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This is a typical day-in-the-life of restoring an Ampeg SVT-Cl/AV/SVT2-Pro bass amp that you find has one or more power tubes that have failed or are way outside the match with the remaining working tubes. Yes, one could always just buy a new sextet and be done with. Most of the time in servicing them, I end up with needing one or two more working tubes to complete a set, and need them to match the rest.
This particular amp started me off with one RED Bias LED lit, and the sound from the amp was a loud hum. AC mains current draw was nominal, so I tweaked the two bias adjust pots to see what I had. To get any hum cancellation, the two pots were set cattywampus, so time to pull power tubes and set it up to see what sort of plate current was going on. And, I didn’t yet know the status of the 12AU7 Driver tubes.
The schematic shows a ‘nominal’ DC level of the cathodes of the two 12AU7’s (@ Pin 8) to be -45VDC, which feed the grids of the power tubes. I saw both driver tubes had a similar working range, so I dialed each in for -45V as a starting point. When I plugged in the first pair of tubes, I had both RED LED’s lit, seeing around 32mA on each tube (320mV across 10 ohm cathode resistors). So, I re-set the bias voltage to -49V, which with these tubes, yielded 20mA)
So, with common bias voltage settings, now looking at each pair of power tubes from the sextet, I’d get meaningful plate current values to show me how well matched the tubes are (or aren’t). I found one measuring 50mA, one at 30mA, and the other four at 20mA ea. So, I needed two matching power tubes to mate up with the remaining 4 tubes.
Over the years, I accumulate a lot of 6550’s and KT-88’s from maintaining all the Ampeg SVT’s, Orange AD-200’s, and other amps using those tubes. In this case, the tubes were J/J Tesla KT-88’s from ARS Electronics (a local tube supplier in Van Nuys, CA nearby). So, after first checking with our new inventory to see if they had any of them with the Test Number of 83 or near that painted on top of the tube, they didn’t’ have anything close. I dug thru the packages of pulls on hand, ended up with a couple packages that actually had the same numbers, and weren’t marked as bad or show any physical defects. Both measured 21mA after having warmed up. So, I was in the ballpark. I loaded all six back into the tube sockets and let the amp warm up @ idle before making the bias adjustments to land it. I ended up with an average of 23mA on each half of the sextet, (22mA, 23mA, 24mA & 22.5mA, 22.5mA, 24mA). I’m used to seeing a much wider variation on used tubes that have been in service for a couple years, so that was a lucky shot.
I don’t make it a habit of mixing different manufacturer’s tubes to cobble a working set, and don’t mix KT-88’s with 6550’s either. I haven’t read up on all the pitfalls that could lead you to…just doesn’t sound like a wise thing to do. It would be an interesting study, given enough time, resources and proper instrumentation to record all the dynamics to see just how the load sharing goes. Life is too short to go down that road, and, I assume it’s been done before, and some details no doubt published.
At any rate, now having a freshly biased output stage, I find the GRN LED on one of the two meters wasn’t lighting up. After probing the Window Comparator circuit, it all pointed to a bad GRN LED. Rats. T-1 size GRN Defused LED. Ampeg uses a dual RED/GRN Array whose lenses show thru the little chassis holes next to the Bias pots. To get at them, you have to tear down the power amp chassis. I confirmed it was a bad LED, so had to pull the chassis apart to get the array removed.
No maker’s marks on the rectangular black plastic block that had the two LED’s installed, with their respective leads bent down at a right angle. I forgot to take some detailed photos of the array, but, it appeared the only way to replace the LED is if they are ‘front-loaded. They were. Using T-1 size LED’s with NO base flange. The GRN LED’s I had on hand were Clear lens, not diffused, so you just see a tiny green dot when powered. After cutting away the bass flange of the part, I sanded the lens with some 600 GRIT emery paper which gave a diffused result. Noted the Cathode position, installed the new LED, bent the leads down, did the same with the existing RED, and re-installed the array into the PCB. Then re-assembled the chassis, loaded the tubes, re-connected the preamp and tried it out.
The amp came in with the complaint of ‘popping and crackling noises. And, I found two of the chassis screws missing from the case, and no cage nuts in those locations to receive the replacement mtg screws, So that was first on the agenda…finding that hardware so they didn’t find theirs way into the wiring for later distribution into the live chassis!
