Preamp Gain Factor

[JMguitars]

An interesting read ........

Preamp tubes have different gain factors, and can therefore be used to alter the tone and "feel" of the amp.

Using the 12AX7 as a base value of 100, you can get an idea of how much less gain each type produces by the following chart. Keep in mind that there are slight variations between tubes within each family. This list is not comprehensive and may not be perfect.

100: 12AX7, 7025, ECC83
70: 5751
60: 12AZ7, 6201, 12AT7, ECC81, 12DW7
47: 5965
44: 6072, 12AY7
41: 12AV7
21: 5963
20: 5814, ECC82, 12AU7
16.5: 12BH7

 

[Damo]

Hi JM if I change from a 12AX7 to a 12AU7, what differences am I going to hear in my univalve

 

[JMguitars]

Hi JM if I change from a 12AX7 to a 12AU7, what differences am I going to hear in my univalve

Hi Damo,

If you check ur Univalve, u should notice 2 preamp tubes.

The official description of these tubes are:

Input: The first preamp tube (to the left) governs the gain and tone of the preamp section, and thus can be said to create more or less preamp distortion. Reducing gain here will also affect the overall volume level of the amp (of course, in addition to your choice of output tube).

Driver: The preamp tube to the right "regulates" signal driving the output tube(s), and thus can be said to influence output tube distortion.

The signal path is as follows:
The rock or more channel uses both sides of the INPUT tube and then goes to the first half of the DRIVER tube -> tone section -> second half of the DRIVER tube -> power tubes.

The roll or less channel uses only the second half of the INPUT tube and then follows the rest of the path above.

----------------------------------------

In layman English, this means that your "Rock" channel will show a more pronounced change than the "Roll" channel, if you were to swap to a different preamp tube.

A swap of the "input" tube (from an 12AX7 to a 12AU7), decreases the preamp gain and is akin to the analogy of switching from an LP (with humbuckers) to a Strat (with single coils). As in the analogy, you should notice a volume drop.

A swap of the "driver" tube (from an 12AX7 to a 12AU7), decreases the signal to the output tubes. Hence, you would experience a later breakup; i.e. ability to crank the Uni for greater power distortion yet enjoy lower actual volumes.

--------------------------------------------

Thus, without even changing power tubes, you could appeal to 2 different playing styles.

A shredder would choose a high gain input tube (12AX7) and a low gain driver tube (12AU7) to enjoy a tighter distortion with greater compression.

A blues guitarist would choose a low gain input tube (12AU7), and a high gain driver tube (12AX7) to get a more of a rock/blues type power distortion, i.e. break-up when strings are plucked hard, and clean when picked lightly.