A (short) catalog of tubes you are likely to see in a guitar amp:

• 12AX7[A, WA] and substitutes - preamp and driver tubes
• 12AT7, 12AU7 and subs, preamp and driver tubes
• 12AY7 - driver tubes
• 6EU7 - dual triode used in some older amps for preamp tube
• 6L6 types - power output tubes, up to 50 watts/pair, a mainstay of Fender
• EL34 - Euro power pentodes, up to 50 watts/pair, many Marshalls
• 6V6 - smaller, lower power cousin of the 6L6, 10-14 watts per pair; used in smaller Fenders
• EL84 - fits a 9 pin socket like a 12AX7 but twice as tall; miniature power pentode, good for 12-18 watts per pair; used in smaller Vox amps, and a quad of these drives the Vox AC-30 for 30 watts.

Substitutions:

* means appropriate for parallel filament circuits
# means may not work in all circuits

Preamp and driver tube substitutions:

• 12AX7 (high gain dual triodes with pinout 9A)
• 12AD7
• 12DT7
• 772912AU7#
• 5751*
• B33912AU7A#
• 5751WA*
• B75912AX7
• 6057
• CV400412AX7A
• 6681
• E83CC12AX7WA
• 6L13
• ECC80312BZ7*
• 7025
• ECC8312DF7
• 7025A
• M813712DM7*
• 7494

12AU7 (moderately high gain dual triodes with pinout 9A)

• 12AU7[A,AW,]
• 6189
• 773012AX7* and subs 6670
• ECC1865814[A,AW]*
• 6680
• ECC8025963
• 7316
• ECC826067
• 7489
• M8136

12AT7 (medium gain dual triodes with pinout 9A)

• 12AT7[many suffixes]
• 7492
• E81CC6201
• 7728
• ECC8016679
• A2900
• M8162ECC81
• B152
• QA240612AZ7[A]*
• B309
• QB3096060
• B7396671
• CV4024

12AY7 (low gain dual triodes with pinout 9A)

• 12AY7(and suffixes)
• 60722082

Power tube substitutions:

6BQ5/EL84 (miniature pentode with pinout 9CV)

• 6267
• 7189
• EF866BQ5
• 7189A
• EL846BQ5WA
• 7320
• N7096P15
• E84L
• Z729

6L6 (beam power tube with pinout 7AC)

• 6L6(many suffixes)
• 7581(A)5881
• WT65932
• EL37

EL34/6CA7 (power pentode with pinout 8ET)
• EL34
• 12E136CA7
• KT777D11
• KT88

6550 (power pentode with pinout 7S)

• 6550[A]
• 7027A#7D11
• KT8812E13

Cautionary Tubes - these are very hard to find

7591/7591A - legend has it that these otherwise excellent tubes were all bought up by an oriental buyer who toured the USA paying cash for all of them he could find, then disappearing. You are likely to only find used ones or the odd pair in some out of the way place. Dealers will in general not have them. I have personally seen trays full of NOS 7591A's for sale in the Akihabra electronics district in Tokyo, lending some credence to the rumor. These were used a lot in old Ampegs. They are very small and high gain for their physical size, so there may not be a lot of room in a chassis for a larger replacement. The 5881 will work in some circuits, but has significantly lower transconductance.

Rumor Update: The rumor mill on the net says that the Russians will soon be making 7591's soon. Cross your fingers...

7199 - combination pentode/triode used as a one-tube voltage amp/phase inverter/driver for a pair of output tubes in some Ampeg amplifiers Note: These were once popular, but are now getting rarer and more expensive. There are a number of other pentode/triodes that can be substituted, but the pinouts are different and this will require require rewiring the socket for the tube. Examples are the 6AN8 and the 6U8. There is a Russian tube that is labeled 7199 which may work, although this is new.

7027/7027A - this is a high power tube similar to a 6550. The supply of these is very poor.

7189/7189A - a higher power/voltage version of the 6BQ5/EL84. Hard to find. A stock 6BQ5/EL84 may work if the power and voltage conditions in the amp are not right out at the limits of the tube design.