Design Notes for 12B, 20A, 20B, 20B-V4, 30A

Description

2A3 Mesh tube

This series of tubes are made for higher gain. Compared to (classical) low gain triodes, any higher gain tube operates at somewhat higher voltage, and lower current. It does not mean, the tubes can not be used at lower voltage, in fact they work remarkable good, just in that case signal output swing will be accordingly lower. Application range is very variable, and many interesting designs are possible.

Miniaturization of tubes, is always on the cost of less nice tube curves. This makes miniaturized tubes develop a lot of distortion at large signal swing. For this simple reason, we prefer our larger anode systems. So here, at high signal, high gain, these tubes come really alive, still at low distortion.

Gain of 12B is appr 14x, 20A/20B is appr. 20x, and 30A is appr. 30x. Depending on the tube type, they can be used as driver tube, pre-Amp Output, Head Phone, Classic speaker amplifier, or OTA amplifier. Please note, tube gain is always specified "unloaded" in data sheets. The unloaded gain will be achieved, if choke loaded, or with an inter stage transformer. In an speaker output stage, the tube is always loaded with the primary impedance of the output tranformer, causing the gain drop, depending on the application.

Some unique applications are possible such as the OTA amplifier, ot the SAKUMA principle. OTA stands for One Tube Amplifier. Such a complete amplifier works with only one tube. Such an amplifier uses a step up transformer at the input, and this gives already enough signal to drive the 20B, which by itself needs not much input signal, because of it's high gain. Sakuma is the name of a Japanese designer, who used this method in many of his amplifiers. By the Sakuma method, the output stage is driven by an identical stage, and 2nd harmomics of the one stage is eliminated by the other.


Applications

 
ANODE TYPE
Driver tube
Line Out
Speaker Output
OTA
Head Phone
Sakuma
12B
Black Powdered
Yes
Yes
Yes
.
Yes
20A
Mesh
Yes
Yes
.
.
.
 
20B
Black Powdered
Yes
Yes
Yes
.
 
20B-V4
Black Powdered
Yes
Yes
Yes
Yes
 
30A
White Powdered One single tube for the first stage and driver


 

Why recommend Lundahl transformers

We think these have the best relation between price and performance, and product portfolio is huge. The quality is hidden, such a frequency range is specified, driving it by signal source with impedance, same as a tube. The Japanese specify their transformers, driven with a zero ohms source, which is to my opinion fake, because it makes frequency curve look a lot nicer as it really is. Moreover, efficiency of the Lundahl in unbeaten, as they produce their own steel cores. Given what an amplifier costs, it is not a good idea to use a higher loss output transformer, just to save 30 Euro.
SPEAKER OUTPUT APPLICATION
LL2735B. Specially designed for us, by Per Lundahl. Such a perfect transformer for the EML20B!

The specialty: When using an 1:4 input transformer, it is possible to build a complete SE amplifier with just one tube. Such concept is called One-Tube-Amplifier (OTA).

We have a programmable (1...4x) gain PCB for this purpose, using LL1544A. .

Some Examples of OTA amplifiers Pic1 - Pic2

Designed for 20B, this transformer works also on 12B. When using 12B, you need a driver tube in addition, but not the step up input transformer any more.

 
CHOKE LOADED APPLICATION

This gives the best electrical performance. The only load is the capacitance of the following tube, and also it's grid to ground resistor. These are generally high values, and the tube works almost in the "unloaded" range. This will result in higher gain, and most of all in very low distortion, even at highest signal, because the load line is horizontal. (Example here). This keeps the signal swing away from the bottem right part of the curves, which is where the distortion occurs.

INTERSTAGE APPLICATION

For 12B, 20A, or 20B, we recommend Lundahl inter stage LL2756.

20A or 20B can be used 1:1.

12B can be used 1:1 or 1:2 step up

With this 1:2 inter stage, and an additional 1:2 INPUT transformer this will be a one-tube driver system, for tubes like 300B, 45 or 2A3. Thus bringing down noise, hum, and distortion to an incredible low level as not possible otherwise. This is because the first tube (as in a 3-tube triode amplifier) is not used. The first tube however is the main source of hum and noise.

