OR120 mod

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tirtir
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OR120 mod

Post by tirtir » Mon Apr 18, 2011 9:54 am

I have a question if it is possible to make a "Mod no2: Opening up your tone" ( [http://users.telenet.be/orangefg/orange_mods.htm ) in my OR120 "Master Volume" reissue from 1995? I want to suggest my tech to do this mod to wake up my amp.
OR120 - OVERDRIVE

a.hun
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Re: OR120 mod

Post by a.hun » Mon Apr 18, 2011 11:55 am

You can certainly get your tech to try this. (The reissue circuits were identical to the vintage amps.) The tech would need to check that the amp stayed stable without that cap as parasitic oscillations while usually inaudible could steal a lot of amp power or cause other problems such as overheating. If they don't happen though and you preferred the sound that way, yeah why not.


Andy.
aNDyH. :wink:

Ever tried to outstare a mirror?

In the bathtub of history the truth is harder to hold than the soap, and much more difficult to find!

tirtir
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Re: OR120 mod

Post by tirtir » Tue Apr 19, 2011 9:57 am

Thanx for replay.
I found another mod that could be easier and not that invasive.
http://orangeampguide.atspace.com/inputgridmod.html

I would like my amp to sound more dynamic and a little brighter.

Do You think this mod will do this?


Thanx
OR120 - OVERDRIVE

a.hun
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Re: OR120 mod

Post by a.hun » Tue Apr 19, 2011 10:58 am

tirtir wrote:Thanx for replay.
I found another mod that could be easier and not that invasive.
http://orangeampguide.atspace.com/inputgridmod.html

I would like my amp to sound more dynamic and a little brighter.

Do You think this mod will do this?


Thanx
Interesting. That looks to be the same as mod #0 on the OFG. Yet while the OFG suggests that removing this resistor will increase the gain, Nick (Santore = 'fiveightandten') says the preamp gain will decrease slightly. Which is true I can't say, or what effect it might have on dynamics. A more dynamic amp than a stock OR120 is hard to imagine and if the amp becomes a little looser as Nick says than I'd personally read that as meaning losing a bit of dynamic range.

Again you'd probably have to try it yourself to see...


Andy.
aNDyH. :wink:

Ever tried to outstare a mirror?

In the bathtub of history the truth is harder to hold than the soap, and much more difficult to find!

OrangePaul
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Re: OR120 mod

Post by OrangePaul » Tue Apr 19, 2011 10:17 pm

I've tried both these mods....

Loosing the 220k resistor to ground made most improvement for me.
I found it didn't change the gain that much but more the tone... and gave the amp a more natural sounding overdrive to me.

I still want to try the PI rewire mod one day when i get around to it
Paul.

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jontheid
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Re: OR120 mod

Post by jontheid » Wed Apr 20, 2011 2:43 pm

The phase inverter rewire mod isn't quite as easy as the field guide makes it sound -It isn't just the phase inverter that is different, it is the gain stage preceeding it also.
It isn't just a rewire, it requires extra components - a 0.068uf capacitor, 1Meg,100k and 15k resistors (you don't need to get a 1k5 resistor as you replace the 1k5 cathode resistor in the previous gain stage with 15k.

If you just change the phase inverter it sounds rubbish. Trust me, I've done it.

Interestingly, the OTR120 schematic reveals that this amp uses a post PI master volume like the OD models but an ac coupled phase inverter like the early OR120s.

Even more interestingly, it looks like the GRO100s used the same dc coupled PI as the post 74 graphics . . .

. . . or maybe it is just me that finds this stuff interesting.

Cheers
Jon
Last edited by jontheid on Mon Oct 15, 2012 2:08 pm, edited 1 time in total.

Gladmarr
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Re: OR120 mod

Post by Gladmarr » Wed Apr 20, 2011 7:40 pm

jontheid wrote:The phase inverter rewire mod isn't quite as easy as the field guide makes it sound -It isn't just the phase inverter that is different, it is the gain stage preceeding it also.
It isn't just a rewire, it requires extra components - a 0.068uf capacitor, 1Meg,100k and 15k resistors (you don't need to get a 1k5 resistor as you replace the 1k5 cathode resistor in the previous gain stage with 15k.

If you just change the phase inverter it sounds rubbish. Trust me, I've done it.

Interestingly, the OTR120 schematic reveals that this amp uses a post PI master volume like the OD models but an ac coupled phase inverter like the early OR120s.

Even more interestingly, it looks like the GRO100s used the same ac coupled PI as the post 74 graphics . . .

. . . or maybe it is just me that finds this stuff interesting.

Cheers
Jon

That is interesting. I never noticed that cap before. I wonder how much low frequency rolloff that causes.

OrangePaul
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Re: OR120 mod

Post by OrangePaul » Wed Apr 20, 2011 10:21 pm

Which Cap?
The 68nF or the parasitic oscillator cap?