The rest of the noises was from a microphonic preamp tube, which in swapping with it’s mate in the preamp, the microphonic behavior ceased. So, a bad input tube, which I replaced.
This particular amp started me off with one RED Bias LED lit, and the sound from the amp was a loud hum. AC mains current draw was nominal, so I tweaked the two bias adjust pots to see what I had. To get any hum cancellation, the two pots were set cattywampus, so time to pull power tubes and set it up to see what sort of plate current was going on. And, I didn’t yet know the status of the 12AU7 Driver tubes.
The schematic shows a ‘nominal’ DC level of the cathodes of the two 12AU7’s (@ Pin 8) to be -45VDC, which feed the grids of the power tubes. I saw both driver tubes had a similar working range, so I dialed each in for -45V as a starting point. When I plugged in the first pair of tubes, I had both RED LED’s lit, seeing around 32mA on each tube (320mV across 10 ohm cathode resistors). So, I re-set the bias voltage to -49V, which with these tubes, yielded 20mA)
So, with common bias voltage settings, now looking at each pair of power tubes from the sextet, I’d get meaningful plate current values to show me how well matched the tubes are (or aren’t). I found one measuring 50mA, one at 30mA, and the other four at 20mA ea. So, I needed two matching power tubes to mate up with the remaining 4 tubes.
Over the years, I accumulate a lot of 6550’s and KT-88’s from maintaining all the Ampeg SVT’s, Orange AD-200’s, and other amps using those tubes. In this case, the tubes were J/J Tesla KT-88’s from ARS Electronics (a local tube supplier in Van Nuys, CA nearby). So, after first checking with our new inventory to see if they had any of them with the Test Number of 83 or near that painted on top of the tube, they didn’t’ have anything close. I dug thru the packages of pulls on hand, ended up with a couple packages that actually had the same numbers, and weren’t marked as bad or show any physical defects. Both measured 21mA after having warmed up. So, I was in the ballpark. I loaded all six back into the tube sockets and let the amp warm up @ idle before making the bias adjustments to land it. I ended up with an average of 23mA on each half of the sextet, (22mA, 23mA, 24mA & 22.5mA, 22.5mA, 24mA). I’m used to seeing a much wider variation on used tubes that have been in service for a couple years, so that was a lucky shot.
I don’t make it a habit of mixing different manufacturer’s tubes to cobble a working set, and don’t mix KT-88’s with 6550’s either. I haven’t read up on all the pitfalls that could lead you to…just doesn’t sound like a wise thing to do. It would be an interesting study, given enough time, resources and proper instrumentation to record all the dynamics to see just how the load sharing goes. Life is too short to go down that road, and, I assume it’s been done before, and some details no doubt published.
At any rate, now having a freshly biased output stage, I find the GRN LED on one of the two meters wasn’t lighting up. After probing the Window Comparator circuit, it all pointed to a bad GRN LED. Rats. T-1 size GRN Defused LED. Ampeg uses a dual RED/GRN Array whose lenses show thru the little chassis holes next to the Bias pots. To get at them, you have to tear down the power amp chassis. I confirmed it was a bad LED, so had to pull the chassis apart to get the array removed.
No maker’s marks on the rectangular black plastic block that had the two LED’s installed, with their respective leads bent down at a right angle. I forgot to take some detailed photos of the array, but, it appeared the only way to replace the LED is if they are ‘front-loaded. They were. Using T-1 size LED’s with NO base flange. The GRN LED’s I had on hand were Clear lens, not diffused, so you just see a tiny green dot when powered. After cutting away the bass flange of the part, I sanded the lens with some 600 GRIT emery paper which gave a diffused result. Noted the Cathode position, installed the new LED, bent the leads down, did the same with the existing RED, and re-installed the array into the PCB. Then re-assembled the chassis, loaded the tubes, re-connected the preamp and tried it out.
The amp came in with the complaint of ‘popping and crackling noises. And, I found two of the chassis screws missing from the case, and no cage nuts in those locations to receive the replacement mtg screws, So that was first on the agenda…finding that hardware so they didn’t find theirs way into the wiring for later distribution into the live chassis!
The rest of the noises was from a microphonic preamp tube, which in swapping with it’s mate in the preamp, the microphonic behavior ceased. So, a bad input tube, which I replaced.
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