Over the weekend I had the pleasure of doing a quick service on and Orange Rockerverb MkIII. The amp is just coming up on two years old and gigged at least every week.
The owner reported that the amp was creating murky distortion and it sounded so horrible it was no longer useable.
So this amp is experiencing a lot of lugging around and used at giigs 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 useage but they also do not like being moved around when hot and they dont like vibration.
Pre-amp valves last longer than output valves so my first thought was to put the pair of EL34’s on my valve testers, fully expecting to see valves that were worn. Surprisingly they were in good condition and still matched. Of course a very slight drop due to wear and tear but they had plenty of life in them
This amp has six pre-amp valves , four 12AX7 and two 12AT7. One of the 12AT7 drives the reverb and this is hard work and I was expecting a bit of a drop it’s performance. I tested them on an Avo Mk2 tester.
The reverb drive 12AT7 was totally worn out !
The other valves were not far behind and all were worn apart from one 12AX7 that was borderline. The combination of a whole collection of worn pre-amp valves.
The owner decided to replace all the preamp valves and we used three Tung Sol 12AX7, one Sovtek 12AX7, to give some variation in the two channels in the V1 position and two sturdy EH 12AT7’s.
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. It is worth remembering that 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/tung-sol-12at7w-preamp-valve/
https://valvetubeguitaramps.com/product/tung-sol-12ax7-four-unit-value-pack/