ODS 124 - revisited for the 21C

[Stephen1966]

One other thought with this circuit... It might be prudent to restore the level pot coming off the plates from the mixer and place this before the master volume in the chain. I figured the Master pot could take the level pots role but I don't necessarily want to increase the gain of the signal in the reverb stage. Maybe the level pot could be a trimmer I could set and forget...?

Stephen

[markusw]

That's a good catch... thank you. I will have a think about the attenuation required. On the mixing side, V3 in the schematic, we have a 1M resistor taking that role. I have a suspicion that I can look to the OD for a suggestion of the usable values. The OD first triode used the 220k+350k trimmer+68k grid stopper. The reverb also has a 68k grid stopper and so, with a combined series resistance of around 320k before the OD, this could be a useful start point, anywhere I suppose, up to 1M. If I knew the required grid voltage, I could work out the value of the attenuation. but failing that, I have a decade resistor box and I could pin it down using the clean channel to measure the signal voltage and avoid clipping.

I really appreciate you pointing this out. Thanks again.

Stephen

Regarding reverb phase.
It has already been discussed here by people much more knowledgeable than me.
https://ampgarage.com/forum/viewtopic.p ... 7&start=30
If you have the impression that there are cancellation effects when mixing clean and reverb you can simply swap the reverb xfmr wires, as suggested edit:"by Gary" in this thread.

Markus

[Stephen1966]

That's a good catch... thank you. I will have a think about the attenuation required. On the mixing side, V3 in the schematic, we have a 1M resistor taking that role. I have a suspicion that I can look to the OD for a suggestion of the usable values. The OD first triode used the 220k+350k trimmer+68k grid stopper. The reverb also has a 68k grid stopper and so, with a combined series resistance of around 320k before the OD, this could be a useful start point, anywhere I suppose, up to 1M. If I knew the required grid voltage, I could work out the value of the attenuation. but failing that, I have a decade resistor box and I could pin it down using the clean channel to measure the signal voltage and avoid clipping.

I really appreciate you pointing this out. Thanks again.

Stephen

Regarding reverb phase.
It has already been discussed here by people much more knowledgeable than me.
https://ampgarage.com/forum/viewtopic.p ... 7&start=30
If you have the impression that there are cancellation effects when mixing clean and reverb you can simply swap the reverb xfmr wires, as suggest in this thread.

Markus

Aha! Excellent idea! Thank you. I think Mr Fuchs mentioned this once before as well!

[rootz]

Excellent information on the reverb phase, makes sense. If you want to keep your dry and wet signal in phase, there's another trick to do that job: replacing the mixer triode with a cathode follower. That also means you don't need the voltage divider in the dry signal path anymore.

But still, I've heard some clips of a #60 clone and that reverb sounds simply delicious. I highly recommend to take a look at them (I do not have al link though). If you allow me to pollute this thread one more time with a schematic...

[Stephen1966]

Excellent information on the reverb phase, makes sense. If you want to keep your dry and wet signal in phase, there's another trick to do that job: replacing the mixer triode with a cathode follower. That also means you don't need the voltage divider in the dry signal path anymore.

But still, I've heard some clips of a #60 clone and that reverb sounds simply delicious. I highly recommend to take a look at them (I do not have al link though). If you allow me to pollute this thread one more time with a schematic...

I've got to say it, you really know how to find an elegant solution. Like finding needles in a haystack. I have to say also, ideas of attenuation and brute-force impedance blocking, intuitively speak to me as just ways to bottleneck the signal. Looking at my current design I strongly suspect there is a somewhat narrow window of optimum attenuation possible and a very much broader range of meh attenuation.

