Author Topic: DOD FX55-B  (Read 421 times)

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Yata

DOD FX55-B
« on: September 09, 2017, 11:11:33 AM »
So I've had a FX55B in my possesion for quite a few years but never cared for it since it sounded brittle and harsh with far too much treble. After Reading up several hours worth of internet posts on people who had tried modding various sections I tried the following mods:

Schematic : https://i.imgur.com/47Whmdu.jpg

Replaced D1 and D2 with a green and red LED (this is giving me asymetrical clipping right?)
Replaced D3 and D4 with 1N34A's
Replaced C8 with a 0.047uf capacitor
Replaced R23 with a 1M resistor
Replaced C3 with a 47pf capacitor
Rewired DC trs jack to a encased plastic barrel style dc jack

Picture:https://i.imgur.com/FEOS06F.jpg

I still want to install a true bypass footswitch eventually but I think I might need to find a different enclosure as even the current DC jack doesnt fit in my enclosure and I need to run the power cables out.

The resulting sound is  ALOT different. The distortion it produces sounds alot smoother and less harsh. The tone itself has less of the low end cut out of it and across the different values of the gain pot it goes from an almost lovely messy hard fuzz to a light crunch/overdrive. This is now one of the nicest pedals I've ever used it terms of tone.

The main problem I'm having is that the overall volume of the circuit is very quiet. there is a gain boost with the Gain and tone pots up full but this means you basically need to play full balls to the wall just to hear the pedal. With both these controls up full when I hit the footswitch the pedal sound is still way quieter than the normal bypassed sound, and with the controls dialed back it's almoste inaudible. I'm wondering if anyone can give me some advice on mods I can make to have it so at least when cleanly A/B'ed the pedal isn't quiter. For every other distortion/OD pedal I have, the full effect is at least slightly louder than my clean channel, but for this it is noticebly quiet.
« Last Edit: September 09, 2017, 12:17:49 PM by Yata »

Mark Hammer

Re: DOD FX55-B
« Reply #1 on: September 09, 2017, 05:42:13 PM »
While swapping the Si D3/D4 diodes for Ge types have provided a tone you like, unfortunately the lower forward voltage of the Ge diodes lowers the maximum output level.

There are two solutions.  One is to switch back to Si types.  Will that eliminate the tone you like?  Perhaps not.  By replacing D1/D2 with LEDs, you're not getting clipping out of them until you turn gain way up, but you're still getting clipping out of D3/D4.  So maybe keeping the original D3/D4 won't be so bad and will still yield te desired tone.

The second is to leave all diodes in place as they are, and  increase the gain of the tone stage that follows.  That can work, but may alter the functioning of the tone control.  That might not necessarily be a BAD thing, but it might also make the control behave a little weirdly, requiring multiple component changes.

My preference would be to put the Si diodes back for D3/D4, see if you like it and take it from there.

Yata

Re: DOD FX55-B
« Reply #2 on: September 09, 2017, 10:25:28 PM »
While swapping the Si D3/D4 diodes for Ge types have provided a tone you like, unfortunately the lower forward voltage of the Ge diodes lowers the maximum output level.

There are two solutions.  One is to switch back to Si types.  Will that eliminate the tone you like?  Perhaps not.  By replacing D1/D2 with LEDs, you're not getting clipping out of them until you turn gain way up, but you're still getting clipping out of D3/D4.  So maybe keeping the original D3/D4 won't be so bad and will still yield te desired tone.

The second is to leave all diodes in place as they are, and  increase the gain of the tone stage that follows.  That can work, but may alter the functioning of the tone control.  That might not necessarily be a BAD thing, but it might also make the control behave a little weirdly, requiring multiple component changes.

My preference would be to put the Si diodes back for D3/D4, see if you like it and take it from there.

Would switching the diodes and putting LED's in D3/D4 work to boost the overall output?

Mark Hammer

Re: DOD FX55-B
« Reply #3 on: September 10, 2017, 10:35:26 AM »
Well it would do that, but then you'd find the output would be so high you'd likely find the volume control wouldneed to stay between 7:00 and 8:00 at all times!

Beginners seem to think that diode type simply changes the quality of the signal.  Whileit does do that as well, it does so by altering how hot the signal is permitted to be before clipping sets in.  Using LEDs in both locations will raise the clipping threshold and level enough that you'll have a harder time achieving the saturation you might want at any level other than "turned way down".

