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1969 SVT restoration project

JimmyM

JimmyM

Apr 11, 2005
127,255
125,174
Plymouth NC
Disclosures
Endorsing: Yamaha, Ampeg, Line 6, EMG
#1
I've got three tube amps, a 1966 B-15N, a 1973 Fender Super Reverb, and a 1969 SVT. All of them look frightfully beaten up, the SVT is broken, and the other two needed some maintenance I've been neglecting to do. Originally I was going to start with the B and the Super, doing a major cap overhaul on both and a blackface restoration on the Super. Well, half a can of contact cleaner and a couple tubes in the Super, and both of them sound great and work well now. The B needs some cosmetic stuff and the Super needs a new bass control pot stem (and I plan to do the Blackface conversion in the near future), but that can wait...

The SVT and 8-10" cab are my priorities right now. I bought it used in 1986 for $650. At one time, it belonged to the organist for Iron Butterfly, Doug Ingle. It was beaten up badly, but it worked for a year. I recovered the cab badly in that cheap fabric store vinyl and replaced the torn grills with black cloth so it looked a little better. And when it broke, I took it into the local amp repair guy, and he fixed it but he also did a mod to one of the tube sockets that bugged me. The power tubes are supplied power by a little cap at the top of the tube instead of the socket, and instead of looking for a proper replacement tube, he put in a Chinese 6550 and wired the socket and cut the cap off.

A few years later, it quit making sound, and I figured it would be a really expensive problem that required a lot more tubes, and this guy led me to believe that I would have to rewire the whole power section and use non-stock tubes. I was pretty broke at the time and sick of moving it, so I just quit using it and bought an SVT 3 Pro. Thanks to the internet, I see you can still get pretty much any tube made if you look hard enough, and since I saved about $450 from what I would have paid for getting the other two amps fully redone, I'm going to completely restore this SVT the right way. Now that Ampeg moved to Vietnam and apparently making even worse stuff than the Crate line, we need all the vintage Ampegs back in action as we can get. So if you like this kind of thing, you may want to subscribe. It'll take a while to get it all done, but hopefully not longer than 4-5 months.

I'll be doing the cosmetic work myself, but first I'll be taking it to this local tube amp guru/blues guitarist named Birddog Bobby for the electronic work. My plan is to get the electronics restored to the totally vintage state it was in when I bought it, keep as much of the original issue parts and tubes as I can, and replace broken parts with vintage parts when I can find and afford them. I spent the last 6 hours taking out the chassis and control panel and cleaning 38 years of dirt and crud out of it. So here's what it needs:

HEAD:

-Knobs uneven, at least 2 knobs need replaced
-2 amp corners
-Fan grill is broken, needs superglued
-Label on back panel missing, functions written in magic marker
-Tolex loose in places, needs reglued
-Last power tube needs restored like all the other ones
-Tube cover for one tube in control panel missing
-New grillcloth

CAB:

-Re-upholster in black tolex
-New grillcloth
-New amp corners
-Go through speakers and recone or replace bad ones (I have a feeling they all need replaced)

I would probably re-upholster the head because it's pretty beat, but it's got this cool stencil on the top that says "Iron Butterfly Doug Ingle," so I'm going to leave it for nostalgia's sake and just let it look beaten up. It would probably have more collector's value leaving it than replacing it, plus it's kind of cool to own an amp that had the organ riff from "Inna-Gadda-Da-Vida" blasted through it a thousand times by the guy who wrote it.

There's a couple things I need a little help with, though. So if any of you can help me find these things anywhere, I would really appreciate it. I've looked on the net and can't find them.

-Tube diagram that tells the function of each tube (found schematics but no tube diagram or function list)

-Period-correct control/jack label for back panel

-Instructions for re-upholstering cabinet (also need one for a '66 double-baffle B-15N cab)

Here are a few pics, taken as I cleaned it up. I ruined 4 rags and finished off the contact cleaner cleaning out the gunk, and I still didn't get it all. I got paranoid about cleaning around the resistors and caps, so I will let Birddog Bobby handle the worst of it. But it cleaned up very well and it still takes a nice picture. I'm really looking forward to getting this amp fixed up...it's a beauty when it works. I might even trot it out on gigs occasionally if I can get my kids to move it for me ;)
 

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#4
An SVT isn't an amp to try and "learn how" to repair as you go. You have an especially rare early one. There is supposed to be a relay tube on the left of the back for warmup delay, but yours has been replaced with a shorting plug. Your "tech" was an idiot to cut the plate cap off of the one 6146 socket and sub a 6550 for it, and most likely, that's at least one of your problems. I STRONGLY reccomend you forget about fixing this yourself and find a competent tech.
 
#6
Yes I agree with everything Psycho said and 6146B's are if anything even cheaper than 6550's nowadays and your power tubes will go quickly without the relay tube. Have an expert fix it for you who knows these amps which is not that common nowadays. That is a really cool amp and I'd leave the Iron Butterfly thing on top too. Make sure you remove the little cap and oil the fan with light machine oil, most of them have been replaced because no one ever oiled them years ago.
 
