VOX AC15 CC1X REPAIR

We cover a VOX AC15 CC1X REPAIR need due to valves being left in the amp for far too long.
A nice compact amp with an Alnico blue speaker. Unfortunately this amp was only suitable for starring in silent movies. The red light was on but nobody was home.  Totally silent.

So what is wrong?

When I open up the amp I see that the current limiting resistors in the power supply are de-soldered. Someone has been inside this amp and tried to repair it. Unfortunately they damaged the delicate pcb tracks.
I test the EL84’s at 400 volts anode. One is is worn but usable however the other one has failed due to the insulation between the electrodes. The valve has short circuited ! I then check the HT volts and find it is a big fat zero.  There is AC volts coming out of the transformer.  The 1N4007  power supply diodes are blown.
I replace the diodes  and then re-connect the limiting resistors . This is tricky due to the damaged tracks and I bridge the tracks with wire as in the picture below.

I replace the EL84’s with a nice pair of Sovtek EL84M because the owner wants a lot of headroom.  The preamp valves were like maracas and microphonic so a pair of EH 7025’s are fitted. These are nice low microphonics and low gain and helps increase the headroom per the owner’s request
The amp is now back in operation! A soak test and then back to the very happy owner.

Why did the EL84 output valve fail?

The answer is very simple.  The amp dates from around 2009 and the all the valves are original. This is a regularly gigged amp. Valves wear out and this is a classic example of what happens when valves are left in an amp for over a decade in a regularly used amp. they not only sound dull but they fail and cause collateral damage in the amp.  In hand wired amps the collateral damage is repairable multiple times. This is not the case in printed circuit board valve amps. You run the real risk of needing a completely new PCB.  Many amp manufacturers are no longer supporting amps from the 2000’s with spare boards so the only alternative is to repair them.
You will have more enjoyment and reliability out of an amp that has the valves changed every 2 years. You know what happens if you drive a car till the oil is all used up in the engine. Bang! New engine needed. Valves are the oil of your amp.

PCB

Here is a picture of the small fragile PCB.

So remember old worn valves spell TROUBLE !!! = £££$$$$