The ‘Fender’ 5f1 ……a kind of Champ kit.

This was interesting, a lovely little ‘Champ-esque’ amp in a nice tweed case. Unfortunately, it’s chief claim to excellence was its chainsaw impression. There was no point in playing anything through it if it didn’t sound like a chainsaw. If I’d been a bit quicker in the brain department I could have come out with the ultimate pedal. The Chainsaw….forget the notes…chop down a tree.

On looking into the design a bit further, I had problems figuring out how even the original could have been used to record stuff like ‘Layla’ and ‘Rocky Mountain Way’. The Monty Python ’Lumberjack Song’?…..maybe. This brings us into the realms of phase cancellation. No, really.

Most valve amp heater circuits have a 6.3 volt ac supply. Which means, on the face of it, that you introduce a big 50Hz signal into the amp. And 6.3 volts rms is indeed a big signal compared with the few hundred millivolts of the input signal. As this arrangement has been working well for a long time, there must be more to it?

The problem with the 5f1 was that the heater hadn’t been grounded. In the schematic, one side of the 6.3 volt supply was grounded. Sorting that out made a big difference to the chainsaw ripping through the speaker. But I also realised that this was not going to be the ultimate in low noise amps even with the ground fitted. I explain.

If we solder a preset pot of around 100ohms, with the two ends of the track (that’s the right and left tags) to the heater terminals of, say, the preamp valve, we produce a hum balance pot. Nearly. The ground has to be lifted from the heaters and replaced onto the wiper contact of the preset. You can then trim out the heater hum by adjusting the wiper position.

How/ why does it work? Before you do this mod, you need to make sure that there is no internal ground connection to the heater winding in the mains transformer. Or your trim pot will short the heater supply to ground. Important, then.

The heater wiring is twisted together for good reason. As one side of the heater goes to a higher voltage, the other goes lower. They are opposite in phase in other words. By adjusting the wiper of our preset pot, we have effectively produced a grounded centre tap which we can adjust so that the positive phase exactly balances the negative, producing a very low resultant signal to upset the amp. Most ac heater supplies have some arrangement of this sort, often with an internal centre tap that is not adjustable. In that case you can’t fit a trim pot.

Although this made the amp useable, and reasonably quiet, it still wasn’t as quiet as it could have been. Which gets us to smoothing capacitors.

There is no doubt that a valve rectifier makes a significant, and positive contribution to the sound of an amp. But it has limitations. The main one being the surge it is able to tolerate. Within the characteristics of the valve a maximum reservoir capacitor value will be stated. In the case of the GZ34, say, it’s 50uF. It’s this capacitor that makes a big difference to the 100z component, which generates the noise in the output stage. More so in a class A amp than a push-pull design (class AB, Ab1 etc) because the class AB amp has inherent noise cancellation properties.

So this little Fender amp could have been quieter with a bigger reservoir capacitor. The original was 16uF and then a load of decoupling stuff after it for the preamps. If that was doubled, the output stage noise would be much less. But you can’t just hang an infinite capacitor on the end of your 5Y4 rectifier, because it will blow the hell out of it.

So 32uF is probably about it. And now…..away with the sensible….!!!! Ha!!!!

I only have one issue with ‘sensible’. There are millions of sensible folks and yet the world is still a miserable place. Which is why I’m very happy to write a loada rubbish. Just so long as it makes me laugh.

I am presently devising a macaroon that will stir tea. I’ve tried it on Costa Copper and McDoodle’s Doughnut Dugout, but they weren’t keen on taking the doors off to get it in.

The Hermit-o-phone is still doing well though. I didn’t get a phone call from the tax office or the gas company. Or anybody else……….

Tea and a macaroon call-eth.

 

DBX 160xt …….under pressure….as Freddy would have had it.

The DBX 160 compressors are definitely a cut above average. They use a chip called a VCA chip. These have been used in analogue synthesisers, more or less from their inception.

You might be wondering at this point why I might want to use the word ‘inception’. Apart from it being a wondrous bullshit word, it reminds me of the Jenson Interceptor, a car with more carburettors than sense. I always thought of it as a sort family saloon AC Cobra. A great way to get rid of your ears, as it would rip them off at the first dab of the gas.

The VCA chip stands for ‘Voltage Controlled Amplifier. There were also, of the same ilk, the VCF and VCO. They were/ are respectively ;Voltage Controlled Filter’ and Voltage Controlled Oscillator’. They all worked in basically the same way. A dc voltage (the control voltage) is applied to the control pin of the chip (Before the advent of chips there were….fish. No, no! Come on, get a grip.) Let’s try that again. Before the advent of chips these devices were built up from discrete transistors. Before that, they were built from valves.

I remember a valve analogue computer at Chesterfield tech college. It had six operational amps and they shifted it from room to room on a fork truck.

The output of a VCA is proportional (I don’t know a car of that name…..wait a minute…..a pro Porche…nal. That’s relief, I thought I was turning sensible.) to the input dc control voltage. In other words, its output amplitude goes up as the dc control voltage increases. On a VCF the filter effect frequency rises or falls according to the voltage, and with the VCO the oscillator frequency rises or falls similarly.

What has any of this to do with the DBX 160xt? Well, this compressor has a VCA chip in it. The control dc voltage is sensed from the input level, and also from the setting of the compression level.

This probably won’t help to fix it, but might help a bit to understanding it.

Here’s a practical fix it bit. The DBX160 xt compressor uses a six pole push switch to bypass the compressor. If this goes bad, it won’t work, and you can’t buy one. I have heard there are plentiful stocks on Calisto, but DHL don’t ship from Mars. But you can get four pole push switches (alps switches). You can use these if you know how to a) solder and b) not panic when tracks peel off.

Take the original, bust, switch out. Tricky, but not impossible. A good desoldering device is advisable. Put the new switch in so that it occupies the 12 holes furthest from the front. Solder it in. Get a piece of heat shrink sleeving, and cut it so that the front bypass switch will push the switch operator forward. I also used a bit of the pipe from an aerosol can inside the heat shrink to make it more solid to the touch.

This will work and also switch in/ out the bypass function. What it doesn’t do is switch the led on the front which remains on. If you can live with that, you’ve fixed it.

I’m now going to phone somebody on this macaroon. It is, of course, my hermit-o-phone for the week. Last week it was a Tesco radish.

Tea…..ahhhh!