#54 (photos)

[Smokebreak]

Thanks Charlie. Those are neat pics!
The OD entrance looks like there's no cap to ground across the 220K, and a mica across the 1M2 spot?
Also at the OD exit, there's a small ceramic bypassing the 1M , headed to the MV.
PI looks like 820r/18K, and the entrance cap there looks big. 22n?

edited for more accurate observations

[vibratoking]

Thanks Charley. You're one helluva nice guy that embodies the spirit of TAG as well as anyone. I appreciate it.

[Max]

typical 3rd generation amp

It's a rather typical 2nd generation ODS chassis with sliding switches. But the higher the serial number impressed into such a 2nd generation chassis, the higher the probability to find a tone stack inside without a 500K treble pot and without the two caps of the "James configuration" soldered to the bass pot.

Slightly unusual to see these earlier OD Specials in a 100W config no?

The date code on the yellow Sprague TVA "5UF" caps on the preamp board is "78 (year) 33 (week)". And I personally wouldn't call an ODS built in late '78 or early "79 an "earlier OD Special".

The general rule based on my experiences: the higher the serial number impressed into a 2nd generation chassis, the higher the probability that it's an ODS 100W. #0055 as an example is a 2nd generation ODS 100W, too (see the attached pictures of #0055). But AFAIR you'll meet 100W amps in some 2nd generation chassis with lower serial numbers (# 0020 e. g.), too.

Picture source: http://personalpages.manchester.ac.uk/s ... 20%230055/

Related infos: http://ampgarage.com/forum/viewtopic.php?t=20316

Cheers,

Max

[jelle]

PI looks like 1M at grids, 820r cathode, 18K tail, 220r, with a 1.8K NFB resistor. 220K powertube grid references. 100k? plate resistors. This would be designed around a 12AX7.

[Charlie Wilson]

PI looks like 1M at grids, 820r cathode, 18K tail, 220r, with a 1.8K NFB resistor. 220K powertube grid references. 100k? plate resistors. This would be designed around a 12AX7.

Yeah Jelle, I should have posted these also.
CW

[jelle]

Even though it is clearly built with a second generation chassis, it also has a number of interesting features that indicate that Dumble was flirting with aspects that would become part of the Classic ODS amps. For one, that phase inverter appears to be transitional towards the Classic and Skyline phase inverter.

The type of coax, although that one is not new to us.

The circuit appears to be a version of a 3rd gen ODS.

I wonder if these treble peaking caps that are parallel to the preamp grids are there to limit the low end response to further focus the OD. Is the value confirmed to be 0.005uf?

[Charlie Wilson]

Probably should have posted this one also .
CW

[jelle]

Thank you!

[Max]

The circuit appears to be a version of a 3rd gen ODS.

The term "generation" - at least as used by me - doesn't refer to some kind of circuit design, but to some kind of chassis design.

Some more detailed infos concerning my personal opinions in regard to the relations between serial numbers, timeline, chassis specs and circuit specs: http://ampgarage.com/forum/viewtopic.php?t=20316

Cheers,

Max

[jelle]

Max,

The categorization you have introduced is a great one. It categorizes the ODS amplifiers in the dimensions of time, cosmetics, and groups of circuits.

I have noticed that most people here are primarily circuit-centric, which is understandable given the nature of this forum.

The way the ODS chassis looks is -in general- indicative of the circuit or group of circuits that the amplifier was originally built with. But many amps have been updated in a way that -in my subjective view- disrupts the categorization above. As an example I would like to mention a silver 3rd generation ODS with a high plate skyliner circuit. I would call it a ,high plate skyliner' and not a ,3rd gen ODS'.

Of course we could be precise and call it a ,high plate skyliner in a 3rd gen chassis'.

We should fight this one out over a beer.

[stevlech]

cap on bass pot .005 or .001?

Also- lower right corner of main board @ PI- RC 5/23/79 - Ry Cooder?

[Max]

Of course we could be precise and call it a ,high plate skyliner in a 3rd gen chassis'.

In my personal opinion there are well known methods in the field of electronic engineering to achieve precision in the communication concerning the specs of electronic devices like guitar amps: pictures, schematics, layouts etc. So IMO calling ODS #0XYZ something like a 'high plate skyliner in a 3rd generation chassis' doesn't leave the level of small talk. But of course small talk can be lots of fun - especially over some good beer.

Cheers,

Max

[Luthierwnc]

This is sort of like archeology. sh

[Max]

The way the ODS chassis looks is -in general- indicative of the circuit or group of circuits that the amplifier was originally built with. But many amps have been updated in a way that -in my subjective view- disrupts the categorization above.

You'll even find a "classic" tone stack in combination with a "standard" OD circuit (trigger entrance) and a ratio control (on the back e. g.) etc. originally built (and not updated) into a 2nd generation chassis.

And because of all these "exceptions" IMO the best way to avoid misunderstandings would be to treat every single Dumble amp as an individual in the sense of e. g.: "On these pictures you see the outside and the inside of ODS #0054". And here is the schematic of its circuit, as far as it can be derived from these pictures" etc.

Also- lower right corner of main board @ PI- RC 5/23/79 - Ry Cooder?

The letters "RC" in combination with some precise date in 1979 you'll find at the same place in some more ODS amps. So I doubt, that "RC" refers to Ry Cooder.

Cheers,

Max

[marcos]

I wonder if these treble peaking caps that are parallel to the preamp grids are there to limit the low end response to further focus the OD. Is the value confirmed to be 0.005uf?

The 3rd generation circuit is very bass-heavy , so I can see the desire to reduce
low frequeny content, especially if you run the bass the control at noon,
which is around 300k to ground with a 1 meg pot.
(BTW I have found I like to set my bass pot around 60k to ground).
I had my personal amp(2nd gen) converted to 3rd generation specs in 1987,
losing the .02 cap on the bass pot and replacing the treble pot with
a 250k, and found the amp too boomy, and went back to the brighter
2nd generation circuit. Later I met a 3rd generation Dumble, which also had a lot of bass.
Still, personally I would prefer more subtle ways to control the bass
content in an ODS style amp.
My 2 cents
Marcos