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

Tube or solid-state, this section goes to eleven!

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:
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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 ...
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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 ...
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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!
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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
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My mission to modify a Peavey Bandit 112 into a boutique amp

Postby microbailey » 15 Feb 2018, 23:52

I'm re-posting and continuing here from the Freestomp Café since I get to upload files in this area :D
Here goes again ...


-----
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)

Bandit_input_stage_orig_schem.png


As you can see (if you read schematics) 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:

Bandit_input_stage_modded_schem.png


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

Fetzer_breadboard.jpg


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

Postby microbailey » 17 Feb 2018, 22:52

I did the mods for the Fetzer input stage on the Peavey PCB - wan't too difficult - mostly just removing and replacing components.
Since the J201 I was fitting in Q2 has a different pinout order (S,D,G) to the original bipolar (E,B,C) I had to bend the Drain and Gate leads over each other into the original PCB holes.
I put some heatshrink sleeving on the Gate lead to stop it shorting on the Drain lead.

I haven't tested the audio yet but I put the pre-amp board on a bench supply (+15V,-15V,+30V - soldered temporary wires onto the power ribbon pins which normally comes from the power amp board).
I injected a 300mV sinewave on the Lo-Gain input jack to check operation and used a scope on Q2 Drain pin and could see voltage gain so I'm happy its working.

If you're wondering where I got the sinewave I bought this el-cheapo signal generator of eBay:
https://www.ebay.co.uk/itm/XR2206-Function-Signal-Generator-DIY-Kit-Sine-Triangle-Square-Wave-1HZ-1MHZ-New/152139329617?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Great little find for £7 - had to build it - but will be really useful in the future for fault finding in amps / pedals etc.

Before I test my new Bandit input stage with a guitar I want to get the Clean channel tonestack mods done to complete the Clean channel upgrade.
I'll describe what I'm going to do there in the next post ...
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 06 Mar 2018, 01:09

The Clean Channel Tonestack mods


Sorry for gap, back from holidays now 8)
So I'm modding the Peavey Bandit clean channel tonestack to sound as much like a Fender "blackface" amp as I can.

Here's the existing Peavey tonestack on the clean channel
Bandit_clean_original_tonestack_schematic.JPG

It looks like a Marshall inspired circuit (the "Mid" pot is wired with all three terminals not just as a variable resistor).
You can also see its switched between "Vintage" and "Modern" which selects some different caps and resistors into the tonestack.

I've decided to use the Fender AB763 circuit as the basis for my mods to the "Vintage" voicing, and leave the "Modern" as is if possible.
This way I can get a Fender-y voice with the switch in "Vintage" position and a more Marshall-y voice with the switch in "Modern" position.

Here's the tonestack from the Fender AB763 circuit
Fender_AB763_tonestack_schem.JPG

The biggest differences I see are 1) the "Low" pot is 250k not 1M5, and 2) the "Mid" pot is 10k and is wired as a variable resistor.
So I decided
    to change the VR2 pot from 1M5 to 250k (except Peavey don't stock a 250k, so I'll settle for 200k which they do supply as Peavey Part No 71190907, same pots as Marshall's I think)
    not to change the VR3 pot to 10k since I'll just get a bit more Mids with the existing 20k
    to remove the R18 since it is changing the taper and reducing the maximum resistance value of the VR4 pot

I plugged the values into the good 'ol Duncan Tonestack calculator to look at the frequency curves - basically to see how close the values I've actually got compare to the Fender original.
I've set the Duncan TSC source resistance (the circle with the wavy line inside which represents the resistance of the driving stage before the tonestack circuit) to roughly R2 in parallel with R5 = 91k, which is what the signal will see.
The Fender AB763 curve is in green, and the "Vintage" position, with the modified values, including a 47nF connected to the "Mids" pot VR3, is shown in blue

Bandit_vintage_tonestack_modded_AB763_fplot.JPG

You can see there is a slight shift in the Mids scoop on the modified circuit. Fender original scoop bottom was at around 500Hz, but mine is around 400Hz. That'll really change the tone so I want to fix it if I can.
The Peavey has got C15 as 22nF always in circuit - the "Vintage" position adds in C16. I'm going to try swapping in 15nF for this which would give 22+15 = 37nF instead fo 47nF.
Re-running the Duncan calculator gives the red plot

Bandit_vintage_tonestack_modded_AB763_fplot_37nF.JPG

This looks better with the scoop bottoming out nearer 500Hz again. I'll go with that.

