Ersatz Fender Pro Vibe Questions for R.G.

[markm]

I have been thinking of building this circuit and had a couple of questions;

Would this circuit still function as well if some more "common" JFETs were used such as MPF102?
Also, what does Q10 actually do?
Although this circuit upon first glance looks intimidating, after looking at it a bit closer, it is rather simple for the most part.
Thanks R.G. 

[R.G.]

Would this circuit still function as well if some more "common" JFETs were used such as MPF102?

Maybe. The problem is that peaky Vgs thing again. The JFETs I suggest are the common ones for JFET phasers and the biasing is calculated for that. Other JFETs won't bias quite the same, so you're into tweaking to start with.

Let me propose another approach. What JFETs can you get easily except the MPF102? Perhaps the BF series?

Also, what does Q10 actually do?

It's a quiet turn-on for battery voltage that prevents LED noises and such from causing audible noise. It's a substitute for the connection of a battery negative wire to the input jack ring contact to turn on the battery. In that, it's my bit of guerilla warfare to start eliminating the connection of a battery (-) to the input ring. The transistor connection does the same thing, costs under US$0.10, and is superior in performance.

[markm]

I see what you're getting at about the bias.
I have 2N5952 but, alas......only 4 
I am assuming (there's that word) that they don't need to be matched?
There are many JFET types I have on hand but, I cannot actually name them right now 'cause I'm not there!
I know for sure I have no BF types.
What I should probably do is, order up some 5952's and do it correctly instead of "half-fast". 

The Q10 idea is GREAT!

[markm]

I've decided the ONLY way to do this is the CORRECT way!
Just placed an order with smallbear for the correct FETS and I'll start working on a Layout for it this evening.
My only concerns now are it's gonna be a "larger" layout and sometimes DIY Creator can run a bit slow when things are larger like this.
Thanks for your help and, I don't think I'll have to but, I may need to contact you again during the project.
Many Thanks R.G.,
   MarkM

[QSQCaito]

Hey mark, this is the tremolo that is also phaser type of thing fender had.. that we discussed not so long ago??

If so, im in, and ask for help in everything you need

bye bye

DAC

PS I couldn't get what exactly Q10 does, what's wrong with battery (-) on the jack? And why is using this tranny better??

Thanks a lot.

[markm]

I just may take you up on that offer Diego!
Thank you.
Work has started tonight.
I will warn everyone ahead of time, it's NOT going to fit into a 1590B! 

[Processaurus]

Here's a thought, on using a pot to blend in the brownness, brownface vibrato on one end, trem on the other.  I tried making a whole audio blending circuit, but it can be much simpler.  Start with a high value pot, with the two outer lugs hooked to the two antiphase LFOs.  Send the wiper to the JFET that is controlling the amplitude of the lowpass, bass side if the filter.  What it does is when the pot is turned to the antiphase LFO, it turns the bass up and down opposite the treble, brownface style, and when you turn it the other way, the bass is in sync with the treble, making it a tremolo (with a slight mid notch as the only quirk).  In the middle of the pot the LFOs cancel, leaving just the treble to change volume, and the bass right in the middle.

Mark, consider inserting a depth control in there, the more subtle, less obvious colors you can dial in are currently some of my favorite with the thing I made based on this idea.  Have fun!

[duck_arse]

yes, an old thread, but I couldn't have thort of a better subject line if I tried.

I'm sure I've got this all backwards, and not understanding it right, and probably doing the calculations wrong, but. some questions.

based on this circuit .....


and these 2 circuits which share the same values.
www.geofex.com/FX_images/FakeFenderVib2.pdf
www.geofex.com/FX_images/FakeFenderVib3.pdf

referring to the jfet version, C5 is shown as 5nF. with the R7 470k this forms a low pass filter set to 72Hz feeding the fet gate, so all the content below 72Hz will be tremmed. the op-amp version has this cap as 50nF, setting the low pass to 7.2 Hz. wouldn't this mean nothing will get tremmed? wrong calcs, wrong understanding or wrong value?

and then, as I'm seeing it, the R6//R18 divider is to make up (down?) for the 2 sigs being mixed later, and the C12 250pF and R35//R19 form a high pass set at about 640Hz. so, nothing below that freq appears at the drain, or gets tremmed?

so do I have my filters entirely arse about, or is there effectively a great big notch in the response between (7.2Hz or) 72Hz and 640Hz? has the question of the differing values been corrected somewhere else?

