OR120 - resistor and bias questions

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Rix
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OR120 - resistor and bias questions

Post by Rix » Thu Oct 22, 2009 3:33 pm

Hi guys.

I recently acquired an OR120 from 1973 (according to the s/n). The main switch was broken so I took it to a good friend of mine who is a tube-amp wizard (builds and repairs them). While there he was doing a complete look-over and found a strange resistor of 1,5 MOhm going from the slave jack to ground (see picture). According to the schematics there should be a 220 kOhm instead but this resistor was missing. Could this be some kind of mod? Anyone seen this before?

Another question, we put in JJ EL34 tubes and I read somewhere that the tubes should be biased pretty hot (approx 70%) but what voltage does this map to?

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Re: OR120 - resistor and bias questions

Post by Tibor » Thu Oct 22, 2009 5:40 pm

the "70%" voltage should be calculated from the voltage measured between pin3 and pin8, the so called plate voltage... call this value AV.... and the "70%" Voltage Y...

Formulars for push-pull amplifiers:
Calculating the range for idle current is
not black magic.
It is really quite simple:
AV = plate dissipation in watts (25 Watts for E34L and EL34)
AS = plate voltage in volts
Maximum value [mA] Y= (AV * 700) / AS

but as an tech your friend should know how to bias the amp :oops:

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Re: OR120 - resistor and bias questions

Post by chrismathewsjr » Thu Oct 22, 2009 5:48 pm

As far as biasing - hopefully before biasing you understand how to measure plate voltage. If not, you might want to let your tech handle it.

Image

This schematic might be a little more accurate than the one you're looking at. I can't find that 220k in my '74 OR120 either.

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'74 Orange OR120 w/Contemporary Avatar 4x12 V30s
'04 Orange AD30 w/Traditional Avatar 2x12 w/G12H30s

http://www.jooniorstudios.com/
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Re: OR120 - resistor and bias questions

Post by Rix » Thu Oct 22, 2009 9:42 pm

Thanks guys, that schematics might be of interest. My amp is the model with both graphics and text, if that helps. Seems like it was kinda chaotic over at the factory these days! :D

Well, I am sure that my tech friend knows perfectly how to adjust bias, I was just curious how to calculate the voltage. I will for sure not try it myself. ;)
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Re: OR120 - resistor and bias questions

Post by bclaire » Thu Oct 22, 2009 10:09 pm

There are a number of mods that people have done over the years and you may be looking at one...

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Re: OR120 - resistor and bias questions

Post by fiveightandten » Fri Oct 23, 2009 2:15 am

Someone likely modified the slave jack so the output was optimal for a rack unit, tuner, or something else they were using with their setup. It won't affect the signal path in the amp so you can leave it without much worry. If it bothers you, replace it back to stock. Though noone uses the slave out anyways. It was designed to be used with the Orange slave heads and really isn't optimal for anything else without some modification, as you see in your amp.

If in doubt...As usual with old amps, a little common sense goes a long way in determining if the amp has been modded. If that resistor is a different type from all the other ones, it's likely not original. The resistors in 70's Oranges should be a light tan in color (aside from the color value bands). I mention this because despite the schematics floating around, Orange was a quirky company and there are amps floating around that don't quite match the circuits they're "supposed" to match.

With respect to biasing, you're free to bias the tubes wherever you prefer them (in terms of tone), as long as it's within the manufacturer's recommended limit. 70% of the max is a common industry standard that works well for most amps. Colder will give you longer tube life, a bit more of a crisp sound, and later breakup. It will also give the amp a slightly more harsh or cold tone to it. Hotter will give you a warmer more organic tone with a little later breakup, though not quite as defined. OR-120's, however, are very jagged sounding amps that have tons of crossover distortion inherent to them, so biasing on the high side to try and dial them out is a little futile, as it's the signature tone of the amp. Personally, i've always prefered my OR-120's on the cold side, and the best sounding OR-series amps i've heard are always biased cold. As usual with tone, however, it's completely subjective, so you should set it however you prefer it.

