Retubing 500C

Does a retubing of a 500C other than the output tubes require new bias adjustments? What are the ramifications of not rebasing?

A stock 500C uses a bias resistor to set bias for the power tubes. Just pop in a matched quad of 7591s and the bias will be adjusted automatically. The other tubes are also biased automatically (in most equipment, including the 500C).

If you use modern production 7591's, it is highly recommended to do a simple mod of of changing 330k grid resistors to ~180k-220k (I used 200k) and change those 4 coupling caps from 0.047uf to 0.1uf 600v. If you stay vintage tubes, you're fine.

So to be clear, I'm not retubing the output 7591's. Just mostly some of the 12AX7's, maybe the 6AU6, etc. So I can just pop new (NOS probably) in without worrying about any further adjustments?

Pop in 12ax7 usually require no adjustment. You can get away popping in 12ax7 as phase splitter without any adjustment unless you are picky to do the AC adjustment. That would require some test equipment. As the MPX, it may need adjustment but may still work without any adjusment. Changing any tube (6ha5, 6cw4, 6hr6, 6au6) in the tuner can detune the alignment some. I've changed out some in mine but all in the front end (6ha5, 6cw4, 6hr6) with all NOS and what a difference to the better. Changing out 6gk5 should not mess anything up, it drives the relay. I've changed out some tube in my 500c some including in the tuner and MPX. It still seem to work same but seem to have better performance because of tube I put in is NOS (like new never used) and old tube were pretty worn. I haven't done any adjustment in mine but I do think it needs alignment otherwise works (been that way before retube). Messing with adjustment in tuner need skills and equipment. What I would do is pop in the new tube and do no adjusment and see how it sounds.

Great info. Thanks.

Why retube?

The 6au6's and others in the FM section are rarely bad. Is you FM ok?, if good leave them. Replacing them will most likely require a realignment.

Several years ago I did some research on the 12ax7's as used in the Fishers. The MTBF (mean time between failure) calculated to be 80 years! The tele's that Fisher used are hard to beat, why replace them if there is no problem?

Just checking to see if I wanted to do some rolling. The FM is fine(only a couple of desirable stations in the Bay Area anyway) and the stations that I use come in pretty well with the help of a Radio Shack antenna.

The only positions worth rolling tubes in are the two phono section tubes and the line stage/tone amp tubes.

The phono stage tubes are the two 12AX7s with shields. I prefer Sylvania long pinched plate 12AX7s or RCA 7025s.

The line stage/tone amp tubes are the two 12AX7s up front and right behind the dial glass. I prefer GE triple mica 5751s in these positions as the most neutral sounding tubes. If you want some tube bloom, they'd not be your pick. That's the most effective position to experiment with in these amps. I've tried most every type and iteration of 12AX7 except Mazda and Brimar.

Your system and listening preferences should be the final arbiter, not my choices.