Selmer Truvoice Twin Selectortone

I’d never seen one of these before, and found a lot of useful stuff on a site called vintagehofner.co.uk, which I can heartily recommend for a lot of vintage British gear. Now that was a relatively sensible start, don’t you think? I think this amp could be usefully instructive to us for more than the problems for which it came in. 

It has an interesting phase splitter, for a start. Here’s a schematic of a usual phase splitter circuit. There are few amps on the market which don’t use some version of this and the versions are very similar except for the odd difference in component values here and there.

The purpose of a phase splitter is to supply the output valves with two different versions of the same signal. So the signal out of output X in the schematics should be the same as that of output Y; but 180 degrees out of phase. Just to get a basic handle on that, anything that comes out of Y, going positive will be going negative at  X by the same amount. A mirror image, if you like to look at it like that. And that, essentially, is opposite in phase.

So we get one input at c3 which goes to the grid. r3 and r4 are called ‘grid leak’ resistors, and serve to reference the grids to ground.  r5 and r6 function to bias the valve so that it can operate to cope with the input/output signal levels.

One advantage with this arrangement, is that we get gain through it. In other words, the signals out are some multiple of the signals in. It depends on the resistor values but maybe around a gain of 10.

This might seem like an advantage at first view, but it has a big drawback. 

This one looks a lot different. For a start it only uses one half of the valve. A phase splitter nearly always uses an ECC 83, once in a while an ECC82 which is pretty much the same valve except it has a lower vg/ia figure which means a lower gain.

The operation of this circuit is different and uses in part an arrangement called a ‘cathode follower’, but the result is the same. But not quite.

In this circuit r1 and r3 are usually the same value, r2 being in there as a bias element. This circuit gives zero gain. In other words you get the same out as you put in. Actually it’s slightly less. So it must be worse than the other circuit, right?

No, actually, wrong. The key word is ‘accurate’. For a class B (that’s push-pull of some description) the ‘push’ has to be exactly the same as the ’pull’. Or the positive half of the signal has to be the same as the negative half looked at a bit differently. It will otherwise introduce distortion of the sort you really don’t want.

It’s more expensive to build an amp with this ‘gainless ‘ phase-splitter, because you have to find the gain from somewhere else, so another valve is needed, the various components associated, hardware etc, etc.

The problem with the first circuit is that, as it has gain, any differences in the resistor values are multiplied, and that situation will get worse as the resistors etc, age.  

This Selmer amp, also had a fault on the trem channel. It didn’t trem. This is well worth another blog, on the subject of ladder/phase shift oscillators, which most amps (certainly British amps) tend to favour. Time for tea.

Biasing and other Black(star) Arts

I recently had an email from a gent regarding a Blackstar amp, and it reminded me why I do this blog at all. I tend to run off at a tangent not infrequently (surprised, eh?), but if I lose the plot altogether I’d be more usefully employed cutting my toenails.

The content of the email was regarding his amp (obviously) and the fact there was nothing wrong with it. My job would be a lot easier, and more profitable, if I specialised in repairs to things that were working fine. On this basis, I could take on anything, if you think about it. Open heart surgery, bridge maintenance, pet continence, deep sea pizza repairs, politics.

No, wait a minute; scrub the last one. That’s too far gone for anybody to take seriously.

Anyway, this gentleman (Kev, his name was) had been on various forums making enquiries about his amp. Just to show the reader the sort of levels of ignorance the writer embodies, I thought that a forum was some place in Rome where Christians used to provide a decent supper for irreligious lions.  No so. What it actually is, is a place on a screen that people who don’t neccessarily know anything at all, can provide advice on the subjects they know least about. 

The problem, is not that there is anything wrong with that (there are plenty of folks out there who do a brilliant service on these forums providing lucid and intelligent opinions and valuable advice), but, that if you really don’t know anything and are looking to be advised how do you know the difference between gold dust and bullshit?

