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This was originally posted in the Vintage forum but only got one reply so I'm trying it out here.
I have a Scott 399 tube receiver with a Scott 335 mpx outboard adapter. When I fire both units up there is a slight warbling/fluttering sound to the FM for the first 10 or so minutes. I've had the pair for a year now and they play perfectly beautiful once warmed up. At first I thought there might be a weak tube in the chain (rectifier or ?) that might be getting weak but am now thinking it's more just the internal "amp, preamp, tuner" of the receiver and the outboard tube mpx decoder (all those tubes) that are all warming up together and simply just takes a little time for them all to get synced up and in spec.
What do you guys think? Normal for what I got?
fwiw, in other systems I have separate tube amps, preamps, even a tube tuner and individually they all sound good and listenable from the start and just improve over the next 20-30 minutes, no "fluttering".
Follow Ups:
I'll guess a marginal electrolytic capacitor that's on its last legs. Its leaky but sort-of reforms to good enough after a few minutes. Again, that's a guess, but it's based on experience.
Update...today I paid more attention to the "time". I had guessed about 10 or so minutes before but timing it today was more like only 5 minutes. My hunch is it's the "sum of all parts" that is at work, especially with the separate outboard mpx unit warming up at it's own pace.
You most likely have a weaker tube in the MPX that corrects itself as it stays on..Do this..When you put it in mono,do you still get the warble on startup? If so,we can safely assume it's in the mpx and the 6BL8 and the 12AT7 in the rear are the most common failures on those.Your alignment may be slightly off if you have to change tubes but not always.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/11/14
No warble in "mono" when I first turn these on. Go to Stereo a minute later and there is warble but goes away in less than 5 minutes. I will pull and test those tubes you mentioned.
I will clean the tube pins and sockets anyway, wonder if that has a chance for the fix?
Let me tell you tho,it could still be alignment issues slightly off and the tester may show the tubes as good on emission but may have slight leakage that cures itself after warmup..See if the test higher after being in the tester for six minutes.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/13/14
I'll try that this evening.
I would be inclined to look for a loose or corroded connection or even a cold solder joint.
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