[edit :] downsized image

[PRR]

> a great big notch in the response between ...72Hz and 640Hz?

"Great big"? These are first-order filters. They fall-off slow. 6dB/octave.

Counting on thumbs, halfway between 72 and 640 is 215Hz, 1.5 octaves either way. The response is about -9dB from "flat". 9dB is quite a bit but not "great big".

If the two paths were in-phase the response at 215hz would sum to a higher level than -9dB, maybe -6 or -3 dB.

In fact they are 90 deg one way and 90 deg the other way and will null *at* 215Hz if mixed equally. So there is potential for a "great big notch", but not very wide. And the two paths are not summed equally.

This null/notch will move up or down in frequency as the mix-ratio is changed.

What you should do is disregard all these later-day re-draws and find the Classic Fender they are attempting to plagiarize. I agree that 72hz seems low for gitar, and 7Hz just sounds wrong, but I'm not curious enough to find the Original ear-tweaked mass-produced plan.

[tubegeek]

[PRR]

Now now, tg, Duck can do his own homework.

It is also worth reading the Patents loudly cited on that plan:
http://www.google.com/patents/US2817708
http://www.google.com/patents/US2973681

For the production Pro, the corners are hi-cut at 72Hz and low-cut at 1.3KHz.

[duck_arse]

I did, indeed, do my own work. even though I don't know my bassman from my pro, I found the 6G5 and 6G12 both use the RG topology, with .00illedgible cap instead of 47nF, and that the 6G5-A and 6G12-A have the voltage divider missing and 2 caps swapped. then I got sick of looking at fender circuits.

paul, when you say the two paths are outta-phase, do you means the resulting envelopes are outta phase and therefore ...?

also - ahh-HA! so, with 1M to ground and 1M to the modulation, the grid shows as 500k load, before bothering with the cap calculations? that shifts some of my pages of numbers a bit.

[tubegeek]

Now now, tg, Duck can do his own homework.

I was just so impressed that it's a Pro amp - most of the amps I know are mere dabblers, enthusiasts, dilettantes!

[PRR]

> with 1M to ground and 1M to the modulation, the grid shows as 500k load

Anything with more than 2 R and 2 C soon becomes a mess to calculate. Since the LFO (the phase-split with 56K at plate and cathode) looks-like much less than 1Meg, and has the quite large (relative to 1meg over the audio band) 0.1u caps, I would approximate the 1Meg+1Meg dividers as "about 500K" to the audio path.

There's really too much stuff here for humans. I think Leo just dumped his cap-drawer at it. But he kept the iron hot. If you read through the lengthy Patents, he thinks that certain details are critical to the sound effect.... he had to be working try-and-try-again (with some experienced insight).

Here's my understanding with idiot assistant plots:


The mix curves are all classic Fender Tonestack curves, only a little low. It is interesting that the trem does this AND there is already a mid-dip tonestack in the amp.

In 50:50 mix the curve is, say, 0dB at 82hz, dips to -8dB at 300hz, +7dB at 4KHz.

For other mixes the dip moves from 120Hz to 300Hz to 540Hz.

If a mix factor went to zero, the curve would echo the Hi or Lo curve-- but this type (two triode) of variable mix probably won't get to all one or the other without huge overdrive at the LFO input.

[tubegeek]

OK, so it's time to build one. Using the "based on the EA Trem" version from GEOFEX, schematic is below.

I have a question, but first some background:

I used the op-amp-held-to-half-battery-voltage method to sort through 15 2N5457's, which was the only JFET I had enough of to bother with trying to sort them. I have some 2N5485's on the way (slow cheap boat) but I figured I'd get my feet wet.

