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

Tube or solid-state, this section goes to eleven!
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microbailey
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Post by microbailey »

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):
Lunch box pedal prototyping chamber
Lunch box pedal prototyping chamber
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 https://www.freestompboxes.org/viewtopic ... ad#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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Post by microbailey »

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):
Original Bandit post-gain dirt channel EQ
Original Bandit post-gain dirt channel EQ
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)
Duncan calc on Bandit stock dirt channel tonestack
Duncan calc on Bandit stock dirt channel tonestack
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
Modified Bandit post-gain dirt channel EQ
Modified Bandit post-gain dirt channel EQ
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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Post by microbailey »

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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Post by microbailey »

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:
Bandit pre-amp dirt channel board mods
Bandit pre-amp dirt channel board mods
Bandit pre-amp dirt channel board mods
Bandit pre-amp dirt channel board mods
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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Post by microbailey »

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.
Pre-amp board partial schematic showing all mods to Clean and Lead channels
Pre-amp board partial schematic showing all mods to Clean and Lead channels
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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Post by microbailey »

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.
New channel indicator LEDs wiring
New channel indicator LEDs wiring
Bandit_channel_LEDs.jpg (13.42 KiB) Viewed 8920 times
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 ).
Pack of JST pre-wired connector cables bought on eBay
Pack of JST pre-wired connector cables bought on eBay
I used these to pre-wire the LEDs ready for installation on the front panel
LED assemblies
LED assemblies
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 harness connections to back of S3 on pre-amp PCB
LED harness connections to back of S3 on pre-amp PCB
LED cables fitted to pre-amp board
LED cables fitted to pre-amp board
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
Location of drill holes for channel LEDs on Bandit front panel
Location of drill holes for channel LEDs on Bandit front panel
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 old op-amps off the PCB
Lifting old op-amps off the PCB
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 1/4" jack sockets fitted to power amp PCB
New 1/4" jack sockets fitted to power amp PCB
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 caps added on underside of power amp PCB
Extra decoupling caps added on underside of power amp PCB
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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Post by Cub »

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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Post by microbailey »

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.
Testing both modified boards with a speaker and guitar
Testing both modified boards with a speaker and guitar
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 solder pins on jack socket
Forgotten solder pins on jack socket
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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Post by microbailey »

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.
Getting the speaker baffle off - you can see the bolts
Getting the speaker baffle off - you can see the bolts
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.
Baffle off showing bottom edge.  Grill cloth is stretched over the baffle
Baffle off showing bottom edge. Grill cloth is stretched over the baffle
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 fixing screws and nuts
Speaker fixing screws and nuts
There's a video someone has put on Youtube of how to remove the speaker from a Bandit Red-Stripe identical to mine here
(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 removed
Peavey stock speaker removed
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 the Red Stripe from a Red Stripe Peavey Bandit!
Removing the Red Stripe from a Red Stripe Peavey Bandit!
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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Post by microbailey »

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) ).
The pre-fx loop gain cut/boost part of the switch
The pre-fx loop gain cut/boost part of the switch
PreFx_GainCut_switch_m.jpg (21.17 KiB) Viewed 8847 times
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
The post-fx loop gain cut/boost part of the switch
The post-fx loop gain cut/boost part of the switch
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
Fx loop gain switch mod for 10db cut to power amp
Fx loop gain switch mod for 10db cut to power amp
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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Post by Nocentelli »

Very much enjoying this thread and looking forward to pictures of the finished article.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by microbailey »

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 Peavey Bandit with re-sprayed grill cloth, new LEDs, new knobs, new colour stripes etc.
Finished Peavey Bandit with re-sprayed grill cloth, new LEDs, new knobs, new colour stripes etc.
Clean channel with new blue LED
Clean channel with new blue LED
Lead (dirt) channel with new orange LED
Lead (dirt) channel with new orange LED
Front grill showing re-positioned logo, painted speaker cloth, and new cream-colour knobs from Kaish
Front grill showing re-positioned logo, painted speaker cloth, and new cream-colour knobs from Kaish
New Jensen Tornado Stealth 100 speaker
New Jensen Tornado Stealth 100 speaker
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.
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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Post by microbailey »

It took a while but I can now give a tone report having used the amp at gig volume and with some pedals last night for the first time.

