DOD FX55 Distortion mods?

[toddebner]

I have read several glowing reviews for the fx50 preamp/overdrive pedal.

Upon looking at my fx55b, the circuit board is marked fx50b-ht.

Anyone have a schematic for the fx50 ?

Perhaps we can find someone who has modded a fx50 and then we can find a way to improve this pedal.

[Geoff Hunter]

Upon looking at my fx55b, the circuit board is marked fx50b-ht.

Thats funny i've got a FX-52 Classic Fuzz which says its a fx50b-ht!

[ian87]

I have read several glowing reviews for the fx50 preamp/overdrive pedal.

Upon looking at my fx55b, the circuit board is marked fx50b-ht.

Anyone have a schematic for the fx50 ?

Perhaps we can find someone who has modded a fx50 and then we can find a way to improve this pedal.

that's crazy! i'll hafta go back and chk my board.

i've always assumed the FX50 was similar to/the same as the lauded 250 OD/Distortion ... if i could get my crappy FX55  into that realm, i'd be very happy!

[toddebner]

I have tested the 55fxb with an audio probe to determine where the sound degrades.

I found that the input and output buffer and the switch circuit work fine with clean signals.

When I get to the gain circuit, the sound is clean.  Then when I increase gain to about 9 oclock, the distortion sounds fine.  But beyond that, it gets nasty sounding quick.  The diodes seem to work fine, but when testing around that 3.9M resistor, the sound degrades quickly.

The gain circuit on this looks very funny.  I haven't seen this with a large resistor in parallel with the pot.

Anyway, this is where I'm starting to look for mods.

[cd]

Anyway, this is where I'm starting to look for mods.

Read my post on the 1st page.

[analogguru]

DOD FX50B and FX55B differ only in the gain poti configuration.

FX50B has a jumper-wire to the output (pin 1) of the OP, and no 3M9 in the feedback-loop.

FX55B has no jumper to the output and a 3M9 in the Feedback loop.


FX55B and FX53 (claassic tube) differ in another value of a resistor in the gain stage (680 instead of 470 Ohm), and in the tone-control the combination 3k3 and 22n is replaced by 6k8 and 100n.

FX55C is much more different to FX55B than the units mentioned above.

FX55 seems to be a rehoused DOD 555.

Would be interesting to see inside-pictures of DOD FX52B and FX 55.

analogguru

[Geoff Hunter]

Link to DOD FX52 Picture

http://i2.photobucket.com/albums/y41/jolem/Pedal%20Info/DODFX52_Top.jpg (~100k)

[ian87]

DOD FX50B and FX55B differ only in the gain poti configuration.

FX50B has a jumper-wire to the output (pin 1) of the OP, and no 3M9 in the feedback-loop.

FX55B has no jumper to the output and a 3M9 in the Feedback loop.


FX55B and FX53 (claassic tube) differ in another value of a resistor in the gain stage (680 instead of 470 Ohm), and in the tone-control the combination 3k3 and 22n is replaced by 6k8 and 100n.

FX55C is much more different to FX55B than the units mentioned above.

FX55 seems to be a rehoused DOD 555.

Would be interesting to see inside-pictures of DOD FX52B and FX 55.

analogguru

excellent info, guru -- thanks! i will take pics of my 55's guts just as soon as my new digicam shows up.

do you know if the 55 suffers from the same (or similar) bass-rolloff-with-gain scenario as the 55B? if so, how can i rectify this??

many thanks,
ian

[toddebner]

Looking at the picture of the FX52, I noticed that my fx55b has the lf353 chip and the 1458 chip reversed.

Could this be a problem ?

[Ben N]

FX55C is much more different to FX55B than the units mentioned above.

Darn, I thought I had some possibilities there.

FX55 seems to be a rehoused DOD 555.

Except for power supply? (18v for the 500 series pedals.)

Ben

[toddebner]

So in an nutshell, as you up the gain, the low end gets cut off more and more.  What can you do about it - well, again given the 55B schematic above, I would get rid of R12 (remove it completely), then take the wire that goes from the DISTORTION pot to the 27 ohm resistor, and move the pot end to pin 2 (use the hole that previously held the 3.9M resistor).  Then change the 27 ohm resistor to something more reasonable, like 470ohms or 1k, that will reduce the gain slightly but will hold the low end rolloff steady... change D4 and D5 to LEDs or some other higher threshold diode, and leave D1 and D2 in, but connect them to ground with a 1k resistor to soften the clipping (lift their ground ends in the air).  What else... disconnect the tone control completely, all it's doing is cutting more bass - replace it with a 10k resistor to ground.

CD,  thanks for pointing me back here.

A couple of questions

1.   Where is the schematic posted ?
2.   If I connect the distortion pot to pin 2 as you describe above,  where does the wiper of the distortion pot go now ?  (originally connected to the 3.9M resistor).  FYI, The third connection to the pot is not connected anywhere on my board.

