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Thread: Ampeg svt-cl salvaging pwr tubes to complete sextet

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    Senior Member nevetslab's Avatar
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    Ampeg svt-cl salvaging pwr tubes to complete sextet

    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.

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    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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    Last edited by nevetslab; 01-12-2018 at 11:41 PM.
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  2. #2
    Senior Member TimmyP1955's Avatar
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    All house-owned amps. Every now and then I install a new sextet just to increase our stock of pulls - or because none of our previous pulls are close enough to the five good ones in an amp to get it going. When I order tubes (JJs, from Eurotubes), I always request the same 'number', so replacing a single non-OEM tube will hopefully not be an issue - I don't think I've had a JJ die yet.

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    Senior Member nevetslab's Avatar
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    The 'real' J/J tubes that come in the Yellow & Blue boxes, individually wrapped....I have had one fail, and with the Apex-matching, was able to get a matched replacement for it. What CenterStaging had been buying over the years thru ARS Electronics....stated to be J/J Tesla.....I've had many over the years fail. I don't know what matching system they use (over in Van Nuys, CA). I've been having better luck with the J/J's and Apex matching...be it KT-88's, 6881's, EL-34's, 6l6GC's, EL84's.

    At least over time, with sextets...you end up with a sizable collection of pulls when you end up having to start over with a fresh set, adding those left from a complete set. I lucked out yesterday in a short time period. All too often, it could take over an hour to cobble together a new 'matched' set from what you have. So, it's no doubt saved us a good chunk of change over the years, since we have a large inventory of Ampegs, Oragne, Vox, Fenders, Marshalls' etc to keep running.

    Out of curiosity, how much difference do you allow between your matching, if, say, the nominal plate current setting is 22mA?

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    Old Timer Leo_Gnardo's Avatar
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    [QUOTE=nevetslab;476669]What CenterStaging had been buying over the years thru ARS Electronics....stated to be J/J Tesla.....I've had many over the years fail. I don't know what matching system they use (over in Van Nuys, CA). I've been having better luck with the J/J's and Apex matching..[QUOTE]

    First, there's no such thing as a JJ Tesla tube. ARS (and other sellers) may try to relate the companies because of JJ's history. Tesla is/was a Prague Czech Republic company that made the decision to quit making "receiving tubes" in the early 1990. They had a fairly good reputation, and continue AFAIK to make large industrial vacuum tubes. Mid 90's, the founder of JJ bought the manufacturing equipment and moved it to Slovakia. The equipment mind you, NOT the personnel that knew how to use it. Naturally there's a learning curve for new JJ workers. The first ones generally sucked but in recent years have gotten much better thank heavens! We started to see JJ tubes in the USA by late 90's; they have about a 20 year history here. So - it's one or the other not both.

    ARS in Van Nuys, conveniently not too far away from your Burbank location. But I have a history with them and not a good one. Late 80's I started buying mostly Siemens EL34's from them. Excellent tubes, tough and good sounding. Rumor was they were "really" RTF from East Germany but so what, the quality & price were there. Around 1991 they sent me a batch with pointy glass ends - relabaled EI tubes - and the breakdown rate was phenomenal. I dared to complain, and got a call from their office accountant "you have a return rate that far exceeds anyone else's, YOU must be doing something wrong!" No, they substituted crap tubes for good ones and wouldn't own up to it. That was the end for me, I'm not going to waste my time arguing with the accountant. Who knows maybe they have gotten better but my 1991 experience proved to me - at the time - ARS was not much interesting in maintaining their reputation based on delivering top quality vacuum tubes, rather just pass along whatever junk arrives on their loading dock. I suppose ARS expects we're all rich and stupid enough to simply throw out any junk they might want to ship. Again I hope they have revised their attitude in the ensuing 27 years but who knows.

