Orange OR120 high screen voltage melting solder...

[acorkos]

This is an original 70's Orange OR120 w/new correctly tensioned tube sockets, new 2K2 2W power tube grid resistors, new 1K 5W screen resistors, newJJ E34Ls biased at 31mA/510VDC = 63% of max...505VDC on screens. This amp sounds great, and LOUD, but after a couple of hours of heavy action, the screen resistors get HOT and the solder melts around the screen resistor leads.

Would adding a 5-10 watt resistor between the choke and power tube screen resistors, like on a JTM45, drop voltage to keep the 1Ks cooler? If so, what value should I experiment with? Thanks for your help.

OR120 Schematic
http://users.telenet.be/orangefg/OFG...hem_post74.jpg

JTM45 Schematic
http://raw-sewage.net/images/jtm45-readable.jpg

[Chuck H]

Well, that will change the tone of the amp. But it may work. The thing is, that amp, and many like it, worked fine for years without modification. So, what's different now? It could be that the operating conditions are causing the screens to overdissapate and draw excessive current. Or it could be a fault, short or wiring error.

With the unit unter test and working hard (whatever condition is causing the resistors to heat up) measure the voltage across those resistors so you'll know how much current they're dissapating.

When did the amp start doing this? Obviously something changed between when it worked without melting the screens and now. What's the history?

[acorkos]

the output transformer looks to be a replacement....physically smaller, w/3 new mounting holes drilled into the chassis. probably rated for less (half?) output power than the original

[Steve Conner]

If the replacement OT had a higher impedance than the original, that would explain the symptoms, as too high a load impedance tends to cause excessive screen current. You could try mismatching 1 step (8 ohm load on 16 ohm tap) or get the correct OT.

[Chuck H]

You did some work to the amp. Was it working when you got it? If yes, was it trying to cook the screens at that time? If not, did you replace the screen resistors and power tubes because they were bad? Did you measure the voltage across the screen resistors with the unit under test conditions that cause the problem?