Tube pedal project

[blues man]

I built my my tube pedal a couple of year ago and have always had radio or tv broadcast noise in it when not playing the guitar but when it is sitting idle. I was just reading some info on getting rid or rf and not having a grid stopper resistor on the input jack to the first tube could be the problem. So I put a 33k at pin 2 at the socket and totally got rid of the noise. I just wanted to say this for other peoples knowledge since it isn't drawn in on the schematic. I would also say Doug has a great web site and I have learned a ton of info here. 

[jbefumo]

This isn't really Doug's tube pedal project, but it was definitely inspired by it.  I did use that great little transformer from his catalog -- what a cool idea. I initially built it using a 6DJ8 (ECC88) for the cascode and an EF86, but then switched to the miniature tubes using two cross-wired 'socket savers', and actually like the sound better. It currently resides in an old computer power supply case, but I'm going to move it into something less unsightly. Note the NJM2396F63 6.3V voltage regulator! No more using 2 diodes to ground with a 5V regulator. 


I'm still messing with the layout, but attached is what I have so far...

[terminalgs]

do you have a schematic as well?  I kinda drew one out but I might have missed something, esp. in the tone stack area.


What is the total clean gain available?  or is there any? is the EF86 overdriven pretty much all the time? If you dime the volume control, what's the total gain look like (in terms of +dB, or Mu factor).,?

[jbefumo]

Yup -- it's in the .PDF attachment.  Let me know if you have problems opening it.


Joe

This isn't really Doug's tube pedal project, but it was definitely inspired by it.  I did use that great little transformer from his catalog -- what a cool idea. I initially built it using a 6DJ8 (ECC88) for the cascode and an EF86, but then switched to the miniature tubes using two cross-wired 'socket savers', and actually like the sound better. It currently resides in an old computer power supply case, but I'm going to move it into something less unsightly. Note the NJM2396F63 6.3V voltage regulator! No more using 2 diodes to ground with a 5V regulator. 


I'm still messing with the layout, but attached is what I have so far...

[terminalgs]

ah, found pdf.  With gain and level at 10, what's the total gain of this look like?

[jbefumo]

I'll have to check my simulation -- easy answer A LOT -- I had to keep decreasing the value of that level pot until it was even usable.  Granted, I tend to be pretty conservative in my use of overdrive, but I barely crack the level control to get a modest level boost.  The gain control, of course, determines how hard you're slamming the pentode, and that has a good range for my personal taste. I've been side-by-siding it with a Klon Centaur clone I built recently, and find that the boost range is similar, but the overall tone is more complex, as is the touch sensitivity.

[DummyLoad]

there is a DC path to the output: see attached 

might have issue with bias upset if plugged into an amp that has a direct-coupled input stage.

--pete

[jbefumo]

Thanks! My bad! It's actually not in my circuit, but when I modified the amp schematic (which was originally followed by a dc-coupled  cathodyne phase inverter), I was sloppy with my cut paste!


Joe

[jbefumo]

Here's a correction.  BTW -- if you look at the layout (which I drew after I built the board), you'll see that I have a bigass .1/600V in front of the tone control.  I'll have to give my Q/A dept a piece of my mind!

[terminalgs]

Here's a correction.  BTW -- if you look at the layout (which I drew after I built the board), you'll see that I have a bigass .1/600V in front of the tone control.  I'll have to give my Q/A dept a piece of my mind!

I know it's obvious, but when I build a pedal or a preamp,  I build the circuit and layout with a R1 (see "A" diag, below) on top of the output level control.  Once the circuit is up and running, If there are gain or tone knobs, I dime them all such that output is as big as it can be and  use a decade box as R1 to dial it in using a sine wave generator as the input and a scope on the output.  I'll shoot for +12dB or whatever, and I try to make unity  9-o'clock or 'noon'.  I write that down on the bottom, or on the schematic (or something that stays with it) so that when I pick it up in 5 years, I know what I'm dealing with, or... if some lucky soul buys it at a flea market for $1 after I'm gone, they'll know and won't blow up their Peavey Bandit (if those things can be blown up...)





Also, see diag.B,   I've toyed with this arrangement but its tricky to dial in.  P1a and P1b are ganged.  as P1a is turned up, P1b increases resistance.  the tricky part is common ganged pots are 1M/1M or 500K/500K, and relative effect on gain might need a 500K/1M to affect a smooth level output.   You can parallel a resistor on one or the other to change it's overall resistance in the circuit, but it takes some fiddling.  It seems like I saw this on an Acoustic or Kustom schematic once.. and marshall did similar things with switches and fixed resistors.


Another thing I've done on recent preamps with consecutive gain stages like in diag.B is to put a resistor under P1a between it and ground.  Dial that resistor in so that the entire gain of the first gain stage is unity.  When it's turned all the way down, all gain is 100% 2nd gain stage.  Also, the final level pot goes to zero volume, when would you ever set P1a to zero signal volume?

[jbefumo]

I should definitely invest the time in building a decade box.  I typically prototype in MultiSim, but I don't have a good sharp-cutoff pentode model for it, so that's a limitation in designs like this.


I'm pretty sure Orange uses a dual pot like you describe -- I tried it once -- not sure why I didn't use it more.  If you're using CTS pots, you can pretty much replace the element of one with a component from a single pot to get whatever combination you need.  It's a bit of a pain, but not too difficult. I think there are some old Allen Bradley dual ganged pots around with mismatched elements, but they're pretty costly.

[smackoj]

I would definitely use this 'tube pedal' in front of the venerable Peev Bandit. I have seen one blow up when the pyrotech guy got sloshed before the concert and put too many M-80s around the Blue Marvel spkr on the Bandit. Aside from the shrapnel unleashed on stage the crowd went wild. This stunt is NOT recommended for the untrained and/or sober musician. But seriously, I would label this pedal as a preamp/booster/overdrive. I prefer the solid state Klon Klone for this application myself but I would never try to talk a toobey out of making tube effects. Knowing my track record on tube builds, I would probably be chasing unwanted hiss and noise forever on this type of design myself. Here is a design I have wanted to build for a long time but I don't have the confidence I would have a 'working' circuit after weeks of work.   

The chap who designed, drew and built this is named Bancika (somewhere in the EU) and he has a website titled  diy-fever.com