Fender Blues Delux

There is a common fault that keeps turning up with these amps, and also the Hot Rod and Deville amps. The output stage of these amps is very similar, if not identical.

As with many new valve amps (and this includes many of the ‘reissues’) they are not built in any way similarly to the originals. It would just be over the top expensive to build say, a Tremolux, or Super Reverb or whatever, with all the hand wiring that went into them at that time. So most of the components are mounted in the same way as they are in solid state amps; i.e. on printed circuit boards. As everything is always in exactly the same place everytime, you can get your friendly pick ‘n’ place robot to do nearly all the work. Whereas the originals were entirely handwired. That would cost you a lot of money today.

Anyway, there’s nothing fundamentally wrong with this. Often. However, now we get to the DeVilles, Hot Rods etc.

These amps use pcb’s like nearly everything else, but the way that the output valve bases are connected to the pcb is a big problem, and it’s not just Fender that does this, many new Marshall amps with valve output stages are also built similarly. The valve bases (sockets) are connected to the pcb by solid metal rails, the bases themselves being screwed to the metal chasis of the amp, and the pcb these rails are soldered into is fixed onto metal posts (plastic in the case of Marshall). The problem is that the pcb flexes in transport and in use, and your nice amp develops cracked joints where the valve base pins are soldered onto the pcb. So what?

Well, the last one I saw had blown the inside of a 6L6 output valve to bits, because a cracked joint to the grid had taken the bias off the valve completely. The output transformer had survived, but that was bit of luck really.

The first thing to do is resolder all the joints to the output bases. I use silver content solder, and I put plenty on. This strengthens the joints, but you can’t unfortunately strengthen the tracks, and these can break. There’s nothing you can do about this. It’s possible to clean back the tracks and run solder a little way along, but you can’t strengthen the adhesion to the board, so that’s probably time spent that isn’t very effective.

If you take the back off (Unplugged of course!!!!!!) the output valves are at the bottom near the middle, and you can see the rails and the soldered connections. Get a magnifying glass to it, and you’ll very likely see cracking round the joints, even on quite new amps.

Mesa Boogie Vtwin Pedal

This didn’t cost the guy anything at all. It didn’t work. It’s basically a valve overdrive/distortion pedal and very nice they are too. But only when they work.

There was no light from the valves, and nothing out of it at all. Half a second looking at the schematic and another half a second looking at the power supply told me all I needed to know. It runs from a step-down supply, of the sort that runs nearly all stomp pedals these days. But with a difference.

The input supply has to be ac; not the normal dc that nearly all pedals use, of  one voltage or another. It goes straight into a step-up transformer inside the pedal, because the valves need about 200 volts or so on the anodes.

I think the input supply should be around 12 volts, (ac), rated at least at 1500 mAmps, and preferably a couple of amps.  That didn’t take long to figure out.

Vox ac30

  This was a very early one. I’d worked on it before a couple of years ago, and it was good to know it hadn’t given any problems till just now.

There’s nothing very unusual about the fault, but it might help to know about it if you come up with something like it.

First of all, the fault was a blown rectifier valve. This is a GZ34 originally but a replacement might be something like a 5Y4. The characteristics of this replacement are pretty similar, but the glass envelope is smaller. I might be nit-picking, but the extra glass area on the GZ34 gets rid of the heat a bit faster so can be no bad thing.

A faulty rec will often short, which means that it will take out your mains fuse (if you’re lucky); or fry the mains transformer if you’re not. This rec had blown open circuit which meant that everything lit up, but nothing came out. The lights were on but there was nobody home, so to speak. Often a bad GZ34 will look just what it is. No heater, so nothing showing a light. The valve you’re looking for is the chubby one on its own, next to the mains transformer. This is the furthest on the right as you look at it from the back of the amp.

Now you have to go carefully, because there are one or two common problems that might blow this valve. So if you put £15 worth of new valve in, it might just blow straight away, if there is another fault. Although you’re perhaps not going to be able to check everything, there is one or two things you can do. With the amp unplugged!!!!!! of course.

Take out the 4 output valves; they’re the furthest away as you look from the back of the amp. Make sure you know which came from where, because you’ll need to put them back in the same positions. Then put your new  GZ34 in (it’s a good idea to give the base a bit of  contact spray while you’re at it) and keep your fingers crossed. If the main reservoir cap has dried out or shorted, your GZ34 will be trashed. If you’re handy with a multi meter, you can check the voltage as it builds up across that cap, but if it doesn’t do much within 20 seconds or so, switch off and take to a man who can, because you’ve got trouble.

Assuming the GZ34 is ok (they often produce sparks inside the valve if they’re not happy) then switch off!!!!!!!!!! let the voltages colapse for a few minutes. If you’re a betting man/woman you might be tempted to put back the output valves. Replace the pair on the inside two bases, switch on, a keep your eyes on the valves. There should be no glow from the anode itself (thats tha grey box inside the valve) and certainly no sparking. Just a glow at the top and bottom.

If you got away with that put back the outer two el84′s and do the same thing.

Shorted output valves or faulty bias bypass caps are a major cause of rectifier failure. So the safest bet is to treat yourself to a new set of EL84′s.

One other thing. Although there are plenty of amps that will run very happily with two of the output valves removed, an ac30 won’t. So don’t be tempted to convert it to an ac15 in that way.