ODS 124 - revisited for the 21C

[Max]

If I understand you correctly, then you are apparently - at least in principle - interested in an amplifier that - at least to a certain extent - combines properties of an ODR with properties of a Seel String Singer. In this context you could perhaps use this block diagram https://schematicheaven.net/dumbleamps/dblock.pdf for this overdrive special with reverb https://www.vintageguitar.com/31567/dum ... -od150-wr/ as some kind of "structural inspiration", because A. Dumble once commented on this Overdrive Special as follows: "If you switch off the overdrive circuit, you have a Steel String Singer". I have always understood this comment to mean that Alexander Dumble may not consider the filter circuit to be a completely indispensable part of whatever makes a Steel String Singer a Steel String Singer.

All the best,

Max

[Stephen1966]

If I understand you correctly, then you are apparently - at least in principle - interested in an amplifier that - at least to a certain extent - combines properties of an ODR with properties of a Seel String Singer. In this context you could perhaps use this block diagram https://schematicheaven.net/dumbleamps/dblock.pdf for this overdrive special with reverb https://www.vintageguitar.com/31567/dum ... -od150-wr/ as some kind of "structural inspiration", because A. Dumble once commented on this Overdrive Special as follows: "If you switch off the overdrive circuit, you have a Steel String Singer". I have always understood this comment to mean that Alexander Dumble may not consider the filter circuit to be a completely indispensable part of whatever makes a Steel String Singer a Steel String Singer.

All the best,

Max

Thanks Max, that's an affirmative to your opening statement. The heart of the amp is the 124 circuit as clean and unadulterated as I can make it but with switchable mods that borrow rootz's reverb from the 005/High Plains Drifter (or 002 - I haven't fully ruled that one out just yet), the bright switch from the 183, Martin's footswitchable FET (it doesn't make sense to me to have two input jacks when you can only use one at a time), and Martin's precision power supply. A four button footswitch. And now, following the breadcrumbs you linked above, the trigger, taper, level and ratio pots on the OD from the 121.

I don't know about the necessity of the filter circuit for the SSS, but the low plate Skyliner EQ already seems to fit the bill for a HiFi-like clean preamp. I say HiFi-like because it is probably noticably far from being hi-fi but a with a punch and expansiveness that easily surpasses the Fender counterparts. In the imagination, that suggestion of clarity on the precipice of chaos is what draws me to this particular amp. While I love SRV, I still like my rock a little more and life without overdrive is a bit too sanitary... for me.

As I mentioned, I spent the last couple of hours, following the breadcrumbs of the 121 and I'm intrigued by the shaping possibilities of the four pot overdrive. And I'm guessing, it wouldn't be too difficult to implement...

You wrote:

"... The overdrive section of the preamp is very sophisticated. The overdrive tube is a 7247 and not a 7025 as usual. This amplifier has four (staggered) pots to control the Overdrive: trigger (silver ring in the "level" location), taper (Marshall-like knob in the "level" location), level (silver ring in the "ratio" location), and ratio (Marshall-like knob in the "ratio" location).

Trigger adjusts how much of the scale of signal is fed into one triode of the overdrive tube. Level adjusts how much of the signal is fed into the other triode of the overdrive tube. Taper adjusts the scale of high frequencies in the overdrive section. Ratio adjusts how much of the overdrive circuit's signals is fed back into the preamp circuit.

Especially because the Overdrive tube is a 7247 (two different triodes with different characteristics in one tube) these four pots provide an extremely useful control of the harmonic content in order to fine-tune the overdrive circuit's voicing.
...

If I have this straight, the tube swap to a 7247 could be an essential ingredient but from the 124 OD, the trimpot leading to the first triode would serve as the trigger, the level and ratio pots would serve as is and it would only require the addition of a high pass filter for a taper, and that might be placed just before the ratio pot as 500p/250k combination as found in the Bluesmaster https://ampgarage.com/forum/viewtopic.php?t=17251. I couldn't find any schematics of the 121 to confirm my suspicions so it's speculative at the moment. But I see possibilites there. Maybe swapping out the tube would be taking me outside the 124 envelope unlike all the other mods so far, which with switching are all configurable to take me back to the 124 circuit. I see the easily available equivalent of the 7247 is a 12DW7, a mix billed as "1/2 12AX7 and 1/2 12AU7". These dissimilar triode characteristics could be the deal breaker.

