e039028.pdf

GENERAL INTEREST

Valves at

Low Plate Voltages

Interesting and surprising experiments with valves

by B. Kainka

Is it just nostalgia, or are valves really somehow better than transistors?

Recently valves have been making something of a comeback in many areas.

Using valves seems to involve a lot of effort, and in particular the high voltages

frighten many people off. But there are dozens of valves lying about in many

a cellar — so why not try something new with those old valves?

Valves are usually driven using an anode volt-

age of 250 V or more; practically never with

an anode voltage below 100 V. For power

ampliﬁers, in particular for radio transmitters,

several kilovolts can be used. Such inconve-

niently high voltages naturally put many peo-

ple off, as do the special transformers and

high-voltage electrolytic capacitors that are

needed. But things need not be like this. A

series of experiments has shown that most of

those valves nostalgically kept at the back of

the cupboard will work at very low voltages.

Of course, we are not talking about achieving

the ultimate in power or ampliﬁcation, but for

simple applications — and a bit of fun — it

ﬁlls the bill.

We will describe in this article how to

build simple circuits using valves with a min-

imum of fuss. Operating at anode voltages of,

for example, 12 V is not recommended by

manufacturers and is not covered in any data

sheet. So, if we are going to learn anything,

we will need to experiment and make some

measurements.

faction in building a small circuit and

making a simple working device.

And it is not just about feeling the

warm glow from the cathode: it is

like going back in time to the early

days of electronics, when the (rela-

tively simple) technology was domi-

nated by amateurs and everything

was visible. Valves in their glass

envelopes are certainly more ‘trans-

parent’ than ICs in plastic packages.

Of course, we could do things ‘prop-

erly’, and use anode voltages of

250 V; but that would not exactly

make for simple and relaxing experi-

mentation on the bench. A chassis

would be required and everything

would have to be built carefully into

a case. And we would always have

to watch out for those dangerous

voltages. None of this is a problem if

we stick to low voltages.

In order to forestall criticism from committed

valve-lovers, we should say that the aim here

is not to build the last word in hi-ﬁ ampliﬁers

or find the optimal operating point for some

particular valve. We are more interested in

gaining some experience with valves in a

simple and safe way. There is a special satis-

Elektor Electronics

9/2003

GENERAL INTEREST

Types of valve

author’s website (www.b-kainka.de)

includes example circuits from a

12 V headphone ampliﬁer using two

transmitter valves to an Audion-type

receiver. There are also many links to

similar projects.

ECC81

a 1

a 2

a 1

The question frequently arises: ‘are

valves still being made?’ The

answer is yes. Out of the enormous

range of valves that used to be

available, a few are still around,

being made by several manufactur-

ers. As well as valves still used in

radio transmitters there are a few

types specially designed for hi-fi

ampliﬁers. Output stage valves such

as the EL84 (6BQ5) and EL34

(6CA7), and ECC81 (12AT7), ECC82

(12AU7) and ECC83 (12AX7) double

triodes are readily obtainable today,

albeit at rather higher prices than a

few decades ago.

Other sources of unused valves

are mail-order suppliers such as

Chelmer Valve Co., who offer a par-

ticularly good stock of European, US

and Russian valves at reasonable

prices. Especially interesting are the

numerous types of miniature valves,

so-called ‘battery valves’, which are

designed to run on low voltages.

However, we are not limited to

using new valves. Devices salvaged

from the cellar will generally be old

radio or television valves. In the

good old days radios and televisions

that were beyond repair were often

cannibalised for their valves. In tele-

visions we generally find P-series

valves, designed to have their

heaters wired in series, with a

heater current of 300 mA, as well as

the ECC81 and ECC82 types men-

tioned above, and any number of

EF80s (6BX6). From radios we can

obtain interesting devices such as

EL84s and EL95s (6DL5) in output

amplifiers. The E-series devices

require a heater voltage of 6.3 V. All

these devices can be brought back

to life, and can even be used at low

anode voltages.

