I would also encourage you to post photos here. We might be able to spot something by looking over images of the area around that particular tube.
It's interesting that Kevin has experienced blown screen resistors on these; I would measure the DCR of all of them and poke and the one on the offending socket with some force to see if it disintegrates or shows cracks.
It's interesting that Kevin has experienced blown screen resistors on these; I would measure the DCR of all of them and poke and the one on the offending socket with some force to see if it disintegrates or shows cracks.
Bias
Unless I’m wrong. Jadis tells you to measure 4 - 5 v between fuses and ground. According to schematic it drops over 180 ohm resistor (negligible drop over the fuse).
Unless I’m wrong. Jadis tells you to measure 4 - 5 v between fuses and ground. According to schematic it drops over 180 ohm resistor (negligible drop over the fuse).
Pictures
Hi. I will post pictures as soon as I get back to it. I put all the tubes back after cleaning contracts in sockets, except the funky one. I even stuck one KT88 instead of it in a different row. It plays. I’m watching to see if any other tubes will go funky. So far so good.
Hi. I will post pictures as soon as I get back to it. I put all the tubes back after cleaning contracts in sockets, except the funky one. I even stuck one KT88 instead of it in a different row. It plays. I’m watching to see if any other tubes will go funky. So far so good.
I've not had good luck with them either, 250V 40mA about 40 hrs a month at a guess as a pass element a PSU powering one of my phono stages, plenty of getter left, but noisy and intermittent after a year of use. Have not bought any more (went to solid state PSU designs)
When I used 6550 in amplifier designs I recommended replacement at 1500 - 2000 hours for modern production and maybe twice that for vintage tubes. Two months of 24/7 would have hit the replacement target.
180 ohms is a bit high imho, but then 4 to 5 volts makes sense, are these resistor bypassed with ecaps? sorry i have not seen the schematics so i ask probing questions...
isn't that a bit short Kevin?
i built a 6550 years ago with g2 regulated and b+ in the 380 volts, it still worked fine today, said the owner...those were the 6550 Tong-sols new production...
It's a matter of hours of operation, and possibly luck. I have not had good luck with recent Russian made 6550.. (I also run at much higher voltages)
I use GE and SV6550 where I need to, and have stopped designing things that use them. My last tube based regulators used Mullard XF3 EL34 and I expect them to last a couple of decades. All my new PSU are fully solid state.
I use GE and SV6550 where I need to, and have stopped designing things that use them. My last tube based regulators used Mullard XF3 EL34 and I expect them to last a couple of decades. All my new PSU are fully solid state.
That is absolutely brutal. I ran a pair of Sovtek EL34s for two years straight before they started sounding iffy. What is the failure mode you're noticing? I have had a decent number of the new KT88/6550 tubes go gassy on me somewhat quickly.
Funny thing is, same amp running 400V/60mA with 6P3S tubes ran for 2 years 24/7 before the tubes were at 2/3 of "new" and you could start to see through the getter flash.
When you say two years are you talking 24/7 or?
In my case the getter flash was basically gone, and the tubes were only at about 50% of new after 2 months @24/7.
It’s alive!
Hello all
So the latest update:
Last night the amp was on for two hrs straight and no problems. I made it difficult for the amp by sticking a KT88 in place of that funky 6550. No problems. I think there are a few versions of what happened:
1. Tube was bad to begin with and went funky color by itself;
2. Tube is good (it tests as good as the others on my BK747) and just went funky color;
3. Tube went bad because of bad connections in it’s socket (I cleaned all with Radio Shack spray);
I couldn’t find any other problems, except for a couple of resistors in driver stage that have drifted. Replaced them. I don’t think I will restore driver stage to original, will leave it as is, slightly “hotter”. I will use the amp for a few days to see how it behaves, and if all is well, that’s the end of this adventure, for now. I want to thank all for your efforts and help. I learned a lot, from the best )))
Hello all
So the latest update:
Last night the amp was on for two hrs straight and no problems. I made it difficult for the amp by sticking a KT88 in place of that funky 6550. No problems. I think there are a few versions of what happened:
1. Tube was bad to begin with and went funky color by itself;
2. Tube is good (it tests as good as the others on my BK747) and just went funky color;
3. Tube went bad because of bad connections in it’s socket (I cleaned all with Radio Shack spray);
I couldn’t find any other problems, except for a couple of resistors in driver stage that have drifted. Replaced them. I don’t think I will restore driver stage to original, will leave it as is, slightly “hotter”. I will use the amp for a few days to see how it behaves, and if all is well, that’s the end of this adventure, for now. I want to thank all for your efforts and help. I learned a lot, from the best )))
BTW, I looked inside the “funky” tube, and on the inside the getter’s flash is nice, even and shiny.
Yes, 2 years of 24/7 operation.
When they finally died, I put Russian reissue GL KT88s in and they both went gassy after a few months.
Interesting. Russian reissue GL KT88 is what Upscale Audio recomnends for my amp, and they charge $750 for 12.
Wow... I guess I've been lucky with the Shuguang KT88s I have... I don't use them much but at least they aren't gassy.
If I had bought those tubes from Kevin, I bet he would've taken care of me more than Tube Depot did.
With input signal turned up and speakers connected (ie fine with volume completely turned down and input and speakers still connected).
I've read reports that this is where the McIntosh amplifiers are shining - some of them have (had) still the original tubes in them after 30 years and still going strong. It was because of the way the output was configured.
Need help with buying a buck trans
It was mentioned here I think by Kevin that the amp would benefit from lowering ac voltage from NY regular (in my building) 124v to specified 110v. I measured B+ at 124, and it’s 515! Spec is 465. Kevin recommended a buck trans. I don’t know anything about them except what I found online. Anybody could recommend anything specific? Brand, model etc? Amp draws about 4 amps (440 w)
Thanks
It was mentioned here I think by Kevin that the amp would benefit from lowering ac voltage from NY regular (in my building) 124v to specified 110v. I measured B+ at 124, and it’s 515! Spec is 465. Kevin recommended a buck trans. I don’t know anything about them except what I found online. Anybody could recommend anything specific? Brand, model etc? Amp draws about 4 amps (440 w)
Thanks
You can get a 120:12V transformer to use as a bucking transformer. The 12V winding can be pretty light duty, and I'm sure you could find a 15V/5A transformer. The 120:12 transformer wired as a bucking transformer will drop a little over 12V, so you'll settle back down to 112V or so on the primary. If you find a 117:12, then you'll buck a little more.