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

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

Postby microbailey » 18 Apr 2018, 23:26

I finally got time out of my day job to try out the circuit idea in my last post.
I did this using my custom-built pedal prototyping test chamber (a lunch-box) which can be seen here (testing a different circuit):
Lunchbox_prototyping_chamber_f.jpg


I used a 28v PSU (as per the Bandit pre-amp board) and selected some J201s from my "Fisherman's Bait" box (you can just see the corner in the above picture).
I use this box to keep pre-measured JFETs - they're sorted into compartments depending on Idss and Vgs(off), so I can see what I've got.
Anyway I chose the following to try (component refs are from the schematic in my last post):
    Q8 - J201 Idss = 0.77mA Vgs(off) = 1.0v
    Q5 and Q6 - J201s Idss = 0.67mA, Vgs(off) = 0.95v (matched pair)
    Q10 - J201 Idss = 0.70mA Vgs(off) = 1.1v
I picked these using my "Gain Pedal JFET Selection Calculator" (see http://www.freestompboxes.org/viewtopic.php?f=6&t=28769&view=unread#p267856) and using the voltage readings which other FSB-ers had taken from the original Wampler Plexi Drive pedal (which this circuit is based on).

I figured using slightly lower gain JFETs for Q5 and Q6 would be good since I am paralleling 2 together (explained in my earlier post).
Also, I didn't add Q11 onward in this breadboard try-out, I just took the output to my practice amp from R(C26) and R40.

So this is what I found from playing around with the breadboarded circuit:
1. The pair of JFETs Q5/Q6 gave way too much gain for the classic Plexi sound I was after, this gain stage went straight into clipping with no crunch.
2. The gain with just one of Q5 or Q6 is better but needs a bit of a boost - I found a 47uF cap across R45 (to reduce negative feedback at audio frequencies) to be a good solution, but I think it needs to be switchable as I also like the low-gain crunch without it
3. The clipped sound is too fizzy and needs a touch more LP filtering after the Q5/Q6 gain stage
4. C66 = 180pF seems about right for the treble bleed cap (hard to tell until I use a full speaker cab)
5. The clipping sounds better with more bass so I changed my C19 to 4n7, and C24 stays as 470pF (not sure if this looses too much bass, again need to try with a speaker cab)
6. JFET drain voltages are around the predicted values (28v-5v ish for each)

So what I'm going to do on the Bandit pre-amp board mods is:
o Fit only Q6 with a J201 and short out base to emitter on Q5's PCB position. This is to address #1.
o Add a 47nF cap across R45 on a switch (as per #2) - I'll use a spare position of the 4 pole switch S2 which I'm re-wiring anyway. Need to think about how S2 will work :hmmm:
o Address #6 with an increase of C30 to 3n3 (from 2n2) - that gives a 3dB cutoff freq of R28+R(new) and C30 => 30k and 3n3 => 1.6kHz Looks a bit too low, I'll try it and see
o Fit 4n7 for C19 (the pre-stage2 bass boost)

Before I modify the pre-amp PCB for this circuit, I need to take a quick look at the post-dirt gain EQ (there's a bunch of Rs and Cs) and decide what I'm going to do with it.
I'll hopefully do that in my next post, and also sort out that new S2 switch wiring in the process.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 19 Apr 2018, 23:40

I looked at the post-dirt stages "Lead" channel EQ on the Bandit.
Here's what's there (apologies for my crappy handwritten schematics cos I'm too lazy to use schematic software):
Bandit_post_dirt_EQ_original.jpg

There a high-pass formed by C20 + R94 (only when S2C is in Down or Up positions).
There's a double low-pass formed by R34 + C32 (only when S2C in Middle position) and R35 + C33.
So this overall forms a sometimes band-pass (Down or Up), sometimes low-pass (Middle) filter.

The BP filter then feeds a standard Marshall style TMB tonestack with switchable slope resistor R25.

To be honset I'm just ging to take out that BP filter. I'm thinking to simplify this part of the circuit (and I don't like the Bandit stock dirt tones anyway).
So I'll remove R34, R35, R52, C20, C32, and C33 from the Bandit preamp board. I'll short out C20 to route the signal straight to the tonestack.

