1969 SVT restoration project

[bobyoung53]

I think the 6146B amps were rated at 330 watts RMS and the 6550 amps are rated at 300 WRMS.

 

[bobyoung53]

After 8 months, my SVT head is finally done! Birddog just gave me the news. Unfortunately I can't pick it up until Tuesday or Wednesday as I'm going off on a trip (Hershey Park Sunday with Bowzer, Freddy Cannon, Peggy March, and the Comets), but Birddog said that in all his 30-some years as an amp tech, he's never heard an SVT with more power than mine. Which is funny, because at first he thought I should replace the 6146B's with 6550's, but now he's totally sold on them and wondered why they replaced them. I'm sure high voltage concerns were the reason, but I'm glad I didn't do like a lot of you did and replace them. Who knows? Maybe I'll start a fad of everyone replacing their 6550's with 6146B's.

Or maybe not

Anyway, he replaced all the caps, replaced all the top wires for the 6146B's, and tracked down a major short in the controls that caused a lot of leakage. So now my SVT is back at full speed, but I'm at half speed because I can't pick it up until next week. Sheesh! I really want to cancel my gig now, but if I do I can't pay for it, so off I go...

Freddy Cannon? Good Boston boy, make sure he plays Tallahassee Lassie.

 

[JimmyM]

I think the 6146B amps were rated at 330 watts RMS and the 6550 amps are rated at 300 WRMS.

30 watts could make that noticeable a difference? Wow, that 6550 that used to be in it must have crippled it a little because I never really noticed it being louder than other SVT's. But Birddog thinks it's quite a bit more powerful. I guess we'll see when I get back from hershey. Got a 9:30 flight. What the hell am I still doing up???

 

[bobyoung53]

The handle! dammit of course! I've been busting my nut trying to work out what that rattle is.

I have a late 70's B-15S that has a rattle in it and I couldn't figure it out until the other day. When I flip the head, the latches really clamp it down and the middle of the board in the front must be a little warped upward or something as it vibrates. The first thing I'm going to do is replace the gasket and see if that cures it, it drives me nuts, I thought the speaker was loose and tightened that to no avail. I'll figure out the cure now that I've figured out the problem.
I had a serious rattle in an SVT cab once too, in fact I bought it really cheap because of it. It turned out to be one of the baffles separating the pairs of speakers, it was vibrating against the front baffle board, it had come unglued. I got out the old screw gun and screwed them and glued them to the front, no more rattles at all in that cab.

 

[bobyoung53]

30 watts could make that noticeable a difference? Wow, that 6550 that used to be in it must have crippled it a little because I never really noticed it being louder than other SVT's. But Birddog thinks it's quite a bit more powerful. I guess we'll see when I get back from hershey. Got a 9:30 flight. What the hell am I still doing up???

I don't know it shouldn't make that much of a difference, some sound better and louder than others, most likely it's just in tip top shape, and what the hell am I still doing up, haha!

 

[bobyoung53]

Incidentally I'm a ham and there are a million 6146B's out there. I use 6293's in my transmitter which is a more powerful 6146A which is what my transmitter takes. I can't use B's in it and neither can any transmitter which takes 6146's or 6146A's. I'm not sure if you can interchange them with 6146B's in an audio amp or transmitter either but if you can, they last about ten times as long as the regulars 6146A's, much bigger plates.

 

[soundmasterg]

My grandpa was a ham too Bob. I inherited a lot of his 6146B's, some other ham tubes, and some RF coils and other things that I don't know what to do with.

Yah I had heard 330w for the 6146B SVT's, 300W for the 6550 Magnavox SVT's, and as low as 240W for the MTI SVT's with Japanese transformers.

I've measured my early MTI at 300W RMS.....and an 6146B early blue line was in fact 330W RMS. I wonder what your SVT's would spec out at?

Greg

 

[PilbaraBass]

30 watts could make that noticeable a difference? Wow, that 6550 that used to be in it must have crippled it a little because I never really noticed it being louder than other SVT's. But Birddog thinks it's quite a bit more powerful. I guess we'll see when I get back from hershey. Got a 9:30 flight. What the hell am I still doing up???

yeah, up late in HERSHEY of all places...

b4 you all comment...I'm a native Pennsylvanian...I know...

 

[oyobass]

Subscribing. A lot of great info here! I cant wait to see/hear about the results.

oyo

 

[JimmyM]

My grandpa was a ham too Bob. I inherited a lot of his 6146B's, some other ham tubes, and some RF coils and other things that I don't know what to do with.

Yah I had heard 330w for the 6146B SVT's, 300W for the 6550 Magnavox SVT's, and as low as 240W for the MTI SVT's with Japanese transformers.

I've measured my early MTI at 300W RMS.....and an 6146B early blue line was in fact 330W RMS. I wonder what your SVT's would spec out at?

Greg

How do you tell?

 

[JimmyM]

yeah, up late in HERSHEY of all places...

b4 you all comment...I'm a native Pennsylvanian...I know...

