2 x 40 W Valve amplifier



With active grid biasing


Introduction
The design of this valve amplifier has been derived from an 1968 article in Radio Bulletin called
"the last of Mohicans, 40 Watts velvet sound".
So back then they thought vacuum tubes would never survive the seventies......

Specifications :

RMS power into 8 ohms: 30W (peak 40W)
Frequency range (-3dB): 15Hz - 33kHz
Input sensitivity: 300mVrms
Output noise comp to 30W/8 ohms: -93dB

I modernized the design somewhat, using modern semiconductors as humble servants of the sacred Vacuum tubes; some highlights:

-The high voltage supply has a switchon dellay in order to allow the cathodes to reach emission temperature before switchon of the high voltage supply.
-The output stage is class AB biased using MOSFET current sources. I this way the bias currents of all output valves
are equal minimising hum and cross over distortion. Most conventional valve amplifiers are biased using a negative grid voltage which is adjusted
for each valve seperately. As the characteristics of a tube tend to change over life, a conventionally biased amplifier must be tuned every now and then!

Circuit description

In the the valveamp schematic below you see that the amplifier is built with only 5 vacuum tubes!
The circuit is a classic push-pull amplifier using an output transformer with a center tap.
This topology is widely used for tube amplifiers, because it is impossible to make a complementary P-type tube:
you cannot make holes in vacuum as you do in semiconductor materials.

The circuit works as follows:
The first ECC81 triode amplifies the input signal, the second ECC81 triode acts as an 180 degree phase shifter
The amplified audio signal is presented on C3 and (180 degrees shifted) on C2.
The output stage is built with 2 x 2 EL84 penthodes in parallel. The phase shifter drives the high side 180 degrees
out of phase from the low side: when the low side pulls, the high side pushes and the other way around.

Because both high and low side of the output stage draw current form a seperate primary winding, the push pull output current
will cause an AC only flux in the magnetic core of the output transformer which will result in an AC signal on the secondary winding
which drives the speakers.
The quiescent current of the output stage will also pass trough the transformer. This current has a substantial 50/60Hz component because
the power supply of is not stabilised. However, if accurately biased, this current will not result in a secondary signal:
because the bias currents are in phase, the low side bias current will cancel the high side bias current.

In this circuit the current sources ensure well defined bias currents for the output stage, minimising hum and cross over problems.
Most designs use cathode series resistors and an adjustable Negative Grid Voltage to set the bias currents of the output stage.
In this way, the bias current is at the mercy of the valve characteristics which are different per valve, but also change over time!

The current sources (the schematic will follow next) bias all four EL84 pentodes on 20mA DC, regardless of tube characteristics
The AC audio signals pass through the parallel capacitors of the current sources(about 100uF).
The output transformer is a toroid type from Amplimo: type 3A524: www.amplimo.nl

Amplifier schematic: (right click, "save target as" to save the gif image)
The 20mA current sources ensure optimal biasing of the output valves. The operation of the current sources
can be explained on the basis of the schematic below:

Schematic of 20mA current source for active grid biasing:


When a voltage is applied on the + and - terminals of the circuit, the MOSFET will start conducting because
the gate is charged through R3 and R1. When the drain-source current increases, the voltage across R1 will increase
until the NPN starts conducting. This will happen at V(R1) = 4.7V + 0.7 Volt or I(R1) = 5.3V/270E = 20mA.
When the current reaches 20mA, the NPN will gently pull down the gate of the MOSFET. In this way the DC current
into the + terminal is set at 20 mA at any voltage. The AC currents can freely pass though the elcaps and the film capacitor.

The MOSFET and the elcaps I used are able to withstand a voltage of about 200Vdc. In practice, 