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* If you help $100/hr
* If you worked on it first $200/hr
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79 Ampeg SVT Volume Drop
- Thread starter numberfourthree
- Start date
The saga continues. Finally got the proper can caps installed. When I was removed the old ones it seemed the last 'tech' installed 2 50/50 cans and took the label off one of them to hide the fact. As a result I don't have much of a wiring guide to work from. Does anyone have any decent images of the wiring of the cap section?
Attachments
I see what may be a problem in the way you mounted the caps. Is that a metal plate under the cap on top of the isolation washer? If yes, the metal plate is in contact with the capacitor case. The sheet metal screws are connecting the capacitor can case to the chassis. The can needs to be isolated from the chassis. The isolation washer can be mounted to the chassis with the machine screws, normally small rivets are used. The metal plate needs to go. The cap mounts directly onto the isolation washer.
The best advice is to follow the schematic and Power PC layout. The Power PC card layout I posted earlier in the thread shows all of the connections.
I converted my 69 over essentially to the 6550 capacitor scheme. I added on extra capacitor to Node E. I modified this image to make it more like the later 6550 based amps. I don't claim it is 100% accurate, but it should give you an idea of how to proceed.
(DON'T TAKE MY WORD FOR IT) your real reference should be the Power PC layout and the schematics.
I kept the same ground scheme as the 69 originally had in it. I don't know if any of the connections were revised in later amps.
Although metal bases are also available. The SVT does not use them. They would potentially cause a ground loop in an SVT.
The mounts for 100/40 capacitors should look like this and be made out of phenolic.
They can be purchased here: Invalid Link Removed
100/40 cans have four ground tabs. The tabs go through the slots and are twisted to hold the capacitor in place. AFAIK this is why they are called twist lock capacitors.
I have always seen the insulators mounted with screws and nuts in SVTs, rather than rivets.
I would verify what you drew using the service manual as a reference.
https://www.talkbass.com/attachments/svt-manual-5-1-1977-pdf.1116491/
The show the PCB and the connections to it. Note that the [-] voltage used for bias, requires that the [+] of the capacitor (C8) connects to ground. It can be confusing because it appears to be in backwards relative to the other capacitors. It’s important that this be wired correctly.
https://www.talkbass.com/attachments/svt-manual-5-1-1977-pdf.1116491/
The show the PCB and the connections to it. Note that the [-] voltage used for bias, requires that the [+] of the capacitor (C8) connects to ground. It can be confusing because it appears to be in backwards relative to the other capacitors. It’s important that this be wired correctly.
>I would verify what you drew using the service manual as a reference.
Definitely!
Definitely!
You are a saint! Thank you. This should get it sorted out for me. I'll let you know otherwise.
On my amp, this is a peach colored wire from the pre-amp (node 2?) to the ground of a 100uf cap.
AFAIK there should not be an extra wire on C9A. The junction between C9A and D11 is node E.
Capacitors labeled in red. Nodes labeled in blue.
My guess is the orange wire coming of C9A is going to the preamp. But this is node E. Per the schematic the preamp is fed from node B.
Note sure how Ampeg did it with later 6550-based amps. In my 69, a wire runs from node A to R49. Two wires are taken from the other side of R49. One wire feeds the preamp and the other runs up to node B on the Power PC card.
This shows the connections between my capacitor scheme and the Power PC card.
Some important wires I did not include are the connections to R49. A wire runs from node A to R49. Two wires are connected to the other side of R49. One goes to Node B and the other goes to the preamp through the umbilical.
The Power PC connection for H is labeled C13. C13 was used on early bluelines when node H was added. AFAIK, later 6550-based amps did not have this capacitor. Instead C9 was changed from 100uF to a 100/40. So the correct connection for H is C9B.
Please feel free to offer corrections.
@beans-on-toast Look Rivets!
[Invalid or Expired Link Removed]
This is labeled as a 78. It looks like the original capacitor scheme (IMHO). Notice R49 at the lower right corner of the Power PC card. The preamp feed appears to be taken off the terminal strip by R49 (Red/Wht) wire. A 2nd Red/Wht wire appears to go up to the top right side of the card (B).
[Invalid or Expired Link Removed]
This is labeled as a 78. It looks like the original capacitor scheme (IMHO). Notice R49 at the lower right corner of the Power PC card. The preamp feed appears to be taken off the terminal strip by R49 (Red/Wht) wire. A 2nd Red/Wht wire appears to go up to the top right side of the card (B).
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Rivets were standard on their amps including the early B-15's for caps and tube sockets. I wanted to source them, Jess Oliver told me that they had a special machine for installing them, they weren't pop rivets. On the SVT, they used a nut and bolt on one cap can hole because another cap's ring was mounted there. See the images.
The tubular resistor was normally mounted on a terminal strip to give it strength. The one in your image does not look original, nor is the way that it's mounted.
It goes to the plastic connector that feeds the pre amp. I think you can barely see it on the labeled 78 from sweat and low post.
Mine is a 78. I am sure certain things were done different in different years.
I hope you are getting the 3 other caps under there as well. The 100/450, the 90/200, AE does carry a 30/600, or suitable one. I used sprague on the other two. This really is an expensive job. And you may need a sextet still.
Nice to have a variac for charging these.
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This is my amp, which is about as close to a match as the one Wasnex posted as I’ve seen. I also had the same cap on the terminal strip and rivets.
That is a normal application. If the existing terminal strip, like mine, wasn't already loaded with wires, they would have done that. I would prefer it as well. I shorted C10 or C11, and yes mine blew, folded over and burnt a few wires, the board toasted a bit. Yes it would be better if possible.
Ferrite beads are good for removing/reducing unwanted self oscillation or external sources of electromagnetic interference. The later, as a source of noise has increased exponentially in all our lives over the years. Maybe at least part of the reason you see them in the later amps. If one were to take stock of everything that generates noise in the average household today, I think it would be a mind blowing revelation.
