FX53 Classic Tube tone stack questions

[tootsMcgee]

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.



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

[antonis]

Of course, some of them (like around upper J201 and Tone pot..) interact in a complex form..

[tootsMcgee]

Of course, some of them (like around upper J201 and Tone pot..) interact in a complex form..

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...

[antonis]

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.

Noy much (audible) difference..
When J201 is ON, 220k is effectively bypassed so any frequency higher than 16Hz is shunted to GND..

[tootsMcgee]

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!

[ElectricDruid]

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?

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.

[tootsMcgee]

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?

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?

[antonis]

Any suggestions on how to make the damn thing brighter?

By cutting lows, perhaps..??
(starting from C1/R2 ..)

[tootsMcgee]

Any suggestions on how to make the damn thing brighter?

By cutting lows, perhaps..??
(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...

[ElectricDruid]

Any 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.

[tootsMcgee]

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.

[ElectricDruid]

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.

Yeah, me too! I'd read that article. Now all we need is a DOD engineer to write it for us!

[Mark Hammer]

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 (    ), 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.

[ElectricDruid]

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.

[tootsMcgee]

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 (    ), 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

[Mark Hammer]

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 (    ), 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:

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

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.

[Mark Hammer]

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.

[Ben N]

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.
...

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:

"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.

[tootsMcgee]

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.

"Subtle" is not what I'd call the bass end of the tone control 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.

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.

Mine has the hard clippers. Seems to be a popular mod to remove them.

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.

This sounds like a fun thing to try; I'll add it to my experiment list.

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.

I've attached mine for reference. It seems to match the schematic I posted. I didn't look very hard though before work.

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.

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.)

[tootsMcgee]

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?