The Making of a Valve Overdrive Pedal

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I have seen a few people ask questions about electronics usually when they are looking to fix something or build a kit etc.. Thought it might be interesting to post the ongoings of my new pedal build?

I designed it and it may work great or could be a total Turdscreamer. We shall see.

First off the circuit. In a nut shell my idea is nothing groundbreaking just bags of gain and a passive tone control that can change the slope from bassy to trebly by just a few decibels. Dont see the point of complicated tone controls with huge adjustments as they tend to make bass flubby and can really suck the life out of the sound. This is probably the hifi person in me speaking here. The valves are 6/30L2; normally found in audio stages of old TVs. I've not designed the circuit to clip but the valves are not particularly linear. Hopefully a clean snappy sound with some nice euphoric sounding harmonics. 

The rough circuit layouts:





I have used the chassis of the rather terrible Behringer VT999 Vintage Tube Monstrosity; a clone of an Ibanez Tube King. It was £30 and has no resale value.



Here is the powersupply. I bought some inverter circuits from Amazon that boost 12VDC to 220V square wave pulse. A quick hook up just to see if they work OK. I was hoping for around 175VDC and getting around 169VDC under a simulated load, which is close enough. It's all a bit of estimation when designing valve circuits anyway.



This may seem a bit anal but I kitted the parts that I need into a compartment box, but this is really helpful for keeping the work area tidy.



More to follow when I get round to doing some more.
Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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Comments

  • paulnb57paulnb57 Frets: 3044
    Interesting project...
    Stranger from another planet welcome to our hole - Just strap on your guitar and we'll play some rock 'n' roll

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  • ICBMICBM Frets: 71955
    About 25 years ago I built a valve overdrive pedal by the very simple method of copying the schematic of a Marshall 2203 preamp as far as the master volume control. I think I used the PT from an old valve radio to get the right voltages.

    It was great, I wish I'd kept it now.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Adam_MDAdam_MD Frets: 3420
    edited May 2017
    Nice I love threads like this. 
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  • Great idea, looking forward to results! 
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  • equalsqlequalsql Frets: 6084
    I am already loving this thread!
    (pronounced: equal-sequel)   "I suffered for my art.. now it's your turn"
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  • rsvmarkrsvmark Frets: 1373
    equalsql said:
    I am already loving this thread!
    Me too. @paul_l more please, as soon as you can! ;-)
    An official Foo liked guitarist since 2024
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  • SporkySporky Frets: 27573
    This is absolutely something I don't want to do, but very much want to see someone else do. :)
    "[Sporky] brings a certain vibe and dignity to the forum."
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  • MtBMtB Frets: 921
    edited May 2017
    Interesting thread.....

    Righto, I'm off to buy up all the cheapo Behringer VT999 Vintage Tube Monster pedals as I feel there's going to be an ebay run on them soon!

    At least that's what normally happens on TFB when someone does a really interesting make and modify thread. ;)
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  • RiftAmpsRiftAmps Frets: 3113
    edited May 2017 tFB Trader
    Great idea, but a little word of warning...6/30L2 (ECC804) do not like to be run hard or they go microphonic quite quickly. I've got a box full of them which are now useless.

    Pa MAX for each side is 2W BUT the total of both halves combined musn't exceed 2.5W, so aim for 1W each side to be safe. I tend to think of them as a fragile ECC82 without a CT heater - should work great in a pedal though.
    *I no longer offer replacement speaker baffles*
    Rift Amplification
    Handwired Guitar Amplifiers
    Brackley, Northamptonshire
    www.riftamps.co.uk

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  • Paul_LPaul_L Frets: 52
    Thanks @RiftAmps . At idle each side should be running at around 4mA and will have around 75V across them, so should be OK. The plan is to run them at a fairly non-linear bias.



    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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  • Danny1969Danny1969 Frets: 10356
    Paul, what the deal with the inverters, wouldn't it be easier to just put mains volts into the enclosure and rectify ?

    I understand the implications of mains powered products that are going to be commercially sold and the rules \ regs etc but as a one off for your own use that wouldn't apply

    Anyway very interesting project 
    www.2020studios.co.uk 
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  • Paul_LPaul_L Frets: 52
    I am not really comfortable with running un-isolated mains into the pedal. It might be fine to start with and I am sure I could run my tongue all over it after power on. Thing is stuff does go wrong and in a few years it might no be so safe and if someone was to touch it, say someone else's child for instance I would be devastated.