The good:
I'm really impressed with the clean channel! Both settings (the original "Modern" and my new "Vintage") sound great with the Jensen, and very different to each other.
If you recall the switch changes the clean tone stack. The names "Modern" and "Vintage" are just what Peavey called the voicing switch settings.
I found I can get some really usable nice bell-like cleans with my strat on this channel now.
  • 1) On "Vintage" I get a Blackface style scooped sound which I really like - With my Strat in position 2 (bridge and middle pickups out-of-phase) it has that classic quack.
    2) On "Modern" I get a fuller sound with plenty bass which sounds great on Neck pickup.
The Jensen Tornado really shines at this kind of chiming clean tone (it's why I chose it).
Very happy indeed :D :D :D

The bad (needs improvement is more PC):
The Dirt channel you might remember was completely re-designed to mimic the Marshall Plexi and had 3 new voicings: 1) Mid-gain, 2) Hi-gain, and 3) Hi-gain with a bass boost.
The Dirt is working fine with plenty of gain in voicings 2 and 3 (I'll come back to voice 1 in a moment). The main issue is its very dark sounding, quite muddy really.

The best tone I got is in Voice 3 where I added the bass boost before the 1st clipping stage.
This setting gives some nice chunky overdriven Plexi type tones - we're in Classic Rock territory here (a la Paul Kossoff). I didn't have a guitar with 'buckers to try.
Adding my Tube Screamer Mini in front of this gives a nice soloing voice, and the TS's mid-push seems to help overcome some of the darkness.
Oh and by the way, when I turned the Bandit's sag simulator knob "T-Dynamics" up past 60% the Dirt channel really opened up and started to sing.
Switching the "Resonance" control to "Tight" helped too.

I think the tonal darkness is due to some over-enthusiastic top roll-off in the circuit either in the post-clipping RC filters or in the tone stack.
Remember this Lead channel tone stack is the original Peavey one - I didn't mod it. And TBH it sounded bad as stock.
I think I need some experiements around the Dirt channel tone stack and/or C42, C24.

I said I'd come back to Voice 1 (High-gain no bass boost).
When I breadboarded this and tested with my Blackstar Fly pocket amp I got a mild crunch. At gig volume I don't think there's quite enough gain there.
However I'd rather fix the overall EQ problem first before trying to fix this since the EQ could everything.
Now I think of it, the one element (apart, obviously, from the Jensen speaker) which I didn't have during breadboarding was the tone stack so maybe I should start looking there.

Overall I'm pleased with the amp.
Distortion (on Voice 3) sounds like a proper amp now not like a transistor radio on dead batteries. and I love those cleans.

One last thing to report is that the Fetzer 'Valve' input stage is working nicely.
If I really push it (my Fender Fat 50 pickups aren't hot enough to do this) by adding a clean boost (I used my Soul Food on lowest gain and highest output level) I get a bit of hair which sounds very tube-like.
One thing I could have improved perhaps was to design the Fetzer stage for earlier onset clipping, but as it is it gives me bags of clean headroon which is more flexible for pedals etc.

The ugly
- ha you thought there'd be an ugly but there isn't :wink:


I'll have a play with the Dirt EQ circuit and report back ... to be continued
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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Post by Cub »

With those knobs, the blue LED and

Thanks for the update, seems like you're nearly there then. Tweaking the dirt shouldn't be that hard for you compared to what you've done to it already. Both technically and cosmetically.

With that silvery cloth, those knobs and the added LEDs, it really looks boutique now!
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Post by microbailey »

It's been a while but for anybody who's still interested in this project I thought I'd report what I did to fix the tone problems I had on the modified Bandit lead channel.

In my post when I first tested the modified Bandit I'd said the Dirt channel was way too dark and muddy for a Plexi style overdrive - it sounded really muted.
I had suspected loss of upper frequencies.

Well here's what I did to diagnose the problem.
Since the Bandit has an FX send-return loop after the pre-amp and before the power amp, I stuck a cheap Graphic EQ pedal in line to see what playing with the freq bands might reveal.
Actually it revealed the tone I was after was in there somewhere.

