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.