ReVibe with no "Re" -- stand-alone Harmonic Vibrato build (with bias trem mode)

[valcotone]

Hi all,

I'm building a stand-alone harmonic vibrato (with no reverb) and thought I'd share some details. I haven't seen any projects like this but if there I'd love to hear about them.

For years I've been chasing the luscious Harmonic Vibrato in pedal form and found a couple excellent examples that I've enjoyed using. But, something was missing (or at least I thought there was room for improvement). Eventually I picked up a reverb + harmonic trem unit which sounds really good. Then, inspired by the many Revibe discussions here and elsewhere (and since I love my old Fender reverb tank) I embarked on a stand-alone harmonic vibrato build.

This build is based largely on Hoffman's layout but with many ideas from Sluckey's project (thank you!!). Since there is no reverb circuitry, I was able to fit everything into a regular Reverb unit chassis and adjusted the power supply voltages with larger dropping resistors. I just cut off the reverb part from the Hoffman turret board and mounted the filter caps at strategic points in the chassis. Along the way I encountered a design by Mrriggs on TDPRI with a switch to toggle between harmonic and bias trem modes so I incorporated that as well (as a push-pull DPST switch on the Level pot).

It worked at first power-up but had way too much gain, so I used a 12AY7 as the pre-amp tube, and made a couple of other tweaks. I have to thank all the contributors to all the Revibe discussions here... so much helpful information is a bounty when working on a project like this.

Here's the current build so far (I hope the original designers don't mind me copying/editing their schematics):







(The control panel is just a temporary paper printout for now.)

Overall, I'm still dialing in the sounds. It's remarkably quite... like VERY quiet with very little noise or hum, which I like a lot.

I welcome any comments or feedback on the design so far.

My current challenge:

So... one thing I'm interesting in improving is the overall dry guitar tone when tremolo is disabled. It's a little sharp in the high-mids and slightly mid-scooped, and not as natural sounding as I'd like in the mid-range. I realize that the overall dry guitar sound will be influenced by the LO/HI pass filters in the modulator when the vibrato is disabled, so I want to see if I can dial that in.

You'll notice I used the LO-PASS and HI-PASS filter design from the modulator in the 6G12-A 2.5-tube design (using the Concert schematic as a reference), and married that to the 2-tube design for the oscillator and phase-inverter... but I think that was a mistake and I didn't realize that the 250pF cap needs a 1MEG to ground to complete the HI-PASS filter... so that might be messing with the frequency response? Maybe the LO-pass side also needs a 1MEG to ground to operate normally? My next step will be to install a 1MEG resistor from each of the grids of V4 to ground.

Alternately, I could rebuild the LO/HI pass filters as per the Revibe design, but then I'd want to find another way to switch between harmonic/bias modes (I like that feature).

Any thoughts?

Thanks all!

[sluckey]

Looks good. Nice project.

[valcotone]

Thanks sluckey!

Any thoughts on the Lo/Hi pass filter design I used (and how I merged it with the rest of the design), and whether the regular Revibe design would sound much different?

[BadCatCubII]

This is awesome, Valcotone, nice work! 

Did you ever think about applying the unused half of the 12AY7 to include the cathode follower of the 5 triode harmonic tremolo? I ask that question not knowing at all if a 12AY7 would be appropriate for that.

I am really interested in the bias/harmonic tremolo switching, I had never seen that, and was wondering if that would work on a project I'm working on.

Does the flashing red LED get annoying after a while?

Forgive me if this is just a noob perspective (I am a noob) but I would think the only/best way to switch the trem on and off with out the tone loss would be a true bypass switch in the signal path, and you would have to use a relay if you wanted to do that with a foot switch, I think.

[valcotone]

hi BadCatCubII,

To be honest I never thought of building the 5-triode design. I was committed to the 4-triode version because I wanted to use Hoffman's layout and board as a starting point. I've considered wiring the unused triode in parallel with the input stage (with adjusted plate/cathode resistor values), which I understand might offer lower noise. That's not a priority at the moment though - maybe later.

I LOVE the flashing LED! I think this was a great idea from sluckey's build, as is the fast/slow switch. Both work perfectly.

Regarding best-method to bypass the trem, I agree a relay to completely bypass the trem circuit would be the way to go if you wanted 100% dry guitar signal. I have a Road Rage loop-switcher I use for that approach with this unit and the rest of my pedals/effects. But, the tone is enhanced when it's engaged which I really like and I want to optimize. Also, there is a shift in the EQ when the unit is in the loop so I'd like to maintain a consistent tone with/without trem.

I have made some progress in the trem-disabled sound and it's sounding better to me. I lowered the supply voltage to the output modulator and tweaked some other component values... for example, R37 is now 10M to attenuate the oscillator output into the lower-headroom modulator. This circuit is sure a balancing act. Now I need to dig deeper to bring more phasey-ness into the trem sound.

What project are you working on that the bias/harmonic switch might work for?

[valcotone]

Here's a SHORT DEMO CLIP into an overdriving tweed style amp.

Harmonic mode up to about 1:23 then bias mode after that.


Here's the latest revision 5... comments/feedback are most welcome.

[BadCatCubII]

The tone and the playing sound absolutely fantastic! Nicely done! Did you build the tweed style amp too?

My project is a 6V6 power amp with a pentode preamp and harmonic trem. I started with transformers from a Princeton Recording amp (a lemon that was given to me by its frustrated owner) and it is my first foray away from the beaten path of copying someone else's schematics. :) There is probably a 50/50 shot that it ends up just being a box of garbage, haha!

[valcotone]

Thanks. The amp in the clip is an original 1959 Gibson GA-40... which features a 5879 pentode pre-amp.

Your project sounds like fun too. How far along are you?

[BadCatCubII]

Sweet! The GA-40 is a sweet sounding amp, and one reason why I'm building my preamp with 5879s!

I'm building everything from scratch including the cab (it's going to be a head) and the chassis. I just finished the faceplate this weekend, and I'm getting ready to do the wiring, but now you have me thinking about cramming a couple more switches in. :)

Since I'm planning to use the 5 triode schematic, I'm thinking through how best to apply your mods to my circuit....

[BadCatCubII]

Here is the schematic I plan to use for my tremolo. S8, I'm hoping, will switch between a harmonic trem. that has the oscillation of the lower frequencies 180º out of phase with the oscillation of the higher frequencies, and a standard trem. that has all frequencies oscillating together. Do you think this will work? am I missing anything? Thanks!

[Hywel73]

Hey BadCatCubII,
Did this work?

[BadCatCubII]

Hey BadCatCubII,
Did this work?

Hi Hywel73, Yep it works! This is a 4 year old thread, and I can't remember if I got it working using my schematic, or  using a DPDT to bypass the phase inversion like Valcotone does, but it definitely works.