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Tube Driven Overdrive / Preamp Pedal

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 7, 2018.

  1. kleydejong

    kleydejong Tele-Holic

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    I am currently quite intrigued by tube driven overdrive pedals. Specifically the Kingsley lineup like the Jester or Minstrel. I’m curious to see if we couldn’t pool some collective tube amp knowledge and translate that into a pretty cool overdrive pedal / preamp. Some of my design curiosities / goals:

    • Run a 12AX7 at proper high voltage.

    • Keep things relatively simple to start.

    • Trim bass early, probably with switching on the V1A cathode bypass cap to limit bass.

    • Trim treble late with a Tweed style tone control.

    • Have the ability to run this as a preamp and as an overdrive pedal into a typical tube amp.

    • Incorporate a solid state boost before the tube to saturate it more heavily for more distortion.

    • Have a power supply setup that doesn’t suck.

    Let’s address some of these one at a time.


    High Voltage

    In my research on tube driven pedals in general, I think there is a mixed bag. One main point of deviation seems to be whether or not the tube is running at what we in amp land would consider normal preamp voltages (mayb 250-ish vdc) or whether they’re starved. My assumption is that running a tube at starved voltages kind of defeats the purpose. However running a tube at higher voltages in a pedal format creates its own set of challenges. Mainly things like transformers, power supplies, and size. Take a look at the guts of a Kingsley Jester, for example:

    [​IMG]
    http:// https://medias.audiofanzine.com/images/normal/kingsley-jester-1152669.jpg

    I see what may be some kind of induction coil better in this picture:

    [​IMG]


    I don’t see any transformers. My assumption is that you’d need something that can take some sort of reasonable DC voltage on the input and convert it to both a HT 250vdc output and a 6.3vac filament winding.

    Here is a more recent picture of a Kinglsey Jester.

    [​IMG]



    I see some kind of PCB circuit on the bottom.

    Here is a picture of the Harlot’s guts - including another shot of what I believe is the power supply on the PCB at the bottom.

    [​IMG]


    I’m thinking a 12v input with a 20x winding ratio to produce about 240v on the output. Many tube pedals I see accept a 12v input with a pretty high current.

    Any ideas on how to power this kind of device?
     
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  2. printer2

    printer2 Poster Extraordinaire

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    The coil you are seeing in the first picture is the transformer. A toroidal transformer is in the shape of a doughnut. The two other pictures seem to use a switching power supply to supply the voltage. Looking at the connectors of the pedal it seems to need a 12V dc supply so the board does step up the voltage. I wanted to make a little pedal/amp with tubes myself. I want to use a 12V laptop power supply and stepping up the voltage with a board you can buy online.

    This looks interesting.

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=201472584304

    I have one similar to this one. Much higher current and bigger footprint. The smaller one was not around when I bought mine.

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=112265113274

    Once you have the HV basically select the preamp you want to use and figure out how to cram stuff in without having it feedback on itself.

    I really like the standoff method of holding the tube sockets. Think I will borrow the idea.
     
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  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The PCB in the pictures is some type of switch-mode power supply (SMPS).
     
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  4. Badside

    Badside Tele-Holic

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    See this from Hoffman: http://el34world.com/projects/images/TubePedalSchematic.gif

    His is a full blown JCM800 preamp, but one can take the general idea and just use one tube (and thus two stages). A JFET or opamp input stage could indeed be used as a signal booster to get more overdrive out of it, or even a full blown TS type circuit before the first tube stage if you want.

    Hoffman's power supply is elegant and simple: instead of feeding DC to the pedal, you feed it AC! Namely, 12VAC. From there, you can generated 12VDC within the pedal (rectify, capacitate then regulate) and feed that to the filaments (a 12AX7 with series filament requires 150mA at 12V) for super quiet operation, and use said 12VDC to power any SS components you might want to integrate. Then you use a 240V to 12V power transformer in reverse to get 240VAC which can be rectified to 336VDC. Feed that to a capacitor then to a resistor of whatever value gets you the plate voltage you want, and another capacitor of course. Total current draw for the pedal should be around 250-300mA at 12VAC, which can be obtained from an off the shelf wall wart (with some extra capacity so it runs cool).