    I've had much better luck in recent years dealing with CE. There again, was a time when CE (aka Antique and Amplified Parts) shipped rattly & microphonic tubes with a "too bad Charlie" attitude, but that was changed - due to some intervention by MEF members (big thank you guys!) - several years ago. CE improved their game, I respect them for that, and we have been doing lots of business since then.
    At least over time, with sextets...you end up with a sizable collection of pulls when you end up having to start over with a fresh set, adding those left from a complete set. I lucked out yesterday in a short time period. All too often, it could take over an hour to cobble together a new 'matched' set from what you have.
    I've done similarly, and sometimes find I'm chasing my tail. By trying to save the customer money on new tubes, I wind up wasting my bench time instead. Hey, I could be earning a living (sort of) instead! If you're making a salary no matter what, then no problem faffing around trying to save tube money .

    Out of curiosity, how much difference do you allow between your matching, if, say, the nominal plate current setting is 22mA?
    10% ought to be close enough. Some amp gooroos have said it doesn't matter but I really doubt it. "Oh the manufacturer never bothered to install matched sets." That may be true but we can do better, and should do better! When you're throwing around 300 watts it's a very good idea to have all 6 tubes equally sharing the load, no weaklings and no short-lived heroes. I make the analogy to a set of tires or a team of horses. When they're well matched, no problems. If not, expect problems.

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    Senior Member nevetslab's Avatar
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    That's some good history you've shared, Leo! I've been in the engineering and tech end of pro audio electronics since the mid 70's, but only dealing with backline gear as a regular basis since early 2009 here at CenterStaging in Burbank. So, I didn't have the history with tube suppliers and the long-term quality control issues as original tube manufacturing equipment was bought and sold to start new tube mfgrs....and all the problems that occurred not having the experienced staff from the older mfgr's.

    I've had enough failures with the re-marked 'ARS Electronics' tubes stating 'formerly J/J on their label. Recently, a batch of 12AX7's and 6l6GC's arrived from them with NO labeling at all, and I was outraged. I'm not the one doing the bulk purchasing for CenterStaging, so all I can do is complain loudly, while on other client work, I do the purchasing and have ceased using ARS tubes. I haven't tracked how long the new tube sets I install last, but I have all that data in my records to find out. I've simply been finding far more reliability with the J/J tubes I buy thru Antique Electronics in Arizona....and, shipping is cheaper, no sales tax, and....with the APEX tube matching, when I've had one tube fail, I can get a replacement that will match up nicely with the remaining tubes in a set, regardless of it being an SVT or Fender, or what. I think I've only had to return one noisy 12AY7 to them. So, I hadn't seen the earlier problems you had with them, and thankful for that!

    I trade 20 hrs a month labor for the shop space at CenterStaging, working as contract labor and NOT a salaried employee. If in trying to cobble a working set of tubes when one tube fails, if that effort takes up to an hour, I'm replacing the set....assuming the Gtr Dept manager has inventory on hand. All too often, there are no tubes to maintain their gear! I won't get into my feelings about that practice!

    I too hold to the 10% figure in having a decent match for proper load current sharing. Sometimes, I have to juggle all the tubes to get there....easier when there's a pair of bias pots instead of a single one.

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    Old Timer Tom Phillips's Avatar
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    I was buying tubes from ARS back in the 1990's. They were selling NOS USA tubes with white ARS silk screened markings on the glass but left the original making on the base. (They were octal tubes shipped in plane white ARS boxes) Those NOS tubes were good but the ARS "matching" must have been done with a dart board. Fortunately, I had purchased enough to make up sets by doing my own matching.

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    Senior Member nevetslab's Avatar
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    Ampeg svt-cl protection circuit benefits

    Having just taken in one of the bass amps from NBC’s The Late Late Show, with the amp (late generation) going into protect, I first pulled it completely apart, as I do with all SVT-CL’s that haven’t been set up. For once, no issues found on the preamp PCB, so put that back together and set aside.