Indeed, there are so many differ-

ent types of valve that we do not

have space in this article to cover

the technical details and socket

pinouts for all of them. All the nec-

essary information can, however, be

readily found on the Internet. There

are also sites that give example cir-

cuits and hobby projects alongside

such data. Even the use of low

anode voltages is mentioned here

and there, and it is becoming some-

thing of a hobby in itself. So, for

example, the hobby corner of the

g 1

k 2

g 1

g 2

k 1

g 2

k 1

k 2

f M

a 2

The ECC81 (12AT7)

The ECC81 is readily available both

new and second-hand. From the

double triode series

ECC81/ECC82/ECC83, which all

have the same pinout, the ECC82 is

also suitable, but the ECC83 has too

low an anode current at low volt-

ages. The ECC81 was originally

intended for HF applications and

sweep circuits in televisions and

oscilloscopes. They are therefore

capable of operation at high fre-

quencies, and best performance is

obtained with anode currents of

between 5 mA and 10 mA. For HF

applications the valve is often found

in a cascode configuration, where

the two triodes are in series and

therefore share the available anode

voltage. This is why the ECC81 has

adequate anode current and

transconductance at low voltages.

Once one has obtained a used

valve, the first question is naturally

whether it works or not. This does

not require a complete circuit to be

built: a couple of simple experiments

can be carried out on the bench with

the aid of crocodile clips. First a

heater voltage must be applied. Fig-

ure 1 shows the socket pinout of the

ECC81. Almost all valves with a

nine-pin ‘noval’ base have heater

connections on pins 4 and 5. The

ECC81, ECC82, and ECC83 are a bit

special, however: the heater element

has a centre tap on pin 9. This

means that the valve can be used

with a heater voltage of 12.6 V (with

a current of 150 mA) or with a heater

voltage of 6.3 V (at 300 mA). This is

very convenient for our purposes,

since we can use 12.6 V (12.0 V will

also do!) for both the heater voltage

and the anode voltage.

First connect the heater voltage of

12 V to pins 4 and 5. After about half

a minute the cathode will start to

glow. If no current ﬂows, the valve is

probably burnt out. This case is rela-

tively rare. More frequently, the valve

is not burnt out but rather badly

aged and will have rather poor char-

bottom view

f M

030063 - 11

Figure 1. Pinout of the ECC81.

ECC81

+12V

030063 - 12

Figure 2. Measuring the negative charge on the

grid.

acteristics. For simple experimental purposes,

however, it will probably be perfectly usable.

The second thing to test is whether the

vacuum in the valve is still hard. Connect a

voltmeter between cathode and grid (see Fig-

ure 2 ). If all is well, a voltage of approxi-

mately –0.5 V should appear on the grid

(assuming the voltmeter has an input resis-

tance of 1 MΩ ). This is already showing the

effect of free electrons. The hot cathode ejects

electrons into the free space around it, and

some land on the grid, giving it a negative

charge. If, instead of measuring the open-cir-

cuit voltage, the short-circuit current is mea-

sured, a value of around 20 µA will be found.

This effect is used in many circuits to auto-

matically create a negative grid voltage,

including in the headphone amplifier

described below.

Whether the vacuum is still hard can often

be determined by inspection. The ECC81 has

a silver-coloured speck at its end, called the

‘getter flash’. When the valve is manufactured

the air is pumped out of it through a glass

tube and then it is sealed. In the end of the

valve there is a ring-shaped groove which is

filled with a metal having a low melting

point. This metal is heated through the glass

using a powerful HF magnetic field: this

evaporates the metal onto the inner surface

of the glass below. The result of this whole

process is to permanently trap the last

9/2003

Elektor Electronics

GENERAL INTEREST

Headphone ampliﬁer

better results. The theoretically opti-

mal operating resistance for the

valve is in the region of U a /I a , which,

at an anode current of only 0.17 mA

is around 70 kΩ . The exact imped-

ance is not critical in this applica-

tion, and so even headphones with

an impedance of 600 Ω can equally

well be used in conjunction with the

same transformer. The impedance

seen by the valve would then be

about 60 kΩ , practically ideal for the

given anode current.

