Bias Tremolo

[Tyler]

I'd like to make a tremolo effects pedal that emulates a Fender Vibro Champ.
I've seen boost/OD pedals that simply use the preamp of an admired amplifier.  I wanted to use this...
http://www.fenderholic.com/schem/champ_vibro_aa764_schem.gif
...and delete everything after the 20nF cap just before the 6V6.  The thing that gives me pause is the 2k7 feedback resistor.  If I'm able to treat the second half of the 12AX7 as having an unbypassed cathode with a value of 47 Ohms, then the gain would be 55 as opposed to the 57 the first half achieves; so it seems to me that there isn't any over-zealous gain in the second 12AX7 half that needs to be checked.  It might be more gain than I need, but I can tinker with that once the pedal is in the physical world.  Right now I'm just trying to figure out if this would be stable without feedback (and without the 47 Ohm resistor)?
I'm sure I'm missing something, thanks for any suggestions you can give me.
Tyler

[adamasd]

The feedback is more for taming the pentode, which you are dumping anyways.

Just copy it exact as you see it and throw a 1Meg pot on the output and away you go.

Oh, and yes, dump the 47 ohm, it is not needed if you have no feedback.

[Tyler]

Thank you.  I wasn't sure what to expect with the tremolo messing with everything    I've found some good information on oscillators but not much of anything about pre-amp bias whatsis (what can you expect with a vocabulary like that?  ).  I'll post some pictures and sounds when I finish it.
Thanks again!
Tyler

[The Radium King]

hah! i just posted in another thread on this page about making a tube-driven vibrato pedal, then i find this. +1 on deleting the feedback resistor as per adamsd; it's designed to tame the power amp and not required in a pedal. you might want to consider some type of mix control to blend clean (bypass) signal with the vibrulated signal; check out a fender 6g15 reverb unit for ideas. another idea would be an effects loop on the pedal (ie, you could send the vibrulated signal to a delay or chorus pedal, then mix with the clean signal). finally, check the signal levels out of the pedal; you may wish to attenuate the vibrato signal before the mix or level control (otherwise you might get big hits in volume when you engage/disengage the pedal).

[Tyler]

Thanks for the tips, my Liege.
I was thinking that the "Intensity" control would essentially blend clean signal with the tremolo.  Were you speaking from experience with this particular amp or with guitar effects in general?  I'll try it both ways eventually, I'm sure.
One other thing I've been wondering about—could I put two LEDs in parallel but with opposite polarity after the "Intensity" pot and thus get something that will give a visual on the speed of the tremolo (I'm thinking "pre-fade" at gigs)?  I'm not sure of the voltages flying around in general.  Would the LED voltage drop harm the tremolo or would the tremolo harm the LEDs?    Do flashing lights actually help as much as I've convinced myself they do?
Tyler

P.S. Maybe I could use electroluminescent material?  I know they have fairly fast peaks and people-who-put-stickers-on-their-PCs-and-call-themselves-modders use it so it might be commonly available.  Hmmm.

[The Radium King]

hah! hey, they're just tips, worth what you pay for 'em ...

i've no experience with that particular circuit, but it is a full-blown guitar preamp, designed to have enough input to drive a power amp. if you are using it as a pedal, then it's going to be throwing a lot of signal (volts) at your amp that's expecting a guitar-level signal (millivolts). ie, it is going to work as a line-driver pedal also (and distortion pedal if you turn it way up). given that, and that you want it as a pedal, i presume that you want to be able to switch the whole thing in and out of the signal chain and not just turn the vibrato portion of the circuit on and off.

regarding the led's, try putting an led in place of the cathode resistor/cap on the cathode of the first vibrato oscillator triode. the led should maintain the required bias (the purpose of the resistor and cap in the days before led's) while flashing in time with the vibrato signal.

[Tyler]

I'll do some quick back paddling-- I hope I didn't come across as a ^%$ there.  I do like the tips and I'm grateful.  I just spent a blissful weekend reading about constant current sources.  You may have ruined me, sir.   
Thanks!