So in summary I'm going to do the following changes to the Bandit preamp board

    Remove R18
    Swap VR2 for a Peavey 200k log pot
    Swap C16 for a 15nF cap
    Short VR3 terminals 2 and 3 together so VR3 acts like the variable resistance in the Fender blackface circuit

Off to the solder station again

I've probably missed something blindingly obvious here which someone will swiftly point out! (nice) comments are welcome.

To be continued ...
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 08 Mar 2018, 01:36

Well I did the mods to the Clean channel tone stack - you can see the completed modified pre-amp board Clean section here:

Clean_channel_mods_1.jpg

Clean_channel_mods_2.jpg


These are the mods for JFET Fetzer valve input stage replacement and the tone stack upgrades to the "Vintage" voice position.
It's just visible where I added the sleeving on the J201 leg as it crosses over to match the old bipolar tranny's pinout on the board.

I also decided to up the local supply decoupling cap C2 from 100nF to 220nF just to try to reduce noise in this input stage as much as possible.

I couldn't resist a quick try out, so I plugged the preamp back into the power amp and plugged in my Fender Strat.
:D Wow! It's definitely made a difference! With EQs at 12 o'clock there's a real scoop to the tone with the voicing switch set to "Vintage" which is what I wanted (and I've not even upgraded form the cheapo Chinese speaker yet!)
I can't do an A/B test (cos I've ripped the old components out), but I'm happy with the sound.
There's just a touch of tubey warmth to the clean which I am going to attribute to the new JFET Ftezer input stage (my earlier check on scope showed it working).
My Strat's Fat50's pickups sound suitably quacky in position 2 - I can't resist playing a bit of Knopfler :) :)

Fiddling with the Low, Mid, High EQ I can temper the extreme scoop and top end to bring up the mids and flatten it all out.
The "Modern" position of the Clean voicing switch retains something closer to a Marshall clean sound even though the tonestack changes affect that a bit too.
Both voices, I think, will be good for dirt pedals (to be tried).

I've got a lot more I want to do to this amp - when I'm finished I'll try to post some sound clips.

Next up is the Dirt channel and I plan to be more drastic with the mods here, cos the stock sound is pretty bad.
I'll cover some ideas on this circuit in the next post.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby Cub » 08 Mar 2018, 09:29

Great work! Back in the day, I was quite impressed with the transtube amps. A really nice sound for solid state. You are really taking it to a much higher level! Fender cleans and Marshall dirt, what's not to like?
I'll be looking forward to seeing what you can think of next. Good luck and thank you for sharing your knowledge and expertise! :thumbsup
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 18 Mar 2018, 16:34

Cub wrote:Great work! Back in the day, I was quite impressed with the transtube amps. A really nice sound for solid state. You are really taking it to a much higher level! Fender cleans and Marshall dirt, what's not to like?
I'll be looking forward to seeing what you can think of next. Good luck and thank you for sharing your knowledge and expertise! :thumbsup

Thanks for the encouragement Cub. I agree - I think they are underated and I always had a fondness for them after using them for years in rehearsal studios.
No I've got going with this I've definitely got more ideas for mods.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 19 Mar 2018, 23:52

I've started thinking about modifying the Bandit's Lead channel.
For those who don't know, this is the bunch of knobs I've got to twiddle with (pic is from the Bandit manual)
Bandit_fpanel_lead_channel.JPG

According to the manual the 3-way switch "Vintage / Modern / High Gain" is supposed to give Classic Overdrive / Metal / "over the top" (????)
I've never really found a sound I like on this channel. It's harsh, fizzy, and (to my ears) very un-tube like.
I found Modern and High Gain completely unusable. I want to fix all that if I can, and at least get some dirt tones I can use.

Looking at the original circuit for the Bandit dirt channel, it has 3 bipolar tranny pairs forming gain stages with a pre-gain pot at the start, and a bypass around the last stage (off the 3 way switch).
Bandit_dirt_gain_stages_original_schematic_f.jpg

Each transistor pair (Q5-Q6, Q7-Q8, Q9-Q10) is connected in Darlington configuration to give high gain (Peavey seem to like Darlingtons), with the last gain stage also having symmetrical clipping from a pair of 1N4148s.
The last Q11 isn't providing gain, its just an emitter follower (like the cathode follower in the Marshall) to give low input impedance into the tone network which follows.
I haven't shown it here but there's a tonestack right at the end - and the 3-position switch does a bit of post-clipping tone shaping at that point with Rs and Cs (I'll look at that more later).