First and main thing I found was that I'd created some kind of bass hump which was giving the darkness.
As soon as I rolled off at 63kHz on the GEQ the tone really improved.
Experimenting more, I found that lifting the 1kHz and 2.2kHz bands restored the clarity and gave the bite I was after (this was true for all 3 of my newly created Lead channel switch settings).
I couldn't get this by using the LOW MID HIGH controls on the Bandit.
Here a shot of the GEQ at optimum position.
Graphic EQ pedal in Bandit's FX send loop
Graphic EQ pedal in Bandit's FX send loop
So I set off to see what mods I could do to the Bandit pre-amp board to replicate the same tone-fixing effect of the GEQ.
Here's what I came up with ...
If you just want to know the result without all the circuit design stuff - skip to the end of this post!!!

To get a sharp bass roll-off at 62kHz would ideally need an active filter (op-amp) since RC filters only give a 6dB roll-off.
However I wasn't into adding new op-amps to the Bandit pre-amp board (even I draw the line at some mods :) )
There is a way to get steeper roll-off with RC filters and that's to cascade them so one RC feeds the next RC. Each gives 6dB cut so 2 would give 12dB.
There's a formula for working out the cut-off freq of cascaded RCs (I won't go into why its not just the usual RC calculator).
The trick is to make the R in the second RC stage be much bigger (e.g. 10x bigger) than in the first RC stage and scale the second C value to match.

So looking at my modified Bandit pre-amp schematic
Original modified pre-amp
Original modified pre-amp
I see there's a bunch of RC filtering between Q10 and Q11 - and it's after the clipping stages (right where I want it).
So I thought if I change what I called R(C26) back to a cap, short out R(added) and C42, and change C30 to be a resistor - I'll get a CR->CR circuit which will make a 12db high-pass filter.

Here's a handy website which calculates cascaded RC filter cut-off freq (and other stuff like phase):
http://sim.okawa-denshi.jp/en/CRCRtool.php

I want to keep the second R pretty high since its in parallel with the 1M res (R40) and I'll assume I make the first R one tenth of second R.
Choosing available values of first R=47k, first C=47n, and second R=470k, second C=4n7 gives a cut-off of 72Hz (or with loading effect of the 1M bias resistor about 85Hz).
So that's in the right ball-park.

Would mean I needed to change
  • C25 to 47nF
    C30 to a 47k resistor
    R(C26) to a 4n7 cap
    R58 to 470k
and wire a short across C42 and the Radded (I added this and now I'm removing it again)

What about lifting the 1kHz and 2.2kHz bands?
I played with the excellent Duncan Tonestack calculator, looking at the Marshall style tone stack and playing with value of the slope resistor.
Long story short, changing R33 to 82k and C21 to 470pF moves the effect of the Treble pot down from around 2.3kHz to nearer 1.3kHz where I want to boost.
So in other words I could redesign my Dirt tonestack to boost freqs around where my GEQ pedal was boosting them.

One last change I decided on was to add a new cap across R40 (that 1M bias resistor for Q11).
I did this because I was taking away the low-pass filter I had originally added, and was worried about high-freq harmonics giving nasty fizz from the clipping stages.
I chose to add a 2n2 cap since this, combined with the drain resistor of Q10 = 15k, creates a filter with cutoff around 5kHz.

Here's a schematic with all the values changed in red:
Updates to Bandit dirt channel - on top of existing mods
Updates to Bandit dirt channel - on top of existing mods
And here's the mods on the Bandit pre-amp board:
Latest Lead channel mods to the pre-amp board
Latest Lead channel mods to the pre-amp board
And yes it did mean I had to disassemble the Bandit once again to take out the pre-amp board (which means taking all the knobs off again :evil: )

Results:
The results are actually better than I expected.
The transformation in lead tone is nothing short of amazing - now very usable rythm and lead tones in all 3 (new) voice switch settings.
And the LOW-MID-HI pots on the Dirt channel now give a much wider range of sounds - down to the mod I did to the tonestack.

The distorion has a nice Plexi-like mid bite to it and I'd say is very usable without any pedals. Les Paul into Bandit - wow!! :D :D :D :D
I never thought a Red Stripe Bandit's Lead Channel could sound like this without pedals!