As for the tonetsack, here's what the Duncan calculator says it does (in blue) compared to its default Marshall tonestack (in green)
Bandit_dirt_tonestack_original.JPG

There's a bigger mid scoop around 700Hz, same band as Marshall but much deeper.
I might just leave this as is and see how it sounds when I've done the other changes.
I've got the option to increase C21 and fiddle with R33 in the future if I want.
I'm also going to remove R25 (on S2D) - again to simplify the tonestack.

Now when I breadboarded the changes to the dirt gain stages (my last post) I said I'd like a way to have a low/high gain switch on the 2nd dirt gain stage (now Q6).
That switch S2c will let me do that if I use it to select whether there's a cap across the source resistor for Q6 or not.

Scribbling those changes onto the dirt EQ schematic gives me this
Bandit_post_dirt_EQ_modified.jpg


So what about that switch S2?
Switch S2 (which if you remember is already being used to switch the bass boost cap C19 in and out of circuit) will now have the following overall settings in its 3 positions:
    o Down (S2B at pin 5, S2C at pin 12) => No bass boost, No gain boost
    o Middle (S2B at pin 6, S2C at pin 11) => No bass boost, Gain boost
    o Up (S2B at pin 8, S2C at pin 9) => Bass boost, gain boost
S2A and S2D are not being used in my circuit now.

That's a scary load of changes to the dirt channel (complete re-design :!: ) but I'm fairly happy with the way its going (I'll see what it sounds like when done tho :) )

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

Postby microbailey » 07 May 2018, 01:05

microbailey wrote:I looked at the post-dirt stages "Lead" channel EQ on the Bandit.
...
There a high-pass formed by C20 + R94 (only when S2C is in Down or Up positions).
There's a double low-pass formed by R34 + C32 (only when S2C in Middle position) and R35 + C33.
So this overall forms a sometimes band-pass (Down or Up), sometimes low-pass (Middle) filter.


Just to fix my own mess (and avoid confusing others), a high-pass filter in parallel with a low-pass filter of course forms a notch filter not a band-pass :slap:

Anyway, I''m back from the USA and ready to do these dirt channel mods now so I'll post an update soon.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 07 May 2018, 16:55

I've now done the mods to the Bandit pre-amp PCB for the dirt channel changes I've been describing.
As before I've removed a lot of components (the ones marked with a red cross in my schematics) and added a few others or changed their value.
Here are some close-up pics of the modified board:
Dirt_channel_mods_1_m.jpg

Dirt_channel_mods_2_m.jpg

As you can see I had to fly some components over the board and insulate the leads with heat shrink sleeving.

Before I try this out with the power amp board and speaker I've got a few other minor PCB mods I want to do, namely:
    1) swap out the signal path JRC4560 dual op-amps on the pre-amp and power-amp boards for lower noise ones (NE5532s) - I'll need to de-solder the old ones to do this
    2) add an LED on each channel (Clean / Lead) to show which channel is active (you can only have one on a Bandit)
    3) add some extra decoupling on the power-amp board (to reduce noise)
    4) replace the electrolytic caps in the power-amp with new ones - probably not worth it since the amp is only 15-20 years old but easy anyway
    5) replace the grotty 6.25mm jack sockets on the power-amp board (effects send / return, preamp loop, footswitch etc) - the guy before must have used this amp as an ashtray
All the above are fairly common amp upgrades (except the added LEDs).

I'm also re-drawing the schematic for the pre-amp from scratch as I've made so many changes I'm getting confused now and need a reference :!:
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 10 May 2018, 00:06

As promised here's the re-drawn schematic including all the changes I've done so far to the Clean and Lead channels on the Bandit pre-amp board.
I've used this to fix a few errors which I'd made in earlier posts (and I expect introduce a few new ones :D ).
I've tweaked 1 or 2 values along the way and shown those here too.
Bandit_preamp_full_mods_v1.JPG

I've kept all the component references the same as the PCB legend, so where it says C(R49) for example, it means I stuck a C in R49, R(C25) means I stuck and R in C25, ... - you get the idea. Btw - be careful sticking things in your Rs (British joke).
Basically I re-used the PCB to fit all the new components. As I said before I cut some PCB tracks and jumpered PCB pads as needed.

