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

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Description

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	Yes
20A	Mesh	Yes	Yes	.	.	.
20B	Black Powdered	Yes	Yes	Yes	Yes	.
20B-V4	Black Powdered	Yes	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.

...

SPICE Parameters

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
430V See Chart	-20.9V	4.1V RMS	831R	25mA	14.6	3k1	Choke or inter stage	LL1668-25mA	60V RMS	0,03%	Driver Stage
430V See Chart	-15V	10,7V RMS	257R	58mA	10	2k3	5k	LL1663-60mA	2,2Watt	1.4%	Output Stage
430V See Chart	-15V	10,7V RMS	257R	58mA	10.8	2k3	6k5	LL9202-50mA	2Watt	0.9%	Output Stage
500V See Chart	-21.1V	14,9V RMS	422R	50mA	10	2k4	5k	LL1663-50mA	4,1Watt	3,3%	Output Stage
500V See Chart	-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
350V See Chart	-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
521V See Chart	-12,2V		254R	48mA	16.6	2k9	11k	LL9202-50mA	1,8Watt	1%	Output Stage
580V See Chart	-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

AC heated vs. DC heated. Historically, tube charts were always made AC heated. If DC heated however, the same tube will off-bias with 1/2 of the heater voltage if charts are drawn. This is not important by itself, only we need be aware of this. For tubes with very high grid voltage like 520B has 100V, it was less important because tolerance on grid voltage is higher than that anyway. For tubes with only 6 Volts grid voltage, 1/2 of the heater voltage does make quite a difference. So for such tubes ( higher gain tubes) this becomes important to know. As a rule of thumb, you can recognise AC heated curves by the zero volt Ug line, starting at zero plate Volt and zero plate current. If DC heated, the 0V (diode curve...) looks like the -2.5V curve of AC heated curves. This is so, because all curves are 2.5V shifted. We made the working points here AC heated. This is just to avoid a mess, because design theory, assumes AC heating. However, you probably use the tube DC heated, and we even recommend this. This makes no difference for the use, only the circuit works with 2.5V shift in grid voltage. Take this into account when choosing the bias resistor and the grid voltage. For example, if the AC heated, the grid voltage is -25V, then if DC heated, you will see the tube needs -22.5V. To achieve this -22.5V, the grid resistor must be chosen lower with a factor 25/22.5.

Use as Pre Amplifier

Application info #1

For use in a separate pre amplifier, a DHT must be used with low noise in mind. Typically the wrong way for this would be to build a pre amplifier with more gain as needed, and then attenuate the input signal by the input volume knob accordingly.

Though people LOVE to do this, it is wrong! Such amplifiers will function dissatisfactory because of low signal to noise ratio, and also will sound sterile.

A good pre amplifier, will use an attenuating output transformer, reducing the gain of the complete product, to a number, not higher than needed. Which is 1x for a pre amplifier, or in rare cases 2x. . This will lower the output impedance of the pre amplifier with the square of the transformer attenuation which is the essential real reason for doing so. Now, with the lower output impedance, a 600 Ohms potentiometer can be used directly at the output. Doing it like this, will drastically reduce noise as well as microphonics. Yet maintain triode sound. In this configuration, the plate signal of the output tube itself, will be 10...40 Volts, also at very low output volume setting. This prevents sterile sound. Overall Signal to Noise ratio stays very high, and microphonics are low.
With such a pre amplifier, even a twisted wire pair can be used at the output, to replace shielded cable, and at very low signal the pre amplifier will be 'dead' silent, or in engineering words, have a very high Signal to Noise ratio.

