Greetings friends. I'm attempting to repair a friend's Ampeg Jet J-12T amplifier and have run into the ditch. Take a look?
Here's the schematic. Amp was originally taken to the shop after it stopped working after a loud hum sound. Tech diagnosed a blown OPT but it was decided the repair wasn't worth the cost, so it came to me. As the amp was built in '96 it's time to replace all the EL caps, but all the WIMA film caps should still be in good shape. I was able to find replacement caps with similar specs for all except the preamp tube cathode bias caps; Mouser only had 22uF 35v caps of the Muse-UES type, which turned out to be Bi-Polar. Mistake?
I also found a broken spring in the reverb tank, and a damaged socket for one of the power tubes:
After replacing the damaged socket and all the old caps, I put everything back together and installed a new reverb tank. I used a Variac and current-limiter and slowly brought up the voltage to full. After running for a few minutes I shut everything down, let the tubes cool off, and then plugged the amp right into the wall. Using my phone as an audio source I was getting a clean audio signal thru the amp, no hum. Volume, Tone, Tremolo and Reverb all working correctly, and the b+ was 320v, right in spec, As I was preparing to test and record all the voltages from test points, I heard a Pop, followed by an airy, fuzzy hum, and saw a purplish-pink glow inside the 2nd preamp tube. After a quick shutdown and cooling period I pulled that tube and saw a small crack coming from Pin 1. The getter of that tube is still mirror-shiny and it's been 24 hours since this happened, so it's not an air leak. Any ideas on what happened? Bad tube, bad circuit, or something else? Any suggestions, advice, help welcome.
thanks!
w
Here's the schematic. Amp was originally taken to the shop after it stopped working after a loud hum sound. Tech diagnosed a blown OPT but it was decided the repair wasn't worth the cost, so it came to me. As the amp was built in '96 it's time to replace all the EL caps, but all the WIMA film caps should still be in good shape. I was able to find replacement caps with similar specs for all except the preamp tube cathode bias caps; Mouser only had 22uF 35v caps of the Muse-UES type, which turned out to be Bi-Polar. Mistake?
I also found a broken spring in the reverb tank, and a damaged socket for one of the power tubes:
After replacing the damaged socket and all the old caps, I put everything back together and installed a new reverb tank. I used a Variac and current-limiter and slowly brought up the voltage to full. After running for a few minutes I shut everything down, let the tubes cool off, and then plugged the amp right into the wall. Using my phone as an audio source I was getting a clean audio signal thru the amp, no hum. Volume, Tone, Tremolo and Reverb all working correctly, and the b+ was 320v, right in spec, As I was preparing to test and record all the voltages from test points, I heard a Pop, followed by an airy, fuzzy hum, and saw a purplish-pink glow inside the 2nd preamp tube. After a quick shutdown and cooling period I pulled that tube and saw a small crack coming from Pin 1. The getter of that tube is still mirror-shiny and it's been 24 hours since this happened, so it's not an air leak. Any ideas on what happened? Bad tube, bad circuit, or something else? Any suggestions, advice, help welcome.
thanks!
w
The purple / pink glow does suggest an air leak. The getter may look good for some time after the leak started depending on how thick it was deposited at the original burn. and how hard the tube was run since it was built. I have a large sweep tube that developed a small crack, but still worked for a few weeks. Then it started to glow inside. The glow got worse and the amp began to distort badly. I pulled the tube tortured it a bit just for fun to get the pictures, then set it on a shelf several years ago. It took several months before the getter showed any sign of trouble. Now 6 years later one of the two getters is nearly gone, but the other still looks normal.
Try another tube if you have one.
The first picture is from 6 years ago. The second picture is the same tube 6 minutes ago. The getter on the back side is nearly gone but the one on the front would still pass for good and this tube spent nearly 2 years running over spec in a class A SE amp before it failed.
Try another tube if you have one.
The first picture is from 6 years ago. The second picture is the same tube 6 minutes ago. The getter on the back side is nearly gone but the one on the front would still pass for good and this tube spent nearly 2 years running over spec in a class A SE amp before it failed.