#8
+ 1 on taking it to the tech. The schematic should tell you what each tube does, but, Now, if you insist here are the output functions:

-single tube (12w I think), the one closest to the front of the amp, is the phase splitter(or inverter, call it what you will) .

- The other two are the drivers for the output section, each tube drives 3 of the output tubes.

The preamp tubes , well, if i remember correctly most are used by channel
1, and channel 2 only uses a single tube. Go with the schematic, you'll se that , for example, the mid rocker uses a tube to work. Again, follow the schematic, look at what's before and after the tube, and you'll undersand what it does.

Now, about the power tubes, you should see about getting them all replaced to 655o', instead of finding a replacement for the original tube. The tubes in your amp are hard to find (if you even find them), and were replaced by 6550's for reliability on the later svts. You could do this conversion yourself actually. Basically the tech was right about you needing to rewire the whole output section, but it's relatively simple operation if you can understand what you are doing.

There's a site, fliptops.net, where you could find the rear panel. and the other missing parts, they specialize in everything ampeg.

Keep one thing in mind, the svt has very thing voltages. Make sure to discharge the filter caps before laying hand on the circuit (though I belive the caps have bleeder resistors). There is another filter on the preamp section, make sure you get that one as well.

Finally, decide what you want to do with the amp. A restoration to original specs is a good idea because, like Psycho says, yours is one of the earliest models. If you wan to play out, i'd put 6550s in there because the original output section is not very reliable. Also, do remember that putting 6550s is competely reversible, so if you ever want to sell the amp with original specs, you could rewire it yet again.


About upholstering the cab. I think the general consensus went something like this: remove the covering and use is as a teplate to cut the new one to size.
 
#9
elgranluis said:
Now, about the power tubes, you should see about getting them all replaced to 655o', instead of finding a replacement for the original tube. The tubes in your amp are hard to find (if you even find them), and were replaced by 6550's for reliability on the later svts.
Click to expand...

The supply of 6146's was drying up in 1969. There are more now because they're not used for industrial functions anymore. (They were popular as drivers for big transmitters or finals for small ones.) I strongly reccomend against converting the amp to 6550's now.

You could do this conversion yourself actually. Basically the tech was right about you needing to rewire the whole output section, but it's relatively simple operation if you can understand what you are doing.

...

Also, do remember that putting 6550s is competely reversible, so if you ever want to sell the amp with original specs, you could rewire it yet again.
Click to expand...

That depends on your definition of reversible. It's NOT an easy mod, and to get 6550's to run properly requires changing out more than just the socket connections. If that's done, you can't go back to 6146's until you undo it.
 
#10
Hey, thanks for all the advice, everyone! Just to clarify a couple things:

I will not be doing any electronic work by myself. Birddog Bobby will. Hell, I was afraid to clean around the circuit board! No way I'll get down and dirty with it. I'm not the least bit competent.

6146B's it was born with, and 6146B's it shall be reborn with. I won't even consider replacing anything on it with anything but NOS stock, with the possible exception of dried caps. And only tubes that have gone bad will be replaced, except for that big Chinese 6550 that mocks me, and where tube power matching is important and isn't getting done now.

fliptops.net has front faceplates but not rear. And correct me if I'm wrong, but wasn't the rear faceplate a label?

Psycho, I didn't want to say it because the guy who worked on it still has a shop locally, but in my opinion, he is, indeed, an idiot. But at the time, he was the only game in town. I'm sure mine isn't the only vintage tube amp he butchered. Plus, info wasn't nearly as available as it is now with the internet, online tube stores that carry NOS, and dedicated Ampeg sites so I didn't know any better. But Birddog Bobby is especially known for his great work on tube amps. He doesn't even repair solid state amps. He moved to the area shortly after I retired this head, otherwise I would have gone to him first.

Please keep the info and comments coming. They're very helpful.
 
#12
Thanks Matt!

Got a question...I just read something from a web search about the power tubes that said that they work together in threes (ABCABC). Is that correct? I always thought tubes worked together two at a time push/pulling.
 
#14
You know, this thread gets me thinking about those new Markbass tube heads. They weigh like 49 lbs., and not knowing much about amps, I never knew before how much of the weight of an SVT is in the power transformers. I knew they were heavy, but I never knew they were responsible for the major bulk of the weight. I am really interested in hearing these Markbass heads with the tiny power transformers to see how it compares. Jonas Hellborg based the design of his new Warwick solid state power amps around the big transformers on the claim that they were the reason tube amps get that breakup that sounds so cool, so it would be interesting to hear what the opposite sounds like, i.e., tubes with modern transformers. I probably won't get to try out a Hellborg since they're built on a custom basis, but I expect those Markbass heads to reach GC widely in the next year since the SS heads sell so well.
 