I actually sorted at two resistance (current) values: using a 47K resistor (about 100 uA) and 22K (200 uA). Here are the results:

2N5457
Tag Number	Vgs, 47K (100 uA)	Vgs, 22K (200 uA)
8	0.387	0.241
9	0.399	0.247
12	0.616	0.502
15	0.626	0.504
11	0.634	0.521
14	0.673	0.555
10	0.803	0.638
7	0.813	0.693
6	0.823	0.678
3	0.882	0.743
4	1.253	1.112
1	1.270	1.125
13	1.437	1.308
5	1.491	1.348
2	1.555	1.408

So, my question: which end of the group (or the middle?) should I take my match from? I don't know how to match up the all-over-the-map results from 2N5457's to the desired behavior of 2N5485's, or if that's even possible.

Is the voltage divider made up of Rds versus R22||R24, that is, 5K? And does that mean I want to use the small Vds matches (JFETs 8 & 9) or the large Vds matches (4 & 1) or the middle set (7 & 6.) My gut tells me I want the ones with the smaller Vds because they will provide a larger range of resistances than the larger ones, but I'm worried I'm missing something and/or the parts swap is unsuitable for some reason.

An observation: as predicted, the two-point match didn't add any information: the ones that looked like pairs at 1 uA also looked like pairs at 2 uA, and the order of values gave the same sorting at each of the test points. I've read umpteen posts about JFET matching now and I'm aware of Rob Strand's input that the parameter that matters most for matching is not this one. But it just made sense to me that if we're trying to get a similar swing on two voltage dividers using a pair of equal-but-opposite control voltages, this matching method could predict that behavior OK.

This is the first "bigger" circuit I plan on doing on breadboard. I broke RG's schematic into a few smaller blocks which I think will make it easier to build and test. I have:
Power Supply, LFO, Input Buffer, Hi Filter, Low Filter, and Output Buffer. I wanted especially to test the LFO before I hooked anything up to it just to make sure it works OK.

One idea I thought I'd adopt from other posters' plans, is to make the Low filter section switchable between Opposite LFO, Same LFO, and No LFO. I guess that'll leave me with the residual EQ scoop in the Same LFO (plain old trem) setting, I'll see if I have to bypass the filters altogether when I get to that point and listen to the results.

Thanks as always for the sharp discussions - I have read a LOT already about this and I'm curious to try it.

[Eb7+9]

You’ll want Q5 and Q6 vcr devices having fairly matched Vgs(off) values for balanced operation ... close Idss matching is of secondary importance for these two devices

You only need a fresh 9v battery and a DMM to know these two numbers and properly characterize/match most jFET types used in gtr pedals

https://www.diystompboxes.com/smfforum/index.php?topic=121610.0;topicseen

—-

Ed. one thing worth mentioning about this version of RG’s design is that Q1 - Q4 could be made into typical small-signal NPN devices ... if any of these four jFET’s are chosen with too low Idss (<about 2.1mA in the buffer cases) these stages won’t be sitting inside their intended linear region at idle ... I’d recommend having around 4mA on those four jFET’s if building for bass or keys, 3mA for single coils and common hum buckets; whereas these four stages bias without that kind of current restriction using bipolar NPN devices

[PRR]

The 0.2V devices *may* overload at guitar level.

The 2V devices may not max-out with a 9V LFO.

Not even analyzed, and not accounting for musician's needs or taste.

Is it costly to put a DIP socket in and try a few?

[tubegeek]

Is it costly to put a DIP socket in and try a few?

Certainly not!

Also: I built the other ("Greatly Improved" tester and checked Vp and Idss - I'll expand the table to include this after I get back home.

I'd say my main cause for being Mr. Question Man on this was the fact that none of the JFETs speced for the circuit rang any bells so I was worried about them being "Unobtanium Specials" that'd make it impossible to get working. The hint to go down the middle makes perfect sense to me. More experimenting and less thinking, more perspiration and less inspiration.

[tubegeek]

This is the post that worried me:

Would this circuit still function as well if some more "common" JFETs were used such as MPF102?

Maybe. The problem is that peaky Vgs thing again. The JFETs I suggest are the common ones for JFET phasers and the biasing is calculated for that. Other JFETs won't bias quite the same, so you're into tweaking to start with.

Let me propose another approach. What JFETs can you get easily except the MPF102? Perhaps the BF series?