To AU or DW...7 that is

[Chuck H]

I'm working on a design that needs to use only 1/2 a dual triode tube to drive a reverb pan. A 12AU7 seems like a good candidate since one triode is rated at over 2 watts max dis. The rub is that the other half of that dual triode will need to be in service as a gain stage. I may be able to get what I need from the 12au7 but I know I can get it from a 12ax7. So I looked into currently available tubes and found out Ei and JJ are making 12dw7 and 12au7 tubes. So my question is... Based on whats most likely to continue to be available, should I try to make it work with the 12au7 or just go with the 12dw7. My gut is saying to try to make it work with the 12au7. But I'm putting it up here for trusted opinions.

Chuck

[Steve Conner]

Hi Chuck

I think a 12AU7 only has a gain of about 15x when used as a gain stage. If that's not enough, you'll just have to go for the 12DW7. (or do something really geeky to boost the gain, like cascoding it with a FET)

[jrfrond]

You really need to divulge more info to determine this. Are you driving a higher-impedance unit directly from the plate via capacitive coupling like the old Magnatone? If not, and you are driving it with a transformer, you will not have enough current gain with a single triode unless it was a power triode like the 6G15 Fender design with the 6K6 or 6V6. This is why you usually see a 12AT7 with paralleled sections.

Concerning tube types: lower plate resistance tubes like the AT or AU sound best for reverb. AX can yield some edgy, brash reverb response due to it's higher plate resistance and resultant narrower bandwidth.

[Chuck H]

I think a 12AU7 only has a gain of about 15x when used as a gain stage. If that's not enough, you'll just have to go for the 12DW7.

Thanks Steve. The problem isn't really gain as this stage will drive an effects loop. It's trying to make the 12au7 grid and plate overdrive to duplicate what I'm currently doing with a 12ax7. I know the 12au7 has a much lower gm so slamming into the grid is how I'm going to start. I've never used a 12au7 for a preamp tube and have no idea if I can get the plate signal to distort the way I want it too.

(or do something really geeky to boost the gain, like cascoding it with a FET)

That would be too easy. No sir. This is being designed for them thar "boutique" types. They's don't take kindly to no silicooties.

Chuck

[Chuck H]

Are you driving a higher-impedance unit directly from the plate via capacitive coupling like the old Magnatone? If not, and you are driving it with a transformer, you will not have enough current gain with a single triode

Thanks John. I know there will be some compromise.

I'm using a transformer. I have seen single triode transformer driven reverbs done with 1/2 a 12at7. not bad either. The 12au7 is capable of a tiny bit more wattage too. The typical "Fender" reverb has ALOT of wet signal. More than anyone I know can use by a stretch. It's driven from a 500pf cap. You can get away with a little more than this without distortion. The reverb pot is a megar 50k. You gan get a little more with a bigger pot (at the expense of adding some noise). And I need to use the other half of this tube in the signal chain. I've never been happy with the tone of 12at7 tubes except as a function tube for driving reverb pans. Thats why I'm considering the 12au7.

This idea stems from my want to use three preamp tubes instead of four. For my design four tubes would leave 1/2 a triode unused. I also wanted to trim production cost by eliminating one tube socket, tube and chassis hole. There's also the benefit of chassis real estate and easier layout.

AX can yield some edgy, brash reverb response due to it's higher plate resistance and resultant narrower bandwidth.

I've never thought much about before. But that could be because of the higher turns ratio through the typically cheesy reverb transformers. Bad transformers couldn't get any better by stepping up the turns ratio.

Chuck