Preamp circuit mod for replacing a 12AX7 with a 12AU7 gain stage

[arush84]

I'm hoping someone can help advise whether any circuit mods are necessary when replacing a 12AX7 gain stage with a 12AU7.

The mod would involve swapping a 12AX7 for a 12DW7 tube in V2 of a Marshall DSL401. The 12DW7 is one half 12AX7 one half of a 12AU7.. so the 12AU7 acts to lower the gain of the overdrive channel. This 12DW7 replacement has been reported to improve some of the problems found in the DSL401's over-saturated overdrive channel without affecting the clean channel.

I'd like to try the mod but I have a few concerns over simply swapping the tubes over because the 12AU7 stage (which is lower gain but having a higher current-draw) surely won't be biased up correctly for use within the 12AX7 circuit. I'm hoping someone with more knowledge of preamp design can help advise here!? In particular would it be advisable to change Plate Resistor, & cathode resistor? is this likely to reduce gain over the overdrive circuit or is the gain reduction reported by others making this tube swap simply the result of having an under-biased gain stage in the circuit?

Here is a link to the preamp circuit. The single 12AU7 gain stage would be pins 1-3 of V2. R82 and R123 are the resistors i suspect may need to be changed. I believe the B+ of the stage is somewhere around 400-410v.
http://www.drtube.com/schematics/mar...60-02-iss9.pdf

[Chuck H]

Changing plate and cathode resistors can optimize for a 12au7, but, then the circuit won't work as indicated by those who report positive results. It's safe to just plug the 12dw7 in. WRT guitar amps "safe" is the relevant criteria since guitar amps intentionally color the input signal. So optimizing in this instance is moot. As long as the circuit operates safely, and sounds like you want it too, it's just right.

BTW, if the 12dw7 has too little gain and saturation there's nothing to stop you from pluging the 12ax7 back in and tweaking the circuit with that tube. Since the 12dw7 isn't being operated in any way that utilizes any advantage to the "au" triode the biggest difference with the tube swap is just voltage gain. And that can be controlled with circuit tweaks as well. So if you want an in between amount of gain that should be possible too.

[arush84]

Changing plate and cathode resistors can optimize for a 12au7, but, then the circuit won't work as indicated by those who report positive results. It's safe to just plug the 12dw7 in. WRT guitar amps "safe" is the relevant criteria since guitar amps intentionally color the input signal. So optimizing in this instance is moot. As long as the circuit operates safely, and sounds like you want it too, it's just right.

BTW, if the 12dw7 has too little gain and saturation there's nothing to stop you from pluging the 12ax7 back in and tweaking the circuit with that tube. Since the 12dw7 isn't being operated in any way that utilizes any advantage to the "au" triode the biggest difference with the tube swap is just voltage gain. And that can be controlled with circuit tweaks as well. So if you want an in between amount of gain that should be possible too.

Thank you for the explanation, some very good points!

My worry was in swapping the 12AX7 for a 12DW7 if there was a risk of breaking the tube with incorrect resistor values.

So just to better understand the logic behind this, replacing a 12AU7 in a 12AX7 circuit is potentially one method of greatly reducing the gain of that stage in the overdrive circuit? even less gain than if the 12AU7 was running in conditions it is designed to operate under?

The problem I have with this amp is how the gain is not well matched between channels. The clean channel sounds great once the master volume is up past around 1/3. But there is a big volume jump when swapping to the OD channel, and it's also very saturated/fizzy sort of distortion even with the preamp gain set lower, particularly the OD2 channel (the OD channel has a +3db boost i believe to go from crunch to lead). I'd ideally like the boosted OD (OD2 channel) to be more like the OD1 currently is and the OD1 closer in volume with the clean channel without getting saturated too quickly!

[Guitarist]

I know exactly what you're describing. The deal with the 12au7 is that they shine in barrelling out lower Z sends. They are really full in the lower frequencies almost dulling your tone if if not used in a CF stage. They're too polite when used as a gain stage unless you want just a bit of boost as in a buffer.

Try a 12ay7 (or almost as sweet, a 12at7) and lower the two resistors: the Cathode and the Plate. Lower values and bigger watts if you're into the safety redundancy. Just swapping in those tubes stock, will net a very cold bias, delivering little tone-making current. I hate math but love tone, so if this sounds unimpressive it is only that in wording. The amp will speak for itself