Tube-Rolling For Dummies...

[Gang-Twanger]

OK, guys. I wanted to ask a few things about this before I tried it. My Fisher 800C has all n.o.s. RCA 12ax7's in the preamp except for one original G.E. that tested good, so I thought I might try moving the G.E. around and trying it in different spots until I settle on what I like best. Their tone is similar to the good RCA's from the same era, but RCA longplates have a slight edge in terms of the online comments I've read, so I figured it couldn't hurt to try the G.E. in some of those other 12ax7 inputs and put a 12ax7 in it's place. Before I start, I need to look at the schematic and see precisely which spot has the G.E. (If it isn't in the AUX or PHONO spots, I probably won't do anything, but I still wanted to get a clear answer about the tube-rolling question regardless).

But first, I wanted to be sure exactly what I need/don't need to do before I go swapping tubes. Now, with preamp tubes in an 800C (or 500C), I got the impression that all I have to do is turn the power off, and maybe unplug it too, but I wanted to check with you guys first before I started anything.

And while we're on the subject, what about the 7591 power tubes? I know the 800C has a fixed bias, but I wasn't sure if that made a difference or not.

 

[canuckaudiog]

What I do is keep playing music as it turns off. You will hear the music slowly lose volume, and when it is totally silent the capacitors are discharged and the tubes won't be receiving any more voltage. Of course, the tubes will be hot to touch so you may not want to remove them at that point, but you certainly could (maybe with a cloth). Or, just wait until they cool off.

My opinion always has been that it's the pre-amplifier tubes that make the most difference, so if I were to adjust the sound of a tube amplifier/receiver, this would be where I would spend the money. I'm not at all familiar with the 7591 though, so perhaps someone else could chime in here..

 

[Gang-Twanger]

I specifically-chose RCA's for the preamp section. It originally had G.E.'s in there, and Mike Urban told me he left the one G.E. 12ax7 in there because it tested good (The 800C uses 12ax7's for the primary input tubes for each section... AUX, PHONO, etc.), and I chose RCA's to replace the BAD G.E. 12ax7's (I think there were five bad and one good), but I don't believe Mike tried moving that one G.E. around at all, so it's probably in the same spot as it was when I brought the receiver to him. If the one G.E. is in an important spot like AUX or PHONO, then it's probably a good chance that one of the RCA's would sound better there.