Beware…..the Vox Concert 501 !

I do carp on a bit sometimes. You noticed? Ah, Shakespear had to write a ton of stuff to get noticed, I just have write a few sentences of drivel to get your attention.

This particular carping is regarding the afore-mentioned 1980′s Vox Concert 501. I can only think of one thing worse than not having a schematic for a piece of gear. And that is when you think you have a schematic, but, in fact, you don’t. This drawing had ‘Vox Concert 501′ written on the bottom, I had a combo in the workshop with ‘Vox Concert 501′ plastered all over it; but it may as well have been a ‘Mongoose Training for Beginners’ manual for all the use it was. Unfortunately, being a trusting sort, I thought it was the real McCoy. Until I discovered there were chips in it.

None in the schematic though. “Oh-oh” I belatedly thought. That was when I found that the third valve stage wasn’t there. At all.

Yes, I admit, I watched snails speeding past me as I wrestled with the fact that the drawing just didn’t fit what I had on the bench. From this demoralising situation, I managed to gain a positive thought. “Burn the schematic” was my best shot.

Even worse than all that, there was a switching transistor missing. This dealt with reverb footswitch operation. I hadn’t the faintest idea what it was, so I concocted a hard wired resistive network. I was convinced that this would introduce switching noise. But it didn’t!

So what the transistor (whatever it was) was for, I’ve no idea.

I most definitely get a tea-laden macaroon for that one, even if only as an anti-frustration measure.

Pi in the ski……erm….sky

In the great firmament of amp design, amp designers (Leo Fender, Jim Marshall, Dick Denny, et al) tend to be focused to the point of being obsessive. Whereas, anybody wandering his way through the vagaries of design issues who can chuck it for a couple of months at a time, (me), and then amble back into it as if nothing happened (because it didn’t), must be, at the very least, flippant.

Correct. So off we go again…….






The name ‘Pi type filter’ derives from the shape of the circuit and its resemblance to the mathematical symbol ‘Pi’.

Pi is a constant which relates the circumference of a circle to its diameter. Numerically it is 3.142 to three decimal places. However………………..

That has nothing to do with why the Pi type filter is so called. Sorry to bore you to death for no good reason. The circuit is called a ‘Pi-type filter’ because….erm… looks like it.

Below is a rather grand picture of the symbol ‘Pi’. If you compare the diagram at the top of the page to the symbol below, it becomes pretty obvious that the shape of the two capacitors and the symbol marked ‘L1 or R1′ is the same shape in essence as the Pi symbol below. If it was clever, I wouldn’t know about it, would I?



So, what is it for? What does it do?

Obviously not to make me look clever because I’ve already failed miserably on that one. If you look back at a blog of February ‘Valve rectifier and how it works’ the output of the circuit is unsmoothed dc. Which is ok for charging batteries but no good if you were charging folks to go into a show full of amps with no smoothing circuits.

Great to accompany a buzz-saw convention though. The purpose of the circuit is to convert the raw (unfiltered) dc, from the rectifier (valve or diodes, makes no difference) to constant and stable dc that the electronics it supplies can usefully work with.

Many thanks to Wikipedia for the diagram. I don’t feel too bad about nicking it because I paid ‘em a fiver on their last trawl.


C1 is called the reservoir capacitor because it stores voltage. Strictly speaking it stores charge, in Joules. It charges up as the voltage increases (on the dotted line above) and discharges as the input voltage falls.

The red line is the ripple voltage, and although it is much smoother than the raw rectified dc as depicted by the dotted line, it is unlikely to be much use for supplying circuits that have to supply audio output. That ripple voltage is what happens across C1.The rest of the Pi type circuit smooths the ripple further by the action of the smoothing capacitor C2 dropping the ripple voltage across the circuit element L1 (or R1).

The physical difference between L1 and R1 is that the L signifies an inductance which is usually an iron cored choke (a coil of wire round an iron core), or a resistance. The significant difference in design is that an iron cored choke (inductance) is a lot more expensive than a resistor, and the difference in operation is that the choke is far more effective as a smoothing element in the circuit, and, unlike the resistor, doesn’t tend to drop much dc voltage across it.

So, harking back to the ’Things in my favourite amp’ subject; which afterall is what this is all about, the smoothing / Pi type filter circuit will definitely involve the series inductance, and not the resistor. However, most amps will have a number gain stages, which are often arranged in cascade, (each stage feeding the next), and this is where we can use the dropper resistor effectively, because we often need to arrange different voltages for different stages.

There is also another issue. That of ‘decoupling’. But more of that in due course……

Tea. And I did get a chuckle or two out of this . So, also, macaroon!!