I liked the base sound of the DOD FX53 Classic Tube but didn't like how it was massive on my pedal board. I've been designing a PCB and I am happy to report that the prototype board works!
The circuit though. Oh man, I do not understand this tone stack. It's so bass heavy. Can someone help me figure out what it is and how it's working?
Goal: The pedal doesn't have a lot of high frequencies coming out. I'd like to goose that a little bit.
(https://i.postimg.cc/bZZQsfB9/DOD-FX53.gif) (https://postimg.cc/bZZQsfB9)
My guesses throughout the schematic, starting at the Input section:
C21 is some kind of high frequency roll-off (how? interaction with R1?)
C3 in the first feedback network rolls off frequencies above 340Hz. I changed this to 47pF to raise the frequency to 840Hz. Not much difference. The 3.9M feedback resistor is very large so I might need something even smaller for the cap.
C6 and the 10K resistor roll off around 10KHz. I only had a 1.8n on hand so the frequency was around 8KHz. I changed the 10K to 9.1K because it's close enough. Not much change.
The tone stack is ??????? I haven't been able to find much info on exactly what the tone stack is. My best guess is that it is blending between two different boost filters.
When the knob is at 0 (wiper is at right side), the 3.9n and 100n caps in parallel boost frequencies below 85Hz and the 3.3k/10uF tail roll off frequencies below 4Hz (guessing). I dropped the 10uF as suggested in another mod thread on this forum to 2.2uF but with the calculator I could probably go further to raise that bass roll-off.
When the knob is at 11 (wiper on left side), the 3.9n and 18k resistor boost frequencies below 2267Hz and the 6.8K/100nF tail roll off frequencies below 234Hz
So I'm thinking:
C6 can be decreased to let more high frequencies pass before the filter stage.
C9 can be lowered to raise the boost frequencies of both filters
The 18k resistor affects the gain/effectiveness of the filter along with the resistors on the tails.
C7/C11 can be made smaller to roll off more bass.
Does this make sense? I want to learn more. How does it work. What other pedals use a filter like this? What is C21 doing? Also what happens if I use TL022s instead of TL072s? I had a bunch on hand. LM833s worked too. I haven't done an A/B test.
Ready to learn :icon_cool:
(https://i.imgur.com/i7sGA02.gif)
Of course, some of them (like around upper J201 and Tone pot..) interact in a complex form.. :icon_wink:
Quote from: antonis on December 18, 2023, 01:09:57 PMOf course, some of them (like around upper J201 and Tone pot..) interact in a complex form.. :icon_wink:
edit: Wrong colour for C5/P2..!!
Ahhh I deleted the JFET bypass but in the process also removed the 220K and C13...I'm not sure how much difference that makes.
Veeeery helpful reminder that "grounding" to Vbias is also a viable spot for a filter, gonna check those out...
Quote from: tootsMcgee on December 18, 2023, 01:21:38 PMAhhh I deleted the JFET bypass but in the process also removed the 220K and C13...I'm not sure how much difference that makes.
Noy much (audible) difference.. :icon_wink:
When J201 is ON, 220k is effectively bypassed so any frequency higher than 16Hz is shunted to GND..
I have an audio demo and I'm not sure where to upload it. The pedal has a super fuzzy low end, which is expected since it's two diodes and a few component values off of the FX52 fuzz. The closest sound I can think of is Hotrod Deluxe/Deville Mk 1 and 2 drive channels, which is...a sound of all time for sure.
So maybe I'm asking too much here but I'm in an experimenting mood. What's the strategy for handling lots of bass? Cut first, [distort then] boost later? Buy a different pedal?
I have some TL072s and NE5532s on the way to try too. Are the TL022s (the low power version usually for LFOs) affecting the sound?
Thanks for all the advice!
Quote from: tootsMcgee on December 18, 2023, 03:57:20 PMI have some TL072s and NE5532s on the way to try too. Are the TL022s (the low power version usually for LFOs) affecting the sound?
With clipping diodes in the feedback loop, and then *more* clipping diodes to ground directly after, I'd be *very* surprised if you could hear the difference in that circuit. Any subtleties from op-amp choice are going to be completely swamped by crunchy fuzzy noises. Especially given that the op-amps are not producing any distortion themselves in this circuit, so they're linear amplification, and that sounds the same whatever device is doing it.
Quote from: ElectricDruid on December 19, 2023, 06:40:25 AMQuote from: tootsMcgee on December 18, 2023, 03:57:20 PMI have some TL072s and NE5532s on the way to try too. Are the TL022s (the low power version usually for LFOs) affecting the sound?
With clipping diodes in the feedback loop, and then *more* clipping diodes to ground directly after, I'd be *very* surprised if you could hear the difference in that circuit. Any subtleties from op-amp choice are going to be completely swamped by crunchy fuzzy noises. Especially given that the op-amps are not producing any distortion themselves in this circuit, so they're linear amplification, and that sounds the same whatever device is doing it.