    This one has the base clamps, and have had many M3.5 Truss head screws with their hex drive strip with the 2.0mm hex driver while trying to remove them from the cover plate. Lucked out this time, all came out without a fight. This later generation has a different ribbon cable, no longer the dual row IDC cable that plugs onto the motherboard header. But, they’ve eliminated the 1/4” FAST-ON’s for connecting the heavy heater wires, now soldering those into the PCB. Went to desolder those, one which came free just fine, but the other one didn’t release, and instead, broke the wide trace of the heater circuit in half by the solder pad, and ripped up a good 3/4” inch of the foil as I was removing the second wire! NEVER had that happen before! So, repaired the damage, then tightened all of the M3.5 standoffs and moved on, as I needed to remove the motherboard to get at the bias pots and to tighten down the loose O/T mtg screws.

    Sure enough, solder fracture on one of the two bias pots…re-soldered both pots, along with the XLR pads, and found the usual fractures on the 5-pin headers at the other edge of the motherboard One of the chassis mtg screws was missing, and it’s corresponding
    M6 cage nut was also missing, along with another. Current production uses cheap Chinese cage nuts, made from untempered steel, so they come out with the greatest of ease the first time you remove the chassis mtg screws. So, replaced those with proper ones, and tweaked the frame so there wasn’t any lateral slop that would allow them to come back out.

    Finally put the chassis back together, after first replacing the stiff vinyl garden hose sleeving with Tech Flex, allowing me to stand the chassis’ up on end for going thru the power tubes to get a best match. Preset the bias voltage to -50V, and started with V3 & V4 in place. RED LED on both, so pulled them and reset the bias for -55V and tried again. 17mA on the two, so a good place to start. I got thru with the other two pairs, finding the spread 17-18mA. So, having already marked the tubes per their positions when I removed them, I put them back into place and let them warm up.

    After 5 minutes warm-up, I brought the bias up to set them at 23mA and made minor tweaks to get an average of 23mA. Swapped a pair of tubes for a better fit, then swapped two more. When I powered back up, it went into Protect. Hmmm. Reset and powered back up, now stable. Made a minor bias tweak, then moved the cooling fan into place and turned it on. Amp went into Protect again!

    OK, now seeing the main cause of it being here in the shop. I pulled all but the last two tubes I had just swapped, and powered back up. OK so far. Tapping on the top of each 6550, I found one of them responded with the RED bias LED lighting, then it went into Protect. I pulled that tube out (V5), replaced it with one from the V4-V6 set. Both tubes now stable, neither responding to my finger hammering on their tops. Powered down, tried the next pair, and again, solid, no signs of fault. Tried the last one, it no longer having a mate (which was the suspect tube), so put another in with it. Fine as well.

    I removed the one tube from the V4-V6 set, and put V5 back in, this time in a different tube socket (V4). Tapped on it again, and as before, it responded with the RED bias LED lighting…then flashed inside the tube and the amp went into Protect.

    I have always liked how well Ampeg’s SVT-CL Protect Circuit works, as it responds so immediate to a tube failure, shutting down the system (removing AC from the Power/Screen Supply Xfmr), and prevents so much damage to the circuits that could take place before a Mains fuse or other fuse could respond. In this case, that circuit was a very useful tool in finding the one bad tube in the set. These were all Sovtek Wing-C 6550’s. I had just found a stash of them in a box full of pulls I had set aside, so now, I just needed to be lucky enough to find one that matched with the remaining 5 tubes. It took me trying two tubes to find it, so that was relatively painless. Biggest pain in this repair was the heater foil damage on the power tube PCB!

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    Senior Member TimmyP1955's Avatar
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    Quote Originally Posted by nevetslab View Post
    Out of curiosity, how much difference do you allow between your matching, if, say, the nominal plate current setting is 22mA?
    I've never measured tubes individually in an SVT. If the amp says it's happy over a long burn in, I send it on its way. If I ever see it again, it's because a tube has become useless.

    In less fancy amps, I'd like to stay within + or - 5%, but with a quad made from pulls I often have to settle for + or - 10%.

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