We recommend the programmable (1...4x) gain PCB for this purpose, using LL1544A

Line Out Application

For 12B, 20A-Mesh or 20B, we recommend Lundahl LL1689.

If used as line output transformer, LL1689should be wired 18:1 for all 20B versions, or 9:1 for the 12B. Doing so, will make the line out stage dead-quiet.

...
.

12B Some operating points.

This is AC heated. If DC heated, Grid voltage shifts 2.5 Volt. (See Notes)
Anode Voltage
Grid Voltage
See
Notes
Input
Cathode resistor for Auto Bias
Anode Current (mA)
Gain
Anode Rp (Ohm)
Anode Load
Lundahl Trans former
Output
Tube Dist. Total
Use as
-20.9V
4.1V RMS
831R
25mA
14.6
3k1
Choke or inter stage
LL1668-25mA
60V RMS
0,03%
Driver Stage
-15V
10,7V
RMS
257R
58mA
10
2k3
5k
LL1663-60mA
2,2Watt
1.4%
Output Stage
-15V
10,7V
RMS
257R
58mA
10.8
2k3
6k5
LL9202-50mA
2Watt
0.9%
Output Stage
-21.1V
14,9V
RMS
422R
50mA
10
2k4
5k
LL1663-50mA
4,1Watt
3,3%
Output Stage
-21.1V
14,9V
RMS
420R
50mA
10.5
2k4
6k5
LL9202-50mA
3.8Watt
2.1%
Output Stage
As a driver stage, most power tubes like 300B, need appr 60V RMS on the grid. Using 12B as a driver, only 4.1V is needed. This can be achieved with a 4x step up transformer, which gives this gain free of noise and microphonics, and supplies even a balanced input.

20A Some operating points

Anode Voltage
Grid Voltage
See
Notes
Input RMS
Cathode resistor for Auto Bias
Anode Current (mA)
Gain
Anode Rp (Ohm)
Anode Load
Lundahl Trans former
Output
Tube Dist. Total
Use as
-12.6V
3.14V
1263
10mA
20.8
5k4
Choke loaded
LL1668-10mA
60V RMS
0,17%
Driver Stage
As a driver stage, most power tubes like 300B, need appr 60V RMS on the grid. Using 20A as a driver, only 3.14V is needed. This can be achieved with a 3x step up transformer, which gives this gain free of noise and microphonics, and supplies even a balanced input.

20B Some operating points

Anode Voltage
Grid Voltage
See
Notes
Input RMS
Cathode resistor for Auto Bias
Anode Current (mA)
Gain
(mu)
Anode Rp (Ohm)
Anode Load
Lundahl Trans former
Output
Tube Dist. Total
Use as
350V
-8.3V
 
414R
20mA
19.3
4k9
16k
LL2735B
0,5Watt
0,92%
Driver Stage
-12,2V
 
254R
48mA
16.6
2k9
11k
LL9202-50mA
1,8Watt
1%
Output Stage
-15.7V
 
368R
43mA
16.5
3k
6k5
LL9202-50mA
3,59Watt
3.7%
Output Stage

 

30A Some operating points
Anode Voltage
Control Grid Voltage
See
Notes
Anode Current (mA)
Trans-
Conduc-
tance
(mA)
Anode Rp (Ohm)
Gain
(mu)
300
-3
10
4.6
8k
36
300
-2
15
5.3
6k6
35
300
-1
20
5.5
5k9
33
.
360
-5
10
4.4
8k
36
360
-3.8
15
5.1
6k9
35
360
-2.8
20
5.7
6k
34
360
-2
25
6.2
5k6
34
360
-1.1
30
6.6
5k1
34
.
420
-6,7
10
4.3
8k5
36
420
-5.6
15
5.0
7k
35
420
-4.6
20
5.5
6k
33
.
480
-8.6
10
4.1
9k3
38
480
-7.5
15
4.9
7k2
35
480
-6.5
20
5.5
6k3
35
.