I am totally not into building a historically correct period piece but I am completely at home with the idea of a meticulous even obsessive attention to detail. MrD had all these brilliant ideas but the opportunity to bring it all together into this kind of amp doesn't seem to have presented itself. The 121 close but it is still way out there, probably a little too far for today's audiences, but the ODS, not the SSS is the pinnacle of his achievement to my untutored eye. No-one has seen my full schematic yet but not counting the tremolo, the circuit block diagram is the same.

Thanks again, and good health!

Stephen

[Max]

In the article I linked in my post https://www.vintageguitar.com/31567/dum ... -od150-wr/ Todd Sharp and the current owner both describe #0121 as an amp they like/d a lot. But as there's no schematic available of the 7247 OD-circuit implemented in this amp, IMO it would perhaps(?) (e.g. depending on your time and/or motivation to experiment) be better sticking to the (thanks to Gil Ayan very well documented!) #0124 preamp including its OD-circuit (If you want perhaps with the addition of external trigger and taper pots on the back or front) but otherwise perhaps thinking about the structure shown in this block diagram (inverter/driver, reverb implemantaion, switching etc.) as perhaps an additional source of inspiration for your project.

All the best,

Max

[Stephen1966]

In the article I linked in my post https://www.vintageguitar.com/31567/dum ... -od150-wr/ Todd Sharp and the current owner both describe #0121 as an amp they like/d a lot. But as there's no schematic available of the 7247 OD-circuit implemented in this amp, IMO it would perhaps(?) (e.g. depending on your time and/or motivation to experiment) be better sticking to the (thanks to Gil Ayan very well documented!) #0124 preamp including its OD-circuit (If you want perhaps with the addition of external trigger and taper pots on the back or front) but otherwise perhaps thinking about the structure shown in this block diagram (inverter/driver, reverb implemantaion, switching etc.) as perhaps an additional source of inspiration for your project.

All the best,

Max

Yeah, thinking about the different gain factors of the 7247/12DW7, the plate voltages might be left as they are with a 100k but the bias could be/would be off in the second stage if I didn't dive into the cathode values. Changes there may entail completely rethinking the OD section in a knock-on effect. The chances are, it could sound great or absolutely the worst. Who was it, who said "the more mods you include, the more chances it can all go horribly wrong"? I forget, but they have a good point. There is perhaps another issue, servicability. I don't think the world is going to run out of EL34s or 12AX7s any time soon, but a tube with limited demand produced by a diminishing number of manufacturers... hmm! (pause for thought!). "Better the devil you know," you could be right. Because of that question mark hanging over future availability, it would be safer to redesign a new overdrive section that uses readily available parts. Safer, but a whole lot more difficult... I should keep reminding myself, MrD did what he did for a reason. If the serial numbers are to be trusted, then this predecessor of the 124 gave him all the inspiration to incorporate his experience into the later build. But he didn't... Why not? Hmmm!

I'm obviously parsing all this in real time so you will have to forgive the noob thinking he can reinvent the wheel. There are some exciting possibilities here but I have to slow down.

[Stephen1966]

I've decided to forego the changes to the OD section in this amp - too many variables - and I would like to get a bit more experience under my belt before I venture out on a new design. As an apprentice piece the 124 has more to offer as the circuits are well documented and understood and I'm really not unhappy with the OD in its current form. The examples I've heard, sound just fine, though there was a Ceriotone version where it sounded just too fizzy. Distinctive, but not what I am looking for in an overdrive. It's a relatively straightforward gain stage in the ODS but it seems to rely for much of its smoothness and depth from the earlier preamp circuit shaping. Messing with that, without fully knowing what I am doing, could kill it... to death

I'm having the devil of a time deciding which chassis to use. The "standard" Dumble chassis I can lay my hands on is 490mm wide, and this would allow me to build a head and speaker cab around 19" in width. (I know, I shouldn't mix metric with imperial but I'm fluent in both...) There is another chassis I might use, but it is a full 10cm (about 4") wider and that would mean a wider speaker cab. I'm sure i could keep the internal volume of the speaker cab around the same with some judicious mathematics but it irks me that it would in some ways be "non-standard". I'm toying with an open back or ported 1 x 12 at the moment.