The Vintage / Modern switch is now selecting a deep scooped (blackface style) tone stack in the Vintage position or flatter tone stack in the Modern position.
The Gain switch is now selecting increasing gain and pre-stage2 bass boost in position 3.

I haven't tested all this fully yet since I want to add the LEDs, and change the op-amps and jack sockets for new ones.

Designing the channel indicator LEDs
I've ordered one bright Blue and one bright Orange 5mm LED, each with a black mounting bezel. I figured Blue would be good to show Clean and Orange would be a good colour for Dirt. The amp already had a Red led for power.
The blue LED datasheet shows a Vf of 3.5v at 30mA max, and the orange LED shows a Vf of 2.2v at 50mA max.
If I use the +15v rail to 0V for them I get: Blue 15 - 3.5 = 11.5v, and Orange 15 - 2.2 = 12.8v. I need to work out the resistor values for each.
    For Blue @ 25mA => 11.5v / 25mA => 460R (use 470R)
    For Orange @ 40mA => 12.8v / 40mA => 320R (use 330R)
When the LEDs arrive I'll do a quick test with a power supply to check the brightness since I know from experience different colours can appear very different brightness (or is that just my eyes !!!)

I intend to piggy back the LEDs off the Clean/Lead channel select switch which has some spare ways on it.
One disadvantage of this is that the LEDs wont reflect what the channel select footswitch does, but I don't really care since I don't use a footswitch anyway (and in any case its gone missing!)
This Bandit uses a DG211 analog switch IC to select between Clean/Lead channels and this is driven by a binary voltage (0v or +15v) from the front panel switch and the foot switch socket (via a triac pulse stretch circuit for switch-over noise suppression - it doesn't work very well anyway since I always seem to get a pop when I flip the switch, and I think I know why).

I'd also like to find a small in-line plug/socket pair I can use to connect the LEDs to the pre-amp board, so I can still remove the circuit board and leave the LEDs on the front panel.

Op-amp replacements
I've also ordered some NE5532 8-pin dual op-amps to replace the 4560s on the original board as these seem a bit noisy.
There's no point in replacing either op-amps which are not in the signal chain or op-amps which are part of the power amp output stage as noise in these will have little audible effect.

I'll post soon with a write up of how the LEDs and op-amp (re)fits went.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 15 May 2018, 22:47

So, experimenting with the LEDs (now they've arrived) I find that actually the Orange LED is too bright compared to the blue, so I'm increasing the series current limit resistor to 470R (same as the blue).
Also I found spare contacts on the S3 channel select switch to use for controlling the LEDs so I've wired them as below.
Bandit_channel_LEDs.jpg

Trawling eBay I also found some pre-wired plug socket JST cable pairs which are designed for Remote Control models.
Each has a red/black pair of wires and a very small red plug or socket pre-wired on the end. The other end is pre-tinned.
Maybe I'm late to this but these are brilliant!! for miniature connections inside amps and pedals :applause:
I bought a pack of 8 for less than the cost of 2 bottles of Old Peculier (I still rather have the Old Peculier though :thumbsup ).
JST Plug and Socket connectors Pre-Wired Li-Po battery leads.jpg

I used these to pre-wire the LEDs ready for installation on the front panel
LEDs harness assembly.jpg

You can see how I've atatched them to the back of the pre-amp PCB on S3 pins 4,5, and 6.
I added some (messy) hot melt glue to take the mechanical strain of the flying cables off the solder joints
LED connections to channel switch.jpg

LEDs connected.jpg

It means that in future I can disconnect the pre-amp board from the front panel for servicing / mods without having to remove the LEDs - just what I wanted.

