79 Ampeg SVT Volume Drop

[numberfourthree]

Thank you all as always for giving me more rabbit holes to go down! The 2 bundled cables seem an improvement with many advantages. I’ll see if I can get specific with the issue before diving into a rebuild.

 

[agedhorse]

Thank you all as always for giving me more rabbit holes to go down! The 2 bundled cables seem an improvement with many advantages. I’ll see if I can get specific with the issue before diving into a rebuild.

Fix the original problem first, before creating new problems.

 

[beans-on-toast]

Thank you all as always for giving me more rabbit holes to go down! The 2 bundled cables seem an improvement with many advantages. I’ll see if I can get specific with the issue before diving into a rebuild.

The cable bundle futzing may help with noise but it isn’t your problem.

Start with the channel 2 power supply voltages compared to the references on the schematic. Ensure that the tube socket contacts and solder joints have no issues. A solder joint can look good and it isn’t, the wire can pull out. Try a gentle tug. An issue with these amps. Use known good tubes in that channel.

If all is well, follow the signal path along channel 2. Use a scope or failing that, build a listening device (aka stethoscope, basically a signal tracer). Follow the signal pathway from the input to where the two channels are mixed by R23 100K resistor and R42 100K resistor channel 1 before the 6C4 tube.

https://www.talkbass.com/attachments/svt-manual-5-1-1977-pdf.1116491/

Listening amp diagnostic tool at the bottom.

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