Over the weekend I had the pleasure of doing a quick service on an Orange Rockerverb MkIII. The amp is just coming up on two years old and gigged at least every week.
The owner reported the amp was creating murky distortion and sounded so horrible it was no longer usable.
So, this amp experiences a lot of lugging around and is used at gigs lasting 3 hours or more. He was leaving the amp on standby, switching it off, and then off it went back into the wheels at the end of the gig. I advised that as soon as the gig is finished, totally switch the amp off and let the valves cool down.
Valves wear with usage, but they don’t like vibrations or being moved around when hot.
Preamp valves last longer than output valves, so my first thought was to put the pair of EL34s on my valve testers, fully expecting to see valves that were worn. Surprisingly, they were in good condition and still matched. Of course, there was a very slight drop because of wear, but they had plenty of life in them.
This amp has six preamp valves — four 12AX7 and two 12AT7. One 12AT7 drives the reverb (a hard task), and I was expecting a bit of a drop in its performance. I tested them on an Avo Mk2 tester.
The reverb drive 12AT7 was totally worn out!
The other valves weren’t far behind, and all were worn apart from the one 12AX7 that was borderline. The combination of a whole collection of worn preamp valves.
The owner decided to replace all the preamp valves, and we used three Tung-Sol 12AX7s, one Sovtek 12AX7, to give some variation in the two channels in the V1 position and two sturdy EH 12AT7s.
We tested the amp, and now this Orange Rockerverb 50 has a lovely full-bodied tone without sacrificing the highs, and the reverb is responsive again.
This is the first time I have seen the output tubes outlive the pre-amp valves. The dulling down and murky distortion is not necessarily a sign that the output tubes are beat up!
Here are links to the valves/tubes fitted:
https://valvetubeguitaramps.com/product/sovtek-12ax7wc/
https://valvetubeguitaramps.com/product/tung-sol-12at7w-preamp-valve/
https://valvetubeguitaramps.com/product/tung-sol-12ax7-four-unit-value-pack/