Todd Sharp amp (121) some musings/ideas

[ElectronAvalanche]

Dear Ampgaragists,

As you all know, Dumble ODS #121 is a rare specimen. Read up on the history of #121 and you will see that it was built for Todd Sharp and that it is covered in the Dumble book.

Little to no information is available, but from the little information I derived a draft for a schematic and also did some experiments with a Dumbleator and my ODS preamp (here only using the clean side).

Max wrote some while ago: https://ampgarage.com/forum/viewtopic.php?t=24312

"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.

So you can adjust the overdrive section for a vast spectrum of tonal colors that by far exceeds the possibilities of a normal Overdrive Special. It's really amazing."

The following is speculation, but bear with me:

So we can learn from this that:

1. there is a 12DW7 (=7247) tube used in the OD section
2. there are separate pots controlling the gain structure, one before a first triode, one before a second triode. In essence like in a regular ODS.

Let`s assume that Dumble used the 7247 (12DW7) AU7 side as a CF. I looked at the Ripper and #189 schematic and they show a CF in the OD section, but there is no practical way to implement two gain pots (aka Trigger = OD input gain control and Level (confusingly named Level, but in essence it is the Gain control for the second triode). So I deduct, that Dumble must have not used a DC coupled CF but rather a AC coupled CF. Think Send stage of a Dumbleator. This is the second stage of the OD circuit which feeds into a master volume (called Ratio) with a HF tone cut (post MV as implemented in e.g. #123.

So lets look at the topology:

You might have a "normal" Clean section in #121 (we will come to the anode/cathode resistor combinations later) which feeds into the first triode of the OD section like in a normal ODS. The value of the series resistor before the Trigger control might (!) be 220k, the trigger is part of a dual concentric pot. Most such pots are available in 2x500k versions, some in 1x250k + 1x500k. Let's assume Dumble used an off-the-shelf dual concentric pot found in Fender Jazzbass bass guitars: 250k and 500k. Trigger (=OD input gain) is the silver ring on '121 (see Max's comment above), so the "lower" section of the dual concentric, which is 500k. Maybe Dumble paralleled a 470k to this 500k and would get 242k. Such a 242k trigger value is normal for an ODS. The "upper" section of the dual concentric is 250k and would be the Level (Marshall-like knob as per Max). So the second "gain" pot of the OD section is 250k. 500k Trigger pots are not unheard of, so trigger might be simply 500k side of the dual concentric with no paralleled 470k.

The first triode of the OD section is thus rather plain vanilla ODS: from relay to 220k series, to Trigger to 68k grid stopper into grid of V2a. #183 has a 250k Level pot, and the corresponding decoupling cap from V2a is 0.015 in 183. So let's take this value as a starting point. Let's also take the series resistors from 183, which will give us 100k post decoupling cap and 180k post Level pot. Again plain vanilla ODS. Now the interesting bit:

V2b is the 12AU7 section of the 7247 tube. Again AC coupled I think (= cap -> bias network -> grid of CF). So the 180k is followed by some coupling cap (value, no idea for now). The biasing of the CF is also to be discussed, I need to read more about this, or maybe someone has an idea). Then out of the CF (via cathode coupling cap) into a series resistor (can be left out) into the Ratio control pot (250k) from there to the relay for switching (as per a normal ODS).The Ratio pot has a HF tone cut (Taper) to ground with 500k and 0.001.

So now to my experiment:

I used my ODS preamp ('183 clone sort of) with a 220k/3k3-150k/2k2 Clean section (Skyliner, etc.) and took the signal from the preamp out into the return in of my Dumbleator (with a return control, which in essence functions as the trigger control. Of course there is a LNFB, the grid resistor is 220kII500pF but it is sort of like a V2a stage of an ODS. I fed the Dumbleator out (output of the Returnstage) into the Send in of the Dumbleator and the Send to my power amp. Again an approximation of the above mentioned theoretical ODS section of #121. The Send Level is functioning as the Ratio control. Also note that of course the Dumbleator I used does have a 12AX7 not a 7247 installed, but nevertheless the topology is nicely approximated.

Open questions: Anode/cathode combination for V2a. Bias of V2b (=CF section), output cap of CF.

So how does it sound? Very very very nice. Marshally. Refined. I like it a lot.

Now to the clean section:

'121 shows a Deep switch, which was changed to MID (published information) since it apparently is a MID boost. So let's assume the tone controls started out as Classic TS (=Treble 250k, Mid, 100k, Bass 250k) and was changed to Skyliner (Slope: 150k, Mid cap still 0.05, Bass to ground: 1k8). Mid boost most likely "medium" : 1nF followed by 500pF. Rock/Jazz as usual. Bright switch, most likely 120pF or 250pF (to taste really). Let's assume a LNFB for V1b and also the usual 220kII500pF grid stopper for V1b. Someone on this forum wrote some years ago, that the anode/cathode combinations were 180k/1k and 120k/2k2. Let's assume rather large cathode caps (22uF, not unheard of).

Presto, maybe (!!!) an approximation of the elusive #121 preamp.

Here is a schematic I put down for further discussion. I plan on building this within the next few months as a 1U rack preamp (dual concentric pots etc.).

Looking forward to thoughts, comments, ideas regarding the biasing of the CF etc.

Electron

Addendum:

I never saw a schematic of #121 nor do I have any special other knowledge. I also do not intend to discuss the power amp, tremolo and reverb. I assume the power amp to be extremely close to the Pentode amp, also the tremolo section. There may be some elaborate CF mixing stages for the Reverb and Dry signals which of course will add to the sound of the amp. Maybe this mixing stage is as in a later SSS? I assume that the character of the OD is mainly derived from the OD section so these other parts of the full amp add something but do not make the OD sound. Also this thread is only for the joy of trying to put oneself in the shoes of HAD and to come up with something maybe sounding equally nice as the original amp.

[WhopperPlate]

I had not considered the Cathode follower application , but I think HAD was more inspired by Ampeg SVT than most realize , especially when it can to anything remotely SSS related , ie 121. Look inside an old SVT and you will see mostly Draloric resistors and that familiar preamp design with the boards and components mounted directly over the tube sockets . 12dw7 is the most common tube in that circuit used for the various cathode followers throughout .

I think you are on to something, perhaps just your own good ideas , but if the shoe fits ….staying tuned

[ElectronAvalanche]

Many thanks for your reply Charlie!

Indeed Ampeg amps might have been a huge inspiration for Dumble. I found this Ampeg V9 schematic and this is quite interesting.

https://el34world.com/charts/Schematics ... peg_v9.pdf

Thank you,

Electron

[jabguit]

Most such pots are available in 2x500k versions, some in 1x250k + 1x500k. Let's assume Dumble used an off-the-shelf dual concentric pot found in Fender Jazzbass bass guitars: 250k and 500k.

Electron

These are simple to doctor - I regularly make up whatever I need using CTS 450 series concentric pots and changing elements as needed.


cheers,