Your solution, the cathode follower is just brilliant. For me, it kills two (no three) birds with one stone: it keeps the signal in phase, it makes the voltage divider redundant, and finally, it takes me another step closer in spirit at least, to the holy grail - the 002. The #060 layouts help a lot - I was psyching myself up to figure all that out without realising there was a ready made solution already out there, though if I am seeing what I think I'm seeing the OD section is placed after the reverb in the layout, next to the PI (though not in the schematics). It really is possible to split the 124 board cleanly between the OD section and the PI, so I would do that and place the reverb circuit between the OD and PI: in the logical order of the signal path... There's quite a lot to digest here (and thank you again, you are always welcome in my book) and there aren't enough hours in the day but I will have a new implementation schematic for you shortly.

Stephen

[Max]

The links to the clips of ODR #0060 that I posted here https://ampgarage.com/forum/viewtopic.p ... 07#p206307 about 10 years ago no longer work.

But the article published by Premier Guitar about ODR #0060 is still online: https://www.premierguitar.com/gear/adve ... e-od-100wr

And all the pictures, layouts etc. and the discussion here https://ampgarage.com/forum/viewtopic.php?f=4&t=17935 of course too.

AFAIR 10 years ago the fooswitch was wired in the way that when pressing its button you activated OD and PB at the same time.

Best regards,

Max

[Stephen1966]

The links to the clips of ODR #0060 that I posted here https://ampgarage.com/forum/viewtopic.p ... 07#p206307 about 10 years ago no longer work.

But the article published by Premier Guitar about ODR #0060 is still online: https://www.premierguitar.com/gear/adve ... e-od-100wr

And all the pictures, layouts etc. and the discussion here https://ampgarage.com/forum/viewtopic.php?f=4&t=17935 of course too.

AFAIR 10 years ago the fooswitch was wired in the way that when pressing its button you activated OD and PB at the same time.

Best regards,

Max

Thank you Max, reading that post I had already discovered the premier guitar links were dead so using the new link I was glad to able to read the article. Interestingly, it confirmed the post OD reverb ideas I had been entertaining and that is how I would implement it too. There may be good reasons why a reverb further back in the circuit makes sense (pre OD) but the general consensus here is that the post OD yields better results - perhaps because they are more predictable in part. I've also been studying the cathode follower idea and the fact that one of its greatest benefits is as an impedance buffer (according to Blencowe) seems a distinct advantage over those without. Tonally speaking. I'm thinking about the #060 circuit now and find it can easily be applied to my last iteration utilising the benefits and mechanical advantages suggested by you, rootz and by Markus. Thank you. It will take me some time though... this week is especially busy. But I'll have a new circuit implementation for next week. On a side note, I can't count the number of times I have hit dead links. The old photobucket links are all dead as far as I can tell. If anyone has any idea if it would be legal (or desirable) to archive these files in some way...? Anyway, a topic for another day.

Considering the idea of a shared OD and PAB switch, it's difficult to know why MrD arranged it that way, but we might speculate that because of the inference in the article that the amp was possibly a test bed in some ways, it was some kind of retrofit. Anyway, in my design, it won't be necessary to do that. I'll be using a four button pedal with an additional decoupling switch between the OD and FET. It also just happens that I have a stack of mahogany downstairs and I'll be making it... Dumblish.

Stephen

[rootz]

The 60 is reverb after od, my schematic (essentially a 60 with some minor tweaks) is reverb after od too.

Many links about the 60 are dead. There are some good sources of info left though (mucho thanks Max!). Here on this forum too. You can confirm most of the layout I posted before against the real board layout in the pictures.

I have absolutely no clue how a CF would sound. I took my original design ideas from the wonderland: two triodes mixing the dry and wet signals. It works well and the cleans still are very reminiscent of the Wonderland I build before. Also, you do not need a pre driver and double 12at7 driver for heaps of reverb per se.

I wire my od/PAB pedals a bit different. I use a regular stereo jack and cable to switch both. Take a look at the lower right hand corner of my schematic. The most left switch is for the reverb, but I never use that one. So just ignore that switch for now. Between the lower PAB and od switches there is a SPST switch and a diode. When you close that switch, PAB is auto engaged in od mode. Simple and effective. I must say, I never use this function. It is very easy to engage PAB and od the same time with one stomp of your foot IF you place the individual foot switches just close enough.

