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Discussion Forum for Mesa Boogie Products
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PostPosted: Tue Apr 08, 2014 8:47 pm 
Bottle Rocket

Joined: Mon Apr 07, 2014 2:57 pm
Posts: 9
This would be OK right? No issue with potentially damaging my amp?

PostPosted: Wed Apr 09, 2014 3:33 pm 
Donating Member

Joined: Wed Aug 29, 2007 5:18 pm
Posts: 2400
Location: Ithaca, NY...10 sq miles surrounded by reality!
It won't hurt; it just won't feed the power tubes with as much gusto.


Mk IIA->C+ DRGX head "Cold Sweat" in GTS shell
1966 Fender Pro Reverb
DC-3 head
Orange Dual Terror
EVM loaded Thiele; MS-12 1/2 back
71 SG; 72 Tele; 76 White Strat; 77 LP Custom; 81 Goldtop; 05 Am Dlx Strat; 11 G0 plaintop


PostPosted: Thu Apr 10, 2014 12:42 am 
Mark III
User avatar

Joined: Fri May 09, 2008 4:22 am
Posts: 482
No damages at all. Try it, it's a matter of taste.

PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

PostPosted: Tue Apr 29, 2014 6:07 am 
Triple Recto

Joined: Fri Feb 22, 2013 7:31 am
Posts: 3088
Location: North Carolina
There are a few tubes that are suitable replacements for a 12AX7. That would be 5751, and 12AT7. I would not go beyond 30% to 40% below nominal gain.
Typical gain factors of 12 series preamp tubes
12AX7 being 100 (nominal)
5751 is 70 (30% reduction)
12AT7 is 60 (40% reduction)
12AY7 is 45 (55% reduction)
12AV7 is 41 (59% reduction)
12AU7 is 19 (81% reduction)

Dropping too low will not be all that great. Typical replacement strategy does have limits. Say for example you have a Mark Series amp where the reverb send and return are split between two different tubes and the other triodes are used for gain stages. Dropping from a 12AX7 to a 12AT7 is okay and will alter gain structure and tone.

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