    Some more wiring




    I've actually finished all the wiring and fixed the errors - the biggest being that I numbered the valve pins in reverse, so 1 was 9 2 was 8 and so on - I felt a right turd when I realised what I had done there..

    Anyway it lives!!

    And how does it sound - terrible!!

    I mean really bad....

    The inverter was a bit of a unknown and I think I put it under too much of a dummy load when I tested it. I got 169VDC but now I am getting around 200 volts. However a quick check of the load line with these new voltages and the valves should still operate in a non-linear region. I think this is the reason for the terrible distortion sound, as opposed to a fantastic distortion sound.  Plenty of gain though!

    I think I will change the operating points of the valves to push them in a more linear region. 

    Did I mention how bad it sounds?
    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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  • ICBMICBM Frets: 71955
    Paul_L said:
    I am not really comfortable with running un-isolated mains into the pedal. It might be fine to start with and I am sure I could run my tongue all over it after power on. Thing is stuff does go wrong and in a few years it might no be so safe and if someone was to touch it, say someone else's child for instance I would be devastated.
    I would suggest that a properly earthed mains connection is actually safer than an unearthed unit with a high-voltage inverter inside it, since you will be earthing it via the audio cable to the amp it's plugged into - and hence the internal high voltage is then a hazard if the ground in the audio connection fails but not the signal connection, which then becomes a virtual earth with the casing of your pedal at a high (negative) potential relative to it.

    If your pedal has a proper earthed mains connection to the casing that can't happen. The only problem is that you may need to isolate the output to avoid a ground loop, but you can do that with a small transformer.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Paul_LPaul_L Frets: 52
    Moving forwards I have taken some more accurate measurements and all is strange. I think my datasheet marked as "tentative" may just be that. Plotting the voltages of the gain stages onto the datasheet's curve it reads that the valve should idle at 9mA, but I measure 5mA. Plotting that in the curve shows that the grid should be at -4V, but it measures at -3V.

    The buffer stages are completely screwy. Did anyone spot the error on the circuit? The lower resistors are 680R & 17K, They should be lower.

    I haven't designed a valve circuit for quite some time, so I know I would make a mistake, but I think I must have been drunk when I done this one!!
    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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  • Paul_LPaul_L Frets: 52
    IT'S ALIVE!!

    Turned out I had neglected to make the earth connection on the second valve. Changed some resistors and it works. Operating points are not particularly close to what my datasheet says, but I suspect the datasheet.

    It's quiet - as low noise, bags of gain and very transparent. So much for euphoric sound. It's like not having there except for the gain and a nice punchiness when the gain is up full. very dynamic too and it does open up the volume and tone controls of the guitar.

    The tone control circuit is a little too subtle though, so I will change it probably to something a little more simple that just rolls off the treble. maybe filter off the higher frequencies above 2 Khz.

    Overall very nice and worth investing some time to make it look nice. Might spray paint it but I find that spray paint tends to be quite fragile these days. Might cover it in a vintage looking cloth and lacquer it.




    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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  • Looks amazing! 

    We need a demo clip :) 
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  • Paul_LPaul_L Frets: 52
    I took the pedal to the friendly guitar shop yesterday and asked the guy there if he would give it a try. I made it clear that this is just a hobby and I am not looking to start selling them. He was only to happy. Went down rather well and the comment that the pedal was something that he would gig with made me well chuffed. He also thought it was a good use of a Behringer pedal once he realised that I had ripped the guts and just used the case and fittings only.

    After a bit more work it now looks like this:








    Just a few finishing touches left like cutting the pot shafts down so the knobs sit lower and adding some guards for the valves.
    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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  • Adam_MDAdam_MD Frets: 3420
    Absolutely love it.  It looks fab dude good job.  The only thing I don't like is the white plastic 3pdt washer.
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  • GagarynGagaryn Frets: 1553
    That looks great. Will be even better after the final fettle.
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  • Paul_LPaul_L Frets: 52
    Adam_MD said:
    Absolutely love it.  It looks fab dude good job.  The only thing I don't like is the white plastic 3pdt washer.
    I know, right.  It has to be nylon or something similar as it has to load up on the nut so that it stays tight with all the pressure of being stomped on.  Would be nice to have a colour option but not found anything yet.  Metal would look great but the nut would keep working loose. 
    Maintained it for 20 years. This old guitar's had 17 new necks and 14 new bodies in its time.
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