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79 Ampeg SVT Volume Drop

#61
Wasnex said:
Let me start by saying I really don't think it's the power supply filter caps. I don't believe these are original, which suggests a cap job may have already been done:
View attachment 4790541

Hopefully at this point you have at least narrowed it down between the preamp and power amp. The sensitivity rating for the power amp is 0.25VRMS. In other words the amp should be driven to full clean power (300W) with 0.25V applied.

If all of the DC voltages check out, troubleshooting without a O'scope can be pretty difficult (IMHO). I don't own one either, but so far I haven't had a problem that really needed one. When that day comes, I will either buy a scope or send the amp to a shop.

In the event you push ahead with a cap job, neither of the 100/40 twist locks needs to be insulated if you stick with the original capacitor scheme. It is true that the main filter sections stack a pair of 100uF caps, but C12A (a 100/40 twist lock) should be tied to ground and C10 is a separate axial that should already be insulated and concealed within the chassis. The

Twist locks are not very common these days. A company called CE Manufacturing makes a suitable 100/40 can but it probably won't be easy to track down an insulator sleeve that fits: Invalid Link Removed

If you want insulated twist locks, an option is to used custom caps with the required insulator already installed from HAYSEED HAMFEST LLC

I have bought several insulated twist locks from Hayseed, and they were priced competitively with CE MFG options.

Just checked and Fliptops offers a 100/40 can that is insulated. 100x40/450v Multicap - Product Details (fliptops.net) My notes say I used 100/40s from Fliptops in my 87 SVT. Mounting ring sold seperately.
Click to expand...
I have an "acquaintance" who is always calling me to offer to give me one of his scopes. But the hidden social cost of doing that is way more than I'm willing to pay. It's kind of like, his parents had to put a steak around his neck to get puppies to play with him.
Besides... Mrs. OGB has a size limit on anything I drag home these days. The paper-work to justify it would be overwhelming. :laugh:
 
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#62
Ok. Cans are properly grounded and wired. I followed the schemes you all posted. I am still uncertain on where the light orange wire from the preamp goes.
If one is looking at the pre-amp connector it is the middle on the bottom row. In my messy past cap job it went to a 100uf, I believe. The wire is original and quite long suggesting it would land up by C9.
6BE0C0EE-7509-4B7F-8AF0-60022EFA736C.jpeg
BE109135-CBF4-444B-AC6E-610CA9772918.jpeg
 
#63
I believe this is the factory connections.
upload_2022-9-16_13-39-13.png


Please double check the connections (in red). I am eyeballing this from your photos, and shadows can make the connections appear in the wrong place.

A goes to R49
R49 goes to the terminal strip.
The terminal strip goes to the umbilical. It appears to be pin 5.

upload_2022-9-16_13-46-57.png


There should be a second wire that goes from the R49 connection on the terminal strip to B.
upload_2022-9-16_13-57-1.png
 
#64
I just shake my head when folks wonder why an amp that’s been previously worked on or “modded” costs more (sometimes a lot more) to correction repair and bring back to proper condition. IMO, this is a good example.
 
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#66
numberfourthree said:
Completely agree. It’s been quite the project! But it’s also gratifying to correct the myriad of sloppy work.
Click to expand...
Imagine the cost of a (real) qualified tech going through an amp like this… could get into the $500-$1k range pretty easily.
 
#67
agedhorse said:
Imagine the cost of a (real) qualified tech going through an amp like this… could get into the $500-$1k range pretty easily.
Click to expand...
Indeed, it's why several local techs were not at all excited about the prospect of working on a vintage SVT. We're also seeing a lot of subtle differences in wiring throughout the various eras. The help here has been crucial. Thank you all again.
 
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#68
Wasnex said:
I believe this is the factory connections.
View attachment 4815970

Please double check the connections (in red). I am eyeballing this from your photos, and shadows can make the connections appear in the wrong place.

A goes to R49
R49 goes to the terminal strip.
The terminal strip goes to the umbilical. It appears to be pin 5.

View attachment 4815982

There should be a second wire that goes from the R49 connection on the terminal strip to B.
View attachment 4815990
Click to expand...

