2. amps-preamps
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Leave tubes in phono section if not using it?

Hi,

I just bought a mid 1980's Moscode Minuet preamp (not Minuet in A) on AudioGon. I haven't received it yet, but I think it uses 4 12AX7's (2 in phono section, 2 in line stage).

I don't plan on using the phono stage. I was thinking of just pulling the tubes in the phono section with the hopes of reducing noise/hum and saving myself two tubes. But, I know I could use shorting plugs on the phono imput jacks to possibly eliminate some noise that way.

My question: Is it possible that pulling the tubes can upset some critical design parameter and create a detrimental situation, or even do some damage?

I will not have a manual or schematic for this unit.

Any thoughts would be much appreciated.

Thanks,

Rich
richgib
mechans
3,048 posts
 
I like the cheapo tube idea. I have a few stock tubes that arent worth the glass the're made of. Just buy some 5 buck Sinos and don't worry about unless a friend wants to try a TT on your pre.
hifiharv
351 posts
 
I Don't know about Moscode, but I asked Cary about this on my SLP 90 preamp. The answer was, Yes, leave the tubes in place. Replace them with something less precious if they are special. Since your pre is older these days, I wouldn't take any chances.
hifigeek1
770 posts
 
Some preamps not only want phono tubes left in, but if the tubes are not quiet they can cause issues with the power supply and can actually cause problems for your high level/line stage as well. Best bet is to call George Kaye and ask him. Then you will know for sure.
richgib OP
16 posts
 
Thanks, Newbie. I'll just put cheap ones in the phono section. I was wondering if it could affect the line stage, maybe even send the unit into oscillation. I had an early Audible Illusions preamp that oscillated when I unknowingly put in some Hungarian tubes. They even warned about this in their manual. I was just curious if the circuit could be affected in some way, as Hifigeek1 suggested.
robetozian's avatar
robetozian
8 posts
 
Leave the tubes in. If you operate this preamp without tubes in place, it will "un-load" the regulators. Voltages in some places will rise, and may rise above the rating of some components. The repair bill will most likely be higher than the cost of a couple of cheap tubes.
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