Wow, can you believe I haven’t posted in this thing since last year? Golly how time flies!

Anyhow I’m mostly writing this post as a smaller update as I realised today that in my previous (rather dry) post last night, I completely neglected the voltage multiplier at the bottom of the amplifier. I guess it partially makes sense that I did, as I spent all day messing and twiddling and testing the linear regulator so the multiplier somewhat slipped my mind, however I’m here now to clear up some things about it as it is equally as important as the low voltage rails that the regulator will be supplying.

What is a voltage multiplier?

For the people reading who haven’t spent the past 3 months reading about the necessary components of a valve amplifier and how those components function, a voltage multiplier does what it says on the tin. It takes an AC input, and doubles it giving a DC output. What makes it a multiplier is that you can chain them together in sequence to give even multiples of the input voltage. Below is a schematic of the multiplier I’m using.

On the left is the input transformer, taking 230VAC in and giving 24V out. The transformer has a centre tap, which we are using as the ground reference throughout the amplifier. The above multiplier incorporates a bridge rectifier in each multiplier section to allow for full wave rectification of the signal. A basic voltage multiplier uses only two diodes, which only gives a half wave rectification (meaning it passes the positive part of the input signal, however gives 0V out when the signal is negative). By using full wave rectification it allows for less strain on the capacitors when there is a load, meaning there will be less of a voltage sag and more power can be drawn.

How does a voltage multiplier work?

To simplify the explanation a little bit, I’m only going to focus on the multiplier on the left, which takes an input of 24V and gives a 48V output. To start, let’s assume capacitors C2, C3 and C4 are all fully charged and as such have their maximum voltage across them.

During the positive portion of the cycle, the voltage on the left of C2 will be +12V, however because it is fully charged the voltage on the right of C2 is also 12V. This means that that 12V difference across C2 is shunted up by 12V. Voltages in series add up, and this in turn gives us a total of 24V on the right side of C2 relative to the original ground.

As for the bottom half of the wave, when the voltage at the top of the transformer is +12V, the voltage at the opposite end is consequently -12V. Given that C3 is charged, this means the voltage on that is also -12V. Just as with C2, the voltage on the left of C3 shunts down the voltage on the right of C3, thus giving two voltages in series; both -12V. The maths works in the same way, and as such -12+-12=-24V on the right of C3. This in turn gives a total potential difference at the output of 24V–24V=24+24=48V. This is then the voltage that is across C4, with the left plate still being at the original 0V and the right plate being at the new 48V.

This can then be chained, using the new input ground as being the output of the last multiplier at each step, giving an additional 48V out with each section. These of course add up relative to the original ground at the transformer’s centre tap and, in this case, give six times the input voltage to provide a total of 144V across the load, when it’s connected to the centre tap ground.

That’s pretty much the principles behind voltage multipliers. If that didn’t make much sense, don’t worry. I fortunately stumbled across this video from EEVblog on this very topic just the other day which should make it much easier to understand. I do wish I’d seen that before I bought the capacitors for this project however, as one thing I didn’t realise when choosing the caps was that you only need them to be rated for the voltage they’re working at in their section. I ended up buying 10 22uF 400V capacitors for this as I was thinking they’d have to be able to withstand the whole 144V each (which is daft now that I think about it) and I went with 400V to underrate them slightly should their voltage ratings drift down over time. Ah well, at least now I definitely won’t have to worry about that!

So why do I need a voltage multiplier?

That’s probably the easiest question I’ve set upon myself yet. In order for valves to function, they require a much higher voltage than their solid state counterparts. People who work almost exclusively with valve amplifiers would use the term “hot” for talking about the high voltage, which is honestly slightly annoying as it took me a while to realise that they didn’t mean literally hot (however they do actually need to run somewhat physically warm as they have a filament in them to free the electrons for conduction, however I’ll talk about that in a later post).

Anyway, valves work using a varying electron flow through a vacuum (hence the American name “vacuum tube”) and, in order to induce any significant current the voltage on the anode has to be quite high relative to the cathode plate in order for the electrical field to have a strong enough pull on the electrons to get them to flow with any significant volume or speed.

That’s why I need a voltage multiplier.

Hopefully that fills some gaps from my last post!

Also I’ve just realised now I’ve actually put it down as the transformer being 24V peak-to-peak, however it’s shown as having 2 12V outputs on the datasheet, which can be connected in series to give 24V with a centre tap (which is what I’m doing). I do think, however, that it might be 24Vrms, which is closer to 34V peak-to-peak, which would give a theoretical output voltage of 204V on the output rather than 144V. This isn’t that big of a deal however, as the output will be less than anticipated anyway due to losses on the diodes as well as electrical impedance in the circuit due to the capacitors and parasitic capacitance between rails and such. As well as these, the voltage on the output is expected to sag and drop when a load is put across the output anyway, so it’ll drop lower still. Needless to say this higher output is no real issue as it’s still high enough for the valve to function as needed.

Also we’re now at 1148 words. So much for a “mini update.”