We measure the bias in static plate dissipation wattage, which is a measurement that takes both the current draw AND plate voltage into account. Measuring bias by the negative voltage alone will not give you a very meaningful number. The max dissipation wattage for a JJ E34 is 25W. 70% of that would be 17.5W, which is a reasonable number to be at. Plate voltage in the amp is likely roughly around 510V or so, loaded (you'll have to measure this). Static plate dissipation wattage = (Plate voltage) x (current draw). So figure 17.5 = 510(i).

i = .034A, or 34mA. Just to give you a rough idea of what ballpark you'll be in. But pay attention to the situation in the actual amp.

Hope that helps. Enjoy the amp!

-Nick
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a.hun
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Re: OR120 - resistor and bias questions

Post by a.hun » Fri Oct 23, 2009 10:38 am

fiveightandten wrote:Someone likely modified the slave jack so the output was optimal for a rack unit, tuner, or something else they were using with their setup. It won't affect the signal path in the amp so you can leave it without much worry. If it bothers you, replace it back to stock. Though noone uses the slave out anyways. It was designed to be used with the Orange slave heads and really isn't optimal for anything else without some modification, as you see in your amp.

-Nick
Nick, GREAT to see you back here man!!! :D

Yeah that Slave mod makes a lot of sense. Will help avoid the sort of situation we discussed here:
http://forum.orangeamps.com/viewtopic.php?t=2835" onclick="window.open(this.href);return false;


Andy.
aNDyH. :wink:

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

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Re: OR120 - resistor and bias questions

Post by Rix » Fri Oct 23, 2009 10:51 am

Actually we removed the slave jack and put a stand-by switch there instead.

A new set of tubes later and.. wow, is this thing rock'n roll or what!
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Re: OR120 - resistor and bias questions

Post by Sooner » Fri Oct 23, 2009 1:04 pm

a.hun wrote: Nick, GREAT to see you back here man!!! :D
Wow! Nick's back. Talking about biasing. Just like the good old days! :P
Sooner
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a.hun
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Re: OR120 - resistor and bias questions

Post by a.hun » Fri Oct 23, 2009 1:36 pm

Rix wrote:Actually we removed the slave jack and put a stand-by switch there instead.

A new set of tubes later and.. wow, is this thing rock'n roll or what!
Yeah that is another mod which makes some sense. Though to be honest I've never actually worried about doing it myself.

And yeah, these amps are indeed a bit special. (Honest but ever so slightly biased opinion!)

Talking about biasing... :wink:

It was actually Ade Emsley (Oranges current designer) who suggested here that OR120s need to be biased hotter than normal (ie with 70% as a lower rather than higher limit). Not entirely convinced myself though - I've really not heard that from others. However for the sake of interest here is what Ade said on that...
Ade Emsley wrote:(http://forum.orangeamps.com/viewtopic.php?t=7296" onclick="window.open(this.href);return false;)

OR120`s need to be biased on the hot side to sound right because of the type of phase inverter used in those amps. The crossover notch will not clear up untill the output tubes are taken to around 70%. You need a really great set of tubes to make the amp reliable!

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!

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Re: OR120 - resistor and bias questions

Post by fiveightandten » Sat Oct 24, 2009 2:44 am

a.hun wrote: Nick, GREAT to see you back here man!!! :D
Sooner wrote:
Wow! Nick's back. Talking about biasing. Just like the good old days! :P
Awesome to see some familiar people around here. It's been quite a while.

Quite a while. :lol:

Hope you guys have been well!

-N
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Re: OR120 - resistor and bias questions

Post by a.hun » Sat Oct 24, 2009 9:09 am

fiveightandten wrote:
a.hun wrote: Nick, GREAT to see you back here man!!! :D
Sooner wrote:
Wow! Nick's back. Talking about biasing. Just like the good old days! :P
Awesome to see some familiar people around here. It's been quite a while.

Quite a while. :lol:

Hope you guys have been well!

