Design Of A Single Ended 2A3 Valve Amplifier - Part 1

This is a simple single ended valve amp I "threw together" a couple of years ago. It was meant for a colleague to try valves at home, but never found it's way out of the house. It's been in the project room ever since (as attested by the obligatory dust to be found in all the best male domains) and makes a particularly good match for the Quasars.

There's nothing fancy about the design - a triode strapped D3a pentode capacitor coupled to a 2A3, both in self bias with AC heating. The HT power supply is a simple LCLC passive filter with a GZ37 rectifier. Here's the audio schematic without part values, all straight forward stuff.

But it has decent parts and sounds very good. Surprisingly good really. Not the best amp I've built (that's a copper GM70 SE amp, but that's another story), but it fits in a single chassis which makes a change from most of my more recent builds... And I haven't felt the need to replace it which speaks volumes. And in the spirit of openness here's a photo of the inside just to prove how thrown together it was. I certainly wouldn't encourage anyone to study my wiring!

The chassis was actually recycled from my very first amp build - a Bluebell Audio 2A3 Loftin White built 10 years ago. Parts were supplied by Philip Ramsey using Shishido san's circuit. I just needed to drill a few more holes to accommodate different output transformers, chokes and a volume pot.

Preamble over, let's talk about the design. Bear in mind this is intended to be a simplified explanation of simple valve amp design - if you want the in-depth theory there are many good sources, try Valve Wizard's website or Morgan Jones Valve Amplifiers. I would also highly recommend reading Gordon Rankin's write up of his Bugle 45 amp - a great amp and really good design primer. First though, recognition must go to Nick Gorham as it was a circuit he posted that inspired me to build this.

The Output Stage

The usual place to start when designing a valve amplifier is the output valve, in this case a 2A3. By looking at the datasheet we can see whether it's likely to be suitable for our system, or what we might need in our system to make it "work". Assuming we have some reasonably efficient speakers a 2A3 makes a lot of sense.

Anode (Plate) Characteristics

Let's look at the anode characteristics graph on the datasheet

This graph shows a series of curves representing the relationship between voltage across the valve and the current through the valve for different grid voltages. Let's consider the classic 2A3 operating point, i.e. how the valve is "set up": 250V, 60mA, -45V on the grid. If we look back at the first page of the datasheet we're even told what might be a good operating point ;-). So looking at the graph above, 250 plate volts and 60 plate milliamps just happens to intersect with where a line representing -43.5 grid volts would be. (Okay, that's not quite -45V but it represents the lower limit of the AC heating, 2.5V.)

If for instance you maintained 250V across the valve but went with -35 grid volts you'd draw around 115mA. And burn out your 2A3 pretty quickly. So we need to check that our proposed operating point is within the valve's rating, and for a class A1 amplifier the datasheet tells us that we shouldn't go above 300V across the valve, and we can plot this line on the graph.

The datasheet also tells us we shouldn't let the anode (plate) dissipate more than 15W. So we can add this to the graph too as a curve with a series of points where voltage x current = 15W.

To draw the 15W curve we could plot a point at 100V and 150mA (which is 15W) say, and another at 200V and 75mA (= 15W), and another at 300V and 50mA (also = 15W), and so on for as many points as we feel necessary. For safe operation of the valve we need to operate in the area of the graph below the curve.

As we're designing a class A1 amplifier the grid should always remain negative relative to the cathode too, so we also need to be to the right of the 0V grid line too.

And if we plot all three on the same graph we find the acceptable area of the curves that we want to work within, the green shaded area below.

 And 250V anode voltage and 60mA is right at the limit of this region, thus maximising the power obtained from the valve. In general it's usual to maximise the power obtained from a valve, if you're lucky enough to have rare old monoplates you might choose to be a little kinder to them and maybe run them at 50mA instead...

Loadline

The next thing to consider is the loadline, an example of which is also shown on the datasheet, rather conveniently. It's the sloping straight line labelled "LOAD RESISTANCE = 2500 OHMS", centered about the -43.5V grid line, ranging between 0 grid volts and -87 grid volts. This loadline represents the load resistance of the output transformer, which in the case of the datasheet is 2500 ohms. But why 2500 ohms? You could just leave it to the RCA engineers and accept they knew what they were doing, which they most certainly did, but the longer answer is it's a compromise.

What do we want from our amplification stage? Usually we want to maximise power and we want to minimise distortion. The relative importance of the two depends on what we're trying to achieve, but as this is an output stage we want a good balance of the two.

If we consider distortion we can see how linear the valve is (or isn't) at our operating point by looking at how evenly spaced the grid lines are along the load line. Between 0V and -60V the spacings look fairly even, but beyond -60V the grid lines tighten a touch. So when the music signal is larger than + or - 15V (so lower than -30V and higher than -60V) the distortion will increase.

