BAJA AC30TB overdrive pedal project 110218

bajaman · Post by **bajaman** » 17 Feb 2018, 08:41

I made a further two changes to the Top Boost version - reduce the input gain stage even further by changing the 2k7 for a 1k2 resistor - this allows the input stage to have a similar headroom to the actual tube amplifier input stage.
Also, I increased the gain of the second stage that drives the passive tone stack by changing the 2k7 to 15k.
Two minor changes that you can make or just use the original schematic - your choice

cheers
have fun
bajaman

and the rev 1 vero for the Top Boost version

HENK · Post by **HENK** » 17 Feb 2018, 16:26

Hello,Hello.
Is it a good Idea to put the
"Celestion Blue Driven by Tube Amp Filter Circuit"(Page Eight)
after this very cool looking Project???

Frank_NH · Post by **Frank_NH** » 17 Feb 2018, 19:02

bajaman wrote:
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!

The attachment Baja AC30 Normal amplifier overdrive 9v low gain rev 1 170218.GIF is no longer available

Thanks for the heads up on the vbias cap! I thought there should have been something there.

Your mods did the trick. Here's the output from LTSpice for a 0.1V 800 Hz input sine wave, with the gain at 50%, tone cut off, and volume at 100%. The clipped wave form has nice rounded corners now.

The 600 mV output amplitude (16 dB gain) is limited by the clipping, but I guess you could mod the final JFET buffer to a simple gain stage if you wanted more oomph.

I'll try to get this built to see how it sounds. Thanks again.

: Capture.PNG (9.16 KiB) Viewed 5653 times

roseblood11 · Post by **roseblood11** » 17 Feb 2018, 20:09

Is the top boost pedal optimized for overdrive, or can it replicate the typical Vox clean sound, maybe with the mentioned Celestion Blue Sim?

If I'd build this, I'd add a booster in front of the circuit that can cut bass, maybe the Boss FA-1 (which I use a lot with real Vox amps).

bajaman · Post by **bajaman** » 17 Feb 2018, 22:10

Is the top boost pedal optimized for overdrive, or can it replicate the typical Vox clean sound, maybe with the mentioned Celestion Blue Sim?

Yes

This latest revision 1 performs just like the actual amplifier - at low gain settings it will "cut your ears off" when the treble control is on max - then the tone cut control warms it up nicely

Very clean until you turn up the gain

cheers
bajaman

bajaman · Post by **bajaman** » 17 Feb 2018, 22:21

link to the above mentioned Celestion blue speaker response simulator which I had completely forgotten I had designed back in 2013

https://www.freestompboxes.org/viewtopic ... it#p225487
If using with the Top Boost or Normal circuits in this thread you can just connect the vbias point to the existing voltage divider or op amp buffered divider vbias point

cheers
bajaman

bajaman · Post by **bajaman** » 18 Feb 2018, 03:45

Here you go - built it and verified - works very well - not a cabsim but an a speaker response load sim designed to duplicate the frequency response caused by the speaker load on a tube output stage.

I had completely forgotten this design, so many thanks to HENK for pointing this one out

cheers
bajaman

HENK · Post by **HENK** » 18 Feb 2018, 04:46

Frank_NH · Post by **Frank_NH** » 20 Feb 2018, 03:49

Hey all,

I finished my Normal Channel vero build (the layout is attached below). Fired up first thing and didn't need any debugging.

I did make one modification, changing the 470n cap to 47n (which is the coupling cap most AC-30 schematics show before the normal channel volume pot). It reduces the gain on the bass somewhat, which is a good thing (see below). Also added a reverse polarity diode, FWIW.

The sound is pretty gainy and exactly what you would expect from the AC-30 Normal channel - which is to say "wooly". That is, it allows a lot of bass frequencies through, just like the old AC-30s. It's not a great tone when overdriven (at least for for me) - until you add a treble booster in front. It then sounds GREAT! I am using a ROG Omega Booster pedal I built a while back and it's a good match for the amp sim as it can greatly reduce the bass frequencies going into the pedal. You could also try a Greg Fryer treble booster or a Rangemaster, but make sure you can control the level as you don't want to slam the JFET and first op amp with a huge signal.

With a treble booster in front, the treble cut control becomes useful for taming any shrillness. Works pretty well for that. I didn't try the speaker sim Bajaman introduced above, and that may provide some subtle shading of the tone. I would propose putting that circuit on a separate vero so you can experiment. Other cab sims would be interesting to try as well.

The vero is verified but let me know if any problems are found. Cheers!

bajaman · Post by **bajaman** » 20 Feb 2018, 05:06

Hi Frank_NH

I did make one modification, changing the 470n cap to 47n (which is the coupling cap most AC-30 schematics show before the normal channel volume pot)

Agreed but i am using a 100k channel volume pot (not the 1M as the actual amplifier uses) so the 470n capacitor is the correct value to use in this application.
cheers
bajaman

Frank_NH · Post by **Frank_NH** » 20 Feb 2018, 14:35

Hi Bajaman,

The AC-30 schematics I've seen have a normal channel 500K volume pot (see attached image) and I agree a higher cap value may be technically correct for 100K, but the 47n seemed to tame some of the sub-100 Hz lows (without the treble boost). I suppose I could change to a B500K volume pot to keep the corner frequency the same. However, if you're placing the treble boost up front, it probably won't matter too much since the lows are not present at the input.

