Preamp tubes..............?


Double Platinum Member
God, I hate to sound stupid....!! On preamp tubes, I know there are 12ax7, 12at7, 12ay7......but what about ones that are just numerical? Like 5751, 6025? Are these all interchangable preamp tubes? Can they all be used the same? Thanks for the education. Steve


A 5751 is slightly lower gain than a 12ax7 and may be used where a 12ax7 may be used. I'm preparing to buy a couple myself to slightly tame the aggressive preamp gain of a marshall type amp I recently built. 12at7s are a tad lower gain than the 5751. I currently have a JAN Philips 12at7 in v2 of the marshall clone mentioned above and enjoy what I'm hearing. Don't be afraid to experiment.:cool:


FWIW the "5-star" thing was GE's marketing, I've only seen it on GE tubes.

12AX7 numbered variants:

7025 - spiral wound filaments for reduced hum. Most 12AX7s after the mid 50s will have this as a standard feature.

6681 - mobile-rated 12AX7. Filaments perform well with a wider range of voltages.

6057 - Brimar's special quality 12AX7.

7729 - CBS/Raytheon's instrumentation grade 12AX7, with heavily gold plated pins.

CV492, CV4004, M8137 - British Military designators. Typically Mullard or Brimar.

B339 - Another British equivalent, most of these are regular Mullard ECC83s branded Marconi or MWT.

B759 - The Genalex "custom" 12AX7, a selected, tested Mullard ECC83.

12AU7: 5814, 5963, 6067, 6189, 6680, 7316, 7730, CV491, CV4003, M8136, B329, B749

12AT7: 6060, 6201, 7728, CV455, CV4024, M8162, B309, B739

12AY7: 6072


When considering tube substitutions:

1a) Tubes have a characteristic known as "basing". This captures both the socket style (octal, 9 pin miniature, etc.) and how the internal elements are connected to the external pins. A 12AX7 has basing style "9A". So do all of the tubes in Saros' list. There are more (and no, I ain't gonna type the whole list).

1b) There are some situations where two tubes with different basing may be substituted for each other. Some are no-brainers, some require additional research. Some examples (ignoring ratings)
-- A 6550 (basing 7S) has the same pinout as a 6L6GC (basing 7AC). For extra fun, a 5881 has either 7AC or 7S basing depending on the manual (RC-30 lists it both ways in different places :)). The substitution can work.

-- A 6CA7/EL34 (basing 8ET) differs from a 6L6GC by one pin. In the 6L6 the suppressor grid is internally connected to the cathode (which comes out on pin 8). In the EL34, the suppressor comes out on pin 1. If pins 1 and 8 are connected on the socket (or both grounded) then the substitution can work.

-- A 7027A (basing 8HY) differs from a 6L6GC by two pins. In the 6L6 pin 5 goes to the grid and pin 6 is not connected to anything. In the 7027A both pins 5 & 6 go to the grid. In the 6L6 the screen comes out on pin 4. In the 7027A the screen comes out on both pins 4 and 1.

The upshot is that you can wire a socket to accept both 6L6 & EL34, or both 6L6 & 7027A, but never both EL34 & 7027A

2) Consider tube ratings. Two tubes with the same basing have the same elements connected in the same ways, but may have vastly different ratings. Good examples are 6550 (7S), 6L6GC (7AC), 6V6GT (7AC), 6K6 (7S). So even though the pinouts match up a 6K6's life span in the 6550 slot of something like a Class AB2 Fender 400PS can be measured in microseconds.

Unless you know the circuit specifics a good rule of thumb is to pay attention to maximum ratings for plate voltage and plate dissipation. Also pay close attention to heater voltage (a 6 volt tube doesn't work in a 12 volt socket and vice versa).
More examples:
- A 12AX7 has max plate voltage of 330V and max dissipation of 1.2watts
- A 5751 goes 330V and 0.8watts
- A 12AY7 goes 300V and 1.2watts

So, the 5751 will tolerate the plate voltage but can't dissipate the same heat. As luck would have it, most MI preamps are designed (self-bias, choice of plate load, etc.) such that this substitution doesn't cause problems.

Going the other way, the 12AY7 will dissipate the same heat, but won't tolerate the same plate voltage. In this particular case the 30V doesn't make a lot of difference. Where you get into real trouble is shoving 200V tubes into 350V circuits[*].

Best thing I can suggest is to invest the $30 in a copy of the RCA Receiving Tube Manual (RC-30) if you want to do a lot of tube swappin'

[*] There are exceptions here as well. Most tubes used in MI gear will tolerate exceeding plate voltage ratings much better than exceeding power/current ratings. Particularly true for some NOS stuff. Your standard BF Fender reverb drive circuit throws 410V on the plates of a 300V rated 12AT7 -- just part of the reason cheap tubes die fast in this application.


Silver Supporting Member
5751's are THE BLUES TUBES ..... Not too much gain or too much headroom, just right.

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