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Author Topic: Jordan Professional 440 Solid State IC Amp Head  (Read 240 times)

galaxiex

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Jordan Professional 440 Solid State IC Amp Head
« on: April 18, 2017, 01:17:46 AM »

When I got this it kinda worked, but both treble controls were broken.
The knobs would just spin and no change to the highs.
Both treble control pots have a “pull brite” function.

Nice that they used CTS pots as I have found that you can take them apart and swap parts or repair as needed.  :)

As I suspected the plastic carrier for the pot wiper was broken so it was just a matter to swap in a good one.

I also changed out all the electrolytic caps including the big-ass main filters.
These caps are all from around the early 70’s and the amp had some noise issues and some “fuzzyness” to the sound.

All cap replaced with same values except for those big-ass filters.
The originals were 7000uf and all I could find to fit the holding clamps were 10,000uf units.
Well… I could have fit even bigger but thought the 10,000 was enough.  ;)

Someone had already installed a 3 wire power cord,
I finished the job by clipping out the line reverse death cap.

After all repairs it is very clean and VERY LOUD!
Even at full volume thru a 1X12 cab Celestion G12H there is no hint of distortion at all.

I haven’t tried any pedals in front of it yet.
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galaxiex

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #1 on: April 23, 2017, 11:25:16 PM »

Here's the amp as it came to me.

Original knobs outer plastic cracked and broken and the knob centers badly seized onto the pot shafts,
which may explain why the 2 treble pots were broken.  :grr

I had a helluva time getting those original knobs off without breaking anything.
Each knob has the aluminum center with 2 set screws.
I removed all set screws and squirted a penetrating oil type product into the set screw holes.
Let them sit for a day and still had a fight to get them off.... and ya gotta get them off to pull the circuit boards.  ::)

Once off I polished the pot shafts with some scotchbrite.

The new silver chicken head knobs are plastic with a brass insert and a single set screw.
Hopefully they won't seize onto the pot shafts.

Black chicken heads actually look better.... but I'll post later why I went with the silver ones.  ;)
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J M Fahey

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #2 on: April 24, 2017, 02:08:58 AM »

Excellent work.Please sharevthe schematic when you have it.
I´m specially intrigued by the damping control.
I have my own, but there´s many ways to skin a cat :)
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galaxiex

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #3 on: April 25, 2017, 01:38:43 AM »

Thanks JM, here is a schem with what I think is the Damping control circled in red.

I'm not sure how well this pic will show up.
This is the schematic that is pasted inside the bottom cover of the amp.
It does not exactly match the circuitry.  ::)

U2 is not in this amp, instead it has discrete circuitry in place of U2.
Also the power amp section is slightly different.

If the pic doesn't show clear enough I'll try to get a close up of the PA section.
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J M Fahey

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #4 on: April 25, 2017, 05:06:00 AM »

Thanks.
INCREDIBLY advanced (for 1970) :Op Amp preamp, variable damping output, Op Amp driven power amp, green fluo panel paint illuminted by a UV tube (now I know where did Trace Elliot get the idea from), etc.

That said, power stage is operated pure Class B and has no provision for biasing, so crossover distortion is inevitable.

Which played LOUD in a live situation is almost undetected; big problem at bedroom levels though.
« Last Edit: April 26, 2017, 12:17:26 AM by J M Fahey »
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galaxiex

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #5 on: April 25, 2017, 10:08:50 PM »

Here is a schem from a different model,
but the power amp section more closely matches what is in the amp I have.
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J M Fahey

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #6 on: April 26, 2017, 12:39:43 AM »

Thanks.
Looks like Jordan was quite the pioneer, this amp has the basic structure of a Polytone amplifier.
Amazing.
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teemuk

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #7 on: April 26, 2017, 12:56:53 PM »

When I started to research all this, what I discovered was that many of these kinds of features (current feedback) were actually introduced much earlier than people commonly think.

There's nothing new in mixing in different feedback topologies. A lot of stuff about that was discovered already in the early days of tube amps and some of the inventions are still exploited. Certain Bogen tube amps from few decades past were famous for employing -positive- current feedback because it - like negative voltage feedback - decreases effective output impedance of the amplifier. If interested, you likely find a few related magazine articles and patent documents with a Google search. Anyway, what the solid-state stuff is doing is just inversion of the idea: Apply negative current feedback, because it increases effective output impedance of the amp.

Overall this scheme must have not been -that- uncommon for amplifier designers. Especially for those who earn a living with it.

Anyway, I wonder what groundbreaking happened in the late 1960's, because after that we see a boom of this basic "current feeback" -topology making an appearance in many solid-state amps, and yeah, particularly as a deliberate attempt to emulate tube amps.

Ca. 1969 Triumph introduced amps that employed negative current feedback and had a moderately high output impedance. However, the circuit documentation still describes the associated circuit as a crude short circuit protection scheme.
Then all suddenly... The self-powered cabs of the Fender Super Showman system (ca. 1969) employed current feedback and associated tube/SS -tone switch. Jordan amplifiers (early 1970's) employed current feedback, with or without adjustment. Randall, Risson, Polytone, etc. In early 1970's the scheme is employed all over the place, and I even heard that there is a German book about solid-state guitar amplifiers from the time that describes the scheme. Several early 1980's patents (at least european ones) refer to the scheme as well-known "prior art" so basically every designer for every bigger brand out there must have at least been aware of this stuff, whether they chose to implement it or not. I wonder what happened?

Let's put this to proper context in time line: All this stuff happens about 15 years -before- the famous "Carver challenge".
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J M Fahey

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #8 on: April 27, 2017, 05:40:19 AM »

Quote
However, the circuit documentation still describes the associated circuit as a crude short circuit protection scheme.
Well, that´s what it originally was.
And not *that*  crude, in its original implementation it was functionally the same as the standard current clamping protection with diodes or transistors.
Way back then, we knew it as "the RCA protection" .
FWIW, found in transformer driven SS amps.
The original idea was never to increase output impedance; amp was standard high damping type, as expected in a Hi Fi amplifier, and started limiting current only above a certain preset (dangerous) level.
Will search for some old schematics ... printed on paper of course ;)  , no Internet way back then :O
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teemuk

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Re: Jordan Professional 440 Solid State IC Amp Head
« Reply #9 on: April 27, 2017, 07:58:53 AM »

For example:


As you see, it's pretty much the generic current feedback topology. In just few years this basic circuit would convert from being a short circuit protection to being a tube amp emulation feature instead. Simultanously gaining worldwide popularity as that.

Yes, information was much harder to acquire back then. Therefore I beleive there must have happened something very groundbreaking that explains why this scheme spread to numerous solid-state guitar amps like a wildfire in the early 1970's - and particularly NOT as a protection feature but as a tube amp emulation feature.

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