As requested here’s a vero layout for the Electro-Harmonix Hot Tubes. It’s based on a schematic of the older version with a CD4049 IC, and not the newer version which incorporated real tubes.

Update (04.02.2011): Just finished building this layout and I can confirm it works 🙂

Update (25.10.2011): Some CD4049 chips doesn’t seem to work properly without an additional trace cut between pins #1 and #16. Give that a try if you’re having trouble with this build.

Update (02.11.11): Here’s a quote from Pedalbuilder that might be useful: “It is usually critical to use the CD4049UBE version of this chip — UNbuffered — not the buffered CD4049BE. May explain problems.”

Update (29.11.2011): Redid the layout. Part labels now match the original schematic. Unused pins on the inverter IC is now tied to ground and the electrolytics have been drawn the correct way. I also added a mod suggestion to tame the extreme gain at the end of the “overdrive” pot rotation after having had a closer look at my own build.

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24 thoughts on “Request: EH Hot Tubes

  1. The more I play bass through it, the more I like it. It does require some getting used to though because you have to work with the interactivity of the Tone and Drive knobs. I noticed that the best sounds are to be had when the Tone and Drive are at the same position. Maybe that’s why the Tone gets bassier when you turn it to the right, instead of the other way around, which is way more common for guitar pedals.

    For instance, both knobs at noon gives your clean bass tone some nice warmth with slight breakup, both knobs at 1-2 o’clock give some more grit that works well for bass, both knobs at three o’clock give you a great full sounding bass overdrive and both knobs all the way up give you full on raunch.

    I also tried it with guitar, but it works way better with bass.

  2. I built it with the CD4049UBE chip from TI and it works! The tone knob works the other way around, so clockwise is more bass and counterclockwise is more treble. Or is it supposed to work this way? The Drive knob really is only useful in the last 5% of the turn though, so I’m going to try the R2 = 100k mod. I couldn’t find a reverse log 2M pot. Also, I noticed that the Drive and Tone knob are a bit interactive. With the Drive all the way up, the treble increases.

    I’ve only been playing it with bass yet, but I’m not sure if I like it though, because I have only found one useful setting. That is with the Tone all the way to the bassy side and the Drive all the way UP for a really deep and raunchy bass overdrive sound. It does that sound REALLY well though…

  3. Smashing! Thanks for your help & diagnostics with this mate. Appreciate it! I’ll give your suggestions a go tomorrow night & let you know how I get on! Thanks buddy. Milkit!

  4. Had a quick look at my hot tubes build tonight. Just to get this out of the way I have no idea what this thing is supposed to sound like, but mine sounds similar to what you’re describing.
    1) To eliminate the extreme gain at the last 1/10th of the pot travel you can replace “R2” (the resistor that connects to “Overdrive 3”) on the layout with a larger value. I had success with a 100k, but you may be able to go lower.
    2) With a 2M linear pot I had very little happening at the first beginning of the pot travel. If you change to a C-taper (reverse audio) pot you’ll have more of the action happening sooner. Now, I don’t think those are readily available at 2M, so maybe a 1M is the way to go? With a 1M pot (any taper) you won’t be able to go quite as “clean”, but I think the overall usefulness of the control is worth it. Personally I think I’d go with a 100k resistor on the board and a 1M linear for the pot.

  5. Thanks Harald, I’d appreciate that buddy. I had a 1M Lin in there whilst I was waiting for my 2M Lin to arrive, it gave a similar problem, though not quite as bad as with the 2M Lin. Have tried a 1M log too, but that gave no better results than the Lin. I can’t find a 2M log pot, though my searches haven’t been too thorough. Any help would be great please bud! Milkit

  6. Hmm, maybe the schematic is wrong. Did you try with a smaller pot? Or maybe a different taper? I’ll try to find my old build and do some testing.

  7. Hi Harald, Do you or does anyone here know of any mods to try re: the gain please? My 2M pots finally arrived, but with this installed, the pot turns about 9/10ths of it’s travel with a very, very slight increase in gain & then it JUMPS to full on gain for the last 10th. All my values except for the 120nf (replaced with 150nf) cap are exactly as the layout suggests. Is this normal operation, or have I done something wrong? Any help or advice anyone can give will be very gratefully received indeed. Thanks, Milkit.

  8. Hi Glenn, No worries buddy. It sounds very similar to Craig Anderton’s “tube sound fuzz” which is possibly no great surprise considering the hex inverter. Does sound similar to a “Tubescreamer”, but with less gain I found. It’s a very good sounding stompbox I’m quite pleased with it and the overall tone, I’d definitely stick it on my pedal board & gig with it!

    If you’re thinking about building it dude, just go for it, I doubt you’d be disappointed. Milkit.

  9. Hi Glenn, Sorry to butt in, but I had to use a 1M pot too. It worked with it, but the gain didn’t “start” until around 5 o’clock & I have some strange oscillations with the gain & volume on full. I have ordered some 2M Lin pots, they’re due to arrive soon, would that sort out the sweep & oscillations I’m getting Harald? Glenn, I ordered my pots from Banzai, they’re available at Das Musikding too.

  10. Hi, Harold where do you find a 2M pot? Could I use a a 1M? what does this sound like?
    I have a spare UBE chip that i want use and this would be a good project, but i would like to know what it sounds like, Cheers Bro.

  11. Hmm… It is usually critical to use the CD4049UBE version of this chip — UNbuffered — not the buffered CD4049BE. May explain problems.

  12. Harald, if I wanted to do away with the tone switch, could i just hot wire the points accordingly? If so, how would I go about doing it? Is it just a case of linking 1+3 together & then out to the volume pot or would that not work? Thanks, Milkit.

  13. Found the offending article. It turns out my 4049 was defective, binned & replaced! As a point of interest for others, I used an ST branded 4049 & I haven’t had to make the track cut.

  14. Hi Arca, may I ask what problems you were experiencing with your build please? Was it no sound at all? Mine is intermittent. Could be dodgy wiring/soldering, but before I take it to bits, I just thought I would ask.

    Thanks, Milkit

  15. Hi, Arca. I just double checked my build and I’ve got a working circuit without the cut between pins 1 and 16. I suppose this comes down to the specific make of the CD4049. I’ll add your suggestion in case others are having the same problem.

  16. Hi Harald!
    I notice on 4049 datasheet that pin 16 is “not connected”. During my debug of your layout, I cut the trace between pin 1 and 16 of 4049, and Hot Tubes works. Maybe in your layout there’s this little error.
    Nevertheless, your blog is awesome! Thanks a lot for your DIY attitude!


  17. I’ve only played with it briefly to verify it was working. It sounded more or less like a boost up until about 2-3 o’clock where it started to become real gritty really fast. I can’t give you an opinion whether I liked it or not, I was just happy it worked, but what little I heard was interesting.

  18. Whoops, forgot to mention that C7 is actually 39pF. The schematic mistakenly had 3.3nF.

  19. Verified! Just reverse the Overdrive pot’s wiring and it’s 100%. Thanks Harald for this awesome effect!

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