Ersatz Fender Pro Vibe Bass swing mod?

[ulysses]

hey rg,

where would i have to mod the Ersatz Fender Pro Vibe to make the bass swing longer and the treble swing shorter?

im thinking of a switch to alter the time the bass stay in longer than the treble - similar in concept to processasaurus's mod - but a switch version..

is there a resistor i can add in / change that i can alter with a switch?

cheers
ulysses

[markm]

Hey, what's a Ersatz Fender Pro Vibe?? 
Still "hung-up" on that Vibra-Tone name huh?

[R.G.]

That's not all that simple. The best way would be to switch in a non-matched JFET that's selected to do that.

What you're asking for translates to a static offset in the treble side (or bass side) to make the treble less sensitive to the positive excursion of the LFO.  You could take the JFET gate resistor to a slightly negative voltage - which does not exist in the circuit yet. You could switch one of the JFET sources to a higher positive bias voltage by putting another diode in the string with D1 and D2, then switching the treble side JFET source to that higher voltage. You could unbalance the resistors in the LFO phase inverter to make the treble side smaller than the bass side.

Some combination of those should do it.

By the way, I did some simulation runs on different JFETs in this circuit. It turns out that the Q2 and Q3 gain stages are very sensitive to what JFET you put in there. Get the wrong JFET and you get a big drop in gain. You weren't crazy - just had bad luck picking highly variable parts.

[ulysses]

You could unbalance the resistors in the LFO phase inverter to make the treble side smaller than the bass side.

this is what i was thinking of. could you point those out to me. i couldn't figure out which ones they were.

By the way, I did some simulation runs on different JFETs in this circuit. It turns out that the Q2 and Q3 gain stages are very sensitive to what JFET you put in there. Get the wrong JFET and you get a big drop in gain. You weren't crazy - just had bad luck picking highly variable parts.

yeah i suspected the trannies from the start. i was confident with my build. i tried j201's, 5485's, 5458's. mark said he had no volume drop with 5458's so i gave up on the trannies..

just some really bad luck i guess. its all good now, it sounds great.

out of interest, is there any way to test the trannies for this circuit so other people dont get a volume drop?

cheers
ulysses

[ulysses]

Hey, what's a Ersatz Fender Pro Vibe?? 
Still "hung-up" on that Vibra-Tone name huh?

haha. mark you can call your layout whatever you like

i thought it would be polite to address the circuit as the Ersatz Fender Pro Vibe because i was asking RG a question and that is what he called it

cheers
ulysses

[markm]

Hey, what's a Ersatz Fender Pro Vibe?? 
Still "hung-up" on that Vibra-Tone name huh?

haha. mark you can call your layout whatever you like

i thought it would be polite to address the circuit as the Ersatz Fender Pro Vibe because i was asking RG a question and that is what he called it

cheers
ulysses

Well, I'm just bustin' on ya after the PM you sent to me about the name.....that's all. 
If anything, I would think Fender would be more upset about the "Fender" -or- "Pro Vibrato" in the title rather than "Vibra-Tone"!
The word "Fender" makes it sound as though it's one of their products, even though it is modeled after a circuit of theirs from long ago I don't recall it using JFETs but......I've been wrong before!! 

[ulysses]

Well, I'm just bustin' on ya after the PM you sent to me about the name.....that's all. 
If anything, I would think Fender would be more upset about the "Fender" -or- "Pro Vibrato" in the title rather than "Vibra-Tone"!
The word "Fender" makes it sound as though it's one of their products, even though it is modeled after a circuit of theirs from long ago I don't recall it using JFETs but......I've been wrong before!! 

Ersatz Fender - translates to "Fake Fender" or "Imitation Fender"

i suggested you change the name from vibra-tone not becasue the name "vibratone" was copyright by fender, but becasue fender released another product called vibratone which had a completly different sound to the brownface vibrato. the fender vibratone was a rebadged leslie cabinet. i just though people may get confused thinking they were building a leslie simulator

but thats cool bro, you can call it whatever you like thanks again for your layout. it sounds great.

as far as i am aware the brownface used tubes not jfets for the vibratro section.

cheers
ulysses

[markm]

as far as i am aware the brownface used tubes not jfets for the vibratro section.

