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Author Topic: Asymetrical distortion Orange OR120  (Read 3204 times)


  • Chipper
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Asymetrical distortion Orange OR120
« on: March 05, 2016, 10:10:45 AM »
I have problem with tube amp Orange OR120.
Got it in non working condition, power section is refubished, all ok.
There are only 2 tubes on output, two of them are pulled off, impedance is set appropriately, one level lower.
Amp starts hard clipping (neg.side only) at low input.
Inverter (12ax7) is concertina/cathodyne, known as may be problematic.
Schematic is from early 70', as is seen on the photo.

Anyone with idea where to start?
It looks like too low bias in inverter?

Resistors and caps around inverter and power tubes are new.
Output tubes are new Tad EL34, running at 500vdc at plates, bias is set to 27ma.

Here is scope screen and schematics:

« Last Edit: March 05, 2016, 11:27:38 AM by Amp1 »

J M Fahey

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Re: Asymetrical distortion Orange OR120
« Reply #1 on: March 05, 2016, 01:28:03 PM »
Cathodyne PI ARE problematic, lots of threads about that, specially about the couple Fender circuits that use them.

There are some workarounds, basically increasing power tube grid stoppers so as to even out (somewhat) load seen by plate and cathode ... but that's really aspirin, does not treat the real problem.

On the other side, Orange amps were built that way, and nobody complained, so ...

Personal rant: Orange amps are poor designs, period.

I couldn't believe it myself so I built a couple: cr*p .

So why were they so famous?

Well, they were built with the absolute best "mechanical" parts: incredible (and very expensive) Partridge transformers (Jim Marshal only used two in his entire career: a couple free samples he got) and most important: Fane`speakers, true 101 dB when Celestions were around 97 dB , and true 100W`RMS when standard was 20/25W .

That alone is an absolute game changer.

Not forgetting the killer looks and graphics and the ultra visible "life vest" orange Tolex which made them part of the onstage presence of great bands.

But electronically?  .... meh!!!!  ::)


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Re: Asymetrical distortion Orange OR120
« Reply #2 on: March 05, 2016, 02:56:31 PM »
Problematic?  They work.  A zillion amps use them.  There may be some technical or philosophical arguments against them insofar as they behave such and such a way under such and such conditions.  But that is not the same as flat out not working.   So concentrate on what is wrong with THIS amp, not what might be a better design or something.

If one side of the outputs is not getting drive, go look for it.  Is there signal at all four grid pins?  I don't care that you are using only two tubes, check all four sockets.  In fact, with two empty sockets, did you try stuffing the other two sockets instead of the two you selected?  Might be a bum socket/circuit.  Output impedance is not going to cause your symptom.

You have four screen resistors, so is there B+ on all four pins 4?  And B+ on all four pins 3?  While we check for signal at all the pins 5 also look that bias is there.

If signal is missing from any power tube grids, then is it present on the other side of the coupling caps, ie at the 100k load resistors on the PI?  Your caps might be new, but that is no guarantee they are completing a circuit, nor that you might have gotten a bad one in your new parts.  Never assume.

PI bias?  I have no idea, but you ought to have similar voltages across both 100k load resistors, so do you?

How does the signal look going into the grid of the PI?


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Re: Asymetrical distortion Orange OR120
« Reply #3 on: March 05, 2016, 07:45:24 PM »
Agree with Enzo, check those screen resistors even if they are new.

Hey, you gotta like that he has two scopes!


  • Chipper
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Re: Asymetrical distortion Orange OR120
« Reply #4 on: March 19, 2016, 09:39:41 AM »
Sorry guys for late reply, strong cold hit me down...

Finally found the fault, it was one socket of power tube.
Sockets are old but looks like new.
After I checked all mentioned and unmentioned possible sources I look at the sockets with magnifier and penlight and there were anode pin and bias damaged. Bias pin was stretched and anode slightly burned, black.
After I changed sockets, output signal is symetric and sound is amazing.

Thanks for suggestion!