No, I think you'll be happy just reverting back to 1N4148s for D3/4.  Like I say, try it, and if you don't like it, we can pursue oter strategies.

Yata

Re: DOD FX55-B
« Reply #4 on: September 29, 2017, 10:23:37 AM »
Well it would do that, but then you'd find the output would be so high you'd likely find the volume control wouldneed to stay between 7:00 and 8:00 at all times!

Beginners seem to think that diode type simply changes the quality of the signal.  Whileit does do that as well, it does so by altering how hot the signal is permitted to be before clipping sets in.  Using LEDs in both locations will raise the clipping threshold and level enough that you'll have a harder time achieving the saturation you might want at any level other than "turned way down".

No, I think you'll be happy just reverting back to 1N4148s for D3/4.  Like I say, try it, and if you don't like it, we can pursue oter strategies.

Sorry for the lateness in the reply but I thank you again for helping me.
I blew a speaker on my practise amp so ive not really had time to work on this/test but I managed to socket the diodes as per your suggestion and try it at a practice studio and your 100% right.  The voume difference is ALOT louder with the silicon diodes back in place, the main problem now is that the tone/sound seems to have changed quite a bit and it's alot harsher sounding. I'm basically stuck with having a nicer sound but almost no gain or having more gain than I could ever need but a tone that is sub-optimal.

Mark Hammer

Re: DOD FX55-B
« Reply #5 on: September 29, 2017, 12:09:51 PM »
If you lower the ceiling, more people are going to bang their head against it, and walk around hunched over.  Changing diode type to achieve different clipping thresholds will alter both clipping intensity and level.

If you find that there isn't enough opportunity for moderate clipping at a level that suits you, you can play off gain structure against diode type.  So, you can replace the 3.9M feedback resistor in the first clipping stage with 2.2M or even 1M to lower maximum gain so that you can get a little more dialability.

Yata

Re: DOD FX55-B
« Reply #6 on: September 29, 2017, 01:32:17 PM »
If you lower the ceiling, more people are going to bang their head against it, and walk around hunched over.  Changing diode type to achieve different clipping thresholds will alter both clipping intensity and level.

If you find that there isn't enough opportunity for moderate clipping at a level that suits you, you can play off gain structure against diode type.  So, you can replace the 3.9M feedback resistor in the first clipping stage with 2.2M or even 1M to lower maximum gain so that you can get a little more dialability.

That was actually my first mod, I changed R23 to a 1M resistor but I'm still only really getting the same amount of gain as my bypassed tone with every single pot turned up full.  Is there some components around D3/D4 that could be changed for an increase in level?

ElectricDruid

Re: DOD FX55-B
« Reply #7 on: September 29, 2017, 02:16:24 PM »
You could try putting series pairs of 1N34As in place of D3 and D4. That might give you the soft edged clipping that germaniums are famous for, but it'd also double the forward voltage (since there are now two diodes in each direction) and that might give you enough level.

HTH,
Tom

Yata

Re: DOD FX55-B
« Reply #8 on: September 29, 2017, 02:50:48 PM »
You could try putting series pairs of 1N34As in place of D3 and D4. That might give you the soft edged clipping that germaniums are famous for, but it'd also double the forward voltage (since there are now two diodes in each direction) and that might give you enough level.

HTH,
Tom

Thanks for another suggestion, this info is all learning experience for me.

I have sockets right now for my diodes, I assume hte best way to do this would be to solder two germainium diodes front to back and fit the end pins in the sockets?

Mark Hammer

Re: DOD FX55-B
« Reply #9 on: September 29, 2017, 03:19:13 PM »
Another alternative is to stick a small-value resistor between the clipping diodes and ground.  So, something like 220-470R between the diode pair and ground.  Softens the clipping and raises the level a bit.

ElectricDruid

Re: DOD FX55-B
« Reply #10 on: September 29, 2017, 08:04:27 PM »
You could try putting series pairs of 1N34As in place of D3 and D4. That might give you the soft edged clipping that germaniums are famous for, but it'd also double the forward voltage (since there are now two diodes in each direction) and that might give you enough level.

HTH,
Tom

Thanks for another suggestion, this info is all learning experience for me.

I have sockets right now for my diodes, I assume the best way to do this would be to solder two germainium diodes front to back and fit the end pins in the sockets?

Yep! I don't know about "best" way, but that's what I'd do! For testing I'd leave the diode leads long too, so that if I don't like it I could still change my mind and use the germaniums for something else. Germanium diodes aren't cheap; at least, not for diodes.

T.