Tweet
One of my clients sent over an Ampeg SVT-CL that was now in Protect Mode. I pulled the rear grille panel off, finding both PM 6550C and Sovtek 6550C power tubes (with Ruby stickers on those three). After pulling the power and preamp chassis from the cabinet, I set them up vertically so I'd be able to use the chassis as a power tube test fixture, normally using V3 and V4 for the test sockets. Only I wasn't getting any current thru V3. It's resistor also tested a little low (9.35 ohms instead of 10.0 ohms as the other five measured. I changed to V2, leaving V3's tube socket to figure out later.
I preset the bias voltage to -50VDC so I could record the current readings of the six tubes, done in pairs. Then I happened to notice one of the PM 6550 tubes had one of the grid heat radiators no longer welded in place, and it was sandwiched between the glass and the top Mica Insulator of the tube. With the tubes removed, I checked for Bias voltage, Plate and Screen voltage on the six tube sockets. All read ok, only V3 was reading 70VDC Screen while the rest read 390VDC. I pulled the power tube cover plate off, removed the wiring from the tube PCB and checked the screen resistor. it was open circuit, so I replaced that.
That loose heat radiator had come off at some point and was the culprit in blowing that resistor and tripping the Protect Circuit.
Now I was down one tube. I didn't have any full matched sets of KT88's nor 6550C's, so I had to spend some time sorting thru what I had on hand, and finally came up with two sets of three tubes, one set measuring 19.5mA, the other set measuring 25mA. Four were Sovtek 6550C's and the other two were the remaining PM 6550C's. I was able to bias the two sets of three and got the amp working again, new set within a couple mA of each other at the extremes, centered around 22mA. And let the client know I found a matched set of Psvane KT88's for $302.40 and a matched set of Electro-Harmonix KT88's for $402.10. I didn't charge them for the 6550 tubes I cobbled together, just for labor. All the other tubes I looked at were out of stock or not available in matched sextets. And more expensive than before Putin's war on Ukraine.
I preset the bias voltage to -50VDC so I could record the current readings of the six tubes, done in pairs. Then I happened to notice one of the PM 6550 tubes had one of the grid heat radiators no longer welded in place, and it was sandwiched between the glass and the top Mica Insulator of the tube. With the tubes removed, I checked for Bias voltage, Plate and Screen voltage on the six tube sockets. All read ok, only V3 was reading 70VDC Screen while the rest read 390VDC. I pulled the power tube cover plate off, removed the wiring from the tube PCB and checked the screen resistor. it was open circuit, so I replaced that.
That loose heat radiator had come off at some point and was the culprit in blowing that resistor and tripping the Protect Circuit.
Now I was down one tube. I didn't have any full matched sets of KT88's nor 6550C's, so I had to spend some time sorting thru what I had on hand, and finally came up with two sets of three tubes, one set measuring 19.5mA, the other set measuring 25mA. Four were Sovtek 6550C's and the other two were the remaining PM 6550C's. I was able to bias the two sets of three and got the amp working again, new set within a couple mA of each other at the extremes, centered around 22mA. And let the client know I found a matched set of Psvane KT88's for $302.40 and a matched set of Electro-Harmonix KT88's for $402.10. I didn't charge them for the 6550 tubes I cobbled together, just for labor. All the other tubes I looked at were out of stock or not available in matched sextets. And more expensive than before Putin's war on Ukraine.
Attached Files