Why does unplugging/replugging TT leads from tube phono pre-amp reset dead channel?

Jmfawdofile 12-22-2017
Suggestions on loading to address this issue? It sounds fine as it’s currently set.

Joe, the main reason I suggested that you determine what the loading is set to is that it might help in determining the cause of the problem. I’m doubtful that any change you might make to the setting would constitute a solution, although I suppose it’s possible.

My original point regarding loading was that when you disconnect and reconnect the cables you are not only physically manipulating the cables and connectors, but in addition when the cables are disconnected you are temporarily changing the impedance presented to the input circuit of the phono stage from a very low value (essentially the cartridge’s impedance) to a significantly higher value (whatever the loading is set to). And it seems conceivable to me that doing so might be allowing the input stage to recover from some abnormal condition.

As you can see in the manual resistive loading is set at the factory to 47K, but DIP switches are provided in the unit which can put resistors of 100 ohms, 1K, and 10K in parallel with the 47K, in any combination. There is also provision for paralleling a resistor supplied by the user, which a previous owner may have done if you are not the original purchaser.

So the overall load resistance may be somewhat less than 100 ohms, or it can be as high as 47K, or it can be at various settings in between. If it is at the low end of that range, the likelihood that my hypothesis accounts for the problem is minimal, since whether or not the cables are connected would make a relatively small difference in the impedance presented to the input stage. If it is at the other extreme, 47K, the likelihood of my hypothesis being relevant would be significantly greater.

But to answer your question, if there is any possibility that changing the loading might be a solution, my guess is that the 47K setting would stand the best chance, if it is not already set that way. If it is not already set that way, changing to 47K would change the impedance presented to the input stage during normal operation in the same direction (i.e., making it higher) as disconnecting the cables has been doing, albeit to a smaller degree.

Good luck. Regards,
-- Al

No advice, only a comment. I’m surprised that when mentioned your TT was an LP12, that everyone didn’t  immediately blame the TT. I have an LP12 and just it’s mention seems to bring out all sorts of haters. It’s gotten comical. Seriously, good luck resolving your issue. 

Well, Merry Christmas and Holiday from an LP12 owner here;
Joe, I just went through 3 months of a similar problem with an intermittent drop out with my older LP 12/Lingo/Troika/Ekos.  One channel would cut in and out with music at times. Then it would stay out for a longer periods; I refitted cables and connectors, which helped for a few weeks and I thought little of it.  As the channel dropped out for longer periods of time eventually the cable jostling didn't help as it did on some occasions before. 

Testing the Troika cartridge leads was not revealing or helpful . Though stored and used little, it may have seen too many years and developed suspension problems.  I do not have duplicate components for everything to swap out and check and I could not just put the Troika in another turntable setup.  Swapping channels isolated the placed the problem before the preamp. So when I buy a cheap MM cartridge and put it the LP12 and find both channels now working fine the next 3 days I'm convinced the Troika is faulty and need to figure a hefty budget for replacing it, and perhaps going ahead with silver or Cardas front to back TT rewiring for the LP12, which had been one of the potential problems. 

After weeks of researching cartridges, and tonearm rewiring, etc (and fortunately before buying any) with sporadic listening the channel again starts dropping out, but now it stays out---steady, consistent, predictable---to where it is easy to troubleshoot and isolate--NOT to the LP12 but rather downstream to the American Hybrid Technology phono stage.   It turned out that there was a cracked/loose solder joint(s) in the connections  power supply umbilical into the board on the main unit. Being 15-20 years old both R and L connections were weak and needed reworked.  Whether an initial cold solder joint or just aging and strain from connecting/disconnecting the umbilical was impossible to tell and didn't matter.

[btwThe Troika works fine it turns out when the phono stage is working, as is the entire LP 12 setup.  I must admit it can be easy to jump to conclusions and blame older gear- especially wading through the dialogue and diatribe between all the Linnies and LP12 Bashers-kinda makes you want to tear your table apart and rewire it...though it might be a good upgrade for a later time it shouldn't be an emotional knee jerk reaction I felt at first...]

I came to a reasonable conclusion early on that was entirely wrong  when swapping out cartridges  because of the intermittent nature of the problem and not being able to confirm the Troika as the problem in another system, and nearly spent thousands to fix a $150 problem.  I was lucky to have waited looking at options and have things worsen and declare itself further; it also helped immensely having some duplicate components --so when an older Adcom processor phono stage was substituted in the system and resolved the the drop out for a perfect 10-14 days  it was then clear to take the AHT phono stage in for testing in another system.  A few dabs of solder and cable anchor and all is well. What a vexing problem from a tiny, inconspicuous spot.   Whether the source for your problem ends up similar or not I wish the best of luck, perseverance and much patience.  and Happy Holidays.
Lary

Hi All,
Thanks for your patience with the silence during the holidays.  I have a few things to report.
- First, I'm grateful for the suggestion to look at the load on the Shelter 901.  The recommended loading is 100.  I had it set at 47K.  I dropped it down from 10K, to 1K, to 100.  Seems to balance the treble/mids so much better...but also, the dropouts seem to have stopped.  This may have been the cause of the issue, Al.  Thanks.

- I also examined each tube carefully.  One of the 6N7Ss had a hairline crack on the base.  I replaced it.  Could this have caused BOTH channels to be randomly dropping??

- I have been listening all week...and the drop out occurred only once after those changes.  I did notice a big dustball on the stylus.  Removed it, did the unplug/replug...and no more issues.  Could that be a factor??

- I appreciate the LP12 support.  I like the table...other than the click in the bb at the bottom of the sub platter spindle. ( I can hear it on very soft passages or before the music starts.)  I think in your situation, the MM change from MC might point to Al's solution.  

- I don't know what a cold solder crack would even look like.  But the Phono preA was at BAT about a year ago...so I assume they would have noticed.

- The system sounds the best it ever has.  I'll keep you posted.

Happy New Year.

Joe

None of the glitches you discovered leads to an easy explanation for what you experienced, in my opinion, but it's good that you found them.  And it's even better that you are now able to listen for longer periods without that aggravating problem, no matter what the cause.  I don't think that, because your unit was at BAT a year ago, we can rule out a cold solder joint that is intermittent.  If the unit functioned flawlessly at BAT on the workbench, they would never have had cause to look for such a problem.  And such problems can be near to invisible by inspection.  Sometimes you just have to bite the bullet and re-solder everything, in order to get rid of an intermittent solder fault.  (I don't suggest you need to do that, since you are doing fine as is.) Happy New Year.