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Jordan Professional 440 Solid State IC Amp Head

Started by galaxiex, April 17, 2017, 09:17:46 PM

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galaxiex

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.
If it ain't broke I'll fix it until it is.

galaxiex

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.  ;)
If it ain't broke I'll fix it until it is.

J M Fahey

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 :)

galaxiex

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.
If it ain't broke I'll fix it until it is.

J M Fahey

#4
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.

galaxiex

Here is a schem from a different model,
but the power amp section more closely matches what is in the amp I have.
If it ain't broke I'll fix it until it is.

J M Fahey

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

teemuk

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".

J M Fahey

QuoteHowever, 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

teemuk

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.


phatt

Interesting stuff :o
Oh where would we be without the collective knowledge and experience base of you more senior chaps.  :dbtu: :dbtu: :dbtu:

When time permits, Could I ask how it works as a form of protection? ???
I'm happy to redraw the circuit with part numbers if that helps.

Phil.

teemuk

One can tweak the feedback circuit to produce lower voltage gain to lower impedance loads. This will simultaneously limit current delivered to those lower impedance loads, and thus power dissipation of the output devices. The "current sensing" resistor additionally prohibits a complete short circuiting of the load.

It's not maybe best of protections, most likely far from it. But at least it limits lowest load impedance to resistance of the current sensing resistor and does not try to deliver almost full rail voltage to low impedance loads. I think one could expect the circuit to handle momentarily short-circuited loads but it probably fails rather quickly if the condition is sustained for long periods of time. Anyway, it's certainly better than nothing.

---

I'm not too sure if Fahey's earlier comments about no bias provision are correct. This does not look like a class-B design to me. Correct me if I'm wrong, but I believe that "diamond buffer" is providing the class-AB bias: In reference to circuit's input (the opamp output) voltage at base of driver transistors is one emitter voltage higher (or lower), pretty much like it would be when dropping over a a generic diode junction. Diamond buffer simply exploits the base-emitter junction of the cathode follower. The output pairs are Sziklai so they introduce just a single junction where voltage drops, thus two diode voltage drops per whole complementary pair should be enough. At least in theory.

Adequate temperature tracking could be an issue with such a simple setup though. Especially because that output stage is configured to introduce voltage gain, which means it likely amplifies its thermal coefficiency.

phatt

Hi Teemu, Thanks for the explanation. :tu:
Sorry I've been busy moving house so a lot to take care of in the last couple of months. :duh

So can I assume changing the value of the current FB resistor (not the low value CS resistor) will alter the amount of current passing through the output transistors? So if it limits the low freq to some degree then it obviously alters the frequency response curve as that FB is increased/decreased.

Also does a Current FB network work the same as Voltage FB where a lower R value = more FB?

I notice the Jordan 440 schematic sweeps both types of FB (via that pot in red circle) allowing for some adjustment either way. I think I've seen Peavey circuits with similar ideas.
Sadly no time for Breadboards until We get the new house sorted out but I might pull up some of my simfiles and have a fiddle with some of these ideas.
Phil.

teemuk

QuoteSo if it limits the low freq to some degree then it obviously alters the frequency response curve as that FB is increased/decreased.

Well, the point is pretty much altering the frequency response. Similarly to high output Z tube power amps, voltage gain to load increases when load increases. With loudspeaker, a reactive load, the amp boosts gain at (moderately low) resonant frequency of the speaker and towards upper high frequencies where impedance increases due to voice coil's inductance.
Yes, RC networks can also be implemented to the loop itself. They commonly serve at least one of these functions: Phase correction, "damping" highest impedance peaks.
QuoteI notice the Jordan 440 schematic sweeps both types of FB (via that pot in red circle) allowing for some adjustment either way. I think I've seen Peavey circuits with similar ideas.
Yes, it basically adjusts between low output Z (where voltage gain is not interacting with load Z) and high output Z (where voltage gain is affected by load Z and amplifier's frequency response is altered by the load). So with generic loudspeakers is goes between "level response" and response with boosted highs and lows.
IMO, the effect is quite often rather subtle so continuous control (e.g. potentiometer) usually adds no more benefit than a simple three-way switch that adjust damping between "high", "medium" and "low".

galaxiex

#14
QuoteIMO, the effect is quite often rather subtle so continuous control (e.g. potentiometer) usually adds no more benefit than a simple three-way switch that adjust damping between "high", "medium" and "low".

Yep, the Damping control pot on this amp is a rather coarse adjustment.

With the control on "10" the amp is somewhat louder compared to "0".
As you would expect, Damping on "5" is half as loud as "10".

A 3 way switch would have worked as well.

Here's pics of why I used the silver chicken head knobs.
They show up better than the black knobs.

The UV tube light must have been an option on these amps. (JM noted above.)
This one didn't have it, so I installed some LED strips and a 12V regulator.
On the back panel there is a place for the UV lamp "start" button.
I installed a mini 3 way toggle to switch between green, red and off.

Maybe I need to source a UV tube.....  8)
If it ain't broke I'll fix it until it is.