1970's Orange OR120 Screen Resistor Value/Question

[thenorthernpike]

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? 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/34258368@ ... 441202099/

Schematic is:
http://www.orangefieldguide.com/OFG_SCH ... hem_74.gif

[martin manning]

I don't know why someone would want to double the double screen resistance on half of the power tubes and delete the others. I'd vote for returning to the stock 1k on all tubes.

[Leo_Gnardo]

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? 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 seen exactly the same thing in other old Oranges. The "ant coffin" resistors were left with long legs, and the wire was very brittle, so a little shakin' back & forth in travel, they fell off. It IS odd that they're 2K not 1K but the smart move these days is to use 2K's in Orange because the high voltage is more than modern tubes can stand on the screen grids. Maybe someone twigged onto this a couple decades ago at the Orange factory and started making the change all the way back then. I'd put fresh 2K on ALL output tube sockets. Why wait for those brittle leads to break off.

Go 10W with that 470R. I'm guessing it's the voltage dropper to the 2nd hi voltage filter stage. An output tube probably shorted and sizzled that resistor.

Better to steal a hole for a mod rather than drill thru that heavy steel Orange chassis. Those things were built to last, mechanically. Some attention to the electronics and they can do the same electrically.

[martin manning]

Do I hear 1k5 for the screens? Four 2k's used to be there, but two of them broke off at the solder lugs? I don't see a 470 dropping resistor on the schematic, but I see 470V. Looks like someone added an MV too.