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One of our Ampeg SVT-CL rental amps came over with a note on it stating the -15dB Input Jack was giving intermittent output. I removed the preamp chassis, enough that I could mount it extended out the front (Fan Panel removed) by attaching the chassis with the Rear Cage Nuts, exposing the insides of the preamp. I powered up the amp (always have a Variac & Power Analyzer to do this with), and when I saw the amp was only pulling 130W Operating, when normally, they're pulling around 210W without the fan panel installed, I looked around the back, and saw one of the power tubes (V2) wasn't lit up.
I first addressed the input jacks, ended up having to remove the PCB assy, then the jacks, then push out the rear Tip contact to retension it (Cliff Jacks), and bend up the Normal, then apply some contact cleaner and burnished the tip/normal surfaces so restoring the milliohm DCR. Put the contact back into the body, soldered them back into the board, now having proper continuity.
Then, I removed the rear panel, powered it back up, and sure enough, no bias LED's lit and the middle rear tube (V2 was cold). Pulled the two chassis out, set the cabinet aside, removed the tubes, removed the hold-down clamps, as I also needed to complete the treatment of hi temp silicon rubber sleeving on them.
Then, I turned my attention to V2 tube. Measured open circuit between Pins 2 & 7. I unsoldered those two pins, which took some time to draw out the solder with solder wick, as well as jarring the solder from the heated pins on the alignment peg....not enough to break the pin, of course, but enough to aid in getting the solder out of the pin cavity.
I checked the heater wires inside the now empty pin shell, and was thankfully reading heater DCR of 0.62 ohms....same as the other five tubes were reading. It took a little while to seep in solder, feeding it into the heated pin to let it wick up inside. I've managed to restore some power tube's heaters in this fashion before, but never know. When the heater's open, dead tube, period.
Now that J/J KT88 Sextet's are selling for around $1000, if you can find them, this was well worth the effort!!
I just got the amp back up and running, standing on the power xfmr end, support block to keep it vertical, with the preamp also standing on end, so I could see what the rest of the tubes were doing as far as their plate current balance was. My records indicated the last time I had replaced the tubes and set it up was in Aug 2011. Not sure when V2 went cold...may have been recently. All my maintenance on this particular amp were minor issues.
I swapped one pair of tubes in the sextet to get a better balance, and it's now burning in...average cathode currents around 22-23mA each.
I first addressed the input jacks, ended up having to remove the PCB assy, then the jacks, then push out the rear Tip contact to retension it (Cliff Jacks), and bend up the Normal, then apply some contact cleaner and burnished the tip/normal surfaces so restoring the milliohm DCR. Put the contact back into the body, soldered them back into the board, now having proper continuity.
Then, I removed the rear panel, powered it back up, and sure enough, no bias LED's lit and the middle rear tube (V2 was cold). Pulled the two chassis out, set the cabinet aside, removed the tubes, removed the hold-down clamps, as I also needed to complete the treatment of hi temp silicon rubber sleeving on them.
Then, I turned my attention to V2 tube. Measured open circuit between Pins 2 & 7. I unsoldered those two pins, which took some time to draw out the solder with solder wick, as well as jarring the solder from the heated pins on the alignment peg....not enough to break the pin, of course, but enough to aid in getting the solder out of the pin cavity.
I checked the heater wires inside the now empty pin shell, and was thankfully reading heater DCR of 0.62 ohms....same as the other five tubes were reading. It took a little while to seep in solder, feeding it into the heated pin to let it wick up inside. I've managed to restore some power tube's heaters in this fashion before, but never know. When the heater's open, dead tube, period.
Now that J/J KT88 Sextet's are selling for around $1000, if you can find them, this was well worth the effort!!
I just got the amp back up and running, standing on the power xfmr end, support block to keep it vertical, with the preamp also standing on end, so I could see what the rest of the tubes were doing as far as their plate current balance was. My records indicated the last time I had replaced the tubes and set it up was in Aug 2011. Not sure when V2 went cold...may have been recently. All my maintenance on this particular amp were minor issues.
I swapped one pair of tubes in the sextet to get a better balance, and it's now burning in...average cathode currents around 22-23mA each.