If the cathode glows, a grid current

can be measured, and the getter

metal is still bright, the valve is in

good order and you can use it to

build this simple headphone ampli-

fier. The circuit is presented here in

two variations. The first circuit, in

Figure 3 , requires just two compo-

nents per channel in addition to the

valve and headphones.

The anode current of 0.17 mA

measured experimentally in a used

valve is relatively low. In specially-

designed low-voltage valves the cur-

rent was somewhat higher: for com-

parison, the low-voltage ECC86

(6GM8 or CV5394)) has an anode

current of 1 mA in this circuit. Note

that if by chance you do have an

ECC86 available, it cannot be ﬁtted

directly in the same socket: it has no

centre tap on the heater, and so

requires a voltage of 6.3 V between

pins 4 and 5.

The first simple amplifier circuit

works perfectly well with high-

impedance (600 Ω or 2000 Ω ) head-

phones. However, it is not good

practice to maintain a DC current

through headphones, although there

is no danger of overloading the sys-

tem, since the anode current is very

low. The sound quality can suffer,

however, at slightly increased cur-

rents and, furthermore, high-imped-

ance headphones are relatively rare.

If we want to use ordinary head-

phones from a personal stereo, hav-

ing an impedance of 32 Ω , the output

level will be very low. The problem

is that the impedances are severely

mismatched: the impedance of the

valve output is of the order of kilo-

ohms. An impedance converter will

do the job: for example, we can use

a small 230 V/24 V 1.8 VA mains

transformer. This has a voltage ratio

of about 10:1. The transformer we

used in our experimental circuit ( Fig-

ure 4 ) had a primary with a DC resis-

tance of 2.5 kΩ and a secondary with

a DC resistance of 100 Ω . Using a

larger transformer has the advan-

tages of lower loss and hence higher

volume. With a voltage ratio of 10:1

the effective headphone impedance

is increased by a factor of 100. If the

headphones have an impedance of

32 Ω , the valve sees an impedance of

3.2 kΩ , which will give considerably

600

...2k

see text

0.17mA

–100mV

50V

ECC81

+12V

030063 - 13

Characteristic curves

The oscilloscope traces in Figure 5

show that the valve does indeed

work with a low anode voltage. The

voltage gain of about 8 was obtained

at high impedance using an imped-

ance converter and 600 Ω head-

phones. In general, the following

relation holds for voltage gain:

Figure 3. A simple headphone ampliﬁer.

10 : 1

32 Ω

see text

0.17mA

–100mV

V = S ×

R a

50V

where V is the voltage gain, S the

transconductance and R a the output

resistance. Given that the output

resistance is 60 kΩ , the transcon-

ductance of the valve must be

0.13 mA/V. This broadly fits in with

the general observation that the

transconductance of a valve at any

operating point is approximately

equal to the anode current divided

by 1 V. The official data sheet for the

ECC81 gives, for example, an anode

current I a of 3.0 mA at U a = 100 V

and U g = –1 V, with a transconduc-

tance of 3.75 mA/V. This comparison

also shows a disproportionate fall in

current and transconductance when

operated at an anode voltage of only

12 V. This means that for serious

applications the anode voltage

should be as high as possible. An

acceptable compromise between

safety and gain might be around

24 V.

In order to learn to understand

the properties of the valve at low

anode voltages, we need to study its

characteristic curves. Manufacturers’

data sheets are of no help here, since

they do not cover operation at such

low voltages (it was apparently at

that time not of any interest). For the

same reason ordinary simulation pro-

grams do not give realistic results.

To measure real data all that is

ECC81

+12V

030063 - 14

Figure 4. Using an output matching transformer.

