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Ampeg SVT makes after reassembling

P

PhantomBox

Aug 22, 2006
41
5
#1
Hi guys.
Recently I received an elder Ampeg SVT with an issue. Channel 2 had a ringing noise when you plug an instrument to it.
The only way to get the preamp out of the (completely messed up) wood enclosure was to take out the power amp chassis first. Needless to say, I removed all tubes first.
This beast weights a ton!
I traced the wires from input channel 2 jacks and resoldered the joints of tube #2. Then I connected preamp to power amp outside the enclosure first to hear if the noise went away. It did....
Ok, so far so good.
Then I disconnected prep and power amp and put it back into the enclosure. After tuition it on, now I hear a hum that is not the 60 Hertz from 120 VAC but higher. I wouldn't call it a "hiss" jet...
Obviously I messed up something when maneuvering the power amp back into the enclosure, but I cannot figure out what it is...
Any ideas?
Thanks guys.
 
#2
IMHO, it's way easier to remove the preamp after you take the power amp out of the headcase, so you followed the normal procedure.

Check the molex connectors to see if a pin was bent over or a wire came loose.

If the problem is not in the connectors, you will need to take the amp apart again.
 
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#3
Hopefully you removed the preamp tubes before soldering in that chassis.

Easiest thing is to reseat all your tubes. Make sure that you didn’t mix up the tubes.

Those old umbilical cables can cause problems if you repositioned the shield (gray) cable. You might have broke the shield connection.

If none of that helps, recheck where you were soldering.
 
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#7
The pin that carries this shield on the molex connector got pushed out of its position. I reseated it and cleaned the whole connector. The amp is still humming. Since someone before removed the hum ballance pot, I'll fit in a new one. All I have on hand is a 600 ohm pot, and according to the schematic it should be 100 ohm. I'll try it anyway...
 
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#8
PhantomBox said:
The pin that carries this shield on the molex connector got pushed out of its position. I reseated it and cleaned the whole connector. The amp is still humming. Since someone before removed the hum ballance pot, I'll fit in a new one. All I have on hand is a 600 ohm pot, and according to the schematic it should be 100 ohm. I'll try it anyway...
Click to expand...

The amp will hum without the pot connected or if it is broken. Until you get a proper replacement pot, you can make an artificial center tap with two resistors.

The original was made by CTS, part number 026T419S101A1A1, 5W, 100 ohm, wire wound.

Link Removed
 
Last edited:
#9
Here is an audio file containing this noise. Nothing is connected to the amp. First the volume on channel 1 and 2 is set at 12 o'clock (half way up), then it's turned down completely. Maybe someone can identify it...
 
#11
I don’t see an audio file. There is a ground point at the instrument in jack. A gray cable connects there. Look for oxidation behind that jack. It could be black or white/gray. It the jack to chassis contact is not clean, the amp will him.

Different amp but this is an example of oxidation on the washers between the jack amd the chassis. Be careful, this gray wires are often brittle and break easily.

IMG_2148.jpeg
 
#16
I still can't attach an audio file... Could you tell me what formats are accepted in this forum? I don't have my computer online and all I have at this time is my cell phone, which is not exactly the latest edition.
 
#17
We have really good technical folks on this site. I have learned to follow their guidance to the letter. They will probably walk you through the troubleshooting procedures that they would perform. What you are seeing are the first procedures they would follow. I notice a suggestion about a hum pot. This is a "procedure." ;)
 
#18
PhantomBox said:
I still can't attach an audio file... Could you tell me what formats are accepted in this forum? I don't have my computer online and all I have at this time is my cell phone, which is not exactly the latest edition.
Click to expand...

Click on contact us at the bottom. There are acceptable file formats but also limitations on the length. They’ll let you know how to do it.
 
#19
Hi guys.
I got rid of the the hum. Turns out that I tighened all nuts on the amp, particularly the speaker jacks. And one was missing an isolating washer. Noise is now gone.
But now I got another issue: when tuning up the treble controls on either input channel, the amps starts to squeak. When dialing the control back it stops. I really don't know if it did that before, neither does the owner.
Any ideas?
 

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