Bigfoot FX Magnavibe  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
User avatar
pinkjimiphoton
Transistor Tuner
Information
Posts: 1242
Joined: 12 Sep 2010, 19:03
my favorite amplifier: my own, a redesigned princeton
Completed builds: so fucking many goddamn pedals it would make your ass hurt just looking at 'em all!!!!!
there's videos that totally suck of about 90 of 'em here:
http://youtube.com/666pinkster
gimme a like: http://facebook.com/pinkjimiphotonrocks
Location: new world order land (formerly la-la land)
Has thanked: 358 times
Been thanked: 479 times
Contact:

Post by pinkjimiphoton »

nice one, nocentelli!!!

gonna have to take this for a spin. ;)
"When the power of love overcomes the love of power the world will know peace."
Slava Ukraini!

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Excellent Nocentelli


shall let you know how it goes

thanks again! :D

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Nocentelli


I have some 47K Linear - would they be suitable for the build, also would i need to change the value of the 20K trimmer for balance - if i were to use them.

could i use other value trimmers, say a 100k trimmer?


Thanks

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

For the pots, you can get quite close to 25k by wiring a 47k resistor across lugs 1+3 . 47k in parallel with 47k = 23.5k: 1/Rtotal = 1/R1 + 1/R2 for resistors in parallel:1/23.5 = 1/47 + 1/47. You can reduce the value of pots easily this way, I tend to use this online calculator to save my brain: http://www.electronics2000.co.uk/calc/s ... ulator.php

The taper of the pots will no longer be linear, but I'm not convinced this is super-critical for this build. You may find that the useful speeds or depth levels are bunched-up at one end of the pots' travel, but it will certainly work until you can order the correct spec. There's some good info on pots in general here:

http://sound.westhost.com/pots.htm
http://www.geofex.com/article_folders/p ... tscret.htm
http://t1.gstatic.com/images?q=tbn:ANd9 ... q7JBdWvtCg

For the trimmer, you could pull the same trick to get the exact value specified (i.e wire a 25k resistor acroos pin 2+3 of the trimmer: 1/20 = 1/100 + 1/25), but since this total resistance is operating itself in parallel with that 6k8 resistor, the effect of the out-of spec trimmer value is greatly diminished. You would probably find you have to turn it halfway before it becomes useful, but after that, it shouldn't be too fiddly to set vibrato/tremolo level at unity gain. I'd try out the trimmer without the parallel 25k first and see.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Hi Nocentelli


completed the build following you instructions regards 47k and 25k resistors - however, and sadly, am getting a huge pop! when engaging the foot-switch, also a considerable volume drop when the pedal is on. When flipping the DPDT switch I get another huge pop! - and only the Tremolo side works... and like you said the gating is bunched up at the end - no noticeable magnatone style pitching wobble in the tone when in the other switch setting - i have also tried separating the LDR and LED resulting in no change of pitch, purely depth.

Basically its sound like it has gone to bypass when flipped up :hmmm:

any ideas?

Oh! and to mention i didn't have any 4M7 resistors, so ran x2 2m2 in series.

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

f you're not getting vibrato there may be some problem with your build since the vibrato mode is pretty much identical to the verified ver3 layout. If the trem's working, that's something: Does adjusting the trimmer alter the level at all?

Footswitch pop can be due to several things, you could try reducing the input resistor to ground to 1M. if you've got an LED wired up (it sounds like you've got it boxed up) increasing your LED resistor often helps. I often use 12k or 15k and the LED is still plenty bright enough. Also, it helps to wire the LED + directly off the dc jack, and power the board from a separate parallel connection.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Using reverse logic - i might try removing the switch/wiring and see if the Vibrato works without it first. I currently have a 2k2 on the LED + so will also switch that to a 15k. The LED + is running directly to the 9V DC. Regards the input resistor - can you confirm is that the 2k2 in the bottom left of the board? - also not exactly sure what a separate parallel connection is, or how i would wire it.

BTW - For the 3PDT wiring I used KindaFuzzy's sketch from page 1

https://www.freestompboxes.org/download/ ... &mode=view

The 100k trimmer with/without 25k resistor currently does nothing when turned.

Cheers for all your assistance Nocentelli :)

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

The input resistor to ground is the 2M2: You could try reducing this to 1M, but his is a bit of a shot in the dark, as my footswitch doesn't pop with the 2M2. By separate parallel connection to +9v, it's always good practice to run two separate wires to the dc jack positive, one for the indicator LED and one to the board. You can run into popping if you, say, have the LED take it's power from the board, or run a single wire from the jack to the LED and then power the board from there.