Heh, I clipped both pairs of didoes as an experiment. The first op-amp has more than enough gain to go into distortion without the diodes clipping the signal. It's actually not a shabby sound IMO. Max gain has a sort of ticking sound though which might be because it's right next to my computer or might be some flaw in the homemade PCB that picks up extra noise. Soft clipping only is pretty nice too.
Any suggestions on how to make the damn thing brighter?
Quote from: tootsMcgee on December 19, 2023, 01:20:16 PMAny suggestions on how to make the damn thing brighter?
By cutting lows, perhaps..?? :icon_wink:
(starting from C1/R2 ..)
Quote from: antonis on December 19, 2023, 01:38:04 PMQuote from: tootsMcgee on December 19, 2023, 01:20:16 PMAny suggestions on how to make the damn thing brighter?
By cutting lows, perhaps..?? :icon_wink:
(starting from C1/R2 ..)
Heh a great place to start.
From probing around I can hear highs being rolled off as soon as the first gain stage, so C3 and the 3.9M resistor are going to get tweaked a bit. 10pF and 2.2M will give me around 7k (RAT-esque, not that I'd compare this to a RAT). Stay tuned...
Quote from: tootsMcgee on December 19, 2023, 01:20:16 PMAny suggestions on how to make the damn thing brighter?
Go through Antonis' list of LP filters and see which ones are really hurting. If the top end is heavily rolled-off and you start taking the bass out too, you're going to be left with very little!
I started with R7/C3 3M9/120p. That gives cutoff at 340Hz, so that's a lot of treble cut straight away. Put 10pF or 22pF in instead, or leave them out entirely. Normally I wouldn't recommend such a drastic course of action - the high end should be limited somehow, but I wouldn't recommend using massive values like 3M9 and 500K in a gain circuit either, so...
I can't be bothered to sim the tone stack right now, but that'd probably be my next move - See what it's doing and how. Alternatively, you can try dropping the value of those LPF caps Antonis has identified by a factor of ten or so, and see which do something serious. That's an "experimental" approach, rather than an "analytical" approach, but they both get you to the same result eventually.
I think 22pF will be the sweet spot. I've been doing some experimenting. 10pF in there right now.
For the fuzzy bottom problem (heh), I noticed that the HPF from C2 and the distorton knob tops out at 340Hz at max gain. I'm going to see if I have some 470 or 680 nF on hand to see if bumping that a bit helps. Slightly higher HPF and slightly lower LPF might be enough to get me where I want without massively changing the pedal design.
BTW, your article at https://electricdruid.net/designing-the-hard-bargain-distortion-pedal/ is immensely helpful for understanding some of the choices that go into pedal design! Thank you for writing that.
For the original DOD design, I wonder sometimes what led to parts being chosen with certain values. Not in a "haha that was a bad choice" way, but more like, what records were they listening to? What sound did they have in their heads? Did they run out of 2M2 so they put a 3M9 in there instead? Stuff like that. I love learning things like that.
Quote from: tootsMcgee on December 19, 2023, 03:19:33 PMFor the original DOD design, I wonder sometimes what led to parts being chosen with certain values. Not in a "haha that was a bad choice" way, but more like, what records were they listening to? What sound did they have in their heads? Did they run out of 2M2 so they put a 3M9 in there instead? Stuff like that. I love learning things like that.
Yeah, me too! I'd read that article. Now all we need is a DOD engineer to write it for us!
Are we sure this schematic is accurate? If I look at schematics for others in the FX5xx series, (e.g., the FX50b and FX52), they tend to have max gains in that first gain stage FAR below what this schematics shows. As shown here, that first gain stage has a max gain of 5700x ( :icon_eek: ), which is not what you'd expect for something that claims to emulate a tube amp; especially when you consider that the FX55 Supra Distortion has a max gain of 3600x in that same sort of stage.
Where these other FX5x pedals differ from what we see here is that the unconnected end of the 500k gain pot is tied to the output of that stage, placing the 500k in parallel with the 3.9M feedback resistor. That reduces the max feedback resistance to 443k, which turns the initially calculated max gain of 5700x into 652x. That's still pretty high, but achieves usable gains for producing a tube-amp tone. It would also raise the high-end rolloff of that stage up to a smidgen under 3khz, which also seems more reasonable.
Good spot, Mark. It's true that the "unadjusted" gain seems way too high, now you mention it. I wasn't focussing on that, so I didn't see it, even though I noticed the extreme values.
DOD's schematics are fairly notorious for "errors" (perhaps deliberate), is that right? I seem to remember hearing such a thing somewhere - round here, probably.
Quote from: Mark Hammer on December 19, 2023, 04:39:37 PMAre we sure this schematic is accurate? If I look at schematics for others in the FX5xx series, (e.g., the FX50b and FX52), they tend to have max gains in that first gain stage FAR below what this schematics shows. As shown here, that first gain stage has a max gain of 5700x ( :icon_eek: ), which is not what you'd expect for something that claims to emulate a tube amp; especially when you consider that the FX55 Supra Distortion has a max gain of 3600x in that same sort of stage.