I’ll be back to post the idea I had about the cf mixer stage.

Edit. Why wire PAB and od switches together? Simple, if you ask me: when you always use PAB with od. I have a HRM amp here wired that way and I think it can be/is a useful solution for such amps.

[Stephen1966]

The 60 is reverb after od, my schematic (essentially a 60 with some minor tweaks) is reverb after od too.

Many links about the 60 are dead. There are some good sources of info left though (mucho thanks Max!). Here on this forum too. You can confirm most of the layout I posted before against the real board layout in the pictures.

I have absolutely no clue how a CF would sound. I took my original design ideas from the wonderland: two triodes mixing the dry and wet signals. It works well and the cleans still are very reminiscent of the Wonderland I build before. Also, you do not need a pre driver and double 12at7 driver for heaps of reverb per se.

I wire my od/PAB pedals a bit different. I use a regular stereo jack and cable to switch both. Take a look at the lower right hand corner of my schematic. The most left switch is for the reverb, but I never use that one. So just ignore that switch for now. Between the lower PAB and od switches there is a SPST switch and a diode. When you close that switch, PAB is auto engaged in od mode. Simple and effective. I must say, I never use this function. It is very easy to engage PAB and od the same time with one stomp of your foot IF you place the individual foot switches just close enough.

I’ll be back to post the idea I had about the cf mixer stage.

Edit. Why wire PAB and od switches together? Simple, if you ask me: when you always use PAB with od. I have a HRM amp here wired that way and I think it can be/is a useful solution for such amps.

You have more ideas!... Please, do! I'm more than happy to build it. I'm not sure English (or any spoken language) is good for describing the sound of tone we might expect, that's where we get into crystals, and marshmallow fields I think The adjectives available to us are like butcher's knives, when what we really need is a scalpel. But that said, I expect that the cathode follower will allow for a much softer clipping. As the input signal grows the signal generated by the cathode already starts to clip and maybe Blencowe has a point that these things are best seen on an oscilloscope. It can be very difficult describing that in the sound we hear but it is a clearly visible phenomenon. I think the loosest analogy I can apply to it is a fuzz pedal. The signal at its driest, so closely resembles the input signal as to be indistinguishable but as you dial in the fuzz, the spikes appear, and grow to be the dominant characteristic of the sound. It [the CF] also generates a just slightly less than unity gain so there is no danger of overt compression or off the rails harmonic distortion... a smoother, more representative tone and highly linear. Hi-fi sound in all but name. A flatter response? Maybe. But given the tone shaping that occurs by virtue of the circuit topology in the preamp stages, it sounds exactly right for my purposes... unadulterated, rich, complex, very dynamic, and great touch sensitivity all on display without any discoloration.

You have a good point about the shared OD and PAB switching... makes absolute sense, but within reason, I prefer the greater options of keeping them separate. I haven't really got into your schematic yet (no time at present) but I appreciate the notes and I'll definitely be referring back to them when I do finally start on the schematic. Soon, for sure...

Stephen

[rootz]

Haha yes those descriptions! Honestly I'm not sure what to expect from the next twi ideas. The follower has some advantages, but are they big enough? I expect the follower to stay clean no matter what you throw at it. It should be able to handle the maximum voltage v1b is able to put out. It is not a follower in the way the one in a Bassman or jtm45 is.

The other idea is fun too I think. Ive never seen a cathodyne splitter used like this before (and there's probably a good reason for that). I thought I might share it, maybe you guys have seen this idea before or have experience with it.

[Stephen1966]

Haha yes those descriptions! Honestly I'm not sure what to expect from the next twi ideas. The follower has some advantages, but are they big enough? I expect the follower to stay clean no matter what you throw at it. It should be able to handle the maximum voltage v1b is able to put out. It is not a follower in the way the one in a Bassman or jtm45 is.

