Bogner Alchemist

This is a beautifully presented amp; if you like that sort of thing. I don’t go too far into the commercial side of these things, because I tend to glaze over when they want me to sign a cheque with that many zeros after the first number.

First, I’ll apologise for the dearth of photos. My little fugi S5000 has given up all pretence of  sanity and I know for a fact that the Bonger Alchemist will have disappeared under a tanker load of printing ink of various colours, topped off tastefully by a mud slide. So no photos.

The fault was that it was low power and lacked punch and depth.

Describing a sound is no easy matter. I had one description (of a Marshall AVT if I remember right) which was memorable. “It sounds like my dog wiping its butt on coconut matting.”  As he didn’t have his dog with him, I didn’t get to check that out. Moving quickly on.

In the case of the Bogner, one of the pair of output 6L6′s was giving considerably less gain, which effectively puts a kind of distortion onto the output which is called ‘even harmonic’ distortion, and what this means is that the positive half of an output signal is larger (or smaller) than the negative half. This is very obvious on an oscilloscope, and not at all obvious to the ears. To the ears, it’s just an annoying sound that is lacking in ‘something’. It is frequently accompanied by crossover distortion, so-called because the top and lower halves of the signal don’t ‘cross-over’ smoothly, and this often comes over as a roughness in the sound at all volumes, but especially noticeable at low volumes. Anyway, there were two issues with the amp. First it needed a new pair of valves.

The biasing arrangement on this amp was very sensible, and I usually nip outside and run up a flag whenever I find anything sensible in anything new. If ‘sensible ‘ isn’t dead altogether it sure smells funny. (Frank Zappa-ish quote). There was a seperate bias preset for each output valve. Before we get too excited about this and throw a party, Leo Fender did the same thing, but simpler and no less effective on his delux reverb, vibralux, tremolux, and no doubt a heap of other ‘luxes’ of one sort or another, with a bias balance control.  Around 1950.

Apart from that there were dry joints on the phase-splitter which meant that the signal was uneven before it ever got to the output valves. So that had be sorted out first.

Biasing seems to be taken as some sort of black art. It’s not, but there is a fair amount of leeway in its setting. This where we get into the ‘Class system’ . Class A, Class B, Class AB1, Class AB2, Class C, Class D.

Class A is of no use to us in this case. It means that the valves are biased to work on the whole of the signal. If somebody says that an amp is Class A, the first thing to look at are the transformers. A 10 watt Class A amp will have transformers in it as big as you would expect to find in a 50 watt push-pull amp. If it hasn’t then it’s not Class A.

Class B is push- pull, where one of the output valves does the positive half of the signal and the other the negative. AB, AB1 and AB2 are different biasing values where the signal has some degree of cross-over, where the upper valve and the lower valve share a portion of the signal. It’s this shared portion of the signal that the element of choice or preference comes in. Basically what it does is smooth out the cross over bit of the signal. The lower the negative bias voltage the higher the current flow through the valve, and the smoother the signal, especially at higher power outs. Unfortunately, this puts a higher load on the valves, so there has be some trade off.

The way I bias an output stage is to bias it into cutoff at a low signal, bring the signal up to maybe half power, and trim the bias till it smooths out the cross over. Then wind it up full and re-adjust till it just smooths out. Then I knock it back to a point where the signal is just at the clip point and check that it clips similarly both sides. That’s where you can adjust it to be even, if you have seperate biasing or a balance arrangement. Then you’d have to go back and check the cross-over situation.

Opinion of the Bogner? Within the limits of current thinking (how to put something together without using people) and current financial philosophy (never give a sucker an even break), if I’d got that sort of money I’d think hard about who it was I was going to hand it over to.

The Dangers of Soundproofing

You might wonder what it is that could possibly be dangerous about soundproofing. Let’s look at the issue like this; you have a twenty-five thousand watt stereo system in your lounge. We have to assume that you are, therefore, deaf; but nevertheless conscious of the fact that your stereo system has the capability of taking out  light aircraft at  25,000 feet.

With this in mind, the volume control on your demolition system is nailed to the setting marked ‘-1000 db’. Which means that, effectively, you wouldn’t be able to hear it at all, even with your head inside the bass woofer, and even supposing you weren’t deaf. So far, so good.

Your next move is more dangerous. In order that you might be able to jack up your volume control, the word ‘soundproofing’ suddenly leaps into your head as if from a great height. That’s where the danger surrepticiously rears its threatening head.

You have ‘soundproofed’ your lounge. Probably by sticking egg cartons, apple boxes, polystyrene t.v. packaging,  onto everything, until it becomes quite difficult to find the way out. Confident that you are sonically on your own desert island, you jack up your volume control until things jump around; and that’s when you hear the knock on the door loud enough to drown your sound system into oblivion.

Outside stand a queue of neighbours a mile long, holding bits that have fallen from their respective properties, and a rope, the end of which looks suspiciously like a noose.

Lo and Behold! You have conjured forth your very own lynch mob.

This little example is merely to illustrate the minimal consequences of getting it wrong. It could have been worse; you might have brought down a jet, or something. 

The issues with soundproofing are simple, but inconvenient. We first of all need to divide sound into its spectrum. Low and high is good enough, for now. The biggest problem is definitely the low end. We also need to clarify the difference between ‘sound proofing’ and ‘acoustic conditioning’. The former is involved with stopping  the stuff getting out, and the latter with controlling what it actually does inside.

To stop bass getting out, you need mass and stiffness. Bass frequencies carry a lot of power and are capable of ignoring your egg boxes altogether, if not actually tearing them to bits. ‘Transparency’ is a good way to think soundproofing. Things that are transparent to bass frequencies are things that don’t move. At all. A sheet of glass for instance, although rigid, will vibrate like a drum, as it is only supported by its very edges. Two sheets of glass with an air gap between; and the sheet furthest from the sound source will vibrate much less, partly because of the cushioning effect of the trapped air. Put another sheet in there and make sure there is a good air gap, and that arrangement makes a good sound proof shield. In that case it’s the stiffness that does the job.

A door is different. Obviously, because it’s not a window. A soundproof door is heavy. And when shut, it’s airtight. Which gets us to another big problem with sound proofing. You have to learn how to do without breathing for long periods. Any space that will let in/out air, will also let in/out sound. Alright in big room up to a point, but if your studio is a cupboard under the stairs? Not easy to play guitar in an aqualung.

Now for the relatively easy bit. High frequency sound. Any hard surface will stop high frequency sound, and any soft material will soak it up. So a normal plastered wall will stop high frequency sound getting out, but it will also have the unfortunate effect of bouncing it back, which creates reverb of the sort that ( usually) we can do without. This where your egg boxes, acoustic tiles, carpets, soft furniture will have a positive effect. Uneven surfaces will diffuse the sound that bounces from hard surfaces, and soft materials will soak it up. So this comes into the realms of acoustic conditioning and is relatively straightforward with high frequencies.

Bass frequencies are a different problem altogether. Any room (or enclosure of any kind, for that matter) will have certain frequencies which it will be ‘tuned ‘to, producing extremely annoying things called standing waves; (which have nothing to do with football), in which a particular frequency is amplified by the room itself to objectionable proportions. And to control or eliminate that can be a big bucks operation. Tuned enclosures can be built to soak up bass frequencies, and ceiling spaces can be modified in various ways to adjust these standing waves.

Acoustic engineers earn their money, make no mistake; there is a lot of maths involved in soundproofing.

But the above stuff gives you some idea of what kind of principles are involved, hopefully enough to avoid the lynch mob.

Time for tea.