Vox Dual Channel Full Plate Voltage Pre-amp

[JakeFuzz]

Now I am hooked    I couldn't get enough after Marsha so I decided to make another one. What is it?

This is a preamp modeled after the famous Vox AC-30 top boost. In addition to the 12ax7 input, second stage and cathode follower I have included a switch to bypass all of that and switch in an EF86 pentode input stage. These both go into a phase inverter "approximation" which is the same as the Marsha pre-amp; it is simply a Fender triode gain stage with 22nf coupling caps. The style of wiring is tagboard and PTP. This uses a Weber cap can for filtering and I went with bridge rectifiers to save a little space. This runs off of a Weber 12VAC to 260VAC step up transformer and ends up with about 380VDC under load. Unfortunately I had to run two regulators to run the EF86 and 12ax7 filaments (I was a little close to the max current of a single and the EF86 runs on 6.3 volts only anyways). This is all wired into an anodized red Hammond 1590DD.

How does it sound? Well the top boost stage has a bad connection somewhere so it is a little fizzy and needs one more run through. Easy fix. But the EF86 stage is awesome. Just crazy cool sounding. I never realized how much the EF86 would be so compressed in a good way. It gets squishy and fuzzy gritty sounding with enough drive. Just on the edge of breakup is the best sound. It makes my roland cube test amp sound incredible. Demos this weekend when I get the TB stage fixed. The PI approx. is running a JJ ecc803 which I never really liked that much so it may change to an ecc83 later.

Between this and the Marsha I am set for awesome pre-amps. These will both eventually run into my 5F2A Princeton clone to bypass the Fender preamp and go directly into the 6V6 stage. I can't wait!

[culturejam]

That's bitchin! 

[sgmezei]

Duuuuuddde. Unbelievable. I want to hear it!!!

[stecykmi]

very sweet build.

what are you using the regulators for exactly?

[night-B]

This is sick! Awesome job 

[nzCdog]

OMG.  (I said 'F yeah' out loud...) Scuse me but thats friggin awesome! Nice PTP action Paul... love a demo when you're done tweakerising... Boom!

[JakeFuzz]

Thanks guys. Demos this weekend when I get a chance. I went through the TB channel with a fine tooth comb and can't find anything totally obvious   Ill have to start doing component checks.

Misha, the regulators are for the tube filaments. I could probably run the 12ax7 filaments on 12VAC without the center tap but I don't want tons of high current AC lines running all over the place. The other regulator is tuned under load (LM317) to 6.3 volts to run the EF86. Those suckers get hot too, I hope they last...  

Oh and thanks to Timbo BTW for suggesting this project. You were dead on about the EF86 sounding killer!

[Om_Audio]

Nice build! I love all those big caps and resistors.
C

[raulduke]

 

Really cool dude.

I am super impressed that it is all point-to-point/tagboard as well!

Nice build! I love all those big caps and resistors.

Agreed; especially because they are actually necessary, and aren't just shoehorned in to add to the 'mojo' factor .

[midwayfair]

Oh man I want to hear this. That's a LOT of stuff crammed into such a small "head." Hope the top boost circuit gets better.

[JakeFuzz]

Thanks! I actually did a layout before hand, rather than just wing it like I did the Marsha. It turned out a little nicer this time around.

The top boost works up to about halfway on the gain knob then something weird happens and it gets kind of fizzed out. What I have hear of it though it is super bright; I thought that 500pf cap was a little small and holy smokes it is ear piercing! I may swap the 500 out for 22nf if I can't hang with the shrill. Anyone have a real AC-30 to confirm I am not crazy?

[GrindCustoms]

That looks terrific!! Love it! Great fawking job Sir!!

And his that a 1590DD(Tall)? if yes, where did you source it? Would be perfect for my Firefly project!

[JakeFuzz]

That looks terrific!! Love it! Great fawking job Sir!!

And his that a 1590DD(Tall)? if yes, where did you source it? Would be perfect for my Firefly project!

It is the tall 1590DD. I got this one from pedal parts plus. It was 17 bucks before powder  but Ive seen it elsewhere for more. Is the firefly the one that uses a single ended 12AU7 as a power tube?

[lincolnic]

I haven't heard it yet and I already want it.

[GrindCustoms]

That looks terrific!! Love it! Great fawking job Sir!!

And his that a 1590DD(Tall)? if yes, where did you source it? Would be perfect for my Firefly project!

It is the tall 1590DD. I got this one from pedal parts plus. It was 17 bucks before powder  but Ive seen it elsewhere for more. Is the firefly the one that uses a single ended 12AU7 as a power tube?

Thanks for the info! I may choose a fancy color it makes a really long time that i did'nt bought a painted enclosure.

Yup, it's that amp, my first amp project