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Just wanted to share my recent experiences with trying a 6AU6 input in a 5E3 type amp. This is actually a 5E3X2 amp kit from Weber. I originally planned on just building a pretty straight 5E3 circuit, but when I found the chassis already had an extra preamp tube hole punched I decided to experiment a little. A couple of recent threads here on pentode preamps wetted my appetite. And a past very successful amp project where the circuit was based on the Matchless Clubman but using a 6AU6 pentode instead of an EF86, combined with the fact that I've got more than a dozen vintage 6AU6 tubes and they seem relatively plentiful and cheep, I decided to go with a 6Au6 input for one channel, a standard 12AX7 input on the other channel. I did a few other mods as well, such as reducing the input cathode resistor value to 4.7uf and the coupling cap values to cut bass; separated the Volume controls and wired up more common style with the wiper as the output not the input; fixed bias phase inverter; and a couple others.
The first problem I had with the 6AU6 channel was a low dc voltage on the grid making the guitar controls scratchy. I ended up using a .1uf cap to block the dc off the input and this took care of the scratchy guitar controls. But then I had problems with just a lot of noise at idle, and a lot of microphonics at high volume.
The 12AX7 side is fine, nice & quiet and sounds just like a 5E3, only a lot louder, and a lot more headroom, a lot more usable volume control, but it never quite breaks up like a normal 5E3, it doesn't get the same grind even when dimed. And this with a 12AX7 input instead of a 12AY7. Sure sounds a lot better with humbuckers, for sure, but missing that great 5E3 grind.
The 6Au6 channel kicks major ass! Sounds incredibky awesome with tones of gain, plenty of grind, sustain, feedback, everything. But, the high noise floor is bothersome. To quell the microphonics I installed a rubber Oring under the tube socket, then I installed small rubber orings under the socket mount screws. Then I installed some self adhesive foam insulating strips on the cabinet wherev er it contacts the chassis, as well as the back panel. And I installed rubber grommets in the chassis where the mount screws pass thru it, and rubber washers under the heads of the chassis mount screws. I've done everything I can think of to quell the vibrations. And it was about 90% successfull in quelling the vibrations, but that last 10% is really bugging me. The tube doesn't ring with microphonics on the note attack, but as the note or chord rings out and shakes the cabinet the tube then emits a low frequency loud rattle that overwhelms the note/chord. This with the volume set to around 5 to 7 or so. With the volume cranked up even higher it seems the sheer volume of the note/chord covers it up, but it's still there and it bugs me.
With the Clubman type amp I built I do not have these problems at all. The amp sounds great, with no microphonics and no noise issues. But in that case it isn't the input, it's the second stage.
I'm about to give up on the idea and strip it out and try something else. Again, I love the sound of the amp, incredibly good tone, very full, lots of bass; cleans sound fantastic, much better than the 12AX7 side, and when turned up it pushes the amp hard with lots of great grind. Just what you want with this type of amp. Where a 5E3 will give a big amp a bloody nose, this one will cut a bigger amp off at the knees. But, the noise issues are too much for me to live with.
I'm thinking of trying a 12A_7 tube wired up as a cascode for the input. I might run into similar noise issues, but perhaps microphonics won't be as much of an issue. Seems like something worth trying though.
The first problem I had with the 6AU6 channel was a low dc voltage on the grid making the guitar controls scratchy. I ended up using a .1uf cap to block the dc off the input and this took care of the scratchy guitar controls. But then I had problems with just a lot of noise at idle, and a lot of microphonics at high volume.
The 12AX7 side is fine, nice & quiet and sounds just like a 5E3, only a lot louder, and a lot more headroom, a lot more usable volume control, but it never quite breaks up like a normal 5E3, it doesn't get the same grind even when dimed. And this with a 12AX7 input instead of a 12AY7. Sure sounds a lot better with humbuckers, for sure, but missing that great 5E3 grind.
The 6Au6 channel kicks major ass! Sounds incredibky awesome with tones of gain, plenty of grind, sustain, feedback, everything. But, the high noise floor is bothersome. To quell the microphonics I installed a rubber Oring under the tube socket, then I installed small rubber orings under the socket mount screws. Then I installed some self adhesive foam insulating strips on the cabinet wherev er it contacts the chassis, as well as the back panel. And I installed rubber grommets in the chassis where the mount screws pass thru it, and rubber washers under the heads of the chassis mount screws. I've done everything I can think of to quell the vibrations. And it was about 90% successfull in quelling the vibrations, but that last 10% is really bugging me. The tube doesn't ring with microphonics on the note attack, but as the note or chord rings out and shakes the cabinet the tube then emits a low frequency loud rattle that overwhelms the note/chord. This with the volume set to around 5 to 7 or so. With the volume cranked up even higher it seems the sheer volume of the note/chord covers it up, but it's still there and it bugs me.
With the Clubman type amp I built I do not have these problems at all. The amp sounds great, with no microphonics and no noise issues. But in that case it isn't the input, it's the second stage.
I'm about to give up on the idea and strip it out and try something else. Again, I love the sound of the amp, incredibly good tone, very full, lots of bass; cleans sound fantastic, much better than the 12AX7 side, and when turned up it pushes the amp hard with lots of great grind. Just what you want with this type of amp. Where a 5E3 will give a big amp a bloody nose, this one will cut a bigger amp off at the knees. But, the noise issues are too much for me to live with.
I'm thinking of trying a 12A_7 tube wired up as a cascode for the input. I might run into similar noise issues, but perhaps microphonics won't be as much of an issue. Seems like something worth trying though.
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