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Swapping Preamp Tubes

4/20/2000 2:18 PM
Ray Ivers	Swapping Preamp Tubes Hello to All, First off, my name is Ray Ivers, and I've got a small company here in Connecticut (R.A.G.E. Electronics), modding and repairing guitar amps since '84. I'm new to Ampage, and so far I've found it to be an incredible forum for guitar amp knowledge. I'll try not to screw it up. I've been seeing references to swapping preamp tubes in amps for many years, and until recently I thought it was a cool thing to do. I no longer think that. I realize that 'if it sounds good, it is good', but it's possible that the person who plugs a 12AU7 into a 12AX7 circuit thinks that he's hearing the sound of a properly-operating 12AU7. He (or she) is not! I've read that 'preamp tubes do not require biasing', but in my opinion this should read 'preamp tubes do not require biasing when replaced with the original tube type and/or if you agree with the original biasing of the tube'. How many of you agree with the original biasing of your power tubes, from the factory? Granted, this is a design issue and not a tweak, but lots of amps are designed with non-adjustable fixed-bias output stages, something I've never considered a good idea. Pluggin a 12AU7 into a Marshall's first gain stage (100K plate resistor, 2.7K cathode resistor, approximately 135 plate volts) results in a current flow of less than 2 mils thru the 12AU7. This means that the 12AU7 is operating in a very non-linear part of its characteristic curves. There are circuits that use devices operating in this region to convert triangle waves to sine waves! This may be a cool effect, but clean it ain't. To properly use a 12AU7 (or 12AT7, or most other 12A_7 tubes) in a circuit designed for another tube will require at the very least a change of the cathode resistor value (along with bypass cap, if any, to maintain the same frequency response), and in the cases of a radical type-change like the 12AU7-for-12AX7 swap mentioned above, a change of plate resistor as well. This is necessary to maintain symmetrical signal-swing and maximum output voltage as well as low distortion, assuming that's what you want. Most guitar amps seem to run the preamp tubes at low currents, whereas hi-fi amps usually run tube currents higher (the 12AX7 runs on super-low current, and is kind of a different animal from the others, although current flow through a 12AX7 will still affect tone). My guess is that users of amps with 12AT7 driver tubes would like the sound of their amp better with higher-current operation of that driver tube - the gain increases slightly but more importantly, the output impedance of the driver stage drops so you can hit the output tubes a little harder. Most people don't realize that most of what is called 'output tube distortion' is not that at all - it's your driver tube clipping when it finds it's just not up to the task of operating the output stage in Class A2 or AB2, where the grids are driven positive with respect to ground (i.e. a 46 volt peak signal applied to a grid with -45 volts bias). At any rate, probably no harm will be done to the amp's circuitry by swapping different tube types, but tube life can be greatly affected by it, especially in the Fender reverb drive stage. There's a lot of potential for getting different sounds out of your amp by swapping preamp tubes, but it's not quite as cut-and-dried as plug'nplay. C'mon - admit it - didn't it always seem just a little too good to be true? And power tubes - that's a whole 'nother story. I would welcome any comments on this subject, and also I should add that there are plenty of tubes that CAN be substituted freely (7025/ECC83/5751/12AX7, etc.). Ray Ivers

4/20/2000 2:18 PM

Ray Ivers

Swapping Preamp Tubes

Hello to All,

First off, my name is Ray Ivers, and I've got a small company here in Connecticut (R.A.G.E. Electronics), modding and repairing guitar amps since '84. I'm new to Ampage, and so far I've found it to be an incredible forum for guitar amp knowledge. I'll try not to screw it up.

I've been seeing references to swapping preamp tubes in amps for many years, and until recently I thought it was a cool thing to do. I no longer think that. I realize that 'if it sounds good, it is good', but it's possible that the person who plugs a 12AU7 into a 12AX7 circuit thinks that he's hearing the sound of a properly-operating 12AU7. He (or she) is not! I've read that 'preamp tubes do not require biasing', but in my opinion this should read 'preamp tubes do not require biasing when replaced with the original tube type and/or if you agree with the original biasing of the tube'. How many of you agree with the original biasing of your power tubes, from the factory? Granted, this is a design issue and not a tweak, but lots of amps are designed with non-adjustable fixed-bias output stages, something I've never considered a good idea.

Pluggin a 12AU7 into a Marshall's first gain stage (100K plate resistor, 2.7K cathode resistor, approximately 135 plate volts) results in a current flow of less than 2 mils thru the 12AU7. This means that the 12AU7 is operating in a very non-linear part of its characteristic curves. There are circuits that use devices operating in this region to convert triangle waves to sine waves! This may be a cool effect, but clean it ain't. To properly use a 12AU7 (or 12AT7, or most other 12A_7 tubes) in a circuit designed for another tube will require at the very least a change of the cathode resistor value (along with bypass cap, if any, to maintain the same frequency response), and in the cases of a radical type-change like the 12AU7-for-12AX7 swap mentioned above, a change of plate resistor as well. This is necessary to maintain symmetrical signal-swing and maximum output voltage as well as low distortion, assuming that's what you want. Most guitar amps seem to run the preamp tubes at low currents, whereas hi-fi amps usually run tube currents higher (the 12AX7 runs on super-low current, and is kind of a different animal from the others, although current flow through a 12AX7 will still affect tone). My guess is that users of amps with 12AT7 driver tubes would like the sound of their amp better with higher-current operation of that driver tube - the gain increases slightly but more importantly, the output impedance of the driver stage drops so you can hit the output tubes a little harder. Most people don't realize that most of what is called 'output tube distortion' is not that at all - it's your driver tube clipping when it finds it's just not up to the task of operating the output stage in Class A2 or AB2, where the grids are driven positive with respect to ground (i.e. a 46 volt peak signal applied to a grid with -45 volts bias).

At any rate, probably no harm will be done to the amp's circuitry by swapping different tube types, but tube life can be greatly affected by it, especially in the Fender reverb drive stage.

There's a lot of potential for getting different sounds out of your amp by swapping preamp tubes, but it's not quite as cut-and-dried as plug'nplay. C'mon - admit it - didn't it always seem just a little too good to be true? And power tubes - that's a whole 'nother story.

I would welcome any comments on this subject, and also I should add that there are plenty of tubes that CAN be substituted freely (7025/ECC83/5751/12AX7, etc.).

Ray Ivers

Replies:

Rebel420 Ray, first off, welcome aboard. I'v... -- 4/20/2000 2:50 PM
dale ray,your comments m... -- 4/20/2000 5:11 PM
Dave James Again, welcome. I too have followe... -- 4/20/2000 9:28 PM