In general, if we wanted to reduce distortion we could increase the load resistance which would flatten the loadline i.e. make it closer to horizontal. But if we do this we will lose some power. And that's the compromise. We're considering the classic 2A3 operating point, and there's good reason for doing so as it's a good balance.

The rule of thumb for an output valve's load resistance is 3 x the anode (plate) resistance. So if we look at the datasheet again, the plate resistance is given as 800 ohms, which multiplied by 3 gives us 2400 ohms, or our 2k5 output transformers. 3k5 output transformers are often seen used in 2A3 schematics and trade a little output power for slightly lower distortion.

If we look back at our graph of anode characteristics with our limiting conditions marked on we can see the loadline is right at the very top of the 15W line. In fact part of the loadline actually crosses the 15W line but it's transient and overall the valve dissipates 15W.

Cathode Resistor

Okay, that was a bit heavy, sizing the cathode resistor is much easier now we've decided how we want to operate our valve. As we're designing a simple valve amplifier we're going to use self bias. We know that there will be 250V across the valve and we need to bias the grid at -45V relative to the cathode. Ohm's law is our friend

voltage (volts) = current (amps) x resistance (ohms)

or

V = I x R

Rearranging Ohm's law gives us R = 45 / 0.060 = 750 ohms. The datasheet suggests 750 ohms, there's a surprise!

We also need to calculate the power rating of the resistor.

power (watts) = current (amps) ^2 x resistance (ohms)

or

P = I^2 x R

(Blogger doesn't seem to allow superscript for the squared term)

Which gives us P = 0.060^2 x 750 = 2.7W

But we must derate this as a 2.7W resistor (even if we could find such a thing) would burn up very quickly. Typically we derate by 3 to 5 times, so our 2.7W becomes 8.1W to 13.5W. Even at 3 x derating the resistor will get very hot and I prefer to go to 5 x if I can find something suitable. In this case I would go for at least 12W, maybe a nice Mills if you're feeling flush.

Our output stage is starting to take shape now. Under the cathode we have a 750 ohms 12W resistor which raises the potential at the cathode to 45V. As there's 250V across the valve (i.e. between the cathode and anode) there will be 250 + 45 = 295V at the anode.

Cathode Resistor Bypass Capacitor

If we don't use a capacitor to bypass the cathode resistor the stage will have lower distortion and higher headroom, but more importantly for an output stage it will have a small fraction of the gain. This is why common cathode stages usually have a cathode resistor bypass capacitor. Calculating the size is a little involved.

capacitance (farads) = 1 / (2 x pi x f-3 (hertz) x R (ohms))

or

C = 1 / (2 x pi x f-3 x R)

f-3 is the frequency at which bass will have rolled off by 3dB. 3dB represents a halving of the bass output in this case so we need to set f-3 somewhere below the frequency at which we want full bass output. If we want full bass output down to 40Hz, not unreasonable for an output stage, let's set f-3 at 20Hz.

R, unfortunately, isn't simply the value of the cathode resistor. It's actually the cathode resistor in parallel with the cathode resistance, rk.

cathode resistance = (anode resistance + load resistance) / (amplification factor +1)

or

rk = (ra + Rl) / (mu + 1)

So rk = (800 + 2500) / (4.2 +1) = 3300 / 5.2 = 634.6 ohms

And therefore R = 1 / (1 / 634.6 + 1 / 750) = 344 ohms

Finally we can calculate the value of the cathode resistor's bypass capacitor

C = 1 / (2 x pi x f-3 x R) = 1 / (2 x 3.142 x 20 x 344) = 23.1uF. So we'd choose 22uF as it's a commonly available size. If you had a 47uf capacitor to hand then f-3 would be 10Hz. And 10uF would give a f-3 of 46Hz. I had a couple of 100uF capacitors handy so used those.

As you can see, if we had simply used the value of the cathode resistor, Rk, instead of R then we would have made the cathode bypass capacitor more than twice the size it actually needs to be.

As there's 45V at the cathode the capacitor needs to be rated higher than this. 50V is perhaps a little too close for my liking and I would choose at least a 63V rated capacitor and possibly 100V.

Grid Leak Resistor

We're nearly there now, just the grid leak resistor to specify. It's purpose is to tie the grid to ground and provide the cathode bias via the cathode resistor. The datasheet usually specifies a maximum value, in our case 500k ohms. Higher isn't necessarily better, but it can't be too low otherwise it would draw significant current and we want to keep the current draw under a milliamp. 100k is a reasonable value for the 2A3, and 1W should be more than enough as it should see very little current.

Final Output Stage

And here it is, our finished output stage. If you're wondering why there are two resistors between the cathode resistor and the cathode, this is in lieu of a humpot. A humpot allows any hum caused by AC heating of the valve to be minimised. But instead of a hum pot two resistors will put the cathode resistor at the centre of the potential difference between the two ends of the filament. In practice 2A3s are virtually hum free with decent construction of the amp and I've never felt the need for a hum pot.

In a later blog I'll look at design of the driver stage.