My chief concern is how a nine-tube chassis in the "standard" size, would interact with the EM planes of the transformers. I would need someone with hands-on experience of this to tell me if I am opening up a can of worms here.

Here is a layout with the transformers positioned as proposed by Martin Manning.

Dumble chassis - 9 tubes and tx.pdf

There doesn't seem to be room for the reverb transformer inside the chassis. This is kind of back to front (viewed looking into the chassis and through), and all the transformers except the relay power would be underneath (or on top of the chassis depending on how you look at it). I would be interested to hear what you think of the OTs proximity to the reverb tubes and PI tube. These will have the can type shielding of course, but is it going to be able to effectively shield the tubes from from EM field being generated by such a beefy transformer? Maybe the signal is so strong there, the noise floor is very low and will make little if any difference. But this is pure speculation on my part. If anyone has any first-hand experience of this or even simply, a theoretical know-how, I would very much appreciate it.

Good health!

Stephen

[Max]

In regard to some larger chassis/cabinet combination with a practically proven positioning of all transformers and the reverb tank you could perhaps think about:

ODR #0060 or SSS#002 chassis
in combination with
ODR #0060 1x12 pole inversion combo cabinet http://www.amparchives.com/folder/2483/
or
ODR head (many pictures online)
in combination with
1x12 pole inversion cabinet (dimensions similar to the bottom part of the #0060 combo cabinet)
or
2x12 pole inversion cabinet (Rick Vito combination): https://www.gbase.com/powered/overland- ... ive-reverb
or
4x10 pole inversion cabinet (Lowell George combination): http://www.amparchives.com/folder/2513/

All the best and lots of fun,

Max

[Stephen1966]

In regard to some larger chassis/cabinet combination with a practically proven positioning of all transformers and the reverb tank you could perhaps think about:

ODR #0060 or SSS#002 chassis
in combination with
ODR #0060 1x12 pole inversion combo cabinet http://www.amparchives.com/folder/2483/
or
ODR head (many pictures online)
in combination with
1x12 pole inversion cabinet (dimensions similar to the bottom part of the #0060 combo cabinet)
or
2x12 pole inversion cabinet (Rick Vito combination): https://www.gbase.com/powered/overland- ... ive-reverb
or
4x10 pole inversion cabinet (Lowell George combination): http://www.amparchives.com/folder/2513/

All the best and lots of fun,

Max

Lots of inspiration here. So you're saying the wider SSS style chassis is no big deal. That it even seems to be the only way to go if we include reverb. It's amazing how the simple mechanical act of introducing 2 tubes, seems to affect chassis size because of tfr emissions if nothing else, and head size affects cabinet size.

I looked up pole inversion, and found a lot of links to pole dancing (not an entirely unpleasant experience) but I guess you mean reverse polarity on the speakers? When the time comes, that's an easy experiment to run but for now I want to get the chassis size right.

Stephen

[Max]

... So you're saying the wider SSS style chassis is no big deal. ...

Yes (ODR/SSS#002), because the weight of the chassis depends largely on the weight of the transformers and less from the required amount of aluminum and cable length etc..

... That it even seems to be the only way to go if we include reverb. ...

Probably not the only way, but with regard to the most interference-free positioning of all transformers and the reverb tank, it has been successfully tried and tested by a number of very well-known musicians.

I looked up pole inversion … I guess you mean reverse polarity on the speakers?

"Pole Inversion Speaker Enclosure" is Alexander Dumble's name for his speaker cabinets with the famous "oval hole" (obviously not "oval" at all) on the back. Here (beginning with "Instead of increasing ... ") Alexander Dumble explains the background for his naming:

"How does your philosophy on speaker enclosures contrast with other companies?