I drilled holes in the Bandit front panel to take the LEDs - I decided in the end to put them at the end of each channel section
Drill holes for LEDs_m.jpg


I've also changed the signal chain op-amps for NE5532s.
In case you've never done this, my preferred method is to snip all 8 leads on the IC package, leaving enough sticking up to be pulled with a pair of pliers / tweezers while heat from a soldering iron is applied
Lifting_off_opamp_preamp_board_m.jpg

I then use a cheap hand desolder pump to clear the PCB holes out ready for the new op-amp.
This is the same method I used to remove the PCB mount pot which I changed.

Then I took the power amp board out of the chassis. This is really messy since there's loads of old sticky thermal grease between the big heatsink and the metal chassis
(to help conduct heat away from the power amp trannies when they're running at high wattage).

I pulled the power amp board out so I could change those grubby PCB mount mono/stereo 1/4" jack sockets for shiny new ones.
These were a bit tougher to remove than op-amps or pots so I had to use strong snips to cut the legs.
Here's the new power amp sockets fitted (for Line In/Out, Effects Loop In/Out etc)
New_sockets_on_poweramp_board_m.jpg

I took the opportunity to add some extra 0.1uF decoupling caps on the power amp supplies for the op-amps to reduce noise. These are across C43 and C45 (the +/-15v supply rails).
You can also see the (cleaned up) power transistors heatsink here, which fits to the chassis with screws and thermal grease
Extra_decoupling_poweramp_board_m.jpg

At this point I'm going to try a sound test with the new pre-amp board and power amp (and old stock speaker) to see if everything is working ... more to follow ...
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby Cub » 16 May 2018, 11:40

This thread isn't nearly getting as much attention as it deserves! I really appreciate the thorough documentation of all the steps you're taking to make this one heck of an amp and I'm always looking forward seeing updates.
Thank you very much and keep up the good work! :applause:
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 20 May 2018, 23:01

Thanks Cub :cheers It's getting close to the end now I think!

I thought I'd try some basic testing of everything together now (modified pre-amp and power amp boards).
As you can see this was just wiring up a speaker on a table, with the pre-amp and power amp boards mounted back into the amp chassis
You can see I pushed some cardboard into the chassis to cover the exposed 240v fuses - I've had 250v+ shocks off amps before - I'm not in a hurry to do that again :shock:
You can also see the new channel indicator LEDs fitted.
Final_testing_1.jpg


Powered on - no smoke or loud noises - good. Started strumming, turned up the volumes and - yes its all working! Clean and dirt channels working. (New) dirt gain switch working too.
All good.

Of course I can't tell anything about tone with a speaker on a table like this - it needs to be in a cab.
Like this there is absolutely now bass at all, just upper mids (sounds like a cheap AM radio). Still a good enough test that the circuits are functioning. And some promising distortion sounds on the gain channel.

All was going well then suddenly the sound went! Well it dropped right down in volume and sounded very faint and crackly.
Hmm - either a poorly made connection or an inadvertent short to the metal chassis somewhere - maybe around those new LEDs?

So I tried the scientific method of whacking the amp with my hand and - boom - volume came back.
I went around the pre-amp (the front PCB) tapping each part with an insulated screwdriver, listening for crackles - nothing.
Then I tapped at the power amp board (the back PCB) and suddenly the audio disappeared again. That narrowed it down to power amp board.
I tapped around and found the Power Amp in socket was causing loud crackles when tapped lightly.
Grrrr - I had to remove the power amp board again (including separating that heatsink from its messy sticky thermal grease base).

When I got the power amp board out and flipped it over - this is what I found
Forgotten_soldering.jpg

Idiot :slap: I'd forgotten to solder 2 of the new socket pins when I had changed the mono and stereo jack sockets.
So these pins were making electrical contact only sometimes. And the signal from the pre-amp passes through this socket on its way to the power amp input.
In fact (as you will see if you search the web) poor connection on these jack sockets due to dirt and corrosion is a top cause of failure of Peavey Bandit's, and one that is easy to fix (if you can solder).

Re-soldering these pins and re-assembling the power amp in the chassis confirmed the problem was fixed and audio was now fine.
While the soldering iron was out I decided that Blue LED was now too bright and so inserted a 100R 1/4W resistor in series with one of the legs to reduce the brightness. I covered this in some heatshrink sleeving. That's done the job of reducing the brightness to around the level of the Orange LED (so Blue now has series R of 570R total).