Yes I think this is also a result of the various changes in the later 70s. R49 connects to the terminal strip but it on the end point of the connections - normally where the wire to C10 goes. This appears factory.
So my 3 connections are:
R49 above D5
White to C10 above D4
Red / Red White to OT above R47

And then R48 to C12

All Pin 4s connect to Screen Resistors. Pins 5 are the grid resistors.

It all seems to match the 78 you posted. B in that picture goes to C11 +.

The Pre Amp wire I'm unsure about seems to go to C9 in that image too.

IMG_0580.jpg
 
#72
Let's explore this
upload_2022-9-16_16-14-46.png


You indicated you were measuring 408V at the connection between C12A and C10. With 663V at node A it should read right around 331.5V.


With the preamp connected here, I would expect the voltage to read low rather than high. The extra current the preamp pulls through R47 would increase the voltage drop. If the voltage drop across R47 exceeds the cap's 450V rating, then the cap will start to break down.

As the voltage goes over the 450V rating, I believe it would cause C10 to start leaking more and more current over time. AFAIK, If the overvoltage situation is sustained it will eventually cause the capacitor to short. However, as the leakage increases it would reduce the voltage drop across C10, which could explain why you were reading 408V. The voltage drop across C10 was 663-408=255V
 
#75
I think I have solved the mystery.

Relooking at the image, it appears the orange wire goes to pin 2 on the umbilical (not pin 5). Red goes to Pin 5, and is the Preamp supply connected to node B.
upload_2022-9-16_17-32-30.png



The 77 service manual shows pin 2 going to E which is C9A. However, the preamp schematic shows no connection on node pin 2.

Here's a composite of the two connectors:
upload_2022-9-16_17-18-7.png



So ( @churchman ) C9A/node E to pin 2 is a valid connection, but it is not used.


If you are curious, the 69/70 schematics shows a ground on pin 2 that goes to the polarity cap.
upload_2022-9-16_17-28-11.png


This wire is gray in my 69 and it is connected to one of the ground tabs on C12, as shown on the diagram I posted.
 
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#77
numberfourthree said:
Ok. Tested out the amp, powered it up, let it warm up, hit the standby switch and blue flash in V9. Took out R42 and R27.

I know, let the beatings commence @agedhorse ;)

Though I curious about how some of the plate resistors are working under load.
Click to expand...

IMHO double check you work. Rebuilding the capacitor network is complicated and confusing. There may yet be mistakes to discover, that you had nothing to do with.

I would bring the amp up with no tubes. Ideally you would use a variac and a current limiter.

Back in the day you could rig up a lightbulb limiter with incandescent bulb. I used a lightbulb limiter to troubleshoot my skunk SVT when it shorted one of the main filter caps. Bought an insulated light socket and clipped it across the blown 10A pigtail fuse inside the amp. I think I used a 40W bulb since the amp had blown the fuse.

This is the light socket I bought. $9.59 at Ace
43cff52f-590d-4171-8fcf-7aa180bf37e5

Ace Plug In 150 W Spot Light Kit 1 pk - Ace Hardware

Measure DC voltages at the power supply nodes, to make sure everything is configured properly. Then measure screen, plate, and bias voltage on all of the output tube sockets.

Before you bring it up with output tubes, adjust the bias for max negative voltage.

If you use a lightbulb limiter, I believe you need 70V (or a little more) before the tube heaters will warm up enough for the cathodes to start emitting electrons. This will require a bulb rated for considerably more than 40W. On 120V line and adjusted to spec, both of my SVTs pull just shy of 300W on idle. If you can find them, try 100W and 150W.
 
#80
That sucks. Pull all the tubes. Get your parts ordered. Double check all your cap work. Fire it up without tubes. I would slow down on this beast. What you are doing is a great gift to restore it. When I first did mine I got some Chinese coke bottles to fire it up. I talked to Fat Willie and he chewed my ass out. But they were just to service it. I eventually bought original Winged C, they cost 25 bucks back then ,94. I about choked. My SVT blew GE tubes 3 times. So the sudden rush of current might of taken it out. Or did you have a load hooked up to it? Never turn a SVT on without a speaker hooked up to it. Or 4 ohm load. Ask me how I know.
 

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