12AX7 Hum

LucSulla

LucSulla

Well-known member
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So I retubed the 800 with some Mullard EL34s (new version, not the real deal) and three new 12AX7s.  Left V4 (verb) and V5 (phase inverter) be for now. I put a new Mullard in V1 and Electro Harmonix in V2 and V3.  I noticed right off the bat that I had a low cycle hum, worse on the clean channel.  

After swamping around, it was the EH in V2.  Both work fine in V3, and neither work in V2 without the hum.  However, if I throw one of the millions of Mesa 12AX7s I have sitting around here, everything is fine.  

My question is what is it about some 12AX7s that make them hum in some sockets when they don't hum in other sockets, and what makes other 12AX7s not hum in the same sockets where the problem was occurring before?

 
some tubes are matched, balanced on both sides of the used grids, some are not. most amps use only one side.

this could be the case of.

 
Does the amp have AC or DC heater supply for the tube filaments? older amps are more likely to have AC, which can cause noise.

This is important because some tubes have spiral heater filaments, which cancel out the AC noise. other tubes do not.

also, have you cleaned and re-tensioned the tube sockets lately? Some tubes have smaller pins than others, so they may not be making as solid of a connection.

 
murkat said:
some tubes are matched, balanced on both sides of the used grids, some are not. most amps use only one side.

this could be the case of.
Click to expand...
Makes sense.  I believe this posted on another forum is true - 

Normal Channel: V1A and V2A and V2B

Boost Channel: V1A and V1B and V3A

That may also explain why it is more of an issue on normal channel. 

 
tbonesullivan said:
Does the amp have AC or DC heater supply for the tube filaments? older amps are more likely to have AC, which can cause noise.

This is important because some tubes have spiral heater filaments, which cancel out the AC noise. other tubes do not.

also, have you cleaned and re-tensioned the tube sockets lately? Some tubes have smaller pins than others, so they may not be making as solid of a connection.
Click to expand...
V2 is a cathode follower.  Might that play into this?

 
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