finished Harmonic Vibe-in-a-box

[Danskman]

Hi,
I just finished this build. A standalone 6G13-A harmonic vibrato. I mixed a 6G15 preamp with the original circuit, using half an 12AX7 for the instrument input, a Mojo 779 PT and a whole bunch of components! I tried to get the same PS voltages as in my 6G13-A and has been successful with this; the effect is gorgeous!
Will try to post pics and schema this week-end, or at least ASAP  Next step: building a small cab for it.
Best regards,
Danskman

[jjasilli]

 

[joelap]

This is great news, as I have been planning a Brown Deluxe inspired amp with the harmonic vibrato instead of the bias modulated trem.  Were you able to find a source for the 4MRA and the 10MRA pots, or did you modify the circuit to accept standard value pots? 

Really looking forward to your pics... and soundclips please too!

-JL

[Danskman]

Hum... Soundclips? I need to ask a friend to play with it, as I'm really a poor guitar player, I'm basically a cool drummer!
Doug sells the pots (3M-RA and 10M-RA), the tranny and all the nuts'n bolts you need to build this box. Chassis is a Hammond 1444-22.
Pics and schematic:

http://s269.photobucket.com/albums/jj63/denferdan/Standalone%20Harmonic%20Vibrato/?albumview=slideshow

PS: it took me 4 hours to get the things done; I'm not fluent with Jschem...

Please find the attached pdf with voltage chart in Note format.
This circuit is based on a Fender Brown "Pro" amp vibrato (core Tremolo effect), a Revibe build (grounding technique, pre-amp and output stage) and on my skill and knowledge for the remaining bits 
Best regards,
Danskman

[sluckey]

I like it!

[joelap]

Looks great!  I'm guessing this is designed to be used at instrument level, hence the 330k->10k voltage divider on the output?

[Danskman]

Looks great!  I'm guessing this is designed to be used at instrument level, hence the 330k->10k voltage divider on the output?

You are right, it was designed as this, plugging a guitar in the Input and connecting the output to Input 1 (high, instrument input) of any amp.
Info: there is a light audible crunch when I strongly hit a chord; I tried to understand the cause and think that V3 could be slightly unbalanced and maybe too "gainy". I've never been able to create this kind of crunch with my sig generator (1000Hz, 100mV). It's not really unpleasant, but it's not the pure guitar sound you get when the axe is connected directly on the amp. From what I found when I 1st built this kind of tremolo, V2 and V3 voltages are critical; you can get bad sound when these tubes are not biased or powered properly. I'm not able to draw load lines and use numerous and complicated formulas to get this stage fine-tuned  maybe some day?? 
Best regards,
Danskman

[bluesbear]

It may work better through an effects loop where it isn't hitting quite as many gain stages.
Dave

[Danskman]

Yes, you are right, it could be easier to insert it in a loop.
The challenge with this circuit is to get the right amount of modulation, with the right output level & tone at the output. I tuned it adding two 0.033 caps to ground after the phase inverter, to lower the huge low frequency pulse that otherwise renders the intensity pot unusable after 6 or 7. In my 6G13-A, these are even 0.1muF... It's all tweak and tune  I must dig into it again until I get what I want.
Note that it works fine as it is today, but, it lacks the small sparkle that makes you say " I want it now right away! " 

regards,
Danksman

[joelap]

Info: there is a light audible crunch when I strongly hit a chord; I tried to understand the cause and think that V3 could be slightly unbalanced and maybe too "gainy". I've never been able to create this kind of crunch with my sig generator (1000Hz, 100mV).

What about changing the order of the 33k an 68k Plate resistors on the first triode stage to reduce the signal strength going into the vibrato effect?  Or replacing both for the same value, like 47k if the reduction is too extreme?  Then, if your output signal is too weak, try increasing the value of the 10k resistor to ground on the output voltage divider to bump up the signal level at the output. 

That might be a good place to start, since you aren't changing the effective anode impedance on the first triode but you are still reducing the signal strength heading into the vibrato circuit. 

[Danskman]

Thanks Joelap for your advices; i will try that this week and will let you know about; I misunderstood the relation between the two res in a voltage divider, I'm afraid... I should open my theory books and re-read the basics  
Best regards,
Danskman

[joelap]

Any reason why you would require the gain stage at all for that matter?  Case in point, if you can do without the gain stage on the input and inject the signal in at instrument level, you can use the now spare triode stage as a Cathode follower to lower the output impedance.  The only thing I'm thinking, and the circuit is admittedly foreign to me, is that you may need the high impedance input of the first triode.

Can anyone shed some light on this?  I attached an image of a snippet of the circuit as I am suggesting without the input stage.  Looks like after the 250pf cap, input would "see" 1Meg to ground.  Is the impedance high enough to plug straight in here?  Highlighted what I'm thinking the input would "see" in red

[sluckey]

Might help with the distortion issue...

You have 220K plate resistors on the output stages. Why? The original uses 100Ks. Try lowering those, maybe even to 56Ks. Or try a 12AU7 tube. You don't need much gain there. Heck, you're using a 33:1 voltage divider on the output.

Another small issue that may not even be an issue...

The 250pF cap that feeds V3B-7 is connected to the bottom side of that 220K input resistor. On the original it actually connects to the bottom top side of that 220K.

[Danskman]

Thanks for all your responses, pals. I will dig it as soon as I can (hard job time, this week). I will work on it before the end of the week and will let you know.
Best regards,
Daniel

[PRR]

> why you would require the gain stage at all

Signal goes to TWO grids. Each grid has 1Meg to ground and 1Meg to LFO phase-splitter.

Ignoring caps (some of which look wrong to me) I count 257K impedance.



I bet the 220K in front has a function also.

So either it has rather a lot of loss or a rather low input impedance. More suited to a plate than a pickup.

[Danskman]

This 220K is misplayed in my schema; the junction of 250pF should be done in top of this resistor.
I used the 6G13A harmonic vibrato circuit and tried to build it instead of the 1st version (used in some of the 6x series Fender amp vibrato) which is part of  the Revibe. The two 33nF caps are there to lower the "pumping" effect of this tremolo; without them, the LFO signal gives a harsh signal and I prefer it smoother.
I'll redesign some parts of my build from scratch and will let you know.
Regards,
Danskman

[Danskman]

Removed the two 33nF caps, cleaned some soldering points, added a 47nF between the input ground and chassis ground (excellent trick!) changed V2 (JJ ECC83S) for a TS RI 12AX7 and all is well. No pump, no more noise, a good range of tremolo speed and intensity, clean sound. Guitar level through the board is slightly louder than direct, but it should not be a problem. It's hard for me to A/B between the guitar and the amp, without a DPDT-equipped pedal, though; my hearing is not what it was 30 years ago. I will ask my friend to check the build. I will correct the schema and repost it soon.
Thanks for the help !
Best regards,
Danskman

[Danskman]

Hi,
I was successful trying to resolve the last problems with this build. It was definitively too muddy and strong in the bass range, plus the output level was too strong.
I made a mistake inverting the two low-pass caps in the input of mixer tube... Added a 68K on the iput preamp socket tube (control grid), used shielded wire to the output socket, changed 330K to 680K resistor in the output circuit.
Now this effect is exactly what I wanted; sound great with a large intensity and speed range, no noise, no buzz, full guitar signal, etc.
I updated my photobucket album, with the new gut shot and the updated schematic.
I join th .sch file, too.
Best reagrds,
Danskman