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Plate Chokes


Plate Choke

There are three types of plate chokes available:
  1. LL1667 for high to medium impedance applications.
  2. LL1668 for medium to low impedance applications.
  3. For low impedance applications (usually output stages) the power supply chokes can be used.
Why a tube with a plate choke works so much better.

For a quick understanding of what choke to take, it is enough to choose the DC current correct. This will automatically point into the direction of high impedance, medium, or low impedance plate choke. High impedance tubes are such, that work at high voltage and low current, and vice versa. Moreover, a device with higher gain is usually higher impedance, and tubes are no exception. So low gain tubes are usually low impedance too. With this approach you can quickly categorize a specific tube, and determine it's intended use.

When choosing a plate choke, it is enough to choose one which matches the bias current of the tube, and automatically things will work out right. Designing a circuit with a plate choke is easier as with a plate resistor.

Design example with 12AU7 / ECC82

Here is an example of how to use the 12AU7 (ECC82) with a plate choke. The datasheet will give this data: Ia=10.5 mA at 250V Anode Voltage. Gain is 17x and Ri=7k7. So far so good. When trying to making a resistor loaded circuit like this, you will find out, it doesn't give the gain of 17x. Moreover you will find, output voltage is disappointing and distortion is high. We cannot begin texts here about how to design amplifiers, but just click on the picture and you will see the result at 250 Volt: Gain is 14x, and and distortion is 6.4% at maximum 34 Volt RMS signal. So all on all not exceptionally good. The reason for this is, the anode resistor of 47k....220k, needed to bias the tube as required. This resistor is a compromise, and the source of the low performance. Due to this resistor, you cannot choose the working point in an ideal area of the curves, and you get compromised by distortion and lack of maximum signal. Furthermore the resistor is a load for the tube signal, and this reduces it's gain. Frankly speaking all of the things we wanted from the tube, are compromised. Of course there are many cases where this is no problem. However, this becomes a problem when high signal is needed at low distortion. This is where a plate choke comes in.

This is the marked operating point of the data table above here. This requires a Vb of 400Volt, but at least we can get 59V RMS that way. The construction is done only quickly by hand, so for precise data, read it from the data table. The intention is only to display it graphically.

Here we take also a 5mA working point, but this time with a plate choke. Since the plate choke has the ability to generate a reversed voltage, effectively we work at twice the Vb. This is interesting, but you need to realize it. So you see here the bias point is with 200V across the tube, and 5mA Plate current. The required choke is LL1667-5mA. The choke has 2400 ohms DC resistance, so 12 Volt drop. It means we need a Vb of 212 Volts only. And not 400Volts as with the resistor design. Also, now look at the maximum signal swing, I read 275V peak to peak, so 97Volt RMS. It has almost doubled, and can be increased much further by simply shift the bias point further to the right. Now comes the nice thing for distortion, at the SAME output voltage of 59 Volts, distortion will be significantly lower as with the resistor design. The gain is unaffected, there is no load, so gain stays 17x.

Conclusion:

Since there is no more compromising plate resistor, this makes the signal construction within the plate curves a lot easier, it is just a horizontal line. As a result, gain is higher, maximum output signal is higher, even so at lower Vb. Distortion is lower, and a better ratio of 2nd. to 3rd harmonics. So basically, all the good things we expect from a driver stage, become a great improvement.

Though plate chokes are specified @ ~RMS, and power supply chokes @ ~100Hz ripple, they are identical core and construction, and full audio range always. In case voltage loss is a problem, lower resistance type exists. For convenient selection, we have put them all here in one table, first sorted by mA, then by Henry.

Connection of two coils
Maximum DC current at full AC signal
Maximum DC current at very low full AC signal
Details here
Henry
Signal

DC resistance
of total choke

Dimensions (mm) and weight
Series
5mA
8mA
810H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
7mA
11mA
580H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
10mA
16mA
405H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
15mA
25mA
270H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
20mA
33mA
200H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
25mA
40mA
168H
Max 390V,RMS.
2400 Ohms
45 x 63 x 73 (0.7kg)
Series
25mA
40mA
100H
Max 285V,RMS.
680 Ohms
45 x 63 x 73 (0.7kg)
Series
60mA
100mA
70H
Max 640V,RMS.
160 Ohms
50 x 75 x 95 (1.4kg)
0,8mA
540H
Grid Choke
5300 Ohms

Is it possible to exceed the maximum plate current? In many cases, the answer is YES! Please CLICK HERE to read more about it.

More Advantages: Not so well known is, a low quality plate choke may easily pick up hum from the mains transformer, which may become audible. When made from a single coil, in fact it becomes a huge pick up coil with very many windings. For this reason, the Lundahl plate chokes are always made of two coils which are in series (in phase) for the electrical signal, but they are placed such that they are out of phase for any magnetic signal picked up by the magnetic core. This prevents the Lundahl plate pick up any hum signal, even though such a magnetic (hum) field is always present at a certain level. This technology was adopted from the input signal transformers, and it works for a plate choke as well.

Technology:

Lundahl plate chokes have a windings technology with maximum sectioning. What does that mean? Plate chokes are made of very thin wire. Suppose you have 4000 windings. You can section it as two coils of 2000, or as 10 coils of 400, etc. You reach a limit, since the smallest section is one layer of single wire. It is just this what Lundahl is doing, and as far as we know they are the only one.

Note, they use the Audio Grade (high frequency) cores + inter sectioned layering also for the chokes. What does this mean? With LUNDAHL, all PLATE chokes can be used as POWER SUPPLY chokes and vice versa.

Detailed pictures of LL1667-7mA plate choke. All plate chokes are constructed this way

Here you see how extremely fine the sectioning is done. You have to look here for a full resolution picture, and you will see the foils are there like the pages of a closed book. Each foil you see represents one layer of very fine wire.

This picture is the outside (final) layer. Click here to see the detail. So this shows you best the coil is not a randomly wound package, as with low cost chokes. This reduces capacitance.

Here you can see they double the sectioning by splitting the coil in two. As you can see the core is almost round shaped. So here all good things of the technology of ring-core is combined with the technology of C -core.

Remember a plate choke is not only for increasing the gain of a tube, it also is very elementary in reducing the tube distortion. (It changes a tube from a not-so-ideal current source into an almost ideal one) So this MUST be a highest quality element, and we think Lundahl made exactly what we need: The BEST.

The symmetrical construction also results in lowest mechanical noise, since the two coils work with opposite orientation to each other. Detailed picture