in the amp so far, replaced all of the electrolytics that were way off spec. The signal caps are all vintage black cat, I haven't tested them individually yet, so I can't say for sure that there all 100% but I will, I'm trying not to take everything all apart at once just yet. I do want it to sound as original as possible, and I haven't seen black cats NOS too much, is there a modern cap that compares if I have to replace?

So this project is my first real tube amp rebuild /overhaul. I'm looking at this one as more of a learning experience, but I'm also intent on bringing this vintage classic American amp back to life.

Thanks again for the help guys,
-Bill

[noel]

Use the google machine to find someone to check the transformer... I've personally had TRS rewind/ test some transformers.  This is one area that I just farm out generally.  Maybe someone else will chime in if they have a method for DIY.

Welcome to the world of audio myths...chapter 1, caps
NOS caps are BS, all electrolytics need be replaced (at this age).  Just build the amp to spec. and it will sounds "as original as possible". 

The first priority is building the amp to spec. and knowing that it is 100% safe.  Speaking of which, install a modern IEC socket and bypass the polarity switch...unless you want to get killed of course.

Good luck with the build.  You're exactly right that the only way to get experience is to actually do it...just be careful and ask questions if you're unsure.  Don't buy in to all this crap about 6550 being different from kt88 or that you have to keep the orginal caps in order to have vintage sound.  There is truth to some of these myths in varying degrees but it ultimately comes down to what is perceptible.  I can tell you that if you build a sunn solarus up to print it will sound amazing... no two ways about that.

[Soundmasterg]

Checking Transformers

To check the power transformer, unplug all tubes, plug in a speaker or resistor load, and turn the amp on. If the fuse blows the PT is bad. If it doesn't blow, then shut it off, put the rectifier tube in, and try again. If fuse doesn't blow, shut amp off, put the rest of tubes in, and turn it on again. If fuse doesn't blow, you're good to go. If fuse blows in either of the last two tests, then you have a socket problem, a tube problem, or a problem with the circuitry around the given tubes. If the PT works, then play the amp. It should work fine from here as the OT and choke rarely fail.

Checking/replacing caps

The electrolytic caps should be replaced. The amp will perform much better with new caps than with old, drifted or leaky caps. You can replace the can with a stock replacement can, which you can get from www.tubesandmore.com . It will work, but with today's wall voltages, the voltage on that cap is over it's ratings and it's life will be shortened. A better solution is to use discrete caps. You can get the 600v Spragues but they are very large and hard to fit in the amp, and are WAY overpriced for what they are. A better approach is to use series-connected 350v caps, which give you a 700v rating. Use this setup for the first two filter stages, and a 500v cap for the third stage, and a 450v or 500v cap for the 4th stage. When wiring caps in series, the uf value is halved while the voltage rating is doubled. I've used 220uf 350v caps in Sunn bass amps for the first stage and 100uf 350v caps for the second stage. This translates to essentially a 110uf 700v cap, and a 50uf 700v cap respectively. You can go with close to stock values but the amp will hum less with larger caps, and the bass response will be quicker and more present. When wiring caps in series, do not forget to add a 220k 1w resistor in parallel with each cap. These allow the caps to share the voltage instead of one cap trying to hog it all and blowing.

The coupling caps generally do not need to be replaced unless they are poor quality or leaking DC. The brown drop caps as used in many Sunns were not the best quality (their ESR is high) so most any modern replacement will be better than them. Likewise the green Cornell-Dubliers aren't the best and most modern ones will be better. The Black Cats sound great and should be left in circuit unless they leak DC. They are paper and polyester caps so if they do leak DC, it would not surprise me. Paper caps attract water and generally will leak, especially when old. You said you were checking the caps? How were you doing this? Checking for value with a DMM won't get you an accurate reading since you are not checking the caps anywhere close to their rated voltage. The only way you should check them is for DC leakage. Anything more than about .3v on a coupling cap can cause the following stage's bias to be off so a cap that leaks more than about .3v should be replaced. To check for leakage, unsolder the low voltage side of the cap, hook your meter to the lead that is in the air and the other lead to ground, and turn the amp on. The voltage should spike and then gradually reduce. When it seems to stabilize a bit, then that is the DC leakage.

Other Things

All of the power resistors should be replaced with 2w metal oxide or metal film. The carbon comps can burst into flames in a failure condition and they contribute nothing to the sound in this position. These are the dropping resistors in between the filter cap sections in the power supply. The death cap should be removed and the ground switch disconnected. The amp should be rewired for a 3 wire power cord, with black coming into the amp, then going through the fuse, then the power switch, and then to the transformer tap. The white wire should go directly to the transformer tap. The green wire should be soldered to a solder lug and hooked under a PT bolt to the chassis, seperate from any other ground. The plate resistors for the preamp and phase inverter tubes should be replaced with new carbon comp resistors. These go bad with age and will crackle and pop. If you replace the sockets, ceramics will work, but many other types are easier to tension and work just as well. www.tubesandmore.com has a nice selection. Get some JJ KT88's and fire itup. These work well and sound great. It will make a great bass amp stock, and an even better one if you go with larger filter cap values in the first two stages. You can also change the 3rd and 4th stage to larger cap values, but this will change the sound there as these stages are RC coupled. The first two stages are not so it only changes the feel if you go with larger cap values here. If you intend to use this amp for guitar, it will work fine and you may like the sound too, though for my tastes, I don't like any of the Sunn tube amps for guitar. They are loud and clean, and the clean is brittle and hard. It could be modded to get a nicer sound for guitar, but if you're going to go to all that trouble, why not get a different amp? Anyway, hope that helps and enjoy!

Greg

[Curran Tone]

Wow, thanks for being so specific man, you just answered my next round of questions before I could ask. I think I'm going to order a bunch of caps from these guys www.justradios.com, they seem to have decent prices on the 600v electrolytic, and also the polystyrene tiny ones, that seem kinda hard to find. I'm gonna order everything this weekend when I get paid, at this point I think I'm gonna get every cap, resistor, socket, maybe some fresh pots and some extra stuff to experiment with too.

Thanks again for all the help guys,
-Bill

[Soundmasterg]

Wow, thanks for being so specific man, you just answered my next round of questions before I could ask. I think I'm going to order a bunch of caps from these guys www.justradios.com, they seem to have decent prices on the 600v electrolytic, and also the polystyrene tiny ones, that seem kinda hard to find. I'm gonna order everything this weekend when I get paid, at this point I think I'm gonna get every cap, resistor, socket, maybe some fresh pots and some extra stuff to experiment with too.

Thanks again for all the help guys,
-Bill

The 600v electrolytics aren't really a good deal. You think they are because they can handle the higher voltage out of one cap right? Well the Sprague Atom series 600v caps are really two caps inside the can, and are WAY overpriced these days as compared to competing brands. No one else does 600v caps though.....because it is cheaper and easier to get two 350v caps and wire in series for a 700v rating as I mentioned above. If you still want to go with 600v caps be my guest but I wouldn't personally. I still have some Sprague Atom 600v 20uf caps around here that I bought years ago thinking I would need them and haven't had a call to use them. I've kept them formed by hitting them with voltage from time to time, but I'd be into selling them if you wanted to buy some for your amp? The only ones I have are the 20uf 600v ones though. If you're interested send me a PM here.

Greg