Are you Hershey Park Happy?

Actually, I just now got to Hershey. I left Orlando at 9:30. Great flights, too! First one I got the first class upgrade, then on the second one I sat with some real sweet 16 year old girls on their way to see The Killers. I may be an old man, but I can still appreciate 16 year old girls, even if they don't appreciate me...

 

[timv]

Hey Greg, how's it going?
Actually, I remember when we tested your MTI and mine and both were right around 270 watts clean. I've tested another since then with the same. I have another coming in a few days so I'll check that one out also.

My 70' with 6146's still tests at a clean healthy 350 watts clean before any sign of distortion on my scope.

I have a 71' that I am fixing up right now for anderbass on here that initially when I got it only had about 170 watts clean with a weird mixed set of sylvania 6550's and one GE. I put in a good set of sylvanias and still got the same power after bias setup and everything. I don't think those small bottle 6550's are meant for an SVT!!!
I then put in a poorly matched set of EH 6550's and with a quick bias setup got 250 watts clean.
I am now going to refurb the amp, new caps, etc... and possibly install a set of GE's or see what we use. I'll have some good numbers after then.
I also have an SVT II here right now of anderbass's and I will get an output from that before I removed the GE's.
I also have an SVT 300 power amp coming so I am curious what the clean power of that is....
You can say I am a little obsessed with SVT's.....

I'm curious what this 69' is putting out too, My 70 has the factory mod done to the driver tubes to lower there B+, i'm curious if his had the mod done and if not, what it is putting out for power without the mod.
Also, I wonder if this mod helped keep the later ones a bit more stable???

My grandpa was a ham too Bob. I inherited a lot of his 6146B's, some other ham tubes, and some RF coils and other things that I don't know what to do with.

Yah I had heard 330w for the 6146B SVT's, 300W for the 6550 Magnavox SVT's, and as low as 240W for the MTI SVT's with Japanese transformers.

I've measured my early MTI at 300W RMS.....and an 6146B early blue line was in fact 330W RMS. I wonder what your SVT's would spec out at?

Greg

 

[JimmyM]

I'm curious what this 69' is putting out too, My 70 has the factory mod done to the driver tubes to lower there B+, i'm curious if his had the mod done and if not, what it is putting out for power without the mod.
Also, I wonder if this mod helped keep the later ones a bit more stable???

What is this mod? As far as I know it hasn't been done. And this is the first I've heard of it. I do know that Jerrold Tiers formerly of Ampeg says it runs better with 12AU7's instead of 12BH7's because of a general lack of quality in 12BH7's, but I know nothing about it, and I generally dislike mods in vintage gear, but I'm still interested in hearing about it.

 

[timv]

What is this mod? As far as I know it hasn't been done. And this is the first I've heard of it. I do know that Jerrold Tiers formerly of Ampeg says it runs better with 12AU7's instead of 12BH7's because of a general lack of quality in 12BH7's, but I know nothing about it, and I generally dislike mods in vintage gear, but I'm still interested in hearing about it.

There are two different schematics for the early 6146 power amps. One is found everywhere on the net and is for the earliest ones and the other I have only found in the Vacuum Tube Valley issue that has the story about SVT's. It is for the later 6146's so I suppose 70's? The early one is dated 5-69, the later one 7-70.
I linked to a copy of them on another thread, or maybe even this thread. But I will find the link again.
Here it is:

the 69 schematic:

the 70 schematic:

Theres a couple pics of my 70 here also: http://s34.photobucket.com/albums/d115/tvogelin/ampeg SVT/

I labeled a close up picture showing the actual factory trace cuts on the circuit board for the modification.

Feel free to ask any questions.

 

[JimmyM]

Feel free to ask any questions.

OK then, what am I looking at here?

I'm not technically inclined whatsoever. I can spot the tubes on a schematic, but that's about it. What I wanted to know was what does this mod do and (mainly) how does it affect the tone? I find it amazing that guys like you can make sense out of technical data and know what happens because my brain so doesn't think that way. I do appreciate you sharing info, but I'm afraid you'll have to dumb it down a little for me to grok it.

 

[darkrider1]

Holy smokes. This was one heck of a project! Amazing! Makes the restore I did on a Silverface bassman head seem like a walk-in-the-park.

Wouldn't a B-15 have been an easier choice for a first restore?

 

[JimmyM]

It was a walk in the park. All I had to do was write checks. BTW, I did a B-15 restore at the same time...that one I wrote a check, plus I did a buttload of sanding and stripping and sent it to Jess Oliver

 

[EviscerateNick]

jimmy how much did the repairs set you back? i have the same head in a similar condition.

 

[JimmyM]

Head repair...$500
New tubes...$225
8 recones...$700

So about $1425. Way more than I had hoped, and I still didn't get the cab recovered.

 

[darkrider1]

Writing the check. That's the hard part for me

$1425 is big bucks for me, especially since my Peaveys & Trace Elliotts work just fine. I would never be able to justify it to my wife (who is also a musician).