ODS 124 - revisited for the 21C

[Stephen1966]

Thanks Martin. Looking at it - in the flesh, so to speak - even this larger chassis shouldn't involve runs more than 12" I was just thinking that the shortest runs in the clean channel might benefit from the lower capacitance cable. If I remember correctly, the photos seem to indicate this... not sure though.

[Stephen1966]

And so here is the latest iteration of the schematic for the whole amp.

ODSR 124 1.4.pdf

On the first page is the 124 circuit with the mods so far. I don't anticipate adding more to this with the parts ready to go bar a few backordered items. I do still need to finalise the power section. I want to nail those voltages in the clean, OD and PI sections. I need to think about the reverb section nodes for a happy medium.

On the second page are the three types of reverb I want to test. The #060 reverb (REVERB A) is probably the most definite at this stage. Thanks to rootz and Aaron and everyone else who participated in the study of the #060 there are many indications it will be a successful circuit. While pondering these though, I became intrigued why the CF was located at the trailing edge of the reverb in the other circuits knocking around, and included in the B and C versions on this page. Unless there are electrical issues I'm not aware of, I would like to try replacing the first gain stage of the #060 circuit with the first stage of the Dumbleator, seen here. The only modification of the latter was to add a 47k resistor to the plate to drop the B6 voltage by about 50V - thus bringing it inline with the values found in the Dumbleator. I don't know if this will reduce the loading on the rest of the circuit but it seems designed that way in the Dumbleator and it makes sense that the Reverb section is like adding FX to the loop.

REVERB B is a kind of hybrid circuit featuring a single CF stage based on rootz's design, and REVERB C (again, by rootz) is the #002 inspired circuit with a dual CF in the mixer tube. Rootz, I've kept the original 100k RETURN pot in favour of the 250k dual concentric pots you were planning. The 250k x2 RETURN is something I haven't fully got my head around yet and to include it at this stage adds a layer of complication that could possible make it harder to assess the circuit against the others. As a later mod, it could be retrofitted without a great deal of trouble. Ideally, I would have liked to have kept the 100k SEND pot as well, but it's position just before the driver probably needs the wider range of the 1M pot.

The idea is build the three (and half!) reverb circuits on interchangeable boards whilst keeping the same voltages, tubes and jacks and pots (that last being the exception). I will need to ask your help when it comes to assessing these side by side. The first job however, would be to have the rest of the amp up and running and dialled in for a good baseline. If anyone has any test methods that sound they might be useful here, I can run them.

I think I have covered most of the circuit to the point where I can start building but before I do that now, I need to finish off my calculations on the power stage and then start on the physical layouts and designs for the boards and front and rear panels. So if any of you see anything amiss in the schematics, as always, I would welcome your thoughts...

Stephen

Edit: the voltages on the PI should be the other way round. From the layout by Tony: "[in] the order of 280VDC for the non-feedback side of the phase inverter plate, and about 10 volts higher for the feedback side."

[Stephen1966]

Could I get some feedback on these calculations I've made? I'm trying to reverse engineer the B+ voltages to get an idea of the components I might need to tweak in the dropping string from the PT. These numbers are based on the assumption of nominal values for Ia and the sum of the resistance in the anode and cathode, which should give me the voltage drop when V = IR. Adding the voltage drop to the desired voltages gave me, my calculated figures:

V1 7025
Ia = 1.2mA
Ra = 100k
Rk = 1.5k
Vd = Ia (Ra + Rk) = 121.8V
Desired Va = 190V
B+8 = 311.8V

V2 7025
*Ia, Ra, Rk and Vd – same as V1
Desired Va = 200V
B+7 = 321.8V

V3 12AT7
Ia = 10mA
Ra = 1.044k (measured across RVB Tx primary leads)
Rk = 2.2k
Vd = Ia (Ra + Rk) = 32.44V
Desired Va = 365V
B+3 = 397.44V

V4 12AX7
*Ia, Ra, Rk and Vd – same as V1
Desired Va = 220V
B+5 = 341.8V

V5 12AX7
*Ia, Ra, Rk and Vd – same as V1
Desired Va = 210V
B+6 = 331.8V

V6 7025
*Ia, Ra, Rk and Vd – same as V1
Desired Va = 290V
B+4 = 411.8V

Rootz mentioned earlier that

In short: your PSU design needs more work. The 12AX7 tubes in the preamp draw approx 1.3mA, PI a bit less. The reverb drivers draw around 1.5mA.
...

When I plug these values into the spreadsheet I made to calculate these figures, the B+ voltages drift even further from the simulated values so I don't know where I'm going wrong. It seems I am miscalculating the current draw somewhere and I'm going round in circles because without the final B+ voltages I can't calculate the bias of the tubes and without the quiescent current, I can't work out the values of the dropping resistors in the PS. These calculations are all derived from the 1.4 version attached above and are based on the first reverb section, based on the #060.

