Difference between ECC83 (12AX7) and ECC82 (12AU7)

[porman]

I'd like to know what the difference between those tubes are and if I can use the ECC82 instead, in an amp that I have an ECC83.

[MWJB]

12AU7 will probably work fine in the phase inverter position (right before the power tubes),or reverb driver.... In most vintage amps with good quality plate resistors it will be OK in the other preamp stages, ...I would be a little careful with some modern PCB/budget amps with this though, 12AU7 draws significantly more current than a 12AX7 and migh possibly burn up a plate resistor. Plate resistors are easily replaced in the worst case scenario (harp players do these kind subs often and rarely have problems).

The biggest factor wil be the lower gain of the 12AU7, the impact of this will vary greatly from one amp circuit to another.

[Robert M. Martinelli]

Hi,
though a drop in replacement is theoretically possible, I would not recommend it, 12AU7s have an anode resistance which is one tenth ( and anode current' s ten times as much ), they can accept larger signal swings at the input but their mu factor is one fifth...so I don' t think a drop-in replacement would yield good results, and, as MWJB wisely pointed out, it could rise some reliability issues about power dissipation on plate resistors.

If I was to replace 12AX7s with a different tube ( e.g. in the case a direct replacement is not available ) I would go for 5751s ( similar ratings and a mu around 70 ) 7025s ( mil-spec version of the 12AX7 ) E83CCs ( Special quality version of the 12AX7/ECC83 ) and ECC803s ( ECC83 with a specially designed spiral heater for low hum ).

Best regards

Bob