Where these other FX5x pedals differ from what we see here is that the unconnected end of the 500k gain pot is tied to the output of that stage, placing the 500k in parallel with the 3.9M feedback resistor. That reduces the max feedback resistance to 443k, which turns the initially calculated max gain of 5700x into 652x. That's still pretty high, but achieves usable gains for producing a tube-amp tone. It would also raise the high-end rolloff of that stage up to a smidgen under 3khz, which also seems more reasonable.
I took the original unit apart to check. The feedback resistor is indeed marked 3M9. I don't have a DMM that goes that high to verify. I actually see the spot you were talking about--there's a missing resistor between the free pin of the gain knob and the output of the gain stage. I read somewhere that they re-use PCBs for different pedals so that makes sense, but on this one there is no resistor there. The serial cap with the gain pot is confirmed 680R and the cap there is 1uF.
The input and first gain stage caps are both 120pF
I see the 3n9 for the second gain stage as well as the 18k resistor, so those are correct at least. I also tentatively spotted the 1n5 that works in conjunction with R5.
Nothing seems outright wrong, but to be accurate I'll have to unsolder the board leads so I can properly examine it from both sides. I'm guaranteed to break one of the wires if I keep flipping it so I might as well get it over with. They're so delicate.
This pedal shares a PCB with the FX52 (fuzz) and FX50B (Overdrive Preamp). Depending on which schematic for the FX50B you look at, there may or may not be a 1M resistor in the first feedback network, which the board supports (by omitting it if necessary) and I assume there's a jumper to hook the gain pot terminal 1 to the output side. I'm looking here for the FX50B: https://www.freestompboxes.org/viewtopic.php?f=1&t=16676&p=186243#p186243 but it seems to match other schematics I've found.
FX52: https://www.freestompboxes.org/viewtopic.php?t=7294 (this one also seems to tie the pot to the output side)
All the FX53 schems I've been able to find have a floating pin on the FX53's gain knob:
https://www.diystompboxes.com/smfforum/index.php?topic=118149.0
https://guitarwork.ru/electronic/DOD/Classic_Tube_FX-53/
Also I think I found a post by Mark Hammer here explaining how the tone control works! https://www.diystompboxes.com/smfforum/index.php?topic=32262.0
Quote from: tootsMcgee on December 19, 2023, 09:36:54 PMQuote from: Mark Hammer on December 19, 2023, 04:39:37 PMAre we sure this schematic is accurate? If I look at schematics for others in the FX5xx series, (e.g., the FX50b and FX52), they tend to have max gains in that first gain stage FAR below what this schematics shows. As shown here, that first gain stage has a max gain of 5700x ( :icon_eek: ), which is not what you'd expect for something that claims to emulate a tube amp; especially when you consider that the FX55 Supra Distortion has a max gain of 3600x in that same sort of stage.
Where these other FX5x pedals differ from what we see here is that the unconnected end of the 500k gain pot is tied to the output of that stage, placing the 500k in parallel with the 3.9M feedback resistor. That reduces the max feedback resistance to 443k, which turns the initially calculated max gain of 5700x into 652x. That's still pretty high, but achieves usable gains for producing a tube-amp tone. It would also raise the high-end rolloff of that stage up to a smidgen under 3khz, which also seems more reasonable.
I took the original unit apart to check. The feedback resistor is indeed marked 3M9. I don't have a DMM that goes that high to verify. I actually see the spot you were talking about--there's a missing resistor between the free pin of the gain knob and the output of the gain stage. I read somewhere that they re-use PCBs for different pedals so that makes sense, but on this one there is no resistor there. The serial cap with the gain pot is confirmed 680R and the cap there is 1uF.
The input and first gain stage caps are both 120pF
I see the 3n9 for the second gain stage as well as the 18k resistor, so those are correct at least. I also tentatively spotted the 1n5 that works in conjunction with R5.
Nothing seems outright wrong, but to be accurate I'll have to unsolder the board leads so I can properly examine it from both sides. I'm guaranteed to break one of the wires if I keep flipping it so I might as well get it over with. They're so delicate.
This pedal shares a PCB with the FX52 (fuzz) and FX50B (Overdrive Preamp). Depending on which schematic for the FX50B you look at, there may or may not be a 1M resistor in the first feedback network, which the board supports (by omitting it if necessary) and I assume there's a jumper to hook the gain pot terminal 1 to the output side. I'm looking here for the FX50B: https://www.freestompboxes.org/viewtopic.php?f=1&t=16676&p=186243#p186243 but it seems to match other schematics I've found.
FX52: https://www.freestompboxes.org/viewtopic.php?t=7294 (this one also seems to tie the pot to the output side)
All the FX53 schems I've been able to find have a floating pin on the FX53's gain knob:
QuoteAlso I think I found a post by Mark Hammer here explaining how the tone control works! https://www.diystompboxes.com/smfforum/index.php?topic=32262.0
Very glad you found that! I had typed out a similar explanation for this thread and somehow accidentally deleted it before posting and didn't feel like spending the time again, so I'm pleased I managed to be "saved by history"!
