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changing tubes in my phono causes hum ?

i have a tube phono that uses "2"-12ax7's one for each channel. with the mullards i get "0" hum. when i change them out to any other tube like sovtek "low noise" , or electro harmonix i get hum. i like to tube roll and dont understand esp with the low noise tubes. suggestions would be appreciated.
thx,tim
bluebottle
zaikesman
3,017 posts
 
Are you sure it is truly hum you are hearing, or could it be 'tube rush'?
gs5556
1,234 posts
 
Your phono section may have AC tube heaters instead of the (preferred) DC. Some 12AX7's don't like AC and will let you know by humming.
zaikesman
3,017 posts
 
If what Gs5556 suggests turns out to be the case, Electro-Harmonix markets a 12AX7 type with an "LPS" (I believe that's correct) suffix, which uses a spiral filament said to avoid the problem. I personally have not tried this tube out, but others on the forum have recommended it as being better all around than the regular old Sovtek 12AX7's alongside which it is made in the Reflektor factory...
fletchj
91 posts
 
Some 12AX7, are sensitive, to the electronic fields, that are present in gear. 12AX7's were regularly shielded, in most vintage gear, for this very reason. If the tube, is not defective, then maybe you could try a tube shield, that you can ground(connect to the ground plane, of the pre)and see if that solves the problem. If interested, I have a source, for that very thing.
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