The case and the sockets/PTP stuff are together but I'm having trouble with the power supply.  I'm basically using Hoffman's Tube Pedal Project's power supply ( http://www.el34world.com/projects/images/TubePedalSchematic.gif ) only instead of a wall wart I have a transformer with two 8v secondaries.  With the secondaries unconnected I get ~10vac on both (normal for unloaded, right?) but when I connect the two in series and load it with the regulator (the second transformer isn't connected) I end up with ~6vac.  Does anyone know what might be happening? 
I can't find shorts on the power PCB... the rest of the circuit rectifies and filters, just at a lower voltage than the maths suggest... any suggestions as to where to go from here would be appreciated.
Thanks!

[PRR]

> mix control to blend clean (bypass) signal with the vibrulated signal

No.

Reverb is different. The sound coming out of the spring has ZERO original sound. It's like the tree that falls in the forest, makes no sound, yet there are these big THUDS coming back from the hills... un-natural.

Tremolo is usually made with a "straight amplifier" except some normally-fixed part (bias or a resistor) is made voltage-variable so the gain is not constant, wobbles up and down.

Then if you dial that varying voltage down to zero, you are back to a "straight amplifier" with constant gain.

AA764 w/Trem and AA764 no-Trem are the SAME plan, except w/Trem has an oscillator and a buffer. The buffered wobble is dumped via a pot into the second stage of the no-Trem amplifier.

If this pot is full-down, that 2nd stage works normal (insignificant bias change due to added 25K in cathode; also minor change in cathode cap). It IS a AA764 no-Trem.

If full-up, +/-1mA is dumped into 2nd stage. Since 2nd stage runs at 1mA normal, +/-1mA forces it to 2mA (higher gain) and to zero mA (zero gain). This is done at the tremolo rate. The sound tremolates.

So Tremolo usually does not need/want a MIX system. It just dials-back the wobble to nothing, leaving a simple amplifier stage.

The "problem" is that this type of simple not-balanced gain control has HUGE 10Hz output. Maybe 100V at tremolo rate mixed with 10V of signal. You can't hear the 10Hz directly, but the 100V will distress the next stage. AA764 uses 0.02u+220K grid-filter into 6V6 which reduces stuff below 40Hz, but you may still have 25V of subsonic slam. Apparently the cheap OT further reduces it so that the cheap speaker did not slap (too bad).

Hmmm... a 0.1u cap from that 2nd stage into a Fender Reverb transformer will shed a lot of subsonic AND reduce that whopping 20V signal down to 0.3V, hot-pickup level instead of smoking-amp level.

[The Radium King]

sorry, i may have been a bit confusing (confused?) before - my understanding is that this was a pedal designed to plug straight into a guitar amp, so 'bypass' in my understanding would be guitar straight into guitar amp (ie, the champ preamp is now out of the circuit). a 'mix'' control would blend champ preamp (and any vibrulated content it may have as determined by the intensity control) with straight guitar. otherwise your vibrato pedal would always be coloring your tone regardless of where you set the intensity control, given how much gain is in the champ preamp. you could rewire the champ as a clearn preamp that works simply to vibrulate the guitar signal (remove tone and volume and relace with an attenuation network designed to not overdrive the second stage, attenuate v2 output, etc.) but a mix control seems simpler nad allows you to shape your tone a bit by blending clean and champy tones, and a bypass would probably be required regardless ...

ps, got no answer on your power supply issues!

[Tyler]

Thanks for the replies-- I figured out the power problem after unsoldering and testing things in sequence.  If I can overcome my embarrassment... okay I didn't check that the Radio Shack voltage regulator had the same pin layout as the regulator I based the power supply on.  Go on, laugh (I know I did  ).

I'm sure I'll be toying with attenuation to get the pedal to be fairly clean.  For sake of gain and impedance consistency I was planning on keeping the pedal in line at all times and only switching the tremolo on and off-- perhaps having two gains I could toggle between.  This is pending experimentation.

In that vein, I suppose an isolation transformer wouldn't be the worst thing I've ever put on stage.  I wonder though what it would sound like to add another tube and make the output push-pull so I could have a fancy-pants balanced volume.  PP is supposed to sound like something, maybe I should find out what.    Science!

I finished the case and power-- my brother wants to make the rest, so hopefully it the tremolo part isn't too buggy on paper.  I posted it here:

http://www.instructables.com/id/Valve-Guitar-Effect-Case-and-Power-Supply/

Let me know if I've shamed you