Posted 25 Oct 2010, 5:54 p.m.

Earlier today Isti queried me about an issue he was having with his amp. I thought it might make a nice little case study for anybody having similar problems. I have included the problem and response. Enjoy :)

I`m having trouble with my amp. It is an ENGL scremer combo. Basically the situation is the following:
1) when I put my delay through the FX loop the effect sounds distorted even in clean channel and its crap.
2) the amp is getting quieter and quieter and the volume just seems to be dying on me more each day.
3) the sound quality coming out of it is becoming worse as well.

It sounds like a tube change for me, but as I have never changed the tubes of an amp before, and because I would like to spare some money it would be cool to know which tubes need changing I would really appreciate it if you could take a look at it and help me out sometime.

Hi there Itsi.

To be frank, changing guitar tubes isn't difficult and is quite useful to learn.

It does sound like a tube issue, but we obviously want to be sure of this. If we find it is we then need to determine whether it is the pre- or power-amp tubes that are the problem.

Generally pre-amp tubes last around 3 to 4 years and power-amp tubes last between 18 to 24 months.

First gain access to where the tubes are in your amp. Make sure that the amp is OFF and UNPLUGGED and has been for a good time. Remember that the capacitors in an amp will hold charge after the amp has been switched off and could potentially give you a nasty or even fatal electric shock.

Have a look at the amp's fuse/fuses to check that they aren't blown. When valves start to die they draw more and more power from the mains and this can lead to blown fuses. This is unlikely in your case because there is still a sound coming from the amp.

When you've done this remove the tube covers (if there are any) by sliding them off vertically. It is relatively easy to tell whether a tube is dead. What you are looking for is whitey/blacky/silvery powder spots on the inside of the tube and possibly burn marks. ( Look at both the pre-amp and power-amp tubes and determine which valves and how many valves are gone. As a general rule if one tube is gone I would recommend replacing all the tubes in that set. Aka one bust power-amp tube should require you to replace the full set of power-amp tubes.

When it comes to changing the tubes, I wouldn't suggest touching the power-amp tubes. If they are the issue, it is worth paying the money to get them changed professionally. You can also get them biased which is something neither you nor I can realistically do. In the case of the pre-amp tubes being the issue, you can do it yourself.

Make sure when you remove the old tubes and insert the new tubes that you do not use bare hands. Use a towel instead. The reason for this is that oil from your hands can cause the tubes to 'explode'. The greasy areas on the tube draw more heat and cause it to be distributed unevenly on the surface which in turn can cause the tube to crack. To remove the tube (gently) pull it up using a SLOW AND SLIGHTLY CIRCULAR MOTION...

When replacing the tube make sure that the pins on the tube are straight. Then line up the pins with the socket holes and insert the tube downwards again using the aforementioned slow and slightly circular motion. The last thing to do is check that the new tubes are working. Plug in the guitar amp and look to see if there is a slight orange glow coming from each tube. Then without plugging in your guitar turn up the gain and master and check to see if you hear the usual hiss. Finally replace the tube covers and everything else, plug in your guitar and see how she plays!

Power-amp (outputs tubes) are a different story. Get back to me with your results.

I hope this helps.


That will be all.