OR120 mod

Orange Amps Technical Q&A's

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LD50
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Re: OR120 mod

Post by LD50 » Sun Nov 10, 2019 7:06 pm

Old thread but well worth the discussion :)
Some points though.
I agree that using a 220k to ground as a voltage divider on the grids is a way of attenuating signal and it was not used on the early Matamps, or the OR series of Orange/Matamps it was used on the Matamp GT100 in the early 70s (which also sported a 470k to ground after the wiper of the volume control and before the grid of V1b). Of course the early tagboard ORSTs had a 1meg pot in series with a 22k to ground (albeit the pot bypassed with a 5nF cap).

Next poin is that the Ormats and later Oranges still kept 100k anode resistors in the PI. The 390k resistor is on the anode of V2a (a 'gain' stage) and was always bypassed with a capacitor, from the early Matamp 2000 which had a 2200pF bypass cap there (and curiously an asymetric cathodyne PI as the plate resistor was 100k and the cathode resistor 82k (remedied on the ORST with 100k and 100k). The ORST used a 680pF bypass cap on the 390k and the OR100 of that era used 1nF on the lead amps and 680pF on the Bass version (at least on those that I have worked on).

Now you mention the directly coupled V2a to V2b being a feature of later amps but that is not the case, the Matamp 2000, the ORST, the OR100 and GROhttp://www.orangefieldguide.com/OFG_SCH ... output.jpg all had directly coupled PI, as well as the Matamp GT100http://bee.mif.pg.gda.pl/ciasteczkowypo ... GT-120.jpg.

The amp that you refer to (the '72 found with pcb mounted preamp valves - referred to as Graphic Mk 11 in the schemshttp://orangefieldguide.com/OFG_SCHEM/OR120schem_72.gif) and mid-lift inductor was the exception having the 0.068uF decoupler and 1M in series to the cathode and then that half of the PI.

Which design is better is a moot point (I like the ORST/OR100 amps for their extra bit of grit and having the depth switch early in the circuit) and have no issue with removing the 220k from V1a grid but felt that I should clarify those points before they enter Orange folk lore (although I suspect that they already have!)

Amperex
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Re: OR120 mod

Post by Amperex » Wed Nov 13, 2019 9:35 pm

thanks for posting guys...there's so many different variations way back then, that I dont disbelieve what your saying, but its not my experience...however I havent been in any GT-100's, only early mattamps ala wishbone ash and orange pics only/text...I also dont know which amp you mean when you say mattamp 2000...it sounds like youve worked on more different models than me

but Ive had countless conversations with both mike soldano, and also with jerry blaha of amp crazy in los angeles (who passed about a year ago sadly) who was a tech for just about everybody in the 70's, and had been through all wishbone ash's black mattamps in the early-mid-70's and had pics/schemes etc...biggest collection of documentation Ive ever seen actually

you did just say early mattamps didnt have the 220K to ground, and thats what I saw too...but the 1000pf bypass cap wasnt there either...maybe WA ordered them that way, I dont know, I do know that andy powell had them slightly tweaked at the factory b/c jerry confirmed this

thanks for posting the schemes, although all the gro schemes by bill hickmott or whoever, he admits in several places he was not saying these were completely verified, and theyre not as far as Im concerned...even the supposed elusive factory mattamp schemes that I used to have, didnt match up to wishbone ash's amps, and they had tons of those tops

I do agree, however, about the depth switch sounding better earlier in the circuit, although those amps have partridges vs parmeko, and that could be the difference we're hearing

mike and jerry both said they must have changed it later b/c of some legal issue or whatever with the later amps using the orange name by itself

cheers
Angelo

LD50
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Re: OR120 mod

Post by LD50 » Sun Nov 17, 2019 5:00 pm

Hi Angelo
Funnily enough I have recently been working on Andy's amps. At present he has two 1970 OR100s on tour with him, he is using the original that he bought new in Denmark St (stolen and later returned) and another that Mark is using. I have also just restored an ORST (50w), that is in the unhack thread in the lower part of the forum, in fact handed it over at the Leeds gig last Thursday. His preference is thick rich mids and clear top end with decent headroom, he will use pedals to get grit when needed. Ed Johnson did the resto work on his original when it was retrieved, I have only had to do some minor massaging on the tour, when a Drive pot died, but it does sound great. Both he and Mark have a Rockverb and a selection of 2x12 V30 loaded Orange cabs to run them through.

yossarian83
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Re: OR120 mod

Post by yossarian83 » Mon Nov 18, 2019 1:47 am

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

Thank you, Angelo! Great analysis here. I'm gonna try it out soon and will report back!

