My mission to modify a Peavey Bandit 112 into a boutique amp

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My mission to modify a Peavey Bandit 112 into a boutique amp

Postby microbailey » 01 Feb 2018, 00:50

Well maybe not boutique but certainly customised beyond what a Peavey Bandit SS usually does!

I love Bandit's since they're indestructible and very loud for cleans (that's another post).
So I picked up a good cond Bandit 112 Red-Stripe on Gumtree and I've decided to re-design it as a new(-ish) amp.
The nice people at Peavey have supplied the schematics for my amp on request and I've set about scribbling over them with ideas.

Here's my plan:
  • Keep the existing power amp board pretty much as is - I like the Tube transformer and sag emulation circuit with the T-dynamics control - so I'll keep all that as its quite usable.
  • Re-use the existing 2 channel pre-amp board by stripping components off and adding new ones to make new circuits. I'll use wire and track cuts to modify the board as needed.
  • Clean channel: Replace the bipolar tranny input stage with a JFET Fetzer circuit and change the tone-stack for a Fender blackface style one. I'm after a Fender Twin sound out of this thing if I can get it.
  • Dirty channel: Rip the whole diode distortion guts out and replace with new gain stages made from JFETs to get a Plexi style distortion. The tone stack is already Marshall style.
  • Replace caps and anything else which looks broken and maybe add some nice status LEDs on the front panel.
  • Give the whole front a face lift
  • Upgrade the speaker from Chinese Blue Marvel to something nice (Jensen maybe for that Twin sound?)

I know this amp has a good spring reverb, a very solid open-backed cab, and plenty of clean headroom, so along with the Peavey "Tube Dynamics" circuit I'm thinking it could sound lush.

Am I crazy?? :hmmm:
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby BIGSMITTY » 01 Feb 2018, 01:10

Sounds like fun project
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby Ice-9 » 01 Feb 2018, 23:54

Yeah , a nice project. It would be a good idea to post the schematic as it will help everyone to give you some ideas as to hoe to go about the mods.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby EddieTavares » 02 Feb 2018, 00:07

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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby FeVeR2112 » 03 Feb 2018, 04:40

I am in on this one.... I recently discovered these Bandits (Bandit 65) and would like to see what we can do with them.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 04 Feb 2018, 19:46

Thanks for all your interest.

I'll try to blog what I'm doing here in case anyone else fancies trying this.

So I've started dismantling the Bandit (good vid on Youtube of where to take the screws out https://www.youtube.com/watch?v=rnMTzi6euxs)
The PCBs are all nice modern looking, with no SMT components, so look fairly easy to work on (my amp I think dates from around 2002).
I've removed the pre-amp board which handily has all the components for Clean and Drive channels and their tone stacks on it, so the mods can begin!

The Peavey input stage is a rather over-designed looking cct with a pair of bipolar trannys in Darlington configuration.
I took a look at the ROG Fetzer Valve page (http://www.runoffgroove.com/fetzervalve.html) so I'll use that circuit as my new first input stage, hoping that I can get a little bit of tube-like sparkle out of triode-like JFET non-linearity.
I'll try to post up the before and after schematics for this part of the cct.
I bought some JFETs off eBay (J201s) so I'll measure and sort these for Idss and Vp and find something which will suit the ROG Fetzer circuit.
I really don't want any trimpots if I can help it - in an amp they'll just fail over time with the vibration.

In answer to someone who asked, I don't really want to post the full Bandit schematics since Peavey give them out to customers FOC on condition that you don't post them publicly which seems fair.

To be continued ...
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 09 Feb 2018, 21:16

Ok so I've started on this project, as I said in the last post by working on the clean channel first.
Here's my sketch of the Peavey input circuit (which is common to both channels)

Image

As you can see (if you know circuits) there's a pair of bipolar trannies in Darlington config to give some initial gain to the guitar signal

After measuring my box of J201s I picked one with Vp=0.35v and Idss=0.83mA
I thought this would do for the 1st stage pre-amp since these parameters allow fairly high voltage gain (+/-0.35v in the Fetzer circuit).
I copied the ROG Fetzer input stage to give an input impedance and LP filter characteristic like a Fender Triode 1st stage, and came up with this circuit as my new input stage:

Image

You'll see what I'm going to do is replace some of the original Peavey components and remove others completely, to make my new gain stage.
I'll bridge the Base to Emitter of Q1 so that my signal arrives at the Gate of my new Q2, my J201 (hope this make sense).

I Googled and read the original AES paper about the Triode emulator, to make sure I understood how the Fetzer circuit actually works.
Basically we're operating the JFET in a very particular part of its curve, using very specific -ve feedback, so its as close to a Triode's Iout/Vin gain curve as possible.
I decided not to put any hi-boost cap across the Rs (source resistor) of my input stage, so as to keep the JFET operating in the most Triode-like region at all frequencies.
I made the Rs out of 2 resistors since the value is critical for the Fetzer to work correctly.

I breadboarded the circuit to test it with a sinewave input

Image

I had to play with the Rd in my circuit to get the output Drain around +15v which is just over half my supply and gives max voltage swing without clipping (this is a clean input stage remember!
The output waveform look slightly mis-shapen - just what I wanted (some gentle non-linearity) !!!

Off to the soldering station to do the mods to the Peavey pre-amp board then :D

To be continued ...
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 13 Feb 2018, 00:19

So something has gone wrong with my images in the last post which I linked from Google Drive :oops: :x
I need to work out how to successfully post an image here before I continue .. any advice appreciated!
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 15 Feb 2018, 23:42

Looks like I need to jump out of here into one of the dedicated areas to continue this thread since I don't seem able to attach photos to my posts in this one.
I'll continue in the "Amplifiers" area since that allows me file attachments
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys
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