    The great thing about this is that the transformer for the plates doesn't have to feed the filaments so it can be much smaller and fit inside the enclosure.

    This is the simple way (even simpler would be feeding 12VAC to the filament), but it can get noisy in such a small enclosure. Care will be required to keep the PSU away from the signal amplification.

    Or you can use a DC-DC converter. I have no expertise on those,

    On the first pictures you posted, there is clearly a toroidal power transformer, so they're probably doing the reverse 12VAC to 240VAC trick (it's not an induction coil, you can see two pairs of wires).

    The newer one does not seem to have such a transformer. I don't know if the chip on the board is a DC-DC converter, maybe. The big transistory thing with a heatsink is a voltage regulator but it regulates to 6.3VDC so that's filament (not sure why they didn't go 12.6VDC in series, that draws half the current). One can assume the other regulator is for the plates, which draw a lot less current which must be why it's not heat sinked.

    Not sure what the WE 221 can is, but WE is Wurth Electronics, although I can't cross-reference "221". This COULD be a transformer because DC DC converters work at a very high frequency so smaller transformers work (inductance does not need to be as high), which is also why cheap one will emit a high pitched noise. I'm guessing this little guy + the chip + the regulator are your plate supply. Not sure how DIY friendly such a supply is, they can be pretty noisy. The brute force approach used by Hoffman is more foolproof, and by using a toroidal power transformer like they did you reduce magnetic interference.
     
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  5. Badside

    Badside Tele-Holic

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    My fear with one of those would be getting some of that SMPS squeal in my audio. I've had phone chargers that I couldn't use while sleeping because of the whine. OP's picture appears to show a small shielded transformer.
     
  6. FenderLover

    FenderLover Poster Extraordinaire

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    You can find the switch mode power supplies for under $10 on eBay (search for High Voltage Boost Converter). 12V in and 250V out is easy and cheap because it only has to supply a few mA of current, while the 12V also runs the heater. They tend to be noisy, but if they operate above the audio range (switching frequency) they are easy to filter.
     
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  7. Wrong way

    Wrong way TDPRI Member

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  8. printer2

    printer2 Poster Extraordinaire

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    The Valvecaster is a joke, using a Fet pedal sounds much better. The magic of the VC is that it is a 'tube', not that it sounds any good. The reason the 12AX7 does not sound good in the VC is with the gain it has it is basically clipping with no dynamic range to speak of.

    Of the two switch mode PS's I linked to the one without a heatsink does not seem to have much of a filter capacitor. The one with a heatsink is bigger which is a downside but it also puts out enough current to feed a couple of 6V6's, the reason I got it. Well one, I have two and I was going to use one for a tube peamp, PWM power section. I was also going to throw in reverb, it was going to be fairly involved, part of the reason I haven't started it yet. Maybe I should do a little two tube preamp just as a teaser first. And I thought I was going to work on my acoustic guitar today.

    Oh yeah, the backward transformer thing does work but it is hard to find a small transformer to fit in a small package. You can use a 120V transformer and use a voltage doubler to get the HV.
     
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  9. kleydejong

    kleydejong Tele-Holic

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    Love this! I think this might be the direction I go. Doug has an AC wall wart and the toroidal transformer for sale in his store - http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Transformers&ORDER_ID=615579741. And his project page has a lot of detail on how he wired up his pedal - http://el34world.com/projects/tube_box_3.htm. I've had a great experience so far both buying from the Hoffman store, and using his projects for guidance. Copying his power supply layout seems pretty optimal.
     
  10. kleydejong

    kleydejong Tele-Holic

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    Overdrive vs. Preamp

    Next question - what is the difference between an overdrive 'pedal' and a preamp, in terms of output? Kinsgley advertises the Jouster or Minstrel as an overdrive pedal. Meant to go in front of an amp. Just like a Tubescreamer or Bluesbreaker would. Conversely the Maiden or Constable are basically the preamp of a Fender or Marshall style amp lifted and placed into a pedal enclosure.

