Classic Hifi Care

I enjoyed the discussion about what TJ had done to his "Frankengrand". That inspired me to fire up the old Imperial. It hadn't been started in quite awhile, as I have the CD changer on the Symphony. I reckon I need to add it to my "to do" list. It has always had zero bass for a couple minutes, then it starts working. IT did that today. I was listening to Christmas music on FM. When it was cold, the eye tube was closed. As it warmed up, the eye tube opened more and more until I could barely receive one of the strongest FM stations in the Phoenix market. Weaker stations didn't come in at all. Something ahead of the eye tube is going away as it warms up. Probably a tube. This may give me the excuse I need, if I EVER get the shop done, to take this thing apart and tinker with it a bit. And give it a new set of wheels. I haven't had my CG plugged in for almost two years. I wonder what won't work on it.

To be fair, there have been lots of hours put on the Imperial since I went thru it in 2014. We used it for four or five years on the CD changer, so it ran 10 or 12 hours a day, every day. It doesn't owe me anything. I may take the time to incorporate some component changes to see if I can improve its performance somewhat. There may well be a weak tube in the amp, hence the "no bass" situation when first started up. It has 6EU7's, so those are automatically suspect. I'll make some plugs for the holes in the speaker enclosures like Bill's 1962 model has. I also have the schematic for his 1962 model, and can compare the two schematics to identify changes made from the time mine was built, until the time his was built.

Sounds like a heat related issue, tubes would be my first item to check, then resistors, then caps. I really hope it's not a case of Silver Mica disease, I have a Zenith with that, and I am not going to attempt that fix...

If it is a tube, you may have to put them on the tester and let them bake a while before testing, heat related tube issues are a pain to track down, especially in this case as the eye tube is the last tube in the chain before going to the preamp tubes, which means there are more tubes to check.

I don't suspect silver mica disease at this point. There is no noise like lightning crashing going on. I've heard SMD can cause fading, which this has. I don't remember if this still has the 6EA8 or if I changed to a 6GH8A. It didn't seen to drift. I had an AA5 one time that acted similar to this. I found it had a wonky IF amplifier. I might let it run until it starts fading, then pull the tubes one at a time and check them while they are hot.