Valve rectifier?…..solid state rectifier?????

Further to the amp design series. This is part three. If I can get to three without suffering brain fade there must be a slim chance I might know something useful. Eh?

a) So what’s the difference? b) Is there a difference? c) Will I notice a difference?

a) One uses a valve. Duh.

b) Not if you’re deaf.

c) See above…..and…..some people wouldn’t notice a difference if a herd of wildebeest stampeded through their kitchen.

A rectifier of any sort has one purpose, and that is to convert (rectify) an ac (alternating) current/voltage to a dc (direct) current/ voltage. But the valve rectifier has certain properties that the solid state rectifier doesn’t. From a point of view of how it affects the sound, the valve rectifier has the property of ‘sag’. I know how it feels. The valve itself has an internal impedance, and what that means is that the current path from anode to cathode (the conducting direction) isn’t a short circuit, it has a certain resistance. (Impedance, strictly speaking.)

This is a very complex characteristic and varies from valve to valve in imponderable ways. This will not endear the rectifier valve to the digital fraternity. They like to put these numbers into this black box and it does this. Every time. No valve will do that, because the valve is a physical device and as such every one is different (within certain tolerances). Not only does it vary from valve to valve, but also depending on the current it is drawing, how hot it is, what is the smoothing arrangement on the dc side. And what day of the week it is, for all I know?

The characteristic of ‘sag’ in the rectifier, means that as the valve takes more current, the output voltage tends to drop, which tends to limit the output of the amp it is supplying, so providing compression. This is not a simple compression. It is affected by frequency, amplitude, phase angle, power amp loading, even speaker impedances. So far as I know, there is no compression circuit that can simulate this form of compression, it is not very predictable and varies (as we’ve already noted) from valve to valve.

A solid state rectifier, on the other hand, has very little impedance in the forward direction. It will run itself without varying the output voltage, to destruction. So it does not compress, and has no sag factor. This is a big difference, as any guitarist will tell you who has wound up a valve amp with a valve rectifier.

But there is a downside to the valve rectifier. It’s expensive and complicated. It needs, for a start, an extra valve base. It also needs a transformer that will give a heater supply voltage for the valve, which is often a different voltage to rest of the amp heaters. In the case of a GZ32 or GZ34 or 5Y4….etc….5 volts. This means that the mains transformer has to have an extra winding to supply this voltage.

You don’t need any of those things for a solid state rectifier circuit. Just two (or four) diodes, or a bridge rectifier. Nevertheless, our amp (should it ever get to that state) will have a valve rectifier. If we keep it small, say 15 watts  rms, we can do the valve rectification with an EZ80 or EZ81. ….So what?…..ah well….these valves have 6.3 volt heaters, so the mains transformer is much simpler; we don’t need a separate 5 volt supply.

So we’ve actually made a decision. Which is quite something in a life of doing my best to avoid them. We use an EZ rectifier valve and design the output to be around 15 watts rms. And that solves another decision. If the amp is going to be some variation on class AB (we’ll look into this a bit further into the proceedings), it is likely to be using a pair of EL84′s. Not unlike the Vox AC 15, or the Watkins Dominator.

The thick plottens………..

Tea! Macaroon! Oh yessssss!

Puctec ZD-987 solder/ desolder station

This is a bit off the beaten track, but might be useful. There’s a first time for everything……

I’ve used a desoldering hand pump device forever, and they work ok. For about a week. You really can’t expect too much from something that costs a couple of quid, can you? And also, if you have a lot of desoldering to do you can get a serious case of ‘pumper’s thumb (?)’ We will not go into that; but it hurts.

The big plus for this soldering/ desoldering station was that it was cheap. I may as well be brutally honest. Most of the other soldering stations (Weller, Antex, etc.) would need me to sell my grandma to drum up the cash. As I don’t have a grandma any more, my style is somewhat cramped in that investment area. It was actually surprisingly good (I’m off grandma’s now) but for one piece of missing information. It was like building a flat pack bookcase from Korea (or somewhere else foreign). On following the instructions to the letter (usually an interpretation from the early Sanskrit) it looks remarkably like a food mixer. A food mixer with a lot of screws missing.

