1970's Orange OR120 Screen Resistor Value

[thenorthernpike]

Greetings....

A friend brought over his 1970's Orange OR120 amp for me to look at. As soon as I opened the chassis, a 2K 5W resistor fell out of the amp. The 1st and 4th power tube have a 2K 5W power resistor on the screen. The 2nd and 3rd have no resistor on the screen. The schematic calls for 1K 5W on each power tube. I've confused as to what is going on here. Can anyone make sense of this? As to why those values have been changed?

Should I just go ahead and add 2K 5W's to the other screens? has I noticed there had been a "presence" control added to the slave output. There was a 470 ohm 5W resistor on there that blew up.

I've posted so pictures here and would really appreciate some help here. I need to get this amp back up and running.

https://www.flickr.com/photos/342583...7646441202099/

The original schematic is: http://www.orangefieldguide.com/OFG_...20schem_74.gif

[Jazz P Bass]

Not knowing why the 2K/ 5 watters are in there, I can only suggest that you contact Orange.
I have found them to be very helpfull.


Tel: 1-404-303-8196
Fax: 1-404-303-7176
info@orangeusa.com
Note: please post the answer.

[The Dude]

Best guess is that some tech at some time had no 1K's handy. I would put it back to the original schematic specs.

[Enzo]

To be clear, the 2nd and 3rd sockets appear to have no connection to pin 4 at all? Like not missing a resistor with a wire instead, but missing a resistor or anything else.

Look at your photos, ther are little X-shaped solder lugs on posts, the screen resistor is soldered from that X to the socket. In the closeup of the 2k resistor, we see three of the four tabs of the X have been soldered. Right now you have one resistor, and the wire off to the power supply, there is a third blob of solder with nothing there. That is where a screen resistor to the other tube used to be.

Here is my opinion. The power tube will not run if the screen is not connected to high voltage. I suspect someone converted this amp to half power - removed two power tubes and made whatevr other changed, like the 2k instead of 1k, and removed the screen resistors at the unused sockets.

So if you want to use all four tubes, you will need four screen resistors, and check for any other missing connections. If you want to add two more 2k, they will work, or you could revert all four back to 1k. Don't over-think it. It is just a guitar amp. If 8 bazillion amps run with 1k screen resistors on all the power tubes, this one would too, regardless of whatever else is going on.

I don;t see a 470 ohm in there, nor do i see a slave output, so either I have no idea, or point them out for me.

[Leo_Gnardo]

70's Oranges had SG resistors with brittle leads that often broke while the amp was being shipped or bashing around in the back of the band van. Seen it happen a couple times. No worries, you just put in replacements & I'd recommend changing any that haven't yet broken their leads off, why wait. Cheap insurance.

Modern Oranges, not a problem.

[Randall]

I believe that amp has been altered. I just did an OR80, which I still have in the shop. Pics in the vintage amp section. The two star connectors each get a wire from the screen grid supply that feeds a pair of resistors to their sockets on each side. The one I just did had one leg of each resistor twisted to the other coming off the star connector with the other sides feeding the screens. I'd use 1K 5 watt.