So last job (I think) is to do a cosmetic overhaul of the front panel and speaker grill, and fit a new speaker to replace the stock unmarked one (doesn't even say Blue Marvel on it).
I know a lot of people have fitted Patriot Texas Heats, and Swamp Thangs to these amps, but I want a Blackface type of clean sound more than the Celestion style sound, so I'm going for a classic Jensen 100W Tornado Stealth. I listed to many samples on the Jensen website (using their tone generator) before arriving at this choice.

Anyway I'll update on the cosmetic changes to the amp in the next post ... there might be someone reading who just wants to give their amp a makeover.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 25 May 2018, 22:59

How to change a Bandit Red-Stripe into a Silver Stripe! (not really but here's how the cosmetic makeover and speaker change went)

First I had to take my Bandit apart - I'd already taken the amp chassis out and disconnected the reverb tank at the beginning of this project.
But now I needed to remove the front grill from the amp cab as I wanted to change the grill colour from black to something else.

Its not obvious at all how the grill comes off, from the front at least.
I found 6 bolts which stick out of the back of the speaker baffle and come through holes in the amp cab and are secured with nuts.
Removing these nuts didn't seem to do anything at first but then I pushed the baffle forward quite hard (assisted by a hammer :)) and it started coming off and swinging forward.
Prising_front_grill_out_m.jpg

The baffle is stuck with some kind of sticky black goo (sealant?) along the bottom - it needed quite a bit of persuasion to come away.
You can see it off here (and lying on top of the cab). You can also see where the bottom was stuck.
Front_grill_off_m.jpg

Now I neded to remove the speaker.
It's held in place by 4 long screws which stick out of the back of the baffle board. Each has a nut you need to remove.
Speaker_screws_m.jpg

There's a video someone has put on Youtube of how to remove the speaker from a Bandit Red-Stripe identical to mine here
https://www.youtube.com/watch?v=rnMTzi6euxs (jump to 1:30 for speaker removal)

Here's my un-branded Peavey stock speaker after removal (and no I don't know why its covered in some sort of paint)
Peavey_stock_speaker_m.jpg

I then did something I wouldn't really recommend ( :o ) I tried to remove the red plastic stripe from below the grill.
I did this because I wanted to completely change the look of the amp.
This wasn't easy though - that red stripe is stuck pretty hard and didn't come away completely cleanly.
Removing _red_stripe_m.jpg

Anyway i got it off and cleaned up as best i could, although I did have to later repair a small bit of damage to the surrounding plastic using 2-part resin filler.
You can also see in the above pic that I took off the Peavey logo which is fixed to the baffle, through the grill, using 2 small black screws.

At first I was going to re-cover the grill, but its a really tough weave which I want to keep to protect the speaker, so I decided to just spray paint it with car paint (which is also much cheaper :) )
The spray painting (Silver Grey for a classic look) went really well (I think) - I gave it 3 coats, letting each one dry completely between.
I also used some enamel paint around the top and bottom of the grill (to add some feature Beige colour) and finished it off with the chrome self adhesive strip you buy to fit on car doors.
I put this chrome strip in the channel which used to hold the red stripe. So I've effectively changed it to a Chrome Stripe Bandit!! - the only one in the world (probably) :applause:
I'll take a photo and put it up next time so you can see the results.

With the speaker baffle off I fitted the new speaker.
As I said in the last post its a 12" Jensen 100W Jet Stealth Tornado which has fixing holes which match the 4 existing mounting bolts.
I wanted a Fender-y clean sound from this amp (if you remember I changed the Clean channel tonestack to Fender Blackface) and I'm hoping the Jensen will help with that.
The Jensen Stealth units are also pretty lightweight and that will help as its a heavy amp for a Solid State (that definitely helps the bass response of the cab though).
I went for 100W for 2 reasons:
    1) Bandit can push out around 80W into 8ohms
    2) 100W is the "Classic" Jensen USA sounding unit - I think I read Fender use it in the GB Twin Reverb
The great thing about Bandit's is they also have a speaker out jack so you can add another external cab for a different sound (and up to 100W max power output).

