"Doug Circuits" VOX AC4 Ver.3

[Attila]

So I have a few spares laying around and was given a fbar  VOX DA10 as platform to build on trashed the digital stuff ready to make something small and nice:


The Credit on the attached circuit goes to "Doug Circuits" who seems to be a authority on  Magnatone vintage amps http://www.dougcircuits.com, unfortunately he does not answer emails  So I thought I would turn to the fundies of the forum.


Please have a gander at the schematic and read some of the tech notes as provided by Doug. Specifically on the Tone control and Vibrato circuit and provide your valuable input   


Build Notes:


Details of the AC4 V3

EF86 essentials
First, I studied Valve Wizard Merlin Blencowe's EF86 analysis. I think this was the key to the ulimate success of the amp. What Blencowe points out is that the original Vox EF86 design is not optimal for a guitar amp. Vox engineers simply lifted the EF86 circuit from the manufacturer's design book which was probably more suited to phonograph and microphone amplification than guitar amps. For my circuit, I followed Blencowe's advice for designing the EF86 for guitar. In this configuration, the EF86 is more stable, noise-free, and has no microphonic issues. The trade off for all this is less gain, however, with a 12AX7 gain stage to follow it, maximum gain was not required.

Vibrato

I wanted to add vibrato back to the circuit, but didn't have the real estate inside the amp for a dedicated 12AX7 plus all the caps and resistors. I had recently been breadboarding an old fashioned oscillator design called the Relaxation Oscillator. This design uses a neon bulb to produce a sawtooth shaped oscillation signal. Read more about that here.

Tone Knob

With both sections of a 12AX7 free for the gain circuit, I designed a tone circuit that might look complex, but it is really rather simple: Both triodes are fed the same signal from the EF86. Each triode has its own tone RC circuit to follow, one focused on bass tone, and the other focused on treble tone. After the two RC circuits, both are mixed together via the mix pot (the tone knob).

EL84 and Power Supply
The EL84 stage is pretty much the same as the original AC4, except a 1KΩ screen resistor was added to lower the screen voltage to be 285V instead of 289V (plate is 283V). The original AC4 output transformer spec'ed a 5KΩprimary, and the TF103-48UL transformer I used is 8KΩ. This is close enough for an SE EL84 amp (close enough for me..).
The power supply needed some beefing. With the extra preamp stages, I wanted to add dedicated filter caps for each stage, and isolate them from the screen supply. I choose 8uf/350V caps for this. After I built the circuit, I found that the EZ80 simply could not keep up with the increased current demand. Since a rectifier tube doesn't introduce much tone-color in an SE amp, I sub'ed in some 1N4007 diodes to take care of this.

Line-Out

I don't think I'll ever build another amp without a post-OT line-out. It is so simple and really opens up the useability of a small 5W amp. The output can safely be plugged into another amp, a house PA, or a recording console. For this one, I chose a voltage divider with R2=150Ω, and R1 set to 4.7KΩ or 3.3KΩ depending on wheather the 8Ω or 4Ωoutput selector is switched.


       

[terminalgs]

that's mine!  I sorry I missed your emails. the whole thing started because I built a Vox AC4 (as per original Vox schematic) into a Pathfinder cab.  While it sounded good, it wasn't loud enough and didn't have balls.  So, I added a 12AX7 gain stage in between.  First, I added a topboost style 12AX7 in front of the EF86., then I tried it like this, with the EF86 out front driving a 12AX7. the amp in the schematic works well except the vibrato, it needs work (it takes the amp's volume down too low when engaged).   My goal was to add a vibrato without using a triode, and while  technically I did that, the vibrato doesn't work as well as  I hoped (maybe if I played with R204 more..) 

the tone controls were an experiment.  while they do work, if I were to do it again, I'd use 1/2 a 12AX7 as an oscillator for the vibrato, and a simple 1 tone-knob control like a Deluxe 6G3 or Magnatone M10.

the EF86 stage works great (all thanks to Merlin's generous web pages).  So well, I used the same EF86 on a 18W build that sounds fantastic.  If you take out the tone controls and the vibrato, you have EF86+12AX7+EL84 and that works so long as you attenuate some of that gain (somewhere) if you look at the two 12AX7 triodes, they each have their gain cut 30%-50% before headed to the el84.  On my 18W build, I put a 12AX7 gain stage in front of the EF86 (which Merlin suggests), and it's great.  Maybe better than 12AX7->EF86.

[kagliostro]

May be a SS Tremolo like in the Mikede amp ?

(full schematic on the attached .pdf file)



or, may be, this circuit that is a SS replacement for the Fender Tremolo ?



(see the .pdf file for info)

Franco

[Attila]

well well what a surprise Thanks for the heads up Terminalgs,  AKA "Doug Circuits"   and to you kagliostro.

Given your comments I think that I will dispense with the vibrato circuit and for the Tone Circuit look at a variable Lo-cut tone filter after the EF86 and maybe a Hi-cut before the power amp. 

Todd Sharp has done this in his 20w amps and seems to work very well, something like this:


Input -> EF86 -> Gain Ctrl -> Lo-Cut -> Vol -> 12AX7 -> Hi-Cut -> El84 -> Spkr


The idea would be to be able to bypass the tone controls in a "wide open" configuration and dial in the desired tone cuts from either/or sides of the spectrum.

If anyone has used this kind of config or has ideas along these lines that will be fantastic, I look forward to your contribution

Ps: see Todds panel below   

[kagliostro]

Todd Sharp has done ............

that is interesting and I suppose it worth a try

only, if you didn't see it, Doug/Terminalgs has planned a simplified version of the amp

http://www.dougcircuits.com/AC4buildnotes.html



Ciao

Franco

[purpletele]

PP  ..... Well for a start you don't hijack someones tread

Sorry,

I'll post it on a separate section