Amperex
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Re: OR120 mod

Post by Amperex » Mon Nov 18, 2019 4:34 pm

LD50 wrote:
Sun Nov 17, 2019 5:00 pm
Hi Angelo
Funnily enough I have recently been working on Andy's amps. At present he has two 1970 OR100s on tour with him, he is using the original that he bought new in Denmark St (stolen and later returned) and another that Mark is using. I have also just restored an ORST (50w), that is in the unhack thread in the lower part of the forum, in fact handed it over at the Leeds gig last Thursday. His preference is thick rich mids and clear top end with decent headroom, he will use pedals to get grit when needed. Ed Johnson did the resto work on his original when it was retrieved, I have only had to do some minor massaging on the tour, when a Drive pot died, but it does sound great. Both he and Mark have a Rockverb and a selection of 2x12 V30 loaded Orange cabs to run them through.
Hey LD, that is awesome! I did just become aware of that in another thread...I know your in the UK, how did you hook up with Andy, as he's been in CT here in the states forever

and also, is the OR100 the model number of the black matamps that he and laurie wisefield were using together, for instance at the 1976 rockpalast germany gig that was just officially released?

If not, what is that model number, and do you by any chance have that schematic, b/c I used to but it was lost with a whole bunch of other rare stuff

thanks much!

LD50
Tiny Terror
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Re: OR120 mod

Post by LD50 » Mon Nov 18, 2019 11:39 pm

The black Matamps were the GT100s
A friend in Canada put us in touch and I've worked on a few things for him since.
Yes he lives in the US but tours the UK and Europe pretty constantly.
I have schems for most Oranges and the early Matamps.

Amperex
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Re: OR120 mod

Post by Amperex » Tue Nov 19, 2019 6:54 pm

thanks LD...I also wanted to know also how your handling the voltage with his amps...at 240 in europe the voltage is correct, but using the same amp here in the US on the 115 tap, the voltage will be at least 30 volts higher and not sound right and need a re-bias

for instance this 73 or120 I just got, I always run them and all my marshalls/voxes, etc here with a step-up at 240, and the plate voltage is 475, but on the 115 tap, its 505-510...sounds like shite, and I never rewire the tranny as was shown on the OFG, b/c the voltage then comes down TOO low...it was down to 440 on this amp, and all the magic is then gone!

thanks
Angelo
Last edited by Amperex on Thu Nov 21, 2019 2:34 pm, edited 2 times in total.

Amperex
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Re: OR120 mod

Post by Amperex » Tue Nov 19, 2019 11:22 pm

I just remembered that jerry blaha said WA used step-ups in america when touring, and when they recorded "new england" at criteria in florida, to keep the voltage constant.

Amperex
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Re: OR120 mod

Post by Amperex » Tue Nov 19, 2019 11:38 pm

yossarian83 wrote:
Mon Nov 18, 2019 1:47 am
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

Thank you, Angelo! Great analysis here. I'm gonna try it out soon and will report back!
right on! :)

here's a pic of the PI mod, you can do it all right at the tube socket!

https://www.dropbox.com/s/tm91ab7ep6v9l ... 1.JPG?dl=0
Last edited by Amperex on Thu Nov 21, 2019 2:34 pm, edited 1 time in total.

bclaire
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Re: OR120 mod

Post by bclaire » Wed Nov 20, 2019 11:23 am

Amperex wrote:
Tue Nov 19, 2019 11:38 pm


EDIT: I keep trying to add a pic and it just won't stick, dont know why.

https://www.dropbox.com/s/tm91ab7ep6v9l ... 1.JPG?dl=0
It's written in the forum's sticky post about first time posters. You have to host the picture yourself and link to it.

LD50
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Joined: Thu May 19, 2016 8:35 pm

Re: OR120 mod

Post by LD50 » Sun Dec 01, 2019 8:53 am


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