SVT Layout Diagram? Anyone?

[bob p]

I'm looking for a layout diagram for an old-school SVT. Rather than re-inventing the wheel I'd be interested in seeing an existing tagboard / turret / eyelet layout if anyone's got one.

TIA.

[soundmasterg]

I doubt you'll find one unless someone has drawn one up themselves. Fender was really the only one who did layouts....

I used to have a 1981 early MCI era SVT that I sold last year. It was very similar to the Magnavox era ones. I never bothered to draw up a layout....the amp worked fine in the 15 years I owned it except for a couple electrolytic cap failures. Right when I bought it I found a factory mis-wire that sent all of the signal on channel 2 through a .022uF cap instead of a .22uF cap which makes a world of a difference when its a bass amp!

Greg

[bob p]

well, i guess that explains why i haven't found any layout diagrams.

factory mis-wires are more common than i had previously thought.

a couple of weeks ago i was working on a phase linear stereo power amp that wouldn't bias properly, and the reason for the problem was that some dyslexic board stuffer at the factory put a 2k1 resistor where a 1k2 resistor was supposed to go, but only on one channel. the result? one channel would bias fine, but on the other bias pot ran out of range and the channel would never bias correctly. the amp had been running hot, consistently blowing output transistors on one channel since the 1980s, and nobody had ever figured it out.

parts substitution errors are easy fixes, once you know that there's a wrong part on the board ... but it's often a real bear to figure out the nature of the problem. i hate trying to trace PCB with a schematic. that's why layout diagrams are so handy.

[soundmasterg]

well, i guess that explains why i haven't found any layout diagrams.

factory mis-wires are more common than i had previously thought.

a couple of weeks ago i was working on a phase linear stereo power amp that wouldn't bias properly, and the reason for the problem was that some dyslexic board stuffer at the factory put a 2k1 resistor where a 1k2 resistor was supposed to go, but only on one channel. the result? one channel would bias fine, but on the other bias pot ran out of range and the channel would never bias correctly. the amp had been running hot, consistently blowing output transistors on one channel since the 1980s, and nobody had ever figured it out.

parts substitution errors are easy fixes, once you know that there's a wrong part on the board ... but it's often a real bear to figure out the nature of the problem. i hate trying to trace PCB with a schematic. that's why layout diagrams are so handy.

Yeah, I agree completely with you. A layout diagram would be nice, but no one did it except Fender. He did it because then he could hire women (small hands) who didn't know much about electronics to wire up the amps because they worked cheaper and just had to follow a pictorial and know a little bit about resistor color codes etc. In my experience SVT's are pretty reliable amps....usually the problems are related to mis-biasing, blown diodes protecting the power tubes, or electrolytics going bad due to age...but you will find the odd factory miswire in there.

Greg

[loudthud]

Aren't all the vintage SVTs on PCBs? The full two channel SVT would be a lot of components for an eyelet board. About the same as the SLO 100. One of the TUT books has a layout for a single channel version. That preamp doesn't have the inductor midrange control. I don't know of any SVT "clones" from companies like Weber or Ceriatone, except for one from Weber, the 6S100. I'm not seeing the 300 watt monster they had at one time.

[Enzo]

I am thinking the same thing LT. A circuit board in the center of the chassis under a large hole, with a removable metal plate covering it.

[loudthud]

I am thinking the same thing LT. A circuit board in the center of the chassis under a large hole, with a removable metal plate covering it.

I haven't seen the insides of an SVT since the early 90's. Seems to me the power amp was on a PCB roughly square about 7 inches on a side and the preamp was a long skinny PCB in a separate chassis. A total nightmare.

[soundmasterg]

I haven't seen the insides of an SVT since the early 90's. Seems to me the power amp was on a PCB roughly square about 7 inches on a side and the preamp was a long skinny PCB in a separate chassis. A total nightmare.

Yep thats pretty much it. They are heavy, lots of screws to undo to open up the amp and get both chassis out, complicated, usually dirty due to the fan and the fact that they are quite reliable so they don't get opened up often. That said, they sound cool for bass.

Greg

[Enzo]

Original SVT power amp board layout:

[soundmasterg]

Original SVT power amp board layout:

Wow, count me surprised there was one of these!

Greg

[J M Fahey]

Original SVT power amp board layout:

Love that PCB.
Brings back old memories of when I drafted PCBs with Bishop Graphics black crepe tape and sticky pad donuts.
The good old pre-PCs times

By the way, that's the real origin of the "rip and reroute" command in PCB drawing software (specially in the autorouter), because that's what you physically did when a PCB became clogged and unsolvable: you had to rip the black tape from the mylar substrate and start again choosing another path.
Oh well.