For a Mk V, if you had a few SPAX7 tubes it seems logical to use them in spots that are earlier in the gain sequence (i.e. V1, V2, V3 to start with). Is my logic wrong? What about V7 (Driver/Phase Inverter)? Does it make any sense to use one there? If you had 7 SPAX7s, would you use one in V7 or just save it?
SPAX-7 priorities.
- Thread starter jaslan
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I would save it. the SPAX-7 seems to have slightly different qualities to it than the standard 12AX7. Sure it is the same tube but selected for its characteristics. I have found it seems to have more roll off on the higher frequencies but not by much. V1 seems to be the logical choice in order to reduce the effects of noise as it should have a lower noise floor. Using the same tube in V3 or other locations will reduce the effects of each stage by a small amount. Keep in mind that the tube number does not mean that is the next tube in order of the circuit path. As it seems the triode pair of each tube is used differently and may not be where you think it is in the circuit path. NOTE: the following is the cascade arrangement for the 90W Mark V only. The mini Marks will be different.
CH3 jumps around (excluding the reverb triodes) this is the circuit path of each tube in order.
V1A-> CH3 tone stack -> V1B-> V5A-> V4B-> V3A->(reverb send/return)-> V6A-> GEQ-> (send)->(return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
CH2 jumps around but not quite like CH3 (excluding the reverb triodes) this is the circuit path of each tube in order.
V1A-> V2A-> CH2 tone stack-> V1B-> V3A-> GEQ-> (send)-> (return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
CH1 is also different
V1A-> CH1 tone stack-> V1B-> V3A-> V3B-> GEQ-> (send)-> (return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
I would only use the SPAX-7 in V1 as it probably would not matter else where. I would much prefer a better tube than the SPAX-7. The only benefit I had when using the SPAX-7 was the slight reduction in gain factor and treble roll off. Depending where you put the tube it may get things a bit muddy. But it may work for you in all positions or none. Tone is subjective at best.
CH3 jumps around (excluding the reverb triodes) this is the circuit path of each tube in order.
V1A-> CH3 tone stack -> V1B-> V5A-> V4B-> V3A->(reverb send/return)-> V6A-> GEQ-> (send)->(return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
CH2 jumps around but not quite like CH3 (excluding the reverb triodes) this is the circuit path of each tube in order.
V1A-> V2A-> CH2 tone stack-> V1B-> V3A-> GEQ-> (send)-> (return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
CH1 is also different
V1A-> CH1 tone stack-> V1B-> V3A-> V3B-> GEQ-> (send)-> (return)V6B-> V7AB. If the FX loop is in hard bypass, the GEQ would feed directly to V7AB (phase inverter).
I would only use the SPAX-7 in V1 as it probably would not matter else where. I would much prefer a better tube than the SPAX-7. The only benefit I had when using the SPAX-7 was the slight reduction in gain factor and treble roll off. Depending where you put the tube it may get things a bit muddy. But it may work for you in all positions or none. Tone is subjective at best.
Thanks for the input.
I have seen that some amps don’t even use 12ax7 for the driver/phase inverter but use a 12at7, which, I think, is a lower gain version of the 12ax7 and basically interchangeable electronically. I am very happy with the amp as is, to be honest. It is very quiet 95% of the time and I have no complaints with the tone. I have a full set of the SPAX7 tubes and another set of 12AX7 (all MESA branded tubes) and was just wondering how I should use them. The SPAX7 are only a couple of dollars more, so I don’t think it is necessary to go to great lengths to “save” them but at the same time, no point in using them where the differences are negligible or nonexistent.
Something else I was wondering about...
When I swap some of them out I was thinking of also swapping the inner and outer power tubes. I pretty much always have the amp in 45W mode (all channels) which only uses the inner pair. I figure it has been about a year or so on those power tubes so the outer should be “fresh” and swapping them with the inner pair would even out the wear on them? Thoughts?
I have seen that some amps don’t even use 12ax7 for the driver/phase inverter but use a 12at7, which, I think, is a lower gain version of the 12ax7 and basically interchangeable electronically. I am very happy with the amp as is, to be honest. It is very quiet 95% of the time and I have no complaints with the tone. I have a full set of the SPAX7 tubes and another set of 12AX7 (all MESA branded tubes) and was just wondering how I should use them. The SPAX7 are only a couple of dollars more, so I don’t think it is necessary to go to great lengths to “save” them but at the same time, no point in using them where the differences are negligible or nonexistent.
Something else I was wondering about...
When I swap some of them out I was thinking of also swapping the inner and outer power tubes. I pretty much always have the amp in 45W mode (all channels) which only uses the inner pair. I figure it has been about a year or so on those power tubes so the outer should be “fresh” and swapping them with the inner pair would even out the wear on them? Thoughts?
Yes, you can swap the inner set of power tubes with the outer set.
As for the phase inverter, it depends on what amp you are referring too. If the amp is designed for the 12AX7 in the PI, you may notice a lower output as a result of using a 12AT7. Even a 5751 can be used in the PI. Some will say the PI does not have much influence on the tone of the amp but I beg to differ. There is a small amount of tonal differences as well as output strength you can achieve with different tubes used in the phase inverter. Also since there are three typical Phase inverter circuits that are commonly used, it may make a difference what tube is used. Mesa generally uses a form of the "long tailed pair" phase inverter. Some Fenders and Orange use the "Cathodyne" phase inverter circuit. And then there is the one called the "Paraphase".
You can look up information on triode phase inverters for more information on how they work.
As for the phase inverter, it depends on what amp you are referring too. If the amp is designed for the 12AX7 in the PI, you may notice a lower output as a result of using a 12AT7. Even a 5751 can be used in the PI. Some will say the PI does not have much influence on the tone of the amp but I beg to differ. There is a small amount of tonal differences as well as output strength you can achieve with different tubes used in the phase inverter. Also since there are three typical Phase inverter circuits that are commonly used, it may make a difference what tube is used. Mesa generally uses a form of the "long tailed pair" phase inverter. Some Fenders and Orange use the "Cathodyne" phase inverter circuit. And then there is the one called the "Paraphase".
You can look up information on triode phase inverters for more information on how they work.
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