Todd

[ian87]

1.   Where is the schematic posted ?

http://bryant02.home.att.net/schematics/dodfx55b.gif

[cd]

[quote="toddebner]FYI, The third connection to the pot is not connected anywhere on my board.
[/quote]

If that's the case, you either have a completely different version from the posted schematic, or the posted schematic is wrong.  My mod suggestions were based on the posted schematic (see link on the first page, or above).  If you have a different circuit, I would suggest tracing it out and then you can change the frequency response accordingly.

[toddebner]

FYI, The third connection to the pot is not connected anywhere on my board

If that's the case, you either have a completely different version from the posted schematic, or the posted schematic is wrong.  My mod suggestions were based on the posted schematic (see link on the first page, or above).  If you have a different circuit, I would suggest tracing it out and then you can change the frequency response accordingly.

http://home.comcast.net/~eric.erickson/DODFX55b.jpg

I have the schematic you reference and also found this one.  I checked them against my pedal and the one posted earlier has many differences.

The link I post here is very close to my actual pedal, the gain/drive pot is wired on my box just like the link here.

The tone controls on mine use the 3.3k resistors (the only difference i found).

Can you suggest how to change the pot connections ?

Thanks,  Todd

[cd]

Interesting, I've never seen that schematic version before.  OK, going by that schem, I would change:

- get rid of the 3M9 resistor
- replace the 470 ohm resistor with something more sensible, 1k-4k7
- run a jumper from the 470 ohm resistor (end that does not connect to 1u cap) to pin 2
- move the wire that goes from the 470 ohm resistor to the gain control to the old 3M9 resistor hole that connects to pin 2 (one end of the wire is on the gain control, the other end on pin 2)
- move the wire that goes from the wiper of the gain control to the old 3M9 resistor hole that connects to pin 1 (one end of wire on gain control, the other end on pin 1)
- NOTE: if the gain control now works backwards, reverse the wiper/lug connections

other things:

Looks like the TONE control cuts lows and highs... I would replace it with a simple high cut filter:

- remove C7, R5, R6, C11, C9, C8
- put C7 where C9 used to be
- insert a jumper where C8 used to be
- if you get no high cut stick a 10k resistor from the LEVEL wiper to pin 5

If you make any $ off of these suggestions donate some of it to the local battered women shelter (or other favorite charity) or may you contract ebola and burn like the dog you are.

[toddebner]

Wow !!

Incredible analysis inside of 15 minutes.

It'll take me hours to just figure out how to do all that.

Thanks,  Todd

[analogguru]

corncerning bass roll-off with gain:

use a bigger electrolytic in the gain stage (to ground) and this will be solved.

concerning FX55 and 555:
This is only an assumption and I am talking about the FX55 3-knob.
There exits also a FX55 with only two knobs.
Sadly I haven´t seen both from inside.

concerning FX55C:
Distortion section is similar, but the gain poti is in the feedback loop like a tubescreamer. Its like a tubescreamer and Distortion+ together.
Tone-control is completely different to the other units we are talking about.
It´s like a Fender tone-stack with fixed bass and mid.  After that a FET- buffer-stage

The first

The 555 schematic you can find here:

http://forum.musikding.de/yabbse/index.php?board=8;action=display;threadid=2291

The FX55 schematic at:
http://bryant02.home.att.net/schematics/dodfx55b.gif
contains some serious mistakes.

The schematic at:
http://home.comcast.net/~eric.erickson/DODFX55b.jpg
is from the first batch. there were made minor changes.

I am working on a schematic which is accurate and shows the (only small) differences between FX50B, FX52B, FX53, and FX55B....so please be patient.

analogguru

[toddebner]

Looks like the TONE control cuts lows and highs... I would replace it with a simple high cut filter:

- remove C7, R5, R6, C11, C9, C8
- put C7 where C9 used to be
- insert a jumper where C8 used to be
- if you get no high cut stick a 10k resistor from the LEVEL wiper to pin 5

I drew this new tone circuit and nothing goes to ground anymore.

Just 18k resistor in parallel with .022uf cap and one end of the tone control not connected.

Is this correct ?

[toddebner]

corncerning bass roll-off with gain:

use a bigger electrolytic in the gain stage (to ground) and this will be solved.

Are you suggesting changing the 1uf cap to ground with something larger ?

Is this in addition to CD's suggestions ?

I am working on a schematic which is accurate and shows the (only small) differences between FX50B, FX52B, FX53, and FX55B....so please be patient.

analogguru

Thanks for doing this,  looking forward to it.

Todd

[cd]

Looks like the TONE control cuts lows and highs... I would replace it with a simple high cut filter:

- remove C7, R5, R6, C11, C9, C8
- put C7 where C9 used to be
- insert a jumper where C8 used to be
- if you get no high cut stick a 10k resistor from the LEVEL wiper to pin 5

I drew this new tone circuit and nothing goes to ground anymore.

Just 18k resistor in parallel with .022uf cap and one end of the tone control not connected.

Is this correct ?

The Tone control stays where it is, you just disconnect/reconnect a couple of things to make it high cut only.

You can increase the value of the 1u cap only to get more bass, but then you still have the effect of increasing drive = decreasing bass.