So there it is: Fender-ish cleans on the clean chan, and up-front Plexi rythm and leads on the Dirt chan with plenty of tweak-ability on the EQ knobs.
What more could I want? (well I wouldn't say no to a Kingsley amp though if anyone wants to donate one)

If I can get time I'll try to properly record some of the available tones.
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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Post by Cub »

microbailey wrote:If you just want to know the result without all the circuit design stuff - skip to the end of this post!!!
Where's the fun in that? :wink:

It's been a while and I figured you abandoned this project. I'm really glad that you didn't!
That must be one great amp now, it will run circles around the stock Bandit it once was.
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Post by L_C »

hello,

just found your post and, man, that´s an amazing job you did there. You did what I just have been dreaming to do to my bandit but, really, I wouldn´t be able to because i don´t have the kind of knowledge you do. This post definitely deserves more views and recognition.

Hope to hear some sound or video clips soon :wink:

Question1: would it be very different to build a preamp and insert it in the loop?

Quastion2: is there a good source to learn about amp and pedal electronics? meaning, learn how to be capable of making decisions when messing with an existing amp or pedal... thanks

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Post by microbailey »

Hi L_C

Sorry its taken me so long to reply to your post! You probably got your answers somewhere else now.
Question1: would it be very different to build a preamp and insert it in the loop?
Nope - in fact that would be a pretty easy way to do what I did. One of the great things about the Bandit is they did quite a good job of modelling (in analog) the way tube amps drive the speakers. So if you stick a good preamp in front of that you'll get a decent amp sound. I wouldn't bother with the loop tbh, I'd just build / buy a guitar preamp and feed the output into the effects return - skip the internal preamp altogether.
Make sure you have a volume on the output of your preamp though as there's no master volume on a Bandit after the effects loop.
Also make sure you change the stock speaker - that made a big difference for me.
Quastion2: is there a good source to learn about amp and pedal electronics? meaning, learn how to be capable of making decisions when messing with an existing amp or pedal... thanks
You can go two ways with that really. Either you learn the tech (which is my background as an EEng) or you pick up stuff as you need by reading sites like this.
For the tech I think the site http://www.electronics-tutorials.ws is a good basis but its definitely on the tech side about the theory.
If your not so theory inclined I think there's other guys on here who can probably better point you at vids and sites etc to get you started. Post in the general forum section.

I was just thinking about making a vid of the amp next time I'm in a rehearsal studio. I'm really happy with it now.
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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Post by steben »

Hi there!

Did you manage to record some tones?
I swapped the speaker with Eminence Swamp Thang and it became such a bodied amp in an instant. Speakers are gold not only in tube amps, even more in SS.
Another thing I did was adding some master volume control AFTER the T dynamics circuit. See below in red rectangle: it is a 47k / 50k pot in series with the feedback tail. At 50k and T dynamics at 10% one now has 0.65 crancked Watts
T dynamics is one of the most misunderstood topologies in the Guitar world. Some players never appreciated fully this feature, because the standard "low" setting of the amp still needs 10 Watts to "break up". I understand peavey wanted a single control with constant percieved loudness, yet a master power control would have made more players understand the concept.
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Post by loidrish »

steben wrote: 18 Jan 2020, 21:15 Hi there!

Did you manage to record some tones?
I swapped the speaker with Eminence Swamp Thang and it became such a bodied amp in an instant. Speakers are gold not only in tube amps, even more in SS.
Another thing I did was adding some master volume control AFTER the T dynamics circuit. See below in red rectangle: it is a 47k / 50k pot in series with the feedback tail. At 50k and T dynamics at 10% one now has 0.65 crancked Watts
T dynamics is one of the most misunderstood topologies in the Guitar world. Some players never appreciated fully this feature, because the standard "low" setting of the amp still needs 10 Watts to "break up". I understand peavey wanted a single control with constant percieved loudness, yet a master power control would have made more players understand the concept.
Old post but, Break Up at 25% attenuation is just too loud for home use, Did you follow instructions or did it by yourself? Because I would like to do the same to my bandit 112 (2 silver stripes), Im pretty good with the soldering iron but my knowledge on shchematics are 2/10, If you have some kind of detailed instructions would be appreciated.

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