Any farmer will advise you on that one straight away. Bullshit is brown and gold dust isn’t.  You get my drift.  But don’t take any notice of me; I’ve been panning for bullshit for years. It’s just that it’s a lot easier to come up with a result than the other stuff.

So the purpose of this blog is usually to give me a laugh, and to blow the odd sacred cow out of the water in the interests of handing out a few items of weaponry that might be deployed against those who might be out to take the unsuspecting for an expensive ride.

Biasing then. What is it? This will stay firmly general, and we might get involved in the particular in a later blog. Bias, in general speech means that we look on some issue more or less favourably, more depending on our own predudice than for any logical reason. Physically, it might mean that we push some item to lean this way or that. Both these loose definitions are not really all that far out.

Electronically, bias usually means that we apply a d.c. voltage/current so that its operation is in some way modified. This actually applies in not dissimilar ways, to semiconductors and valves. Even the internals of chips (integrated circuits) employ bias in various ways.

There is another usage of the word, with regard to recording to tape. It is the high frequency signal that we apply to the record/erase head that carries the intelligence (I use the word in its ultimately broadest sense) onto the tape. A better word for it would be ‘carrier wave’. Not in our area of interest at the moment.

If I apply a signal (say, music, from a c.d. or instrument of some sort) to the input of a valve (usually the grid), unless I do something else to it, it will be unusable. The valve can only amplify positive (or positive-going) signals, and so it ignores everything that isn’t. It’s about here I could do with diagrams, but that’s for a bit more detail later. Anyway, the upshot of the last statement is that, whatever you put into the valve, only half of it comes out. The positive half.

In the case of a preamp valve, a positive bias is applied to grid (effectively) this lifts the whole operating range of the valve so that it can accommodate the entire signal. Physically, all that does this is a single resistor in the cathode circuit. This has the unfortunate terminology of ‘fixed bias’ and ‘automatic bias’ they’re the same thing. Even though they don’t sound the same.

Fixed bias pretty much says it. It can’t be altered, easily.

Output valve bias, although it works in the same sort of way, the reason for it is quite different. Most amps are some version of class B. If somebody wants to sell you a 100 watt ‘class A’ valve amp that you can shift with out the aid of a truck, he probably has a stall down the road selling Rolls Royce’s for twenty quid. Without going into any of the technicalities, a class A valve amp will have transformers in it (mains and output) up to ten times the size of a class B.

Another name for class B is ‘push-pull’. There are a number of different degrees of class B. class AB, classAB1, class AB2, but they all work in the ‘push-pull’ manner. A 50-60watt amp will have two vales in the output stage. They’re usually EL34,  6L6. One pushes and the other pulls. All this is saying is that one valve amplifies the positive bit of the signal out, and the other the negative.

If one does upper and the other lower, it stands to sense (don’t how that got in here) that they must cross over. This cross-over point is nastily audible if it isn’t set well. And it is set by the bias.

So far so good? The bias on many amps can be adjusted (not all; original vox’s amongst them) and this is the ‘biasing thing’. The same amp can often be biased to class B, class AB, class AB1, or classAB2. What’s the difference? It’s all in the bias voltage, which sets the current through the valves WHEN THEY’RE NOT DOING ANYTHING. So the valves have to take some current (a few milliamps to over a 100) to keep them alive. It is this current that sets the cross over point. So what? What difference does it make?

Quite a bit. If negative grid volts are set too high the valves are cut off for a certain percentage of the signal, and you get cross over distortion. If too low, the valves take too much current, and either ‘go soft’ (red plate, in American), short and take out your fuse (or output transformer if you’re unlucky) or just have a very forshortened life.

Somewhere between crossover distortion (and this varies with the power it’s putting out) and overheating, you have fair amount of leeway. A bit less bias you get a few more watts output and smoother cross-over point; a bit more, you get less stress on the valves and associated stuff, but a little less output and the sound tends to lack ‘Punch’, subjectively.

If you got through this, I can recommend a cup of tea; or whatever.