I'll put it all back together now and let you know what it looks and sounds like - project almost done!
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 30 May 2018, 23:35

I know I said it was almost done but I couldn't resist just one more small mod :D

The Bandit is a pretty loud amp and I'd like to be able to drive the Clean and Dirt channels hard yet still reduce the output (for example at home practice), while retaining the option of higher power for gigging,
I thought about adding variable wattage to the power amp, but that's a big mod which would probably involve changing the power amp PSU and likely a new and expensive transformer.
Although not giving the flexibility of variable wattage, I realised there is a simple way to reduce the signal level hitting the input to the power amp board.

The Bandit (well at least mine) has a push switch on the back which introduces a 10db cut on the Fx loop output (the Fx loop is after the pre-amp and before the power amp).
The idea I guess is that you can reduce the drive level to pedals or outboard processing to match their expected input levels.
Looking at the Bandit schematic this switch actually does 2 things:

The first part of the switch is a 10db cut (for the math-heads -10db comes from 20 x log(R12/R1+R12) ).
PreFx_GainCut_switch_m.jpg


In tandem, the second part of the switch is a 10dB boost (or cut depending if your an optimist or a pessimist) to make up the gain from the fx loop. The 10dB this time comes from 20 x log(R9/Rp) where Rp is the parallel resistance of R9 || R13. If you do the maths you'll see this op-amp has unity gain when the first half of the switch is not cutting
PostFx_GainCut_switch_m.jpg


So in summary the switch either cuts 10dB on the Fx send and boosts 10dB on the Fx return, or provides unity gain through the Fx send / return.
So my mod is to just short pins 2 and 3 on this switch so the switch always has unity gain (no boost) on the second half (around the op-amp U1A).
This turns the switch into a selectable 10dB cut switch on the signal hitting the power amp which is a handy way of turning the volume down whilst retaining the pre-amp character.
So its a kind of master volume.

What I lose is the make-up gain on the Fx return, but that's probably Ok since most pedals and outboard gear have output levels with inherent gain anyway.

Here's the mod, which is on the power amp PCB
FxLoop_gain_switch_m.jpg


I think this might be my last mod to these boards - but then you never know ...

Speaker grill is spray painted now and looks just like new silver-grey speaker cloth (and much cheaper!!)
I've changed the knobs too but more on that next time.
I'll take some pics and post.
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby Nocentelli » 01 Jun 2018, 22:22

Very much enjoying this thread and looking forward to pictures of the finished article.
mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.

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microbailey (09 Jun 2018, 00:31)
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Re: My mission to modify a Peavey Bandit 112 into a boutique

Postby microbailey » 07 Jun 2018, 22:56

Thanks Nocentelli.
Sorry for delay - I've been busy building (garden walls that is, not pedals!)

Here are some pics of the finished amp as promised
Finished_Bandit_1.jpg

Finished_Bandit_2.jpg

Finished_Bandit_3.jpg

Finished_Bandit_4.jpg

Finished_Bandit_5.jpg


I bought the new knobs from eBay, about £4.50 for a pack of 20 (pack includes a tiny screwdriver to fit them).
They look to be the same type used on many pedals including Wampler and Joyo.

One problem I had with the knobs was that the they fit the opposite way around to the old ones.
What I mean is the flat on the pot spindle is exactly opposite where the grub (retaining) screw is on the new knob.
This meant I had to take care getting the pointer in the right place so it wasn't off at the ends of the range of pot travel. Also I had to do them up really tight.
I put each pot on 12 o'clock and then fitted the new knob so the pointer was straight up, then tightened the screw.

I also moved the Peavey logo slightly higher and more central as I thought it suited the more traditional look of my made-over amp.

Does it still look like a Peavey?
It certainly doesn't sound much like the Bandit I started with :D
I'll come back with a tone report once I've played with it a bit.
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"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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Nocentelli (09 Jun 2018, 15:24)
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