20B use as OTA

Application info #2

OTA stands for One Tube Amplifier. So yes, this means a complete HiFi amplifier with only one tube. This is a concept where no pre amplifier tube and no driver tube is used. The output tube is typically 20B-V4 in such a case, and it provides almost enough gain by itself. The missing gain of appr 2...4x can be achieved by a passive gain stage, in the form of a small step input transformer from Lundahl. (We supply PCB for this, with programmable gain of 1..4x, using LL1544A). Such a passive stage costs less than a good tube stage, and provides gain at zero hum and zero noise. We much recommend this concept if only a few Watt output power is needed. Sound of the 20B-V4, as speaker driver will be 'sweet' and not dominant. Moreover, tube sound develops with the 20B also at low volume. Due to exact geometry of the anode box, there will be no audible third harmonics. So any influence will be only by second harmonics, thus achieving in a very good way, what is called triode sound.

Search the internet for this phrase: "OTA 20B One Tube Amplifier", or also click on "Professional Users" on the jacmusic website, and search that page for the phrase "20B"

12B Notes

Application info #3

For driving a 300B, it needs appr. 60V AC signal. Quite a lot, but 12B can do this nicely, and a lot higher voltage too.

It is recommended to check at the external website, jacmusic.com, for the EE-08 and EE-20 board, at the Lundahl Section. These transformer boards will supply the needed gain.

In addition, 12B seems IDEAL to build a SAKUMA amplifier. This very clever concept works almost free of distortion, as long as driver stage and output stage are identical. Enter SAKUMA in the search box of the jacmusic.com website. (external website)

About Noise and Distortion

12B, 20A, 20B, 30A tubes are based on a very old principle, where high gain tubes have much wider Anode distance. This is still the ultimate way to make a very linear DHT high gain tube. Miniaturization has always been the enemy of tube linearity, specially with DHT. For this reason, the 20A/20B/30A will outdo any small size tube. We encourage you to check the tube curves of any miniature tube, like 6SN7, ECC88 or ECC82, and you will see this series presented here is much superior. However this results of larger dimension tubes is a higher working voltage. For this reason, tubes like this were not made any more after 1935, when miniaturization of tubes started to become more important. With the new production tubes we make today, miniaturization is no requirement. So for high gain tubes, we can now focus on lowest possible distortion. We build these tubes again the original way, with wide Anodes. Check Anode picture here.

These tubes belong to the lowest distortion tubes ever build. Total distortion for small signal has been measured below 0,08% at 10 Volt Eff output signal, 400V Anode. The distortion will almost not increase at higher signal. (See Note5) . Interestingly, when one DHT triode is driving another, a part of the distortion of the second tube will be eliminated by the first tube. This even harmonics cancellation appears only with triodes, and they must be similar tubes, like drive a 300B with one of the tubes of this series. Since Triodes produce very low uneven harmonics, canceling the overall distortion this way, is an amazing application of the 20B or 30B tube. (See note 4)

For lowest possible mechanical noise with (any) high gain DHT pre-amp tube, use high quality tube sockets that do not pass the chassis vibrations to the tube. Chassis vibrations can come directly from the mains transformer, or by air from the loudspeaker. The best sockets for this are probably the Yamamoto Teflon, because Teflon is a flexible material. Use tight fitting Teflon tube dampers such as the Duende Criteria. They make the best damper for the EML tubes. They can be fit very nicely on the tube top, or one on the top and one on the bottom.

Description

Applications

ANODE TYPE

Driver tube

Line Out

Speaker Output

OTA

Head Phone

Sakuma

12B

20A

20B

20B-V4

30A

Why recommend Lundahl transformers

SPEAKER OUTPUT APPLICATION

CHOKE LOADED APPLICATION

INTERSTAGE APPLICATION

Line Out Application

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

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

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

30A Some operating points

Anode Voltage

Control Grid Voltage See Notes

Anode Current (mA)

Trans- Conduc- tance (mA)

Anode Rp (Ohm)

Gain (mu)

Grid Voltage
See
Notes

Grid Voltage
See
Notes

Grid Voltage
See
Notes

Gain
(mu)

Control Grid Voltage
See
Notes

Trans-
Conduc-
tance
(mA)

Gain
(mu)