So Peavey went for a Marshall style pre-amp with a tonestack and master volume after the gain (compare the nice annotated schematic excerpt from Rob Robinette's excellent website, below)
Marshall_JTM45_plexi_preamp_schematic.JPG


I'm thinking if I keep the 3 gain stages, and the tonestack at the end (like the Marshall) but get the clipping from JFETs or MOSFETs, and play with some cap values, I could get something more like an overdriven Plexi sound.
I'm sure I can do something more interesting with that 3-way switch too. Its got 4 poles (4 sets of 3 way switches in tandem) so rewiring that could give some nice gain/tone possibilities.

Goes for cup of coffee... :scratch:

So I thought about it and remembered that I liked the sound of the Wampler Plexi Drive clone I built for someone a while back, and that circuit has 3 gain stages too (its an AIAB for a JTM Plexi). I'm talking here about the original Plexi Drive.
So maybe I can take the Wampler circuit as a starting point and fiddle with values etc to see what I can shoe-horn into the Bandit circuit board?
I've got a box of sorted J201 JFETs so that's a good starting point too.

Here's the Wampler Plexi Drive pedal (schematic was traced a while back here on FSB). I've shown the 3 gain stages and the location of the tone shaping (some before stage 2 and some at the end).
Wampler_PlexiDrive_gain_stages_f.jpg

I'll see if I can map that type of circuit onto the Bandit schematic to replace what's there, and improve the usefulness of the 3 position switch for different sounds.

I'll come back and post when I've got something more concrete.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 29 Mar 2018, 00:28

Ok, so I finally got time from the day job and life in general to post how I'm going to do this.
I've looked for a long time at that Bandit schematic, and at the 3-way voicing switch on the lead channel, and at the Wampler Plexi drive circuit, and here's how I'm going to start ...

I'm going to strip off selected (= most) of the components from the Bandit's existing dirt channel :D (red crosses below where components removed), and bridge some components with wire (where there's green lines below), fit J201 JFETs (where there's a badly drawn JFET below)
Bandit_dirt_gain_stages_modified1_schem.jpg

This will leave plenty of empty holes in the PCB to do something useful with!

What it will in fact leave me with is shown below. I've kept the same component refs as the Bandit PCB and schematic so you can see how I got here from the original (I hope)
Bandit_dirt_gain_stages_reconfig_schem1.jpg


Since I (mostly) removed come components which feed each gain stage output into the next, I'm left with 3 distinct JFET gain stages which start to look like the Plexi Drive, but kind of in the wrong order.
I can now see 2 caps which can give a choice of pre-clipping high-pass: C19 and C24 (on the output of Q8).

I'm thinking Q8 will become my Plexi Drive gain stage 1, Q5+Q6 will become my Plexi Drive gain stage 2 with the gain pot in front to adjust the amount of overdrive, and Q10 will become my Plexi Drive gain stage 3. Q11 is still the emitter follower (I left it as it was).
I'll need to re-draw this schematic again to show how it will look with the order as it will be connected. The great thing is: I can make the order whatever i want now by jumpering wires between these JFET gain stage blocks.

A word about Q5+Q6. I fancied some extra gain in this stage (which will become stage 2 of the Plexi Drive circuit), so I put 2 JFETs (which will need to be matched for Idss and Vdsoff) in parallel because that will give twice the Id current through R29 and so twice the voltage gain into the next stage. JFETs current share quite nicely in this configuration and you might notice the Wampler Tweed 57 uses this style. And my PCB has the holes to do it since there were 2 trannys there originally anyway.

I know this looks likes a mess :roll: but I think it will come together and its much more fun than adding a new PCB!

I'll come back and post a sorted schematic and take it from there. I've got a couple more ideas to put in to this yet anyway. I've also got to figure out all the new component values for best tone.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 05 Apr 2018, 00:21

I've been working on that dirt channel schematic - this is a work in progress so bear with me - this is what I can now re-draw those tranny stages as (I've kept all the component references as the original Bandit PCB)
Bandit_dirt_gain_stages_reconfig_schem2.jpg

Hopefully you can see where I'm going with this if you compare it to the Wampler Plexi Drive schematic I showed in the last post.

So by swapping the transistors for J201s, making some track cuts, adding some link wire, changing one transistor and one capacitor for resistors (not a mod I would normally recommend :lol: ) I can change the Bandit dirt channel into something very similar to a Plexi Drive which I hope will sound a whole lot better.