Yeah I know PI mod isn't simple....thats why I haven't got around to it :)

And the reason for doing it is also to see if it gives me a more balanced output drive to my power section.
My right side always runs a bit hotter, (regardless of valve position -when run as an OR80) when played at high gain levels

And I agree...different amp designs are fascinating...I'd love to build my own amp one day... maybe just a preamp first
Paul.

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OrangePaul
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Re: OR120 mod

Post by OrangePaul » Mon Apr 25, 2011 7:04 pm

Yeah i'm guessing you didn't mean the parasitic oscillator cap as i'm sure that you would know thats for high frequency roll off now I think about it lol
But yeah, interesting that the 'supposedly superior' PI has that extra cap in there...
Paul.

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Gladmarr
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Re: OR120 mod

Post by Gladmarr » Mon Apr 25, 2011 9:29 pm

OrangePaul wrote:Yeah i'm guessing you didn't mean the parasitic oscillator cap as i'm sure that you would know thats for high frequency roll off now I think about it lol
But yeah, interesting that the 'supposedly superior' PI has that extra cap in there...

No, I meant the cap that isn't common to the two designs.

MrHaroldA
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Re: OR120 mod

Post by MrHaroldA » Mon Oct 31, 2016 11:52 pm

jontheid wrote:it requires extra components - a 0.068uf capacitor, 1Meg,100k and 15k resistors (you don't need to get a 1k5 resistor as you replace the 1k5 cathode resistor in the previous gain stage with 15k.
Hey Jon, can you please elaborate on the 1k5 to 15k cathode resistor? The schematics I found don't mention that one ...

fiveightandten
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Re: OR120 mod

Post by fiveightandten » Tue Nov 01, 2016 1:49 am

MrHaroldA wrote:
jontheid wrote:it requires extra components - a 0.068uf capacitor, 1Meg,100k and 15k resistors (you don't need to get a 1k5 resistor as you replace the 1k5 cathode resistor in the previous gain stage with 15k.
Hey Jon, can you please elaborate on the 1k5 to 15k cathode resistor? The schematics I found don't mention that one ...
The schematic labelled "72 Graphic MKII" has a 15K cathode resistor in the gain stage BEFORE the PI. I believe that's the one he was referring to.

It's this circuit:
http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif" onclick="window.open(this.href);return false;
Image
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MrHaroldA
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Re: OR120 mod

Post by MrHaroldA » Fri Nov 04, 2016 2:53 pm

fiveightandten wrote:The schematic labelled "72 Graphic MKII" has a 15K cathode resistor in the gain stage BEFORE the PI. I believe that's the one he was referring to.

It's this circuit:
http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif" onclick="window.open(this.href);return false;
Ah; I was using this schematic as reference, which also has the 1k5 like the later schematic drawn:

Image
http://www.orangefieldguide.com/OFG_SCH ... output.jpg" onclick="window.open(this.href);return false;

Amperex
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Re: OR120 mod

Post by Amperex » Sun Nov 10, 2019 12:35 am

hello, yes, I KNOW this is an old thread, I'm replying to it to set the record straight on a few things for future gents to be able to sort out their amp easier.

I used to be "Amperex" on the OFG and the plexi palace eons ago, and Im the one who "suggested" to remove the 220K resistor to ground before V1.

It does NOT belong there, period, remove it. Its not on any other oranges pre-73, and not on any mattamp/fender/marshall/vox/hiwatt etc etc etc guitar amps, period.

you do NOT need a grid load here first of all, and secondly the gent who said the gain will decrease, NO, its just NOT true, Im sorry. No offense.

A resistor does not belong there on any amp and just bleeds precious input signal to ground.

This is why they did it.

The reason whoever at orange was, who decided to screw with the original design, really messed things up here. They added it to offset their other bungle, taking a perfectly good balanced cathodyne PI and making it unbalanced by directly coupling the two triodes together, but with NO coupling cap to keep DC from entering the next stage from the previous one! You cannot hear this cap in the audio path.

thats the very much needed .068 cap thats on all pre-72 oranges and mattamps/fender tweeds/ampegs/gibsons, and basically ANY amp that has a cathodyne PI...along with the 1 meg and 1.5K resistors that also belong there.

if your wondering why your post-72 orange with this circuit has two of the EL34's burning out before the other two, now you know why. Unbalanced PI.

That being said, even if you dont rewire the PI (but please do it), removing the 220K resistor from the input, is like taking a blanket off the front of your cab; its just better fidelity ALL around, and not necessarily brighter, just fuller and doing what that stage is supposed to at this point. It sounds ALOT less compressed/mushy and tighter like the orig amps did.

The reason they added it was because directly coupling the PI, and also changing the nominal 100K plate resistor to 390K (!) was rediculous...it completely overloaded the PI and made the gain mushy and unlcear and spongy/untight...so they realized they needed to remove some of that mush, and decided to do so at the input, after the fact...WHY, lord only knows! This is backwards engineering in the classic sense, lol.

these amps dont sound very much like a classic pics-only or mattamps b/c of this, so if you want your post-72 amp to sound like the early ones, you must rewire the PI and remove the dreaded 220K...its the best thing you'll ever do. You'll be 99% there, especially if you use vintage glass, which is absolutely necessary IMO.