Really? You're kidding right? 

[ulysses]

Really? You're kidding right? 

i hope you are taking the piss 

cheers
ulysses

[markm]

Really? You're kidding right? 

i hope you are taking the piss 

cheers
ulysses

[ulysses]

translates to "i hope you are joking"

http://en.wikipedia.org/wiki/Taking_the_piss

cheers
ulysses

[markm]

Of course.
I certainly wasn't "seeing a man about a horse"! 

[ulysses]

Of course.
I certainly wasn't "seeing a man about a horse"! 

so i dont actually need to show you a schem of the brownface with tubes in the trem/vibrato section then?

aw hell, im going to anyway
http://www.ampwares.com/ffg/schem/pro_6g5_schem.gif
http://www.ampwares.com/ffg/schem/pro_6g5-a_schem.gif

cheers
ulysses

[ulysses]

hey rg,

would changing r11 to 1.1k and r20 to 3.3k give me the effect i am after?

could it be r12 and r21 that needed modding?

am i on the right track here?

cheers
ulysses

edit: im now thinking its r24 and r22 that need unbalancing, but totalling 20k. is this correct? my pcb is looking pretty heavily battered at this stage

[ulysses]

Of course.
I certainly wasn't "seeing a man about a horse"! 

of course, of course, a talking horse! 

sorry, i had to do it.

cheers
ulysses

[R.G.]

ulysses -

The resistors I was talking about were R22 and R24. The LFO signal across R24, and therefore the signal to the bass side of the tremolo is fixed, and will be whatever the depth pot tells it to be. The LFO signal across R22, and therefore the LFO signal to the treble side will be in the ratio of R22 to R24.

With R22 and R24 equal, same sized signals. If you want to short the tremolo, reduce R22. Try cutting it in half and see if that's too much.

[ulysses]

haha thanks rg

i had already made the changes to r22 and r24

wish i had posted before you had so i could proove my prowess

i put a dpdt switch in there that changed r22 from 5k0 to 10k0 and r24 from 10k0 to 15k0 - using resistors in series on the dpdt.

it sounds really really nice. it sounds awesome. mark you have to try this.

remove r22 and r24 and wire in a switch using the following diagram.

http://aronnelson.com/gallery/album165/ulysses_brown_swing_mod

cheers
ulysses

EDIT: rg, does that mean that the mod i made to r24 is pointless, that i should make r22 change only?

[R.G.]

does that mean that the mod i made to r24 is pointless, that i should make r22 change only?

The change isn't pointless, it's just that you can get the same result only changing R22.

The way that stage works is that the base of the transistor is held at a certain voltage by the bias resistors. If the transistor has enough gain and the power supply limits let it, etc, etc, the emitter will always be one diode drop below the base voltage. The 2n5088 has a lot of gain, so for a wide range of resistances at R24, the emitter voltage will always be the same. The LFO signal from the depth control is funneled into the base, so the LFO signal the emitter is always unity times the depth control signal.

However, the same current (minus the tiny base current) appears at the collector, and causes a voltage drop across R22. So the voltage drop across R22 is always the current through R24 times R22; the DC part of the current though R24 is fixed at the bias voltage minus the base-emitter drop divided by R24. So the voltage across R22 is always the ratio of R22 and R24 times the voltage across R24.

If you make R22 too big, you run out of voltage in the power supply. But for any value of R22 smaller than R24, the LFO signal will be the ratio of R22/R24 times the signal at the emitter, and inverted in sign.

By making R22 = 5K and R24 15K, you have the base LFO = depth control LFO value, and the treble LFO be 5k/15K = 1/3 as big.

You could have just left r24 at 10K and made R22 3.3k, same result. But changing both is OK too.

You only need to remember that the LFO voltage coming in from the depth control can be as big as 2.2V peak, 4.4V peak to peak. I tried to bias the phase inverter so that it would accept that on the emitter and just barely on the collector. Making R22 bigger than R24 will make the LFO clip and flat spot as the transistor runs out of power supply to swing both the collector and emitter. R22 always being the smaller one will always give you a workable LFO with no distortion up to (I hope!) the top of the depth control.

[ulysses]

hey rg

thanks again for that reply most courtious.

cheers
ulysses