A very excitable Luminescence

[marknoir]

Hello tube gurus

Soldering iron was still warm from recapping my old fave Lumi A (finally getting rid of all the weird noises), as I came across another one, and couldn't pass it by. It was supposed to be a working unit, needing mainly cosmetic work, but - of course - it turned out to be a project with some strange gremlins. Owner was not very familiar with MFA Lumi, told me that tubes are very microphonic, but there is no hum. When I checked the voltages they were all in spec. 6SN7 right channel line tube was the culprit and was replaced. Then I listened to it, and it was OK, with AXONs on the output it sounded like chocolate molasses, but that could be addressed later. I plugged my OC9 into phono and it played fine. Then I plugged m AT33PTG in and the whole hell broke loose!!! Phono went into severe oscillaton, taking out that same line stage tube! Or the other way around... I replaced it again with a cheap Russian one this time, and it was fine, until I plugged in the AT33 - same beating oscillaton, taking out that same tube (because I am too slow to turn the line stage volume down, I suspect)...

Any idea why it hates that particular cartridge and that particular tube? Decoupling cap on that tube is bad? Why does phono go into oscillation? I was planning to re-cap this one as well, or may be even build a whole new four-tube (status B1-C or C) powers supply to bring it to a C status (main chassis has a C faceplate and C circuitry), but I need now to address this issue first - never encountered oscillation in a Lumi...

Any advice appreciated.

Thank you

[kevinkr]

I assume the phono stage is oscillating, the AT-33 has rather low dcr and low inductance as well - perhaps the combination of the two with the interconnect are responsible. You need a scope to look at this. Not sure why it would take out a tube in the line stage section though.

[marknoir]

Originally Posted by kevinkr

I assume the phono stage is oscillating, the AT-33 has rather low dcr and low inductance as well - perhaps the combination of the two with the interconnect are responsible. You need a scope to look at this. Not sure why it would take out a tube in the line stage section though.

Hello Kevin and thank you

It takes out line tube(s) - I assume - because volume control is set high for MC straight in. There may be some other problem there, but for now I'm trying to figure out how to stop it from oscillating. Time to dust off the scope :-)