Would I benefit by putting 12AT7s/ 12AU7s in the V2 & V3

[joey_truelove]

The signal path through a Egnater or Randall pre-amp follows:

AV1(input stage) > MV1 (first pre-amp stage) > MV2 (second pre-amp stage) > AV2 (phase inverter) > AV3 (summing and buffer)

Transfering the valve arrangements of e.g. vintage Fender and Marshall amps we find the signal path goes through two amplifying 12AX7s/ECC83s maximum before reaching the phase inverter or other functional valve. In the case of Marshalls and some earlier Fenders these were AX7s/83s too; in later Fenders these were AT7s/81s

The AT7 offering less gain and more output would give us more headroom and more tranparency compared to the AX7. The AU7/82 has about the same output but even lower gain. I figure using either of these would put distortion to the first three valves or, with one of the above on the module, the first two valves - hence the harmonic contents would decend from the "right place" for these amps.

Does anyone have any ideas as to how much gain the functional valves would require to drive the circuit?

 

[khingpynn]

As for the amount of gain required to "drive" te circuit... you could have all 12AU7 or all 12AT7 the circuit will still perform the question is whether it will be to your liking.

Khing

 

[joey_truelove]

As for the amount of gain required to "drive" te circuit... you could have all 12AU7 or all 12AT7 the circuit will still perform the question is whether it will be to your liking.

Khing

Thanx mate! I'm sure as long as you put a signal into a relatively simple pre-amp design even AU7s could see the signal through from input to output. This pre-amp has, I understand a relatively simple frontend so feeding a high output is of no necessity. My concern is the signal level at the output.

AX7: gain 100, output 1.2mA
AT7: gain 60, output 10mA
AU7: gain 17-20, output 10.5mA
As you can see there's a leap between the output of the AX7 to any of the other two. I understand using both triodes will differ 1/5 in output between the AU and the AX

Yes, it reflects in matters of tone too. I was just asking from a technological standpoint if I can get a usable signal level out of the valves involved. These also being functional valves they would possibly have worked with a gain level at 1.

 

[maximus1]

they will cut your gain big time, which will effect your tone . any where you put them

 

[khingpynn]

A viable option would also be the 5751. I use them a lot and here is why I like them.

The 5751 has about 75% of the gain of a 12AX7 and the output varience is undetectable under a scope... so same output. I find a lot of 12AX7's have little headroom and the preamp cct clips far to early. I like to "drive" the preamp cct a ways before breakup occurs to my ears the bottom end can suffer from a high gain tube at times in that it can be difficult to find the sweet spot on the dial if the cct is clipped early by and agressive preamp tube... the dial becomes very sensative and a hair adjustment offers big change. The lesser agressive 5751 allows for the dial to be more responsive and I find the sweet spot easier. In truth in a high gain preamp cct the maxed out 12AX7 and maxed out 5751 give a comparable amount of gain in respect to audio perception... the 12AX7 is more yes but the 5751 is still a lot.
The sweet spot I'm referring to occurs on the preamp gain dial. As preamp gain is increased first you will hear the clean head room then the beginning of the clipped signal then once you reach that there is a increase in distortion that goes from barely breaking up or low distortion to medium gain/distortion... a more open cleaner distortion. Then the gain starts to take on a thicker more compressed less open tonality and the high gain trip begins... make sence so far. The sweet spot for me is the point at which medium open sounding gain thickens and becomes more compressed and high gain begins. somewhere in there is where I like to be... there is still a note clarity and sustain quality suitable for rythm and lead and it only takes a little nudge to send it over the top for singing saturated leads.

For my money the 12AT7 or ECC81 equivilant is a fine tube to use in a tube boost pedal where no extra dirt is desired or at leat very very little is. The 12AU7 or ECC82 equivilant is an even better tube for a tube boost pedal in that it offers up more gain but is still laid back enough to give a lot of clean headroom and can still add more dirt without becoming over bearing. The 12AX7 or ECC83 equivilant used in a tube boost pedal is simply too much gain and not enough clean head room and funtions better for an all tube distortion pedal.

For my money I like to run 5751's or 12AX7 in my amps and 12AU7's in my pedal for a boost of clean headroom and slight amount of dirt... I use a Soldano Supercharger GTO with Tung Sol 12AU7's.

here is an interesting read... see the info in yellow... http://www.vacuumtubes.com/12ax7.html

Khing