    I'm thinking about level. A preamp is presumably driving a higher level. But are there other variables in play? Impedance? Or can one just add a master volume at the end and dump a lot of that signal to ground so it can run into another amp? Or are the differences more based on circuit design?

    I get the 'preamp' thing. I can grab whatever preamp from my favorite amp schematic and drive it using the power supply indicated above. Perhaps copy a simple solid state buffer and throw that in front to drive V1 into distortion. The output of which is optimal to run into a power amp.

    But I am not sure how one would optimize it to run into another preamp / power amp.
     
  11. printer2

    printer2 Poster Extraordinaire

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    Just for giggles.

    [​IMG]
    Has a grid biased circuit for the input and a three way switch to change it to normal Fender operation with a 1.5k cathode resistor and 100k plate. It has a resistor in parallel on the filter network when in cathode bias to get the voltage up. Want to drop it for grid bias operation to get that old time feel. I didn't bother to calculate what I needed for the resistors, will select them by ear. Tweed tone control on the volume.

    Next looks to be a mess, it has a four pole switch to change it from a gain stage with cathode follower to two gain stages, the third triode is cold biased, oposite of what the cathode follower brings to the table. The tone stack has a two pole switch with an open middle position. In the middle position it approaches a BF curve if you drop the treble down a notch. Otherwise it is a Marshall 33k or 68k curves. After that just the remaining triode, not sure what it has to bring to the table yet.
     
  12. printer2

    printer2 Poster Extraordinaire

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    And if you thought that had enough twists and turns. Will have to think about elevating the heater circuit to keep the heater to cathode voltage within specs of the tube, 200V for the 12AX7 and 100V for the 12AY7. Taken from the Valve Wizard site.

    [​IMG]
     
  13. kleydejong

    kleydejong Tele-Holic

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    @printer2 - Those schematics look awesome! I'm thinking I may try something simpler to start. I know I could tackle that circuit in an amp. But I'm thinking a single 12AX7 with perhaps a simple boost in front to goose the gain. Probably Blackface inspired as that seems to be where my tonal preference had landed.
     
  14. awasson

    awasson Poster Extraordinaire Silver Supporter

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    I posted this to the forum earlier today but it probably belongs in here...

    Nutube 6P1 Double Triode Thermionic Valve


    https://uk.rs-online.com/web/p/double-triode-valves/1449016/

    This thing is an obvious fit for an overdrive pedal. Less that 1 volt for the heaters and somewhere around 70 or 80 for the Plates.
     
  15. printer2

    printer2 Poster Extraordinaire

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    You know, seeing the pedal up there got my interest up and it sort of took over my day. After posting those two I thought, 'Oh I hope the OP doesn't get annoyed at me for making a three foot castle in his sandbox.' I used to draw up designs and post them here to have people think out of the box or use a piece here or there in their design. I don't in any way expected anyone else to try this design out. I already have a change in mind that I need to try to see if it will work, something I haven't seen done yet.

    That is what happens when I hurt my back yesterday getting rid of boxes of books. One was a little too heavy. Kind of sore today so doing some woodworking wasn't in the cards, might as well sit in front of the computer. I did start wanting to do the one triode and cathode follower with a fet in front. How I got the second tube in there I don't know.
     
  16. kleydejong

    kleydejong Tele-Holic

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    I saw a few demos for the Ibanez Tubescreamer using one of those. Intriguing. I'll keep my eye on that.
     
  17. kleydejong

    kleydejong Tele-Holic

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    Haha, no problem. Thanks for chiming in. I would rather be bombarded with lots of good information than not get enough. There are some really cool ideas in those preamps that I may test in a future build - whether in a full amp or in a pedal.
     
  18. DHart

    DHart Poster Extraordinaire Gold Supporter

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    Interesting thread... tagged it.
     
  19. virge0110

    virge0110 Tele-Meister

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    Sorry to resurrect an old thread. I was wondering if anyone has traced down one of his pedals and made a schematic?

    I hope me asking this doesn't rub anyone the wrong way.

    To me, his pedals are unobtainable so I'd like to see what is going on under the hood and take a jab at it!
     
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