To summarise here is what I got:

Calculated B+ nodes
B+3 = 397.44V
B+4 = 411.8V
B+5 = 341.8V
B+6 = 331.8V
B+7 = 321.8V
B+8 = 311.8V

Simulated B+ nodes
B+3 = 368.9V
B+4 = 409V
B+5 = 336.2V
B+6 = 323.6V
B+7 = 312.5V
B+8 = 303.8V

Here is the spreadsheet with the calculations in more detail:

Power nodes calculation.xlsx

[rootz]

Scratch that...

[talbany]

Did you factor in the current draw from the FET? (If you are installing one)

Tony

[Stephen1966]

Did you factor in the current draw from the FET? (If you are installing one)

Tony

grid
No, but I woke up this morning and thought... FET! Also, I didn't take account of the grid leak, tail and presence pot of the PI. Back to the drawing board

[Stephen1966]

Aargh! It's got me. According to the datasheet, the 2N4416A of the FET pulls about 10mA. The B+ supply voltage should be ~300VDC. Beyond that, I'm lost!

When I add the grid leak, tail and presence pot into the equation for the PI, I get nonsense figures.

I must have a fundamental misunderstanding of something, somewhere. It's disheartening.

[talbany]

Aargh! It's got me. According to the datasheet, the 2N4416A of the FET pulls about 10mA. The B+ supply voltage should be ~300VDC. Beyond that, I'm lost!

When I add the grid leak, tail and presence pot into the equation for the PI, I get nonsense figures.

I must have a fundamental misunderstanding of something, somewhere. It's disheartening.

Stephen
IMO these voltages you are trying to calculate should only be considered as a general guide and not absolute. Since preamp tubes are cathode biased so each will be biased individually depending on the current draw of each triode sometimes as much as a 10 V swing. This can also vary depending on wall voltage, power transformer tolerances, tube types/matching, age, manufacturer, transconductance,... etc. Another words try to factor as many of these variables determine the best average from the given operating parameters and overall sonic performance of each section and tweak as necessary after you build it..( I generally use a resistor decade box).This would be my recommendation otherwise you could spend more time calculating than simply tweaking.
It looks like you are real close
Good Luck!

Tony

[Stephen1966]

Aargh! It's got me. According to the datasheet, the 2N4416A of the FET pulls about 10mA. The B+ supply voltage should be ~300VDC. Beyond that, I'm lost!

When I add the grid leak, tail and presence pot into the equation for the PI, I get nonsense figures.

I must have a fundamental misunderstanding of something, somewhere. It's disheartening.

Stephen
IMO these voltages you are trying to calculate should only be considered as a general guide and not absolute. Since preamp tubes are cathode biased so each will be biased individually depending on the current draw of each triode sometimes as much as a 10 V swing. This can very greatly depending on tube types/matching, age, manufacturer, materials, transconductance... etc. Another words try to factor as many of these variables determine the best average from the given operating parameters and overall sonic performance of each section and tweak as necessary after you build it..( I generally use a resistor decade box).This would be my recommendation otherwise your thread could go on forever.
It looks like you are real close
Good Luck!

Tony

Thanks Tony, I appreciate the moral support. Is it safe to use a decade box with resistors rated for 1/2W? In any case, I tweaked the values of the current draw for each of the tubes and expanded it, because it was a mix of single triodes and triodes in parallel (the 12ATX). I was aiming for the simulated voltages but to get there I found several things which I am hoping rootz can clarify.

The preamp triodes (V1, V2, V4, V5) come in at about 1.12 mA which seems on the money (or close enough for jazz) for the 7025/12AX7.
Because of the FET numbers, I know the B+8 node should be about 300VDC
The PI tube (V6) pulls about 1mA (slightly less as rootz indicated) but only if I don't include the grid leak, tail and presence values!
The 12AT7 (V3) can only pull about 2mA to achieve the voltage drop of 6.488V from the supply node. This seems fairly meagre, given the nominal current draw of 10mA found in the datasheets.
Is it possible the B+3 is underated? If I increase the draw of the 12AT7 to 10mA I get a B+3 supply voltage of 397.44V which is about 28.5V higher than the simulated B+3.

Anyway, even if my brain is too old and shrivelled to make sense of the circuit constraints, the circuits don't lie. And as much as I like the banter, back and forth, I'm starting to lose patience with myself and feel the need to just get on with it. Not knowing the operating points of the original's tubes is frustrating because that's where the tone and headroom are and all I have to go on are the anecdotal descriptions of the tone, which suffer from the usual paucity of language. I could use the #060 PS as the model but the photos weren't conclusive and Aaron's schematic involved some very different values, but the same topology in a high plate setting.

Power nodes calculation (2).xlsx

[rootz]

I'm with Tony here, the voltages function as a guidance. Your PSU looks different from my schematics, but the basics are still the same: if clean, od and PI plate voltages are low, decrease the first dropper from 2k2 to 1k8. Order some extra resistors for the PSU to experiment with and you'll get in the ball park rather soon.