There is a glass tube with a spring in it that should fit behind the sucky-pipe-thing. I found that a jack for a Transit van was helpful in installing this. You really have to have powerful thumbs to put this in. It is probably worth an investment in a thumb-building-up course as you have to take the tube out on a regular basis to clean out the solder waste. Except there isn’t any waste if the temperature isn’t effectively set, because it won’t desolder.

Here is the first real crunch. In the instruction sheet, it says that leaded solder melts at 180 degrees C, and leadless solder 220 degrees C. Which it does. However after some experimentation, we find that after setting the temperature to around that recommended level, we pull off the tracks quite well, but don’t actually desolder anything.

You need to desolder at around 335 degrees C. At that level it does work very well.

However, another problem (which is described in the instructions) is that at high heat levels (see workable operating temperatures) the solder tends to clog the intake pipe with a lump of lead. This lump of lead (also according to the operating instructions) you can’t get out. That’s because all the tin content of the solder alloy has been burnt off, which leaves you with a lump of lead that you can’t shift with the poking tool. Fair do’s though; Puctec do inform you of this. And also that your nice desoldering machine is unusable. Decent of them.

There is a way round this. You might have to do this a lot if you are soldering valvebase joints where there is a lot of solder to get rid of. You bring the temperature up above 400 degrees C and thump the cleaning stick in and out repeatedly. As the temperature gets up to around 380+ the solder will shift and you will have a clean tube. Sounds a bit brutal, but it works and is a lot better than chucking your nice new machine away.

Cuppa tea! And I certainly deserve a macaroon for this one.

The Second One in the amp design series.

The general idea in designing/ inventing anything at all, is to get together some wonderful ideas, build them, and then find out why it don’t work. Unless you use a Computer Aided Design piece of soft brain (sorry, software), in which case you can get it to virtually design it, virtually build it, virtually test it, and come up with something you can then virtually poke around with until you get something new. Except it won’t be, because the intuitive human input  has been relegated to accidental poking about and the imagination relegated to accountancy and marketing.

I am not, then, very likely to be a CAD person. In which case I shall have to think a bit. Oh dear.

We’ll start with the power supply. “Why would you do that?” you might enquire. Because if we get that wrong I am not going to get a macaroon. If that part of it is less than it should be, the rest of the amp is unlikely to be worth plugging in. So what do we want from a power supply? At its most basic, some device that takes the ac mains (240-250 volts in UK) and converts it to a useable dc voltage for the electronics. There is little difference in that requirement whether you are supplying a semiconductor (solid state, transistor; call it what you like) or valve circuit. The difference is that semiconductors tend to be higher current, lower voltage, and valves tend to want the opposite. So a power valve will be likely to have several hundred volts at the anode, whereas a semiconductor amp will need voltages of less than hundred volts. The actual voltages depend on the power output of the amp. The power transistors will put out amps, and the power valves, tens (or maybe a few hundred) milliamps.

Clearly an over simplification, and the actual figures will be dependent on the sort of power rating of the amp. But it at least illustrates the general principle.

There are two different possibilities for the power supply, and the main difference between those alternatives is in the way the incoming ac voltage is rectified.

 

 

This is a basic arrangement for a full wave rectifier circuit using solid state diodes. It could also be a bridge rectifier arrangement, but the result is the same, so we will stick with this one.

A diode will only conduct in one direction, and the transformer’s job is change the 240 volt input to a useful voltage for the amp to work with. The primary side is the 240 volt input and the secondary is centre tapped, which enables the ends of that winding to act in opposition. That means that when the voltage increases at one end, (with respect to the grounded centre tap) the other end decreases. This enables the diodes to conduct alternately in their forward direction, so producing a dc voltage from the ac input.

All this sounds fine and dandy, but the output is like a rowboat on a ski jump. Not smooth in other words. And therefore not useable for the amp, as it stands, because what you would hear most is a 100hz buzz. Most tunes really need a bit more than that.

So far there is no difference between that which the valve amp requires from a power supply and that of a transistor amp. The difference comes in with voltage/current requirement, and that brings in a big possibility with the valve amp. It can use valve rectification, and the solid state amp does not have that option.

In the next blog we shall consider the why’s of that, and what the differences are.

And just in case you might think  I’ve turned sensible, I shall, especially for you, dear reader, come up with some really, really daft ones.

Have a good macaroon…..I mean…..day.