The other idea is fun too I think. Ive never seen a cathodyne splitter used like this before (and there's probably a good reason for that). I thought I might share it, maybe you guys have seen this idea before or have experience with it.

I'm still not totally on the ball here, but thoughts so far... the parallel triode (12AT7) I agree, it seems like overkill. It could have something to do with the SNR though. Blencowe called it 'largely academic' in guitar amps and yet it works. Higher performance often equals incrementally smaller changes = I would keep it.

Feeding the input side of the reverb from the mixer plate? Hmm, shouldn't we be looking to keep the voltage and current as stable as possible at the plate...? I don't know, like a buffer of some sort? Also, I would add a grid stopper on the mixer and maybe reduce the value of the build out resistor (1M in your designs?), both to prevent oscillation. I have zero experience of these, it's purely hypothetical. I would welcome better minds to this conversation.

Stephen

[talbany]

Steven
There are 2 main advantages to the (low impedance) tweed mixer AFAIK. 1 being? there is less loading on the mixer itself on both the wet and dry signals. 2nd is the frequency response. The follower gives you a low source impedance that allows the reverb mixer to work as designed vs a mixer that has too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response (typically quite a few dB loss of the highs).The cathode follower prevents this loss, allowing the mixer to retain more of it's "theoretical" frequency response.(Same for driving tone stacks) This helps give the reverb that thicker shimmering effect on the reverb (for lack of better word )
On top of that you get a nice low level output impedance signal supplied to the send jack to help drive any external outboard gear and the only thing you would need is some kind of return amp for the line in.

BTW..Once that signal hits the tank? all bets are off on the phase relationship.


You know at some point your going to have fire up the iron?
Tony

[Stephen1966]

Steven
There are 2 main advantages to the (low impedance) tweed mixer AFAIK. 1 being? there is less loading on the mixer itself on both the wet and dry signals. 2nd is the frequency response. The follower provides a low source impedance that allows the reverb mixer to work as designed. If the mixer is given too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, (typically quite a few dB loss of the highs).The cathode follower prevents this loss, allowing the mixer to retain more of it's "theoretical" frequency response.(Same for driving tone stacks) This helps give the reverb that thicker shimmering like hi end (for lack of better word )
On top of that you get a nice low level output impedance signal supplied to the send jack to help drive any external outboard gear and the only thing you would need is some kind of return amp for the line in.

BTW..Once that signal hits the tank? phasing largely becomes irrelevant


You know at some point your going to have fire up the iron?
Tony

Fire up the iron yeah! For sure, but I have to tidy up first

About the loading on the mixer... are you suggesting, as rootz did, the single tube mixer might not be up to the job? And isn't the Tweed mixer a four tube design? I'm trying to keep an eye on transformer loading as well. I have no real idea of what the source impedance could be but your comments are well noted. If it has to be four tubes, so be it. Dumbleator parts are arriving even as I speak...

[talbany]

Steven
There are pros and cons to everything amp building..I just gave 2 of the Pro's you can decide on the cons and weigh your options..I don't know what else to tell you or a better way to explain the Tweed?..I am not sure what designs your contemplating..Maybe rootz can help with that

Tony

[Stephen1966]

Steven
There are pros and cons to everything amp building..I just gave 2 of the Pro's you can decide on the cons and weigh your options..I don't know what else to tell you or a better way to explain the Tweed?..I am not sure what designs your contemplating..Maybe rootz can help with that

Tony

Thanks Tony. It's much appreciated. And well put... the reverb is really the last piece of the puzzle for me and rootz's latest designs look very promising... I might have to play with the filter caps and add a couple of extras for the reverb to deliver the voltages it needs, but then, with all the elements in place I will be able to show everyone the whole amp design. Sorry, I don't wish to sound coy about it, but I feel I'm pretty close to a workable design now.

Rootz, in your simulation, is there any way to observe the impedance loading on the mixer? The #060 looks similar enough to the #124 to put us in the ball park.