By the way: may I ask why you choose the a PSU with all nodes in series? IMHO that is not the way it is done in #60, though your version will work fine too. It is just a tad more difficult to tweak to taste.

[martin manning]

According to the datasheet, the 2N4416A of the FET pulls about 10mA. The B+ supply voltage should be ~300VDC. Beyond that, I'm lost!

FET current draw depends on the DC operating point. In the ODS, the FET circuit draws ~2mA through the 150k, about half of which goes to the FET.

[Stephen1966]

According to the datasheet, the 2N4416A of the FET pulls about 10mA. The B+ supply voltage should be ~300VDC. Beyond that, I'm lost!

FET current draw depends on the DC operating point. In the ODS, the FET circuit draws ~2mA through the 150k, about half of which goes to the FET.

Thanks, this is useful to know... Do you know if the original B+ was also around 300V? Is this what you mean by the DC operating point?

If so, it could be a breakthough, a cypher to unlocking the rest of the power nodes and the bias of the CL, OD and PI tubes. I have to acknowledge that the voltages could vary very generally around 300V but this ~2mA figure is the closest I've got to a clue as to what the rest of the tubes were pulling in terms of their bias. We know the beginning of the string was delivering around 440V from the CT (B+ possibly a tad more to account for OT resistance - 1300 Ohm on each side of the CT) and your work on the FET was just immaculate. I'm making the assumption that all solid-state components are "made equal, but some are more equal than others", and the silicon transistor - even a replacement one like the 2N4416A - is ~probably~ equivalent to the original.

[talbany]

Thanks Tony, I appreciate the moral support. Is it safe to use a decade box with resistors rated for 1/2W?

Yes anything downstream from the screens 1/2 watt should be fine.(after you smoke test the amp and determine the current draw from each tube) If the voltages are too high you can always leave the existing resistor in add another in parallel find the sweet spot then do the math and replace. Always remember to drain the caps between each resistor swap if you have any.
BTW.If you branch off for the reverb (at screens) this makes it easier to tweak. Oh and try to find a seperate ground point further downstream for your reverb preamp filters away from the output tubes.

Tony

[Stephen1966]

I'm with Tony here, the voltages function as a guidance. Your PSU looks different from my schematics, but the basics are still the same: if clean, od and PI plate voltages are low, decrease the first dropper from 2k2 to 1k8. Order some extra resistors for the PSU to experiment with and you'll get in the ball park rather soon.

By the way: may I ask why you choose the a PSU with all nodes in series? IMHO that is not the way it is done in #60, though your version will work fine too. It is just a tad more difficult to tweak to taste.

I think you are both right, the only way to crack this nut will be to buy a load of spares of different values and tweak until I find the results I'm looking for. It just kind of bugs me that I can't work it out on paper. It's the first time I've tried this though. The ODSR is my apprentice piece. Half the battle is knowing when to run, run, run away, live to fight another day though, so lesson learned, I guess I may not be able to express the mathematical model of it all yet, but I can clearly see that I need to move to the next stage of experimentation now with observation and testing. Depending on what Martin says (see above) I might give the mathematical model approach another shot but I'm likely to make better progress by putting the thing together first and then exploring the formulae. It's an exercise in frustration otherwise.

Regarding all the nodes in series... It isn't super obvious in my schematic but B+3 is parallel to the main string. Like in your schematic and Aaron's. Unless I a fundamentally misunderstanding something yet again

ODSR 124 - PS - 1.5_Page_1.jpg

060 Schematic - PS - Aaron's 060.jpg

Screenshot 2021-05-01 at 19.15.44 (2) - PS rootz.jpg

Thanks again for all the moral support, guys

Stephen

[Stephen1966]

Thanks Tony, I appreciate the moral support. Is it safe to use a decade box with resistors rated for 1/2W?

Yes anything downstream from the screens 1/2 watt should be fine.(after you smoke test the amp and determine the current draw from each tube) If the voltages are too high you can always leave the existing resistor in add another in parallel find the sweet spot then do the math and replace. Always remember to drain the caps between each resistor swap if you have any.
BTW.If you branch off for the reverb (at screens) this makes it easier to tweak. Oh and try to find a separate ground point further downstream for your reverb preamp filters away from the output tubes.

Tony

Thanks Tony, good to know. This design should bleed itself to ground with the resistors in parallel with the filter caps, but you're right... it's always good to double check when it comes to high voltages. Resistors in parallel, very practical approach, less stressful on the boards if I don't have solder and repeat, again and again. Again, I don't think it's very obvious in my schematic but I already have the reverb branching off. [I should look to tweak the program to make the nodes a bit more visible.] Grounding, hmm, this is on my to-do list; I appreciate your advice though. I don't think I could have figured that out from the photos of the 060. The reverbs - all of them - are a work in progress in that respect. I'm moving on to layouts this week. I'll factor the grounding in. Cheers