It can be done without pulling the board, and you only need a 1 meg resistor, the .068 cap, a 1.5K resistor, and a 100K resistor for the plate as additional components, thats it. A good tech can do it in about 15 minutes.

Lastly, removing the 1000Pf cap across the 100K plate resistor in the PI on early amps (if they have it) (and on the changed value 390K on post-72 amps) is also something you should DEFINITELY do. It was put there for possible parastitic oscillation b/c of the gain changes, but Ive never encountered an orange amp that needed it.

For the record, Ive seen several pic-only amps that didnt have it, (even though its in the scheme that everyone has) and Ive NEVER seen one in a mattamp. Its also like taking a blanket off the front of the cab, clearer and better fidelity...we DONT want any capacitance there on the plate!

You wont be able to hear any difference when the amp is played clean with this cap removed; only when driven, and the more you crank it up, the more you'll hear how much this cap was squelching highs away.

There you go. This all being said, I just scored a 73 OR120 a week ago that I couldnt even play b/c it sounded so buzzy/squishy...not anymore :)

Rock on!
Angelo

Jondog
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Re: OR120 mod

Post by Jondog » Sun Nov 10, 2019 1:51 pm

Amperex wrote:
Sun Nov 10, 2019 12:35 am
hello, yes, I KNOW this is an old thread, I'm replying to it to set the record straight on a few things for future gents to be able to sort out their amp easier.

I used to be "Amperex" on the OFG and the plexi palace eons ago, and Im the one who "suggested" to remove the 220K resistor to ground before V1.

It does NOT belong there, period, remove it. Its not on any other oranges pre-73, and not on any mattamp/fender/marshall/vox/hiwatt etc etc etc guitar amps, period.

you do NOT need a grid load here first of all, and secondly the gent who said the gain will decrease, NO, its just NOT true, Im sorry. No offense.

A resistor does not belong there on any amp and just bleeds precious input signal to ground.

This is why they did it.

The reason whoever at orange was, who decided to screw with the original design, really messed things up here. They added it to offset their other bungle, taking a perfectly good balanced cathodyne PI and making it unbalanced by directly coupling the two triodes together, but with NO coupling cap to keep DC from entering the next stage from the previous one! You cannot hear this cap in the audio path.

thats the very much needed .068 cap thats on all pre-72 oranges and mattamps/fender tweeds/ampegs/gibsons, and basically ANY amp that has a cathodyne PI...along with the 1 meg and 1.5K resistors that also belong there.

if your wondering why your post-72 orange with this circuit has two of the EL34's burning out before the other two, now you know why. Unbalanced PI.

That being said, even if you dont rewire the PI (but please do it), removing the 220K resistor from the input, is like taking a blanket off the front of your cab; its just better fidelity ALL around, and not necessarily brighter, just fuller and doing what that stage is supposed to at this point. It sounds ALOT less compressed/mushy and tighter like the orig amps did.

The reason they added it was because directly coupling the PI, and also changing the nominal 100K plate resistor to 390K (!) was rediculous...it completely overloaded the PI and made the gain mushy and unlcear and spongy/untight...so they realized they needed to remove some of that mush, and decided to do so at the input, after the fact...WHY, lord only knows! This is backwards engineering in the classic sense, lol.

these amps dont sound very much like a classic pics-only or mattamps b/c of this, so if you want your post-72 amp to sound like the early ones, you must rewire the PI and remove the dreaded 220K...its the best thing you'll ever do. You'll be 99% there, especially if you use vintage glass, which is absolutely necessary IMO.

It can be done without pulling the board, and you only need a 1 meg resistor, the .068 cap, a 1.5K resistor, and a 100K resistor for the plate as additional components, thats it. A good tech can do it in about 15 minutes.

Lastly, removing the 1000Pf cap across the 100K plate resistor in the PI on early amps (if they have it) (and on the changed value 390K on post-72 amps) is also something you should DEFINITELY do. It was put there for possible parastitic oscillation b/c of the gain changes, but Ive never encountered an orange amp that needed it.

For the record, Ive seen several pic-only amps that didnt have it, (even though its in the scheme that everyone has) and Ive NEVER seen one in a mattamp. Its also like taking a blanket off the front of the cab, clearer and better fidelity...we DONT want any capacitance there on the plate!

You wont be able to hear any difference when the amp is played clean with this cap removed; only when driven, and the more you crank it up, the more you'll hear how much this cap was squelching highs away.

There you go. This all being said, I just scored a 73 OR120 a week ago that I couldnt even play b/c it sounded so buzzy/squishy...not anymore :)

Rock on!
Angelo
Wow, an old resurrected thread with a long post from a new member that doesn’t include links to child labour made goods for us to purchase just by giving up our credit card numbers and birth certificates. :P
Seriously though, great info and thanks very much for sharing. Value-able info I’d say, and welcome! Congrats on the amp!
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