Matched preamp tubes?

Do preamp tubes (12AX7) also have to be matched like power tubes?

tubelix said:

Do preamp tubes (12AX7) also have to be matched like power tubes?

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No.

Boogie amps do not use pairs of preamp tubes in push-pull configurations, and the preamp tubes are individually cathode-biased. So, no worries on matching pairs of preamp tubes.

Where matching does come into play, in regard to "preamp" (loose use of the term) tubes, is with the phase-inverter tube, which some insist must have balanced triodes. The 12AX7 is a dual-triode (two circuits in one tube), and the transconductance of each triode is matched in a "balanced-triode" tube.

- T

Thanks Timbre Wolf for clearing that out. Then it's possible to change one bad preamp tube at a time.

I have a .50 Caliber Combo (EL84 version) and I have noticed an increasing hum from it. I think it’s from a bad preamp tube. I’ll try to locate the bad one switching tubes one by one.

tubelix said:

Thanks Timbre Wolf for clearing that out. Then it's possible to change one bad preamp tube at a time.

I have a .50 Caliber Combo (EL84 version) and I have noticed an increasing hum from it. I think it’s from a bad preamp tube. I’ll try to locate the bad one switching tubes one by one.

Click to expand...

Yes, changing preamp tubes one-by-one is the best way to assess which, if any, is causing the problem. Mismatched power tubes can cause hum too, and sometimes one of a matched pair will drift. If you get to the power tubes, after trying preamp tubes, change them as matched pairs only.

- T

I found my Mark IV likes to eat one side of the preamp tubes while leaving the other side as new. It is weird but I certainly would not spend more for matched tubes being that they will not wear evenly anyway.