Orange Amps Forum

Hey guys, so I picked up an early 70’s Orange OR120 a while back and I noticed that when I cranked up it full 2 right power tubes would start to redplate, I opened up the amp and checked the bias and noticed it was a bit high, I adjusted it down to roughly 70% and put in new JJ El34’s, now the tubes no longer red plate but did notice something weird, if I turn the F.A.C. Knob to the left most position when I increase the volume past around 9:30ish I can watch the two right tubes all the sudden glow a lot less inside and the two left tubes suddenly increase in glow a lot. So my question is; is this normal behaviour for an Orange OR120?


I should note that the amp has been recapped but some of the values I noticed aren’t exactly what the originals were.

That sounds like there's something wrong.

Initially, when the two tubes were redplating, you should have switched sockets with the two tubes that weren't redplating. If the redplating followed the tubes, that would have confirmed those two tubes were bad.

But if the redplating followed the sockets (the redplating tubes worked normally in the other sockets, and the other tubes started redplated), that would have told you there was an issue with the amp.

If you still have the old tubes, I'd put them back in, crank the bias back up, and try that test.
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Did you measure each tube's current draw individually when setting the bias? Did you measure the plate voltage of each tube? Did you measure at the tube pins, or did you measure at some collective test point?

I did swap the pairs of tubes and it did not follow so I knew it wasn’t the tubes, next I check the bias with my probe and it was set well above what it should be. And to awnswer your question yes I measured each tube’s currently and plate voltage individually to make sure none were higher than the others, even though there’s only 1 overall bias pot to adjust.

Bring it to an amp tech. It sounds like it may be beyond what you can do yourself.

No, that's not normal. Post a gut shot of the amp. It sounds like it might be oscillating. This isn't a '73 with PCB mounted pre-tubes, is it?

1) What are the bias numbers you are getting for each socket (plate voltage, control grid voltage, and idle draw)?
2) If you swap the tube pairs, do the bias numbers change notably?
3) Does the amp have any modifications?

OrangePiFuzz wrote: ↑

Wed May 16, 2018 8:20 pm

I did swap the pairs of tubes and it did not follow so I knew it wasn’t the tubes, next I check the bias with my probe and it was set well above what it should be. And to awnswer your question yes I measured each tube’s currently and plate voltage individually to make sure none were higher than the others, even though there’s only 1 overall bias pot to adjust.

Hmmmm, I'm very confused. The redplating stayed with the sockets, but the plate voltage and idle current are the same for all the tubes regardless of which socket they're in?
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Let's look at a different factor. With the new tubes, the heater glow in two of them changes when you turn the FAC knob. Are those the same sockets that caused redplating with the old tubes and hotter bias?

Part of the appeal of the OR120 is the asymmetrical output of the phase inverter as you approach clipping. That’s part of why one side is being driven differently. There’s also been issues historically with high frequency oscillation in those amps which can cause red plating without an obvious cause. The oscillation is above the human hearing band and above the output range of the output transformer, so you’d usually only see it on an oscilloscope. I guess this all means yo should take it to a good tech.