-N
Doing fine here Nick. aNDyH. is 'semi-retired' now and I've reverted to my original (vintage) login name.

Where have you been? Busy making good sounds I hope? Hope all is going well with you in these troubled times.

Biggest amp news with me is that I'm in the process of acquiring a really nice '74 DR103 - the one amp I've really lusted after for many many years. It is in Scotland right now though, so I'm not sure when I'll actually get my hands on it and bring it over. Probably sometime the beginning of next year. Meantime it is in very safe and capable hands. :D
Mentioned it here and the following page:
http://forum.orangeamps.com/viewtopic.p ... 01#p456901" onclick="window.open(this.href);return false;

Well pleased with that find, and also really enjoying my newish Japanese Fender '51 P bass RI. Back to bas(s)ics you could say...

Other than that, yeah working, playing, hoping the sea doesn't rise up and reclaim Amsterdam...

Hope you'll be hanging around here a bit longer this time Nick! Always great to have you around. :!:

Best,
Andy.

PS What are you doing with my Dads car...?
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!

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Re: OR120 - resistor and bias questions

Post by fiveightandten » Sat Oct 24, 2009 2:29 pm

a.hun wrote: Doing fine here Nick. aNDyH. is 'semi-retired' now and I've reverted to my original (vintage) login name.

Where have you been? Busy making good sounds I hope? Hope all is going well with you in these troubled times.

Biggest amp news with me is that I'm in the process of acquiring a really nice '74 DR103 - the one amp I've really lusted after for many many years. It is in Scotland right now though, so I'm not sure when I'll actually get my hands on it and bring it over. Probably sometime the beginning of next year. Meantime it is in very safe and capable hands. :D
Mentioned it here and the following page:
http://forum.orangeamps.com/viewtopic.p ... 01#p456901" onclick="window.open(this.href);return false;

Well pleased with that find, and also really enjoying my newish Japanese Fender '51 P bass RI. Back to bas(s)ics you could say...

Other than that, yeah working, playing, hoping the sea doesn't rise up and reclaim Amsterdam...

Hope you'll be hanging around here a bit longer this time Nick! Always great to have you around. :!:

Best,
Andy.

PS What are you doing with my Dads car...?
I've been doing ok Andy. Up until a few weeks ago, I would've say fantastic...but, as you mentioned the economy, yeah we're seeing the affects of that around here, which has left most of us at my job with reduced hours (and thus less money, and no benefits). Things will pick up, but for now...Boooooo!

I got sucked into the car enthusiast scene, mainly BMWs (though I love my little WRX too!). A bit of a different world when compared to hanging out with musicians, but it's a lot of fun and i've met some good people. If your Dad drives an E36 M3, tell him he's got great taste in cars! Though, the model we got here in the states doesn't quite have the horsepower of the cars manufactured for the European market. We get jealous of those.

And sadly, i'm not playing any music currently. I do miss it though, so i'm hoping to get something started up again soon. I was talking with a friend of mine about this the other day, we'll see how that pans out.

Nice job on the DR103! That must be exciting. Those are hard to come by these days. Had I known, I would've been buying them up when everyone was swooning over old Marshalls and couldn't care less about Hiwatts because they were "too clean" :lol:. '74 is a good year. it's very close to the circuit I based my clone off of. I can tell you though, even the 504s are clean enough to play bass through for a lot of things. They're a little stingy in the low end compared to the Oranges (or a purpose-built bass amp), but I think you'll be pleasantly surprised if you haven't had a chance to crank one up through a bass cab.

Keep us updated on it!

-N
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Rix
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Re: OR120 - resistor and bias questions

Post by Rix » Sat Oct 24, 2009 10:27 pm

Once I heard of a term called off-topic; never got it explained though! ;)
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Re: OR120 - resistor and bias questions

Post by fiveightandten » Sat Oct 24, 2009 11:16 pm

Rix wrote:Once I heard of a term called off-topic; never got it explained though! ;)
Quiet down you...your question got answered. Let some old friends catch up with useless banter. :mrgreen:

-N
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