I've still got to sort out component values, and I've not yet used that four pole switch S2 to best advantage (so far it is only going to switch in an extra cap in one position).
While we're talking about switch S2, what I've done is parallel a cap in one position - this will add more bass frequencies into the distortion stage (Q5+Q6). I've got further ideas for this switch though.

For component values - here goes:
R61, R55, and R40 are just keeping the tranny inputs at ground => 1M
Let's start off with the JFET biasing resistors having the same values as in the Plexi Drive, namely R51, R45, R57 = 1k and R26, R29, and R28 = 15k

I'll just say a bit about what those resistors do. In each case the resistors convert the JFETs drain current into a voltage. In the case of the resistors in the source leg (to 0V) they provide a positive voltage on the source pin relative to the gate pin, so the gate sees it as a negative voltage. This means for a little drain current there is a small negative voltage on the gate, and as the drain current gets bigger there is a bigger negative voltage on the gate. Since these are N-channel JFETs (to get technical) more negative gate = less drain current, so there is negative feedback - which means the gain stage sort of limits its own gain (that's how most amplifiers work folks).
If I chose JFETs with the right Idss and Vgs off then those 1k source resistors will work just fine.

For the 15k drain resistors (which go to Vdd) these give a voltage at the gain stage output again which is proportional to the drain current. In the original Plexi Drive the supply was 9v so they were probably chosen to set the voltage at the JFET drains, with no input signal, to be around 4.5v (which is what you'd probably want). In my case that 4.5v is the voltage below my Vdd which means I'll sit at +28V (the Bandit's supply for the preamp) minus 4.5v = 23.5v. That means when my signal swings up it will hit the rail pretty dang quick. When it swings down though it has plenty of volts before it goes so low the JFET turns off. All of that means really asymmetrical clipping which ain't no bad thing. I'm not intending the Q8 stage to be clipping either.

One further complication. I put 2x JFETs in parallel for Q5, Q6. If they're matched, that will double the drain current of one, however there's plenty voltage headroom down the way (towards 0v) so I'm going to leave R29 at 15k for now. I'm going to breadboard all this anyway before committing the changes to the PCB.

Another thing - these drain resistors affect the tone. That's because each resistor forms an RC filter with the gain stage output cap. So fiddling with the bias resistance will also affects the EQ => affects tone.
By the way (I'm really digressing now :whappen: ) that's why you shouldn't just stick any old JFET in a distortion pedal circuit and then tweak the drain resistor for the drain voltage you want. It works Ok, but as you change the resistor value you change the EQ of that stage and you'll get different tone - maybe not what you really wanted if you;re tryng to do a pedal copy'
Digression over :slap:

For all the caps. I'll start with the values in the Plexi Drive and tweak for tone. So that means:
C27 = 22nF, C25 = 1uF, C30 = 2n2
I'll leave the stuff around Q11 as it is on the Bandit board since its just a unity gain (that means gain of 1) follower to give a high impedance into the next bit of circuit.

C19, C24, and C66 are really going to affect tone so I'll just play with them on the breadboard, starting with C19 = 470pF, C24 = 2n2, and C66 = 180pF (the Bandit original value).
C19 and C24 as said earlier will affect how much bass feeds the clipping stage, while C66 is the treble bleed cap around the gain pot so you get more treble when the gain is down low.

I need to re-draw this think again at some point as the schematic is getting really messy now!!
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby Cub » 06 Apr 2018, 12:33

Impressive! :applause: Do you think the Plexi Drive will need a lot of tweaking?
After all, it won't be going in front of an amp but will be the actual front of the amp.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 06 Apr 2018, 17:00

Hi cub. I won’t know till I try it!

But what I’m aiming for here is to leave some room for various tones, so I don’t want to use too small coupling cap values which end up killing all the bass (once it’s gone you can’t get it back) or using too big values which swamp the clipping stage with bass (which will create mud in the lower frequencies).
Particular factors here are: my gain pot which is different resistance value from the one in the original Plexi Drive pedal (so the RC cutoff will be different), the Bandit dirt channel EQ stage which comes after these gain stages, and the speaker which comes after everything in the amp! (I plan to swap the speaker from the Peavey stock).

I’m not expecting loads of tweaking like you’d have with a pedal trying to emulate a very specific tone, but I’ll probably have to adjust the caps (and maybe some resistors) to allow for the factors above which are specific to my model of Bandit.

I’ll start by just breadboarding these 3 gain stages and using my little Blackstar Fly with headphones just to check I’m in the right ballpark. Then I’ll modify the Bandit PCB and try with the power amp and real cab and take it from there.
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