There's also a 220K resistor at the output (lug 2) of the volume pot which is used with all channels on the amp prior to mixing and sending the combined signal to the power amp. But my LTSPice model showed that this didn't do too much except bump the the signal level down. But if you wanted to more authentic, you could add it in. Your speaker sim actually bumps the signal up a bit, so no problem with the output level if you add that.

Now, I'm off to breadboard a bunch of treble boosters...one thing leads to another!

bajaman · Post by **bajaman** » 20 Feb 2018, 21:29

Hi Frank_NH

my error - thanks - I forgot the Vox uses 500k volume pots, so the correct value capacitor should be 235n when using a 100k pot - 220n will suffice

cheers

bajaman

bajaman · Post by **bajaman** » 20 Feb 2018, 21:31

and here is the schematic for the AC30BM treble boost section

Frank_NH · Post by **Frank_NH** » 21 Feb 2018, 14:06

Thanks Bajaman. That's the classic Brian May treble boost with a switchable gain option. Definitely need an output control pot as it produces over 30 dB gain, and the amp sim will definitely be clipping the first op amp. I suppose you could put some clipping diodes (LEDs) before the first stage in the amp sim, like the Runoffgroove Britannia, to control the signal - I'll have to check that out in LTSpice. Meanwhile, I was been playing around with different clipping transistors, and BC107B seemed to work well - then I realized the BC547 and BC107 are pretty much the same

.

Right now, my Runoffgroove Omega booster pedal is doing a fine job as the front end to the amp sim, so if anyone builds it, that is one viable option for the treble booster. Ultimately, I may finish this up as an all-in-one pedal with the treble boost in front (and always on) and controls for boost level, amp volume (gain), treble cut, and output level.

bajaman · Post by **bajaman** » 21 Feb 2018, 20:24

Definitely need an output control pot as it produces over 30 dB gain, and the amp sim will definitely be clipping the first op amp

okay - just change the 47k output resistor to a 50k linear potentiometer ( or preset potentiometer

) and take the signal out from the wiper - or two fixed resistors as a voltage divider ( try 39k and 8k2 perhaps

)
cheers
bajaman

Frank_NH · Post by **Frank_NH** » 22 Feb 2018, 13:55

Quick update. Tried the Brian May treble booster on the breadboard with a 50K volume pot and piped it into the amp sim, and it worked fine. Interestingly, this is a very similar configuration to that used on the Vox VBM1 amp produced some years back (Vox's take on the solid state "Deacy Amp"). However, I actually think the Omega booster sounded a little better. May ultimately go that route for an all-in-one build. Haven't tried the speaker sim circuit yet.

I also compared the sound to the ROG Britannia, which I had built a few years ago. The Britannia has a wider range of tones but can't quite get the saturated distortion that the present circuit can. So build this a your "Brian May in a box" pedal and you'll be all set to rock out to "Bohemian Rhapsody"!

Frank_NH · Post by **Frank_NH** » 22 Feb 2018, 14:18

The Vox VBM1 Treble Booster...

Frank_NH · Post by **Frank_NH** » 26 Feb 2018, 04:09

OK! Here's the logical conclusion of my efforts to simulate Brian May's rig - the Q-Drive!

I settled for the ROG Omega as the treble booster and integrated it into my previous vero layout for the AC-30 Normal channel sim. Just finished my build tonight and it sounds GREAT! A very versatile distortion. With the Range and Boost set low and the amp Gain up near full, you get a very convincing Brian May sound (well with the right guitar, pickups, Brian's talent etc.

). Can't wait to box my prototype!

You can substitute other transistors if you want (e.g. BS170 for the MOSFET) but mind the pinouts.

Have fun!

287m · Post by **287m** » 27 Feb 2018, 13:06

oh shit, why i just see boost schemo

anyway, this is normal channel for you

===
Question :
Are Sprint user really just use some pict, load it and just tracing? Really easy like that?
and can clone whatever layout in speed of light?

Ask here, because i know there a lot of sprint user here.
And my friend who use sprint never talk much about this.

Frank_NH · Post by **Frank_NH** » 27 Feb 2018, 14:08

In my project I used the ROG Omega as the booster:

http://www.runoffgroove.com/omega.html

Instead of the 4.7u cap at the output, I used just a single 1u cap, which becomes the input to the normal channel amp sim above. In my testing, the normal channel amp sim on its own is (like the real amp itself) very "wooly"; however, with a treble boost in front, it becomes a great distortion with lots of singing sustain and harmonics (like Brian May's tone).

BAJA AC30TB overdrive pedal project 110218 [documentation]