Vibracaster - or "How to turn your Valvecaster into a Tremolo"

[frequencycentral]

A few people have asked me about a single speed pot version of the "Bipolar Disorder", so here it is, the "Vibracaster".

The tube section is basically the Valvecaster schematic. I prefer a 100K 'Gain' pot - but maybe a tremolo doesn't need overdrive too? So, the gain pot can be ditched and replaced with a 100K resistor.

With the 'Depth' pot turned down it's just a standard Valvecaster.

If you want it even 'cleaner' you can replace the 220k resistor at 12AU7 pin 1 with 100k, and the 100k resistor at 12AU7 pin 6 with 47k.

The LFO section is a phase shift oscillator - many thanks to R.G. for helping me to understand how they work.

This is on my breadboard and working right now, I'm using a 6111 submini tube on the breadboard, but it will work identically with a 12AU7.

I wanted to breadboard a phase shift oscillator for another project I'm working on. Then it occured to me that it would be perfect for a "Vibracaster" too. I wasn't going to actually make it into a pedal, but I guess I will - I like it a lot! I'm going to build it with a submini 6111 in a tiny enclosure, I won't bother with the gain control.

Oooooh! I hope someone esle builds it too!

EDIT: Oh, and I hooked up a 'Rate' LED (via a 4K7 resistor) from the emitter of the second transistor (the one on the right). The LED kathode goes directly to earth, obvioulsly.

[kurtlives]

Neat!

I really wanna build one now.

[frequencycentral]

Neat!

I really wanna build one now.

Wow! That was quick work Chris!

[Darkness, Darkness]

I still have to finish my distortion box first (no much time, need to order some parts to try ... again....the project is going ahead slowly...) but that one is second on my list ! Please continue to be creative around low voltage tube design : they sound so good (maybe "not as much" as tubes running at real voltage but they sound damn good !) I really was wanting a tube tremolo thing, something like a Deluxe Reverb or Vox AC30... how this one compared to these others ?

[kurtlives]

Neat!

I really wanna build one now.

Wow! That was quick work Chris!

Haha it was..

Once I finish debugging and building 3 amps and 9 effects and my Valve Caster that are on my bench now I am going to build this unit!

How are you liking it so far? What's it sound like?

[frequencycentral]

I really was wanting a tube tremolo thing, something like a Deluxe Reverb or Vox AC30... how this one compared to these others ?

I think it has a cool vintage sound, but then I'm baised - I'm it's Dad!

I don't have an AC30 or Deluxe Reverb to compare it to (any contributions gratefully recieved), but theoretically is should sound similar. I'm pretty sure most vintage amps used tube phase shift oscillators to create tremolo, the Vibrocaster just has the transistor oscillator because tubes won't oscillate at low voltage. The phase shift oscillator is one of the oldest tricks in the book, invented soon after R.G. first created electricity.

[comfortably_numb]

If one were to want to insert this LFO into say, a vanilla overdrive, which has a bit different topology than the Valvecaster, how would it be done?  Would you connect the wiper of the the depth pot to a cathode resistor in the OD circuit?  Or does it have to be directly connected to the cathode to work properly?  And what difference does it make which triode you use?  I notice that your Bipolar Disorder is almost a backwards valvecaster.  Why did you design this way?

Sorry for all the questions, I just like to have my head around the theory before I undertake a build.

Thanks

[frequencycentral]

If one were to want to insert this LFO into say, a vanilla overdrive, which has a bit different topology than the Valvecaster, how would it be done?  Would you connect the wiper of the the depth pot to a cathode resistor in the OD circuit?  Or does it have to be directly connected to the cathode to work properly?

The last triode in the Vanilla Overdrive is non-inverting, so you would be sending the LFO directly to the output - that might not be good! The reason it works so well with the Valvecaster schematic is that the second triode is normally directly connected to earth, so with the depth at minimum it's just a normal Valvecaster. Also, the output of the Valvecaster is from the plate, so the LFO doesn't go directly to the output. I experimented with bootstrapping (resistor and electro cap in parallel) the cathode which recieves the LFO, but the results were a lot better without. I don't think just hooking up the LFO to either the Vanilla Overdrive or the Pepper Shredder would work as well as this circuit. You would have to breadboard it and experiment.

And what difference does it make which triode you use?  I notice that your Bipolar Disorder is almost a backwards valvecaster.  Why did you design this way?

Yeah, it's the other way round in the BD. That was because I was having problems with LFO bleed, which i combatted by using the 0.01 cap and the 100K resistor between the two triodes as a highpass filter, to get rid of the thump. The Vibracaster doesn't suffer with the same problems, probably because the LFO waveform in the BD was kinda triangle, while the LFO waveform in the Vibracaster in more a sine.

You also have to bear in mind that I'm really a complete idiot masquerading as an elecronics engineer.