#15
JimmyM said:
You know, this thread gets me thinking about those new Markbass tube heads. They weigh like 49 lbs., and not knowing much about amps, I never knew before how much of the weight of an SVT is in the power transformers. I knew they were heavy, but I never knew they were responsible for the major bulk of the weight. I am really interested in hearing these Markbass heads with the tiny power transformers to see how it compares. Jonas Hellborg based the design of his new Warwick solid state power amps around the big transformers on the claim that they were the reason tube amps get that breakup that sounds so cool, so it would be interesting to hear what the opposite sounds like, i.e., tubes with modern transformers. I probably won't get to try out a Hellborg since they're built on a custom basis, but I expect those Markbass heads to reach GC widely in the next year since the SS heads sell so well.
Click to expand...

Jimmy, one of those transformers in the SVT is an output transformer; it matches the impedence of the output tubes to the speakers. Hellborg's power amp adds an output transformer which has been omitted in most transistor amps as they've been able to make low impedence power transistors for many years.

Regardless, I'd be curious to know exactly what's inside those Markbass tube amps.
 
#16
True dat, but the power transformers in the Hellborg are equally old school and equally heavy, and he credits them with contributing to the tube sound too, as well as creating a more pleasing clean sound. But since the Markbass solid state amp sounds so much like clean tubes to me, I have to question the use of the giant power transformers.
 
#17
OK, decision time...after Christmas, I will be digging out the cabinet and starting work on it. I have a big decision to make...recone or replace?

All the speaker baskets are original. Shortly after I got it, I came to the conclusion that it needed new speakers. At the time I opted to get them reconed because I liked the speakers but felt they were a little saggy after 15 hard years of B-3 going through them. I did the top four speakers first. They came back sounding great, but with blue cones. Blue cones? What's that about? I wanted dark gray like the originals! So I left it at 4 and never did anything about the others.

So now I want all 8 cones to be dark gray like the originals. That means they'll all have to be reconed if I use them. It would be so much easier to have a box of 8 new Eminence SVT speakers shipped to my apartment, put in the new ones and toss out or sell the old ones for cheap. But it would be $20 more per speaker to replace them, and they wouldn't be the vintage baskets.

Obviously something has to be done here. Even if they were all stock, nobody wants a saggy-sounding speaker box, even collectors. And sonically, I've thought that recones sound just as good as the original cones. If I'm wrong about that, please inform me. But assuming that I'm not, which do you think holds more appeal to a potential buyer of a vintage amp...all brand new speakers, or the originals reconed? Me, I'd say reconed, but I'd be interested in hearing opposing viewpoints.
 
#18
PSYCHO, I get what you say, it's not only socket connections, but it is just as easy to do as it is to undone. So it is possible that the fella didnt butcher the amp. Unless, of course, he know better than this and he is 1000% an idiot!

Jim, I have the same amp as you do, and nope, it's not just a label, it's a strip of metal, like a faceplate, that has all the labels on it.

About your tech, I don't doubt he is an idiot, but actually, it is possible to install a single 6550 and make it work with 61XX's. Just like you could wire a single socket for el34 in a 6l6-equipped amp. Hell, you could probably wire a socket to work with a 6l6. (ok, maybe way out of spec, but the GE's will probably handle that. Not a practical example, just sort of an example to show that almost anything is possible;))

About the speakers, as long as you keep the originals, a buyer/collector will be perfectly interested. He can decide what to do with them. If you want to use the cab, just get new ones, and if you decide to sell, offer the buyer the chance to get either set.
 
#19
JimmyM said:
It would be so much easier to have a box of 8 new Eminence SVT speakers shipped to my apartment, put in the new ones and toss out or sell the old ones for cheap. But it would be $20 more per speaker to replace them, and they wouldn't be the vintage baskets.

Obviously something has to be done here. Even if they were all stock, nobody wants a saggy-sounding speaker box, even collectors. And sonically, I've thought that recones sound just as good as the original cones. If I'm wrong about that, please inform me. But assuming that I'm not, which do you think holds more appeal to a potential buyer of a vintage amp...all brand new speakers, or the originals reconed? Me, I'd say reconed, but I'd be interested in hearing opposing viewpoints.
Click to expand...


The EH SVT speaker doesn't sound as good as the original CTS speakers. If you can get them reconed, do so. Ted Weber doesn't do recones anymore, but he IS selling his SVT speaker clones now, and if you want new ones, they would be the ones to get. BTW a recone IS every bit as good as the original speaker provied it was done properly using the right parts.
 
#20
JimmyM said:
Thanks Matt!

Got a question...I just read something from a web search about the power tubes that said that they work together in threes (ABCABC). Is that correct? I always thought tubes worked together two at a time push/pulling.
Click to expand...

HI

More info.

http://www.okabass.suntuubi.com/datafiles/gallery/5/Kopio svt rakentaminen 095.jpg

Follow the two white wires in the middle (to the powertube pins 5). Those are the signal wires from the phase inverter (12BH7 tube). Left white wire's going to the AAA-side, and the right goes to the BBB-side

Or look down-left where are the six anode resistors. (six white "boxes"). There are 3 blue wires going to the three leftmost tube anodes (AAA side). And 3 yellow wires going to the rightmost tube anodes (BBB).

As you can see, the tubes are assembled AAA/BBB.
 

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