McIntosh C-20 question

I've just noticed(since i've been playing more Lps this week) I have a hum and noise in both phono inputs on my C-20. You may say the turntable is not grounded, but even when I turn up the volume when nothing is hooked up i can hear it. My question is in the tube layout where are the tubes which service the phono section located? I think i might have a tube going bad. BTW, even when i turn up the volume on any other input, the preamp is dead silent.I have spare tubeswhich I can swap out when i know which ones to check. Any help will be appreciated. Thanks!

 

I'm not sure which tube(s) correlates to the phono section in the C20, but you might not have to put in a new tube(s). Since the phono section has higher gain, and is therefore more susceptible to hum, you might be able to just switch around what tubes are in the C20 now with satisfactory results...provided what you switch into the phono section is quieter than what's in that position now. That might work for a while, anyway. My guess is only one of the 12AX7s handles the phono section, but I couldn't say for sure as I've never owned a C20. Try switching the 5 12AX7s around and see if that does it.

 

I want to say there are 6 12ax7s and 2 12au7s. And Don, you are probably right, a 12ax7 is probably the tube.

 

I had a C22 for a few years, but that's the only McIntosh preamp I have experience with...it had 6 12AX7s and nothing else. Out of curiosity, though, I did some snooping around at the time regarding other McIntosh preamps and found that the C20 had only 5 12AX7s, along with the 2 12AU7s you mentioned...and a 6X4 rectifier. I'm not a McIntosh expert or anything, but I'd bet the old swaperoo would work. Post what happens if you give it a try.

 

Iwill, I also have some more Tele 12ax7s if any are really dead. I'm just too tired to do it tonight. It will probably have to wait til tomorrow when i get off of work.

 

Grab some shut-eye and send those Telefunkens my way if you ever decide you don't need 'em.

 

Had a couple of things going on here. One Bad 12AX7,a can cap 1/4 bad,(tech put a band aid on it until cap arrives) and two other 12AX7's were microphonic(showed me which ones)Been listening w/o changing those sounds much better, but I'm gonna trade those out tonght. Sure glad I got some extras around. Gotta get some more now to keep my stock up!!!!!!!

 

Sounds like you're set. Nice that Telefunken 12AX7's last so long, eh? A few more spares oughta go a looooong way.

 

Of course I need em' for my 240 also. I got a set of Fisher Branded Teles in my 240 right now.