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I just recorded a soundclip, but I have no idea how to post it, I've added it to my 'Builds' Myspace (in my signature bit, below), but it won't be uploaded onto the Myspace page for at least 24 hours. It was recorded directly into Wavelab through my soundcard, an Event Echo Gina 24, with no other processing. If anyone can tell me how, i will add the soundclip to this thread - PM me!

[Renegadrian]

Rick, it seems you do like tubes!!! 
I will try your circuit in a couple of weeks...Gotta wait some parts I ordered...
I will surely report it then!!! 

[kurtlives]

You could wire this circuit to be a double pedal me thinks...

Wire it like standard true bypass and you go a trem pedal.

Add a SPST switch to break the connection to the second triodes cathode and you got a trem pedal that can be switchable to a booster/overdrive pedal.

[andrew_k]

Love it. I'll be trying this one out on a breadboard in the next week for sure.

Would a 2N3904 be a suitable replacement for the BC547?

[dxm1]

Would a 2N3904 be a suitable replacement for the BC547?

I dunno - a 2N3904 would have a max Hfe around 300. The BC547 is about 800.  Maybe a 2N5089 would be a better sub...

[andrew_k]

The BC547 is about 800.

Ah, the data sheet I read said 300hFE for the 547... Maybe I was looking at the wrong column (it was one of those many-items-in-one data sheets). I'll try higher gain transistors first. Thanks 

[dxm1]

Ah, the data sheet I read said 300hFE for the 547...

The OP doesn't specify -a, -b, or -c version.  I think a -b would be close to a 2N3904...

[Jimmy-H]

you have been very busy Rick!!
Now I have to choose between two tremolo's
Great work again!

[frequencycentral]

You could wire this circuit to be a double pedal me thinks...

Wire it like standard true bypass and you go a trem pedal.

Add a SPST switch to break the connection to the second triodes cathode and you got a trem pedal that can be switchable to a booster/overdrive pedal.

Yeah, I was thinking the same as I drifted off to sleep last night. Or you could have no bypass at all - just two footswitches, one to hardwire the gain for maximum resistance or the gain pot, the other to hardwire the depth to earth or the pot.

Love it. I'll be trying this one out on a breadboard in the next week for sure.

Would a 2N3904 be a suitable replacement for the BC547?

Ah, the data sheet I read said 300hFE for the 547...

The OP doesn't specify -a, -b, or -c version.  I think a -b would be close to a 2N3904...

I used BC547C. R.G. says that the oscillator wont start up with low gain transistors. If you use other transistors you should socket them so you can change them out if the oscillator doesn't fire up.

Also, see this thread for the design of the oscillator: http://www.diystompboxes.com/smfforum/index.php?topic=70056.0

you have been very busy Rick!!
Now I have to choose between two tremolo's
Great work again!

Build this one Jimmy, the Bipolar Disorder is a lot more pain for not much more gain. The BD took ages to iron out problems with, LFO bleed and thump, the Vibracaster just worked well first time. Also, the parts count for the Vibracaster is just tiny in comparison with the BD.


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I forgot to mention - I used a 10uf electro cap across the power rails to decouple and smooth. Positive end of the cap to 12 volts, negative end to earth, preferably where the power goes into the circuit.

[Salvatore]

Very sweet design Rick, simple and efficient.
Have listened to you're sound clip on myspace, and I love it.
Bet this could be a great swell pedal to if you replace the lfo by a simple (attack, threshold) env.gen.
On my build list.

[frequencycentral]

Very sweet design Rick, simple and efficient.
Have listened to you're sound clip on myspace, and I love it.
Bet this could be a great swell pedal to if you replace the lfo by a simple (attack, threshold) env.gen.
On my build list.

Thank you. Yes, I have been thinking about using something similar as VCAs in my modular synth system. It would have to be an inverted envelope though, as the tube 'VCA' is open at 0 volts. I have plenty of envelope generators - I just haven't got round to doing that experiment yet. I've also got an idea for a tube filter pedal - I found a great schematic here: http://electro-music.com/forum/viewtopic.php?highlight=tube+vcf&t=23067 I ordered the 5672 tubes, they're here, I just have to get around to it - it's planned in my head if you know what I mean - A/R envelope with initiate momentary footswitch, envelope follower, tri/square LFO, sample and hold.

[Salvatore]

Great !!!

I have been eying Jeroen Baars his designs, he's planning to do a complete modular system, most of it not based on tubes.
He did also made a tube vca based on the same 5672 tube.
http://www.synthforum.nl/forums/showthread.php?t=90425&page=2
(Glad I can speak Dutch)

[frequencycentral]

Great !!!

I have been eying Jeroen Baars his designs, he's planning to do a complete modular system, most of it not based on tubes.
He did also made a tube vca based on the same 5672 tube.
http://www.synthforum.nl/forums/showthread.php?t=90425&page=2
(Glad I can speak Dutch)

Yeah, I'm planning to make the 5672 VCA too - you should translate the whole thing for me - the only Dutch I know is "Let Op! Drempels!"