Figure 5. Input and output signal voltages

displayed on an oscilloscope.

remaining gas molecules in the envelope. If,

after years or even decades, the getter ﬂash

is still bright, then all is well; if it is white or

grey, it means that air has leaked in and the

getter metal has oxidised.

Elektor Electronics

9/2003

GENERAL INTEREST

needed is to apply a variable grid

voltage and measure the anode cur-

rent ( Figure 6 ). The measured char-

acteristic curve shown in Figure 7

indicates a rise in transconductance

with anode current at negative grid

voltages. When the grid voltage is

positive, the transconductance stops

rising, and in the region above +1 V,

it starts to fall again. At the same

time the grid current rises and,

above about U g = +0.5 V, can

exceed the anode current. It is worth

plotting the characteristic curve, in

particular for used valves, in order to

determine an optimal operating

point.

CD player, and a little grid current no

longer matters.

In the circuit in Figure 8 a grid

current of the same order of magni-

tude as the anode current is set up.

The anode current and the achiev-

able output drive are now three

times higher than with a grid volt-

age U g of –0.1 V. This gives almost

ten times the output power, which

should be adequate for many uses.

The grid voltage is set at +0.5 V and

the anode current is 0.5 mA. We are

therefore in the region of the charac-

teristic curve where the transcon-

ductance is constant, and so distor-

tion should be low. The sound of this

simple ampliﬁer is indeed very good,

even though it might not be perfect

from a purely technical point of view.

The inevitable distortions introduced

by a valve stage, especially when

driven hard, are however generally

not regarded as unpleasant.

–2V...+2V

ECC81

+12V

030063 - 15

Figure 6. Plotting the characteristic curve.

900

800

700

600

500

400

300

Positive Grid

If it is desired to build the head-

phone amplifier as a permanent

device, rather than merely experi-

ment with it, it might be found that

the output volume can be too low in

some conditions, especially when

operating from a 12 V supply. What

is needed is to give the electrons a

bit more energy by applying a

slightly positive grid bias voltage.

Old hands will now protest vehe-

mently that this implies that a grid

current will ﬂow, resulting in severe

distortion. That is true in principle,

but it is not a problem if the grid

drive has a relatively low imped-

ance. Previously, in the golden era of

valves, the grid had to be driven

from a high impedance source, since

the output of the previous stage nec-

essarily had a high impedance.

Today we can use the low-imped-

ance headphone output of a small

anode

current

(

200

100

Perhaps you have noticed that in

this circuit the valve can be simply

replaced by two NPN transistors.

Instead of a grid current we have a

base current, instead of an anode

current, a collector current. Of

course, the transistors no not need

a heater: such is the nature of

progress. Which circuit sounds bet-

ter is a matter of taste: try it for

yourself. Most people come to the

conclusion that the valve sounds

better. It is worth putting up with

the fact that the power consumed

by the heater is orders of magnitude

higher than the output power of the

amplifier: in return one can enjoy

the cosy glow of the cathode and

-1,5

- 1

-0,5

0,5

-100

grid bias ( V)

030063 - 16

Figure 7. Characteristic curve for an ECC81 at

U a = 12 V.

the opportunity to warm ones hands (care-

fully!) on the valve.

More power

Readers with a lust for power who are

tempted to try to go a step further and drive

the headphone amplifier to the edge of dis-

tortion might prefer to wait for the second

article in this series. We will be looking at real

power valves such as the EL84, EL95, ECL80

and ECL86, as well as a PL504, which we will

be using in an amplifier with a loudspeaker

output, and at an anode voltage of only 27 V.

We will also describe several miniature Russ-

ian ‘battery’ valves, which not only work with

low anode voltages, but which also dissipate

much less heater power.

10 : 1

0.5mA

ECC81

(030063-1)

+0.5V

50V

Links

Hobby projects and experiments:

http://www.b-kainka.de/

Chelmer Valve Company:

http://www.chelmervalve.com

+12V

030063 - 17

Figure 8. Stereo ampliﬁer with positive grid bias.

9/2003

Elektor Electronics

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