BAJA AC30TB overdrive pedal project 110218 [documentation]

[bajaman]

You asked for it and here it is
The Vox AC30 Top Boost amplifier including driver and output stage in a convenient 9v dc overdrive pedal format
There are three places in the AC30 where clipping occurs at different signal levels - with input gain and tone controls on maximum reasonably symmetrical occurs first on the output stage at approximately 1.3mV 1kHz sine wave signal input level. At 2.8mV signal input the driver stage goes into asymmetrical clip at first and eventually symmetrical clip as the signal level is increased. Finally the tonestack driver and cathode follower stage goes into reasonably symmetrical clip when the input signal is increased to 23mV. the tone control section does clip asymmetrically at high frequencies when the treble control is on maximum and this does add to the distortion flavour somewhat.
I have used a combination of clipping devices to mimic this clipping behaviour - do not be surprised if the blue leds do not light up at normal signal levels - the output stage will go into heavy clipping well before these leds light up
The most difficult part of this design was the implementation of the Tone Cut control - this took a few hours (more likely minutes - lol) to realise in this design but i feel i have got it reasonably close to the amplifier response.
here it is - enjoy and have fun building it

[bajaman]

Here is the normal channel design (for those who want that Brian May sound) - just add a rangemaster treble booster clone in front and you are there
enjoy
bajaman

[bajaman]

Here are some response curves for the Top Boost and Normal channels

[bajaman]

amended Normal channel schematic (4n7 in tone cut section)
The Top Boost schematic is correct with the 6n8 capacitor though!

[FausterCraster]

Baja, you are the Leonardo Da Vinci of DYI community.
Your projects should be exposed at Louvre in Paris !!!



which type of Blue led ? (5mm, normal/ultrabright) ?
is it possible add TopBoost tonestack to the normal circuit with no impact ?

[bajaman]

which type of Blue led ? (5mm, normal/ultrabright) ?
is it possible add TopBoost tonestack to the normal circuit with no impact ?

just standard 3mm or 5mm blue leds

not really, but you could try changing the 3n3 for a 470n capacitor and leaving out the 560p from the top boost schematic to get a hybrid normal channel plus tone stack version

[deltafred]

Many thanks bajaman,

I've spent the morning ordering the components that I don't have in stock.

[Frank_NH]

Wow - that was quick! Thanks again, Bajaman.

Can't decide which to build first, but perhaps I'll wait until Alex posts some vero layouts (he's faster than I am ). I was thinking about building the normal channel with a treble booster in a single box - instant Brain May!

I also have built and boxed the ROG Britannia and am interested to see the difference in that emulation with these circuit designs.

[Cub]

This is the most amazing spree of projects posted that I have ever witnessed in my time here. I'm very grateful for all the time and effort that were put into this and it's hard to pick which one of the high gain pedals to build first. It will be even harder now that Baja mastered how to convert the phase inverter and power amp as well, kicking of another spree?

I'm very tempted to build the JVM pedal so I could take it with me on open mic nights. Whatever amp is present that day, I can always have the sound of my own and it would fit right in the gigbag!

However, there's one amp I'd like even more to fit in a pedal. Once, I got to play this clone of a Trainwreck Express and it was brilliant. It was everything I ever wished a plexi would be. More gain on tap than an actual plexi and incredible touch sensitivity.

Baja, you probably have a long list of legendary amps you'd like to convert. If the Express isn't on there already, dare I ask if you could put it on the list? Please?

[bajaman]

If the Express isn't on there already, dare I ask if you could put it on the list? Please?

stay tuned

[HEAD]

Surely the last couple of days here have been very exciting. Thank you so much Baja for all your affords and contributing!
What I would ask the more than me capable community is to do also some PCB layouts (I'm not a big fan of vero in case it gets more complicated than your ordinary fuzz or simple gadget) and share it so we can develope this further. If I could throw in an Amp I would like to see it would be a Fender 5E3 (I'm already having a clone of the Catalinbread JFET version of it an loving it - but you know? We pedal builders are nuts and always looking for new projects and challenges just because...).

Regards
Helge

[albru80]

I was thinking to draw a layout that switches between Normal and Top Boost.
It would have 2 Gain pots and 1 Tone pot that would switch just the cap from lug 2 (I think it's not possible to have 2 pots unless you omit the LEDs)
Would the Tone pot have the same response by switcing between the 2 modes?

