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This seems to be the month for Ampeg SVT-CL's to come to my shop. The previous one still had the original 1/2W 220 ohm Metal Oxide Screen Resistors installed on the power tube PCB, amp had both low output and level changing, but wasn't in protect mode. I found two of the six screen resistors open, so I replaced all six of them with 1W Metal Oxide MOS package resistors, which fit the pattern for 1/2W resistors. The tubes installed were ARS JJ/Slovakia 6550 / KT88's. When I set up to check the power tubes, which I do with only one pair of tubes installed at a time, using positions V3 and V4 as a general rule, I found not only the two tubes that had been in positions V3 and V5 to be bad, firing the Protect Mode circuit, where their screen resistors were open, but also found V1 tube fire the protect mode circuit. I dug thru what remaining ARS KT-88 tubes I had here at the shop, and found two pulls that matched close enough to the remaining three tubes, but didn't have a sixth that would.
I knew I didn't have replacement KT-88 tubes here, so I went down the street to the Guitar Dept to dig thru our new tube collection, and came back with five ARS 6550 / KT88 tubes, marked JJ/Slovakia and '88' on the top of the tubes. I plugged in one, finding it too was close enough to the five, and then balanced that set, let it burn in for a while, checking later, and had a working set.
Odd that Protect Mode didn't fire with that one tube still having an intact screen resistor. Put that amp back together, ran it for a while, playing thru it aggressively and sounding fine.
This next SVT-CL that I now have on the bench went into Protect and was sent here. I have it on the bench now. Digging thru my service records on that Asset #, I found in the collection of tubes installed....a pair of EH 6550's that are shaped somewhat like KT88's, three Sovtek 6550-Wing C's, and a Sovtek 6550/Fender GT pull from a Super Bassman. Found the EH 6550's found their way into the amp after having two of the six Sovtek 6550's failed in March 2018. Then in August 2019, one of the remaining Sovtek 6550's failed, and I installed that Fender GT/Sovtek 6550 pull, which balanced out. That lasted until this past week. After removing those tubes, I saw enough pigment color change on the surface of the Sovtek Winged C 6550's that I was considering not using them, and, I still had to see what caused the Protect circuit to fire. Normally fires when power tubes fail.
I first checked the amp to see that the Protect Circuit doesn't fire with no tubes installed (removed from the cabinet, standing up on end giving me access to the bottom side of the Power Amp Main PCB). It powered up fine, checked the Driver Tube bias voltages, recorded them on my notepad. I had four of the ARS JJ/Slovakia 6550 / KT88 tubes left, all new, and the two EH 6550 tubes. I noted the plate/screen current reading on those tubes, then removed them. I then checked the remaining Sovtek 6550's in pairs, expecting to find at least one bad tube. None of those remaining four tubes fired the Protect circuit. Those Sovtek 6550's had been in use since at least 2014, so time to put them aside.
That left me with four 'matched' ARS JJ/Slovakia 6550 / KT88 tubes, and these two EH 6550 tubes, which have been in service since March 2018. Beyond that, I'd have to order more J/J KT-88 tubes (will still have to do that, but looking for a current solution).
Normally if I use two dissimilar mfgr's tubes together, I'll place one on each half of the output stage to balance out with the other two pairs. It seemed to work with the plate current balance between the four ARS tubes and the two EH tubes, that the balance was best with the two EH tubes together on one side. Maybe after these have burned in a while, I'll get to swap one to the other side.
That still leaves the question of WHAT caused the Protect Circuit to fire on this particular amp. And, why on the previous amp, finding one of the remaining tubes bad, Protect didn't fire? In the past, I've had power tubes that were sensitive to vibration enough that tapping on them, you could get the plate current to jump and get the RED Bias Lamps to light, as well as if severe enough, Protect would fire. Thus far, not happening.
I knew I didn't have replacement KT-88 tubes here, so I went down the street to the Guitar Dept to dig thru our new tube collection, and came back with five ARS 6550 / KT88 tubes, marked JJ/Slovakia and '88' on the top of the tubes. I plugged in one, finding it too was close enough to the five, and then balanced that set, let it burn in for a while, checking later, and had a working set.
Odd that Protect Mode didn't fire with that one tube still having an intact screen resistor. Put that amp back together, ran it for a while, playing thru it aggressively and sounding fine.
This next SVT-CL that I now have on the bench went into Protect and was sent here. I have it on the bench now. Digging thru my service records on that Asset #, I found in the collection of tubes installed....a pair of EH 6550's that are shaped somewhat like KT88's, three Sovtek 6550-Wing C's, and a Sovtek 6550/Fender GT pull from a Super Bassman. Found the EH 6550's found their way into the amp after having two of the six Sovtek 6550's failed in March 2018. Then in August 2019, one of the remaining Sovtek 6550's failed, and I installed that Fender GT/Sovtek 6550 pull, which balanced out. That lasted until this past week. After removing those tubes, I saw enough pigment color change on the surface of the Sovtek Winged C 6550's that I was considering not using them, and, I still had to see what caused the Protect circuit to fire. Normally fires when power tubes fail.
I first checked the amp to see that the Protect Circuit doesn't fire with no tubes installed (removed from the cabinet, standing up on end giving me access to the bottom side of the Power Amp Main PCB). It powered up fine, checked the Driver Tube bias voltages, recorded them on my notepad. I had four of the ARS JJ/Slovakia 6550 / KT88 tubes left, all new, and the two EH 6550 tubes. I noted the plate/screen current reading on those tubes, then removed them. I then checked the remaining Sovtek 6550's in pairs, expecting to find at least one bad tube. None of those remaining four tubes fired the Protect circuit. Those Sovtek 6550's had been in use since at least 2014, so time to put them aside.
That left me with four 'matched' ARS JJ/Slovakia 6550 / KT88 tubes, and these two EH 6550 tubes, which have been in service since March 2018. Beyond that, I'd have to order more J/J KT-88 tubes (will still have to do that, but looking for a current solution).
Normally if I use two dissimilar mfgr's tubes together, I'll place one on each half of the output stage to balance out with the other two pairs. It seemed to work with the plate current balance between the four ARS tubes and the two EH tubes, that the balance was best with the two EH tubes together on one side. Maybe after these have burned in a while, I'll get to swap one to the other side.
That still leaves the question of WHAT caused the Protect Circuit to fire on this particular amp. And, why on the previous amp, finding one of the remaining tubes bad, Protect didn't fire? In the past, I've had power tubes that were sensitive to vibration enough that tapping on them, you could get the plate current to jump and get the RED Bias Lamps to light, as well as if severe enough, Protect would fire. Thus far, not happening.
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