I think mine's different. I just don't believe in a baffle board with a couple of sides. Everything is designed to respond tonally. Even my open-back enclosures use air to the optimum. It's an ongoing process; I'm still finding out things that are useful. There's a definite technique to developing enclosures. Instead of increasing the output all from the front by feeding more watts in, I designed a special series of open-back enclosures so that there's actually an air pole inversion process--I make the air respond in an in-phase relationship, both in front and in the rear of the enclosure. So, from the same amount of speakers, it's almost a doubling of sound.

Does that change the tonal quality?

Yes. The low end is absolutely luscious. You feel like you're floating on a football field filled with marshmallows. And it gives a singe to the midrange that puts solos right out there. It works great for chords and solos, but especially well for slide. It's the kind of enclosure that Lindley and Lowell George used."

Source: https://ampgarage.com/forum/viewtopic.php?f=4&t=22012

All the best and have a nice weekend,

Max

[Stephen1966]

... So you're saying the wider SSS style chassis is no big deal. ...

Yes (ODR/SSS#002), because the weight of the chassis depends largely on the weight of the transformers and less from the required amount of aluminum and cable length etc..

... That it even seems to be the only way to go if we include reverb. ...

Probably not the only way, but with regard to the most interference-free positioning of all transformers and the reverb tank, it has been successfully tried and tested by a number of very well-known musicians.

I looked up pole inversion … I guess you mean reverse polarity on the speakers?

"Pole Inversion Speaker Enclosure" is Alexander Dumble's name for his speaker cabinets with the oval opening on the back. Here (beginning with "Instead of increasing ... ") Alexander Dumble explains the background for his naming:

"How does your philosophy on speaker enclosures contrast with other companies?

I think mine's different. I just don't believe in a baffle board with a couple of sides. Everything is designed to respond tonally. Even my open-back enclosures use air to the optimum. It's an ongoing process; I'm still finding out things that are useful. There's a definite technique to developing enclosures. Instead of increasing the output all from the front by feeding more watts in, I designed a special series of open-back enclosures so that there's actually an air pole inversion process--I make the air respond in an in-phase relationship, both in front and in the rear of the enclosure. So, from the same amount of speakers, it's almost a doubling of sound.

Does that change the tonal quality?

Yes. The low end is absolutely luscious. You feel like you're floating on a football field filled with marshmallows. And it gives a singe to the midrange that puts solos right out there. It works great for chords and solos, but especially well for slide. It's the kind of enclosure that Lindley and Lowell George used."

Source: https://ampgarage.com/forum/viewtopic.php?f=4&t=22012

All the best and have a nice weekend,

Max

Thanks for clarifying Max, the oval hole, open back design is one I'm looking at. A good weekend to you too.

[Stephen1966]

And so, some progress here. I've settled on the wider chassis which should give me a head and cabinet around 24" wide... I've just ordered the speaker and chassis which, with the transformers makes up all the heavy parts... There's no going back now

The wider chassis, immediately relieves the strain of placing a reverb after the OD and before the PI, in fact, I figure I can split the main board cleanly at the junction between the two opening up a pathway between, for hot DC lines feeding the pedal and providing me with enough room for not one, not two, but three tubes for the reverb and its accompanying circuit.

Up to this point I had been scratching my head over how I might implement the #002 reverb. I think I kind of found the answer by reducing the mixer stage down from 2 tubes to one, courtesy of designs by M. Hartman "SSS Preamp and Reverb" https://ampgarage.com/forum/viewtopic.p ... d053c4c9bc and the documentation of the #002 (drawn by Aaron?) https://ampgarage.com/forum/download/file.php?id=54132 from https://ampgarage.com/forum/viewtopic.php?f=5&t=32047.

Here is the fourth version of reverb:

ODS RVB 4.0.pdf

I have carried the bypass switching over from version 3.0 - I would still like to completely isolate the reverb and reroute the signal directly from OD to the master. Also, I still have no plans to use the SSS filter but as a consequence I've also included the 1M mixing resistor/voltage divider net, from rootz's reverb. Intuitively, I assume we are dealing with a similar amount of signal as would make it past the filter if it were included, though if there is likely to be an adverse affect on high frequencies it might be worth reducing the mixing resistor with 820k with a 100k to ground as per the #002. The component differences between Hartman's reverb and the SSS002 (revision 7 144 1) are minor - a lower value grid stopper on the input side triode, marginally different cap values on the cathodes of the first and second tubes, the resistor on the second from 2.2M (#002) to 4.7M (Hartman) is perhaps the biggest component difference. A major difference however is absence of the cathode follower in my design. Tony makes the Tweed mixer sound like mannah from Heaven but while this version may not carry off the elan of the Tweed mixer, the intention is to give the reverb a little shine. Do I understand correctly, Tony, that this kind of mixer presents a minimum of impedance to the PI? There is a side order of Dumbleator being developed alongside this amp so I'm not concerned about disparity of signal to and from pedals.