To be honest, i'd be more interested in getting the vibrato working. If you've got slight trem (or can see the LFO LED flashing, and the rate and depth controls affect the speed and brightness) Q2 seems to be ok, maybe look carefully at the values and soldering around Q1, check all cuts, cap and transistor orientation etc the usual stuff. I say this because it's a single transitor audio stage, and with that biasing arrangement, it sounds like unity volume to me whatever the depth setting: If you've got a volume drop, Q1 can't be right (if you're confident of the wiring). Then if that all looks fine, have a play with the alignment of the LED/LDR pair - Several people have mentioned it being tricky to get enough depth. I'll be honest, mine isn't even heatshrinked, i just have the LDR flat to the board facing up, and have the LED bent down over pointing at it like some demented goose-neck botch-job. It sounded great on the breadboard and when testing the board, but when i boxed it up (in a 1590A - never again) it lost all depth. I had to open it all up again, discovered the LED had been nudged aside. it took a fair bit of fiddling to ensure it stayed put but I played whilst tweeaking the position listening for the deepest position.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Right Got you! - yeah! that's how i wire all my pedals ( parallel LED and board separately wired to power ) didn't actually know there was another way of wiring, am very thorough when it comes to bridges and debris when soldering etc - and didn't spot anything on an addition look over. The tremolo is quite gated, and used Madbeans 1k5 on the x2 2k2 and 10k instead of 27k approach to depth. I also used an ultra-clear red LED - and like yourself, I don't have it shrink wrapped at this stage and am getting a good signal from the LDR, as a Tremolo - usable, though not as slow as i had hoped, but the volume drop is considerable. It could well be Q1 so will switch that one out.

Shall try the board without the DPDT, and minor changes tomorrow and get back to you.


thanks again :thumbsup

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

Reducing the value of Q1's emitter resistor should increase the volume - That's what the trimmer does.As a vibrato, it should be unity with the 7k5 (or 6k8): When switched to trem, the LDR now bleeds some signal to ground whenever the LED flashes: This causes the rhythmic dip in volume we call tremolo, but also reduces the overall perceived volume level: The other half of the switch brings that trimmer into play and reduces the Q1 emitter resistance to raise the level.

Good luck!
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Hi Nocentelli

changed out the Q1 - no difference - changed the resistor to 15k on the LED positive wire to DC ... still have the Pop! again i went over the board with a surgical blade, checking for solder bridges, debris etc - nothing found. Removed the DPDT from the circuit - now i get bypass - LED flashes as before - I can hear a faint beating of the LED flashing and still have the volume drop and that's about it :hmmm:

Am wondering if i also have a grounding problem somewhere, and feel to resort to my standard configuration for wiring - this is how i wire my pedals for true bypass - would this method work with the circuit.

Image

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

oh - forgot to mention - do the pots 1 lug need to go anywhere?

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

Zombie_Crush wrote: Removed the DPDT from the circuit - now i get bypass - LED flashes as before - I can hear a faint beating of the LED flashing
...And reconnected the LDR leg to the Q1 emitter?

Wiring diagram looks fine - that's exactly how I do it - nice diagram btw.

Lugs 1 are traditionally wired to lugs 2, but I don't think it makes any difference if you do or not in this circuit, the pots are just used as variable resistors.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

...And reconnected the LDR leg to the Q1 emitter?


lol good point :wink:

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

vol drop - still pops - barely audio-able flutter :shock:

think i may try making your original version2, as i had a 9 to 15 hole vero board for that version - and see how that goes

staying calm :|

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

transistor pinout? i'm sure you've checked several times, but i'm grasping at straws now...
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

:slap:

It has been working the whole time! - I wired the 1K5 from H to F instead of H to E! it was hidden behind a .22 poly-box and the leg was bent slightly backwards so appeared in line with E

Can confirm your Vibrato/Tremolo Mod switch works! veeeeery nice little vintage vibe! there's a bit of a click when switching modes - wonder if there's a way I can ground the switch.


All said - its working! :applause:


Many thanks Nocentelli

User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

Post by Nocentelli »

Excellent news! How effective is the volume balance trimmer? I'm going to breadboard a mash-up of the Q1 section of the Magna (with trem/vib switch) and an opamp-based LFO that goes much slower than the Magnavibe Q2 that might be more useful for the trem mode. I'll post in the DIY designs section when i've got a layout verified.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
Zombie_Crush
Breadboard Brother
Information
Posts: 81
Joined: 26 Sep 2010, 02:05
Has thanked: 18 times
Been thanked: 1 time

Post by Zombie_Crush »

Cool! - yeah, would be nice to get it a little slower and would be very useful if successful :)

regards the trimmer .. Yep! worked very well, didn't need to use the 25k resistor to get it set with the 100k i had to hand, just less room to set it.
its a nice little tremolo too - quite choppy - so long as the LDR is set directly facing the top of the LED lens

All the best with that Nocentelli :wink:

back to a bit of Enter The Dragon - Sampans

User avatar
greenskull
Resistor Ronker
Information
Posts: 253
Joined: 03 Jul 2007, 22:47
Has thanked: 33 times
Been thanked: 8 times

Post by greenskull »

Zombie_Crush:
Did the flutter/beating background noise go away after fixing the mistake? I built one up a while ago using a pcb layout and have that problem.... or is that just the nature of the beast?
fuck smooth tone, fuck eric johnson - Seiche

Post Reply