What makes no sense to me is the amount of calculated gain on tap, going by the posted schematic. Not even the Proco Rat has that much. If the FX53 was the Super-Duper-Ultra Distortion, maybe that would make sense, but not if it purports to emulate a tube amp. I should probably fire up my own stock FX53 down in the basement, and give a close listen, especially since - as mentioned - several in the FX5x line use the same board.
My curiosity was piqued, so I trotted downstairs and brought up the FX53 that one of my oldest friends gave me for my 65th birthday. Taking it apart, I see there are MANY differences between what is on the posted schematic in this thread, and what is actually on the board.
Now, my friend would not have modded it, and I certainly didn't, but neither of us have any sense of what any previous owner (he gave me a box of pedals he had bought at local yard sales and Value Village) might have done.
First off, it sure as shooting does not sound like it amplifies 5700x at max gain. It's dirty enough, but really seems to fall more in SD-1 territory, although the Tone control does not yield the sort of tonal extremes found in any of the TS-9 derivatives, and tends to be subtler.
The dual op-amps are a 1458 for the input and output buffer stages, and an LM353 for the clipping and tone stages, and NOT a pair of TL072 chips. Quite possible the change may have been done for later or prior issues, but this is what I see on the board.
The (2nd pair of) clipping diodes to ground (D3/D4), over on the right, are not there. It does appear that something might have been soldered to those pads in past, but there is much about the board that has the sloppiness of construction in that era.
The diodes in the feedback loop of the 2nd op-amp (D1/D2 on the left side) actually are in series with a 4k7 resistor, to "soften" the clipping further.
The feedback cap in that stage is 82pf, and not 120pf. And although the 680R/1uf ground leg is there, there is NO 3M9 feedback resistor. Instead, it is 1M5. In tandem with 82pf, that yields a high-end rolloff beginning around 1.3kz. Yes, one of the outside lugs of the 500k Drive pot goes to nowhere. Well, it goes to a pad that is neatly soldered over, and no further. You see that empty slot just below the 1M5 resistor. The wiper of the Drive pot goes to pin 2, as per the drawing. That yields a max gain of 2200x, which is still pretty dang high, but I gather the 4k7 series resistor and the high-end rolloff, takes much of the aggressive quality away.
There may well be other differences, but that's as far as I looked.
(https://i.postimg.cc/645fkKbk/100-9214.jpg) (https://postimg.cc/645fkKbk)
Quote from: Mark Hammer on December 20, 2023, 09:54:37 AMTaking it apart, I see there are MANY differences between what is on the posted schematic in this thread, and what is actually on the board.
...
The (2nd pair of) clipping diodes to ground (D3/D4), over on the right, are not there. It does appear that something might have been soldered to those pads in past, but there is much about the board that has the sloppiness of construction in that era.
The diodes in the feedback loop of the 2nd op-amp (D1/D2 on the left side) actually are in series with a 4k7 resistor, to "soften" the clipping further.
The feedback cap in that stage is 82pf, and not 120pf. And although the 680R/1uf ground leg is there, there is NO 3M9 feedback resistor. Instead, it is 1M5. In tandem with 82pf, that yields a high-end rolloff beginning around 1.3kz. Yes, one of the outside lugs of the 500k Drive pot goes to nowhere. Well, it goes to a pad that is neatly soldered over, and no further. You see that empty slot just below the 1M5 resistor. The wiper of the Drive pot goes to pin 2, as per the drawing. That yields a max gain of 2200x, which is still pretty dang high, but I gather the 4k7 series resistor and the high-end rolloff, takes much of the aggressive quality away.
Huh. So this (https://teardownit.medium.com/the-undeservedly-forgotten-classic-tube-%D0%BEverdrive-dod-fx-53-guitar-pedal-diy-aac7d7102031):
Quote"The overdrive section now has symmetrical soft clipping on two silicon diodes in the negative feedback circuit.
"Hard clipping DOD was not abandoned; that's why the pedal is called Classic Tube Overdrive-Distortion. Moreover, the asymmetrical hard clipping of the Overdrive Preamp 250 was replaced with a symmetrical one.
"This was all for nothing; the tech-savvy crowd modified the pedal by removing hard-clipping diodes, and they got a nice overdrive. This DOD FX-53's hard symmetrical clipping distortion ain't pretty and can't be turned off without modifying the pedal."
The various Youtube demos don't sound like a gain godzilla either, FWIW. It might be worthwhile to put the hard clippers on a switch, or to experiment with LEDs in that position, John Greene Glasspak style, so the hard clipping doesn't kick in until the gain stage is cooking.
Quote from: Mark Hammer on December 20, 2023, 09:54:37 AMFirst off, it sure as shooting does not sound like it amplifies 5700x at max gain. It's dirty enough, but really seems to fall more in SD-1 territory, although the Tone control does not yield the sort of tonal extremes found in any of the TS-9 derivatives, and tends to be subtler.
The dual op-amps are a 1458 for the input and output buffer stages, and an LM353 for the clipping and tone stages, and NOT a pair of TL072 chips. Quite possible the change may have been done for later or prior issues, but this is what I see on the board.