[Frank_NH]

I was thinking to draw a layout that switches between Normal and Top Boost.
It would have 2 Gain pots and 1 Tone pot that would switch just the cap from lug 2 (I think it's not possible to have 2 pots unless you omit the LEDs)
Would the Tone pot have the same response by switching between the 2 modes?

Hi Alex,

I would do a combo design exactly like the modern AC-30 (below) - without the reverb and tremolo. The Normal channel has a single Volume control (Gain) whereas the TB channel has Volume + Treble + Bass.

Looking at the schematics, the two channels could share the first gain stage (IC1a) then split via a switch before the Gain control (and thus have two Gain pots). This would allow switching between the two channels with different gain levels (i.e. rhythm and lead). The Normal channel would then go directly to the 47nF cap at the input of IC2a whereas the TB Channel would pass through the active tone stack/boost. If you wanted, you could add a simple op amp buffer to the Normal channel. The remainder of the circuit is basically the same (except for the cap at the Tone Cut control, but that's a minor difference - I would just use one value - e.g. 4.7nF).

Hope this helps!

[albru80]

Hi Frank.
That was my idea.
Started a layout this morning but I don't have much time at the moment.
Wasn't sure if the tone would change much when switching.

[bajaman]

I built the Top Boost version last night and made one small change - I decreased the first stage gain (changed 8k2 resistor to 2k7) to allow a more useable gain control range.
I was very happy with the sound of the pedal following this small change.
cheers
bajaman

[bajaman]

Some pictures of my build - the enclosure was recycled from an earlier AC30 simulator - (the hole is where there used to be a cabsim switch).
cheers
bajaman

[Frank_NH]

Some pictures of my build - the enclosure was recycled from an earlier AC30 simulator - (the hole is where there used to be a cabsim switch).
cheers
bajaman

IMG_3513.JPG

IMG_3514.JPG

Nice! So how does it sound? Smooth, saturated, chimey overdrive? I'm thinking it would be cool to do a Brian May pedal with a transistor treble booster into the Normal channel circuit. In any event, I have a bunch of TL064 now so I can experiment.

[bajaman]

Here is the vero layout component placement and cut traces side views. The tone stack components and the tone cut capacitor are mounted directly to the control potentiometers
Do not forget to snip the BC547B collector legs off
cheers
bajaman

Baja AC30TB vero cuts.png (6.78 KiB) Viewed 6411 times

essentially the vero layout above follows my build - I moved the leds and electrolytic slightly and added the 22k resistor to the board which is mounted directly to the tone cut pot on my actual build.

[Frank_NH]

Hey Bajaman - I have a quick question. For fun, I decided last night to simulate your AC-30 Normal channel circuit in LTSpice. I was surprised to see that the signal (a 1 kHz sine wave at 0.1 V amplitude) was becoming a square wave after the power section op amp stage (with the 10K resistor in the feedback loop), and the waveform reminded me of a fuzz circuit response. The gain pot resistance was set relatively low. Is this how the real amp behaves? I've seen online demos of AC-30s and they seem somewhat cleanish until you put the channel volume past noon.

Thanks! And nice vero layout!

[bajaman]

Hey Bajaman - I have a quick question. For fun, I decided last night to simulate your AC-30 Normal channel circuit in LTSpice. I was surprised to see that the signal (a 1 kHz sine wave at 0.1 V amplitude) was becoming a square wave after the power section op amp stage (with the 10K resistor in the feedback loop), and the waveform reminded me of a fuzz circuit response. The gain pot resistance was set relatively low. Is this how the real amp behaves? I've seen online demos of AC-30s and they seem somewhat cleanish until you put the channel volume past noon.

hello Frank_NH
You are quite right - i originally designed this project as an overdrive / distortion pedal but to obtain the same onset of clipping behaviour as the actual amplifier requires the op amp gains to be a lot lower - one of the penalties when one is working with 9vDC versus 300vDC (or thereabouts) in the actual vacuum tube amplifier. To give a much better approximation to the amplifier involves reducing the input and driver stage op amps gains considerably. Here is the revised schematic for a far more accurate representation - it still retains the same important phase and frequency response correlation to the actual amplifier but at a much lower output level.
enjoy
bajaman

ps: It is most important ito add the 1uf capacitor in the low impedance vbias op amp stage (to the original posted schematic as well ) - without this the pedal becomes a great white noise generator!