This is hugely experimental for me, but why not? I would appreciate any feedback

Good health!

Stephen

[rootz]

Hi Stephen, that bigger chassis sounds like a very good choice! This makes room for a whole lot of other ideas, also a reverb that is much more like the one in the 002.

However, you can’t implement the 002 reverb like you did afaik. Normally in a 002 the reverb would be taken from the volume pot wiper. The reverb pre driver will be parallel to v1b. This way the signals at the mixer will be in phase. If I’m correct the signals in your design will be out of phase though. Correct me if I’m wrong on this.

There is a design floating around here that is essentially what you are after: ODRS #60! This design can be adjusted to a reverb after the master, in original form it is that way to an already large extend. The #60 has a very different preamp setup, with rather unusual plate and cathode resister values and combinations. The reverb section afaik could be added to any preamp though.

[rootz]

Here’s a layout for a #60 derivative. Needs some adjustments, but looks mostly correct.

[markusw]

Hi Stephen, that bigger chassis sounds like a very good choice! This makes room for a whole lot of other ideas, also a reverb that is much more like the one in the 002.

However, you can’t implement the 002 reverb like you did afaik. Normally in a 002 the reverb would be taken from the volume pot wiper. The reverb pre driver will be parallel to v1b. This way the signals at the mixer will be in phase. If I’m correct the signals in your design will be out of phase though. Correct me if I’m wrong on this.

There is a design floating around here that is essentially what you are after: ODRS #60! This design can be adjusted to a reverb after the master, in original form it is that way to an already large extend. The #60 has a very different preamp setup, with rather unusual plate and cathode resister values and combinations. The reverb section afaik could be added to any preamp though.

I'm not sure that phase matters when mixing clean and reverb signal.
In the latest version of the schem the signal is fed into the reverb circuit from the plate of V1b (CL2) without attenuation. Therefore, I fear it will overload the first stage of the reverb circuit (V1a in the schematics).

[Stephen1966]

Hi Stephen, that bigger chassis sounds like a very good choice! This makes room for a whole lot of other ideas, also a reverb that is much more like the one in the 002.

However, you can’t implement the 002 reverb like you did afaik. Normally in a 002 the reverb would be taken from the volume pot wiper. The reverb pre driver will be parallel to v1b. This way the signals at the mixer will be in phase. If I’m correct the signals in your design will be out of phase though. Correct me if I’m wrong on this.

There is a design floating around here that is essentially what you are after: ODRS #60! This design can be adjusted to a reverb after the master, in original form it is that way to an already large extend. The #60 has a very different preamp setup, with rather unusual plate and cathode resister values and combinations. The reverb section afaik could be added to any preamp though.

Thank you rootz. Yeah! When I first looked at the #002 I didn't immediately see the signal coming from the volume wiper and where that's fine when we are at, or at least towards the end of the signal chain in the preamp, it wouldn't work with the switching channels without some fancy switching which doesn't seem necessary anyway. And thanks again, for pointing me in the direction of the #060. I will definitely check that out and see if it offers a way around the problem of phasing reversal. I suspect you are correct because V2 although using both triodes, is behaving like a single triode in my understanding. It may be a concern because of secondary effects on the second order harmonics. I would really need to study up on that though and so the simplest solution is to avoid the out of phase relationship if possible. Perhaps #060 can offer some clues there.

Good health!

Stephen

[Stephen1966]

I'm not sure that phase matters when mixing clean and reverb signal.
In the latest version of the schem the signal is fed into the reverb circuit from the plate of V1b (CL2) without attenuation. Therefore, I fear it will overload the first stage of the reverb circuit (V1a in the schematics).

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