"Subtle" is not what I'd call the bass end of the tone control :D but the treble side certainly doesn't do much. It's far less shrill than, say, a Bluesbreaker.
I have the same opamps; I'm not sure where the TL072s came from. I may have put them in as a placeholder. I've experimented with a few different ones and haven't noticed much change. It would be interesting to put in some deliberately limited ones (i.e., OP07) for some testing, since I socketed my board.
QuoteThe (2nd pair of) clipping diodes to ground (D3/D4), over on the right, are not there. It does appear that something might have been soldered to those pads in past, but there is much about the board that has the sloppiness of construction in that era.
Mine has the hard clippers. Seems to be a popular mod to remove them.
QuoteThe diodes in the feedback loop of the 2nd op-amp (D1/D2 on the left side) actually are in series with a 4k7 resistor, to "soften" the clipping further.
This sounds like a fun thing to try; I'll add it to my experiment list.
QuoteThe feedback cap in that stage is 82pf, and not 120pf. And although the 680R/1uf ground leg is there, there is NO 3M9 feedback resistor. Instead, it is 1M5. In tandem with 82pf, that yields a high-end rolloff beginning around 1.3kz. Yes, one of the outside lugs of the 500k Drive pot goes to nowhere. Well, it goes to a pad that is neatly soldered over, and no further. You see that empty slot just below the 1M5 resistor. The wiper of the Drive pot goes to pin 2, as per the drawing. That yields a max gain of 2200x, which is still pretty dang high, but I gather the 4k7 series resistor and the high-end rolloff, takes much of the aggressive quality away.
I've attached mine for reference. It seems to match the schematic I posted. I didn't look very hard though before work.
(https://i.postimg.cc/G8P0h5Tj/fx53-front-toots.jpg) (https://postimg.cc/G8P0h5Tj)
Quote from: Ben N on December 20, 2023, 10:36:04 AMThe various Youtube demos don't sound like a gain godzilla either, FWIW. It might be worthwhile to put the hard clippers on a switch, or to experiment with LEDs in that position, John Greene Glasspak style, so the hard clipping doesn't kick in until the gain stage is cooking.
Yeah, that's on the list for experimentation! As well as different LED types. I don't have many hard clippers in my collection yet.
My serial number in https://serial-number-decoder.com/dod-pedals/dod-serial.htm says it was made in 1983 but I think I may have mixed it up either either the backplate for the flanger or the delay pedal, since the classic tube started in 1989. Actually no, that one says 1984. Who knows. I wish the model number was on the plate as well. I got these all off eBay and the delay and classic tube were both listed for parts. The delay pedal backside looks hand-drawn, very curvy and sparkly, while the classic tube has the usual hard angles.
The classic tube shows signs of repair, mostly in the form of a new battery connector and LED repair. Nothing looks disturbed on the board.
I wanted to thank you all for the wonderful conversation around this project. I'm an electronics newbie and the pedal building is a side hobby to relax and learn. And to get some cool new sounds on the board of course. (Shoutout to the Aion Vulcan octave fuzz, by far the coolest thing on my board so far.)
Here's my custom board prototype. It's a bit of a mess. And I ordered the wrong DC jack so it's hardwired for battery at the moment. AION bypass board used because I like them.
Personal information redacted...I should probably make a pen name for my boards or something. Hear me out: Toot Sounds. Too immature? ;)
(https://i.postimg.cc/p5LMyBH5/board.jpg) (https://postimg.cc/p5LMyBH5)
Quote from: tootsMcgee on December 20, 2023, 12:33:01 PMQuote from: Mark Hammer on December 20, 2023, 09:54:37 AMFirst off, it sure as shooting does not sound like it amplifies 5700x at max gain. It's dirty enough, but really seems to fall more in SD-1 territory, although the Tone control does not yield the sort of tonal extremes found in any of the TS-9 derivatives, and tends to be subtler.
The dual op-amps are a 1458 for the input and output buffer stages, and an LM353 for the clipping and tone stages, and NOT a pair of TL072 chips. Quite possible the change may have been done for later or prior issues, but this is what I see on the board.
"Subtle" is not what I'd call the bass end of the tone control :D but the treble side certainly doesn't do much. It's far less shrill than, say, a Bluesbreaker.
I have the same opamps; I'm not sure where the TL072s came from. I may have put them in as a placeholder. I've experimented with a few different ones and haven't noticed much change. It would be interesting to put in some deliberately limited ones (i.e., OP07) for some testing, since I socketed my board.
QuoteThe (2nd pair of) clipping diodes to ground (D3/D4), over on the right, are not there. It does appear that something might have been soldered to those pads in past, but there is much about the board that has the sloppiness of construction in that era.
Mine has the hard clippers. Seems to be a popular mod to remove them.
QuoteThe diodes in the feedback loop of the 2nd op-amp (D1/D2 on the left side) actually are in series with a 4k7 resistor, to "soften" the clipping further.
This sounds like a fun thing to try; I'll add it to my experiment list.
QuoteThe feedback cap in that stage is 82pf, and not 120pf. And although the 680R/1uf ground leg is there, there is NO 3M9 feedback resistor. Instead, it is 1M5. In tandem with 82pf, that yields a high-end rolloff beginning around 1.3kz. Yes, one of the outside lugs of the 500k Drive pot goes to nowhere. Well, it goes to a pad that is neatly soldered over, and no further. You see that empty slot just below the 1M5 resistor. The wiper of the Drive pot goes to pin 2, as per the drawing. That yields a max gain of 2200x, which is still pretty dang high, but I gather the 4k7 series resistor and the high-end rolloff, takes much of the aggressive quality away.
I've attached mine for reference. It seems to match the schematic I posted. I didn't look very hard though before work.
(https://i.postimg.cc/G8P0h5Tj/fx53-front-toots.jpg) (https://postimg.cc/G8P0h5Tj)
Quote from: Ben N on December 20, 2023, 10:36:04 AMThe various Youtube demos don't sound like a gain godzilla either, FWIW. It might be worthwhile to put the hard clippers on a switch, or to experiment with LEDs in that position, John Greene Glasspak style, so the hard clipping doesn't kick in until the gain stage is cooking.
Yeah, that's on the list for experimentation! As well as different LED types. I don't have many hard clippers in my collection yet.
My serial number in https://serial-number-decoder.com/dod-pedals/dod-serial.htm says it was made in 1983 but I think I may have mixed it up either either the backplate for the flanger or the delay pedal, since the classic tube started in 1989. Actually no, that one says 1984. Who knows. I wish the model number was on the plate as well. I got these all off eBay and the delay and classic tube were both listed for parts. The delay pedal backside looks hand-drawn, very curvy and sparkly, while the classic tube has the usual hard angles.
The classic tube shows signs of repair, mostly in the form of a new battery connector and LED repair. Nothing looks disturbed on the board.
I wanted to thank you all for the wonderful conversation around this project. I'm an electronics newbie and the pedal building is a side hobby to relax and learn. And to get some cool new sounds on the board of course. (Shoutout to the Aion Vulcan octave fuzz, by far the coolest thing on my board so far.)
The marker scribblings at the bottom of my board say "FX53" as well, but say "35 H" instead of "7 H". Does that mean anything? I have no idea.
It IS quite possible that mine was modded by the 2nd-to-last owner (or even before then), but we have no way of knowing. My puzzlement and urge to check the board on mine was really spurred by a seeming mismatch between the
name of the pedal and the characteristics suggested by the components and values. The pedal name suggests it was aiming for something capable of brute force, but also a little tamer than other pedals in the series.
Finally, I'll just note that stickers with the names of what I assume were either assemblers or QC testers line one side of the inside of the enclosure. They are: Crystal Barlow (484), Donna Ramos (211), "Silver" (208) and Cesar Robles (576). Using the dating-by-serial-number app you linked to, it says mine was made in or around January 1983. But the site also indicates the FX53 was made between 1989 and 1997, that DOD recycled serial numbers and that the FX series were made in the USA and China. So I don't know what to believe.
Quote from: tootsMcgee on December 20, 2023, 12:56:05 PM(https://i.postimg.cc/p5LMyBH5/board.jpg) (https://postimg.cc/p5LMyBH5)
I think that looks very nice and much neater than DODs, as well as not taking up half an acre! :icon_cool:
Quote from: Mark Hammer on December 20, 2023, 02:56:57 PMFinally, I'll just note that stickers with the names of what I assume were either assemblers or QC testers line one side of the inside of the enclosure. They are: Crystal Barlow (484), Donna Ramos (211), "Silver" (208) and Cesar Robles (576).
I see Crystal Barlow and Silver in mine!
I hear if you get a set of all 10 you get a free Happy Meal (tm).
As for the huge gain, I've noticed that on the stock unit in particular, the gain increases massively at the very end of the drive knob, rather like a fuzz pedal. My assumption is that because the opamp gain is given by 1 + 3.9M / (P1+680), that as the resistance of the gain pot approaches 0, the rate of change of the opamp gain increases (geometrically?)
e.g., going from 500K resistance to 450K changes the gain from around 7.8 to around 8.6. but going from 50K to 0 on the gain pot changes the opamp gain factor from 78 to 5736ish. So the last bit of knob travel is doing quite a lot. The last 30 degrees or so of travel are labeled as a "Boost".
The manual from https://www.synthxl.com/dod-fx-53/ says "The FX53 Classic Tube duplicates the warm, smooth distortion and long, singing sustain and harmonics of the early 60s amp stacks made famous by superstars like Jimi Hendrix and supergroups like Cream and Led Zeppelin." I guess that explains what sounds they had in mind.
Quote from: ElectricDruid on December 20, 2023, 03:10:04 PMI think that looks very nice and much neater than DODs, as well as not taking up half an acre! :icon_cool:
Thanks! The whole reason I wanted to do this was to make it smaller (and to practice my Kicad skills)
Quote from: tootsMcgee on December 20, 2023, 05:33:25 PMI hear if you get a set of all 10 you get a free Happy Meal (tm).
:icon_lol: :icon_lol: :icon_lol:
You know, reassembling the pedal now, I think it may well have been me that removed the second pair of diodes. When I saw and recalled that I had installed an additional mini phone jack to enable remote switching, it dawned on me that I may well have removed those diodes. The timestamp I have on the different FX53 schematics in my DOD directory is well before I acquired the pedal, so I may well have thought "I don't really need thoseones there, because I have these other ones over here".
Would I have changed the 3M9 to 1M5 and the 120pf to 82pf? Would I have stuck the 4k7 in series with the diodes? Absolutely no recollection of doing so,but the diode-pair removal seems in keeping with my approach back then.
So, let me put a stop to any possible rumours right now, and just say that the use of a MC1458 and LM353 chip-set, instead of a pair of TL072 chips IS a change on the production, but I cannot vouch for anything else I might have noticed is different in mine. Should it be the case that somebody else finds the same changes in theirs that I have on mine, well THEN we can start up the rumour mill. But for now, I'll just make a 90% retraction. Apologies for unintentionally misleading.
Quote from: Mark Hammer on December 20, 2023, 09:54:37 AMThe dual op-amps are a 1458 for the input and output buffer stages, and an LM353 for the clipping and tone stages, and NOT a pair of TL072 chips.
LF353. as good as exactly the same as a TL072.
Yes, although the 1458 not so much.
Quote from: duck_arse on December 21, 2023, 10:09:01 AMLF353. as good as exactly the same as a TL072.
Ancient days industrial standard.. :icon_wink:
1458 actually is a dual 741..
Is there a particular reason they chose that combo of opamps? I put TL072 out of convenience in mine because, well, it's basically what you see in the dictionary when you search for dual op-amp. Then I switcherooed the clipping stage one and flirted with others to see how they saturated without any diodes at all. The rc4558 at least sounded nice. This might be something to slap on a breadboard instead of working on a "completed" pedal.
I need to get an IC puller...
Quote from: Mark Hammer on December 20, 2023, 09:54:52 PMBut for now, I'll just make a 90% retraction. Apologies for unintentionally misleading.
No worries at all, it was enlightening to see the inside of someone else's unit. I have some mod ideas now!
1458 chips can be useful in high-gain situations that are intended to produce clipping in the chip due to headroom limitations. In this instance, however, it is being used for the input and output buffers, in unity-gain mode. FWIW, a TL072 would be okay in that slot,but maybe a 5532 might be even quieter.
Quote from: duck_arse on December 21, 2023, 10:09:01 AMLF353. as good as exactly the same as a TL072.
In the day, the LF353 was different from a TL072. Different circuitry and process. Overlapping specs. '072 quickly got a penny cheaper than '353, and everybody jumped. I understand now everything is an '072 with varying code-stamps.
Some "dual 741" were literally that but I *thought* the early 1458 were improved 741-like guts. But then, late 741s consistently ran to the high side of original '741 specs.
Get some very high quality "gold" nail-polish, that always sounds better.
Quote from: Mark Hammer on December 20, 2023, 02:56:57 PMFinally, I'll just note that stickers with the names of what I assume were either assemblers or QC testers line one side of the inside of the enclosure. They are: Crystal Barlow (484), Donna Ramos (211), "Silver" (208) and Cesar Robles (576).
So the Tadeo Gomez, Lydia Sanchez, Lupe Lopez & Maybelle Ortega of DOD?
I've been experimenting with different component values alongside an LTSpice sim. The two big positive changes so far have been a reduced cap in the feedback loop (10p) and a reduced cap in the tone stack (1n instead of 3n9). The pedal still has more high end rolloff than I want, but it is getting me in the right area.
It's hard to do this without messing up the charm of the original pedal. When I hear that kind of organic, ragged, almost fuzz-ish distortion, it makes me think of a gain or clipping stage getting slammed with lots of low end. Removing too much of that turns it into something else. I just want a little more note definition in the upper registers so that my guitar doesn't sound like a beeping synth.
One nice thing about this circuit is that the sustain is prone to subtle but pleasant feedback. I don't know if it's inherent to the circuit design, or a byproduct of playing with higher gain designs than usual.
I tried a few different clipping diodes in the hard clip section--1n4148 and BAT46 are fuzzy, 3mm LEDs are a little clearer but not much. I think a switch to toggle soft, hard, and soft+hard would be a more fruitful thing to explore next than swapping more parts right now.
Sound demos eventually...I did put the clone with the schematic values next to the original pedal and in an informal A/B test I doubt you could hear the difference in a mix, so I know the circuit is as designed at least!
I just found out that PedalPCB actually has a PCB already for this series 😅 https://www.pedalpcb.com/product/fx5x/
That would've saved me some time...but designing my own PCB from the schem was great for education.
I added a soft/hard+soft/hard clipping selector switch, which is a fun addition. I'm not sure which I like best. The tone control is more useful than I thought. On the original, it sounds a bit more treble-y near or slightly right of midpoint. The boost on the high side feels more centered on mid-highs. I really feel that the gobs of bass response is a key part of the original's sound. It's not a tight-sounding pedal and never will be without more significant value changes.
Time to build another one with different values to experiment again!
NECROPOST
Picking up this project again after a break. I'm pretty happy with it. My diode mod has soft/soft+hard/hard clipping toggleable by an on-on-on DPDT switch. I've found that either soft or soft+hard is the sound I like, hard-only is kinda meh.
The biggest problem I've found with this pedal is that it's just not loud enough. Unity gain is pretty high on the volume knob and turning up the gain after a point just shreds the signal and doesn't boost the volume much. What kind of shenanigans can I do to make it, well, louder?
There's a non-inverting buffer at the very end at unity gain. I'm thinking of modifying that to add a few dB of gain there by cutting the trace and adding a 10k resistor across the feedback loop, as well as another 10k resistor to ground off the inverting input. In theory I think this should go from unity gain to 2, which I think is a simple 3dB boost? From there I can tweak the values.
Any pitfalls I should watch out for? Whatcha think?
Just for grins, to keep us on track, could you post an 'as-built' schematic showing that output buffer? 8)
I was being pretty cagey with the schem/layout before I found out that PedalPCB did it better (https://www.pedalpcb.com/product/fx5x/) so I don't have to worry anymore ;) Still, please don't yoink this for a commercial project. You have better choices and I don't want anyone to think I'd offer support for this mess! I'm a hobbyist.
The board layout is...brute force to say the least. I had help from someone much more talented than me, so it's not a complete disaster, but still.
Dev board:
(https://i.postimg.cc/3dkhfFJD/devboard.jpg) (https://postimg.cc/3dkhfFJD)
Layout:
(https://i.postimg.cc/V552zLyx/layout1.jpg) (https://postimg.cc/V552zLyx)
Schem:
(https://i.postimg.cc/Y43TCvCH/schem1.jpg) (https://postimg.cc/Y43TCvCH)
The current experimental changes, as wired, are:
120p in feedback loop is 47p to raise treble cut-off a bit.
Added switches to disconnect diodes
Questions:
Do I need the 2M2 pulldown at all? The original FX53 schem does not have it. I blindly added it and now I don't remember why.
Planned changes for future board revision:
1. Add jumper for the gain knob pins 1 and 2 connection so it can be wired like the other FX5x pedals
2. If the buffer changes work out, scoot some stuff around to add fixed or soldering-adjustable boost there.
Overall though, even the original FX53 I have just isn't a particularly loud pedal. It can get very very distorted towards the last travel on the gain knob, but that doesn't actually raise the level much. It gets lost in a mix easily.
For fun (I don't know what I'm doing) here's my sim. The buffer *appears* to do what I want with that fixed resistor config, by moving the graph up without modifying it.
The gain knob is fixed at "halfway" (250k/250k voltage divider)
The graphs represent full bass boost, flat, and full mid boost (left, middle, right position)
By far the biggest offender for the treble rolloff is the 120p that is usually in the feedback network. The 1khz point is as follows with the tone knob in the middle (mostly flat) position
120p: -10dB
47p: -5dB
10p: 0dB
(https://i.postimg.cc/0rpXk6WH/sim2.jpg) (https://postimg.cc/0rpXk6WH)
Weather update:
I put the 120pf back in place in the feedback network. 47pf did make it brighter but also removed some of the oomph that makes these pedals awful/fun/awfully-fun.
It's still a little bit quieter than the original FX53 I have at full blast and I can't figure out why. At this point I have two guesses:
1. Something inherent to the switching mechanism (clean signal bleeding in?)
2. TL072 in place of the LF353 is clipping or otherwise not able to push the signal as high. (Yes, I know the schem I posted says TL072 for both, but the unit in my hand has a LF353).
About switching. Part of the switch mechanism seems to double as a filter? Looking at the original schematic again:(https://experimentalistsanonymous.com/diy/Schematics/Distortion%20Boost%20and%20Overdrive/DOD%20FX53.gif)
I removed Q2, the upper J201, but as part of that I also removed R? (220k) and C13, which may or may not have been a good idea. The diagram you guys made for me on pg1 notes that that area around J201 is actually part of a filter. But in my head I've always treated it as basically a way to short the signal to ground on the upper side. Should I have left the 220k + 1uF C13 in?
I'm waiting on my 353s so we can find out definitively if that's the issue.
Oh! And I found my intermittent switching problem. Whoever had a go at this pedal before me had rotated the rectangular power jack housing so that the contacts were facing up instead of sideways. This made the board contact it and bent the board a bit (not good!)
Finally, I was experimenting with taking photos of the board that I took out for cleaning and resoldering the offboard wiring. This isn't very useful as a technique because you can't see the cap values very well, but it looks cool so I'm sharing it.
(https://i.postimg.cc/d7tWL91V/fx53-artsy.jpg) (https://postimg.cc/d7tWL91V)
Found it. Turns out BAT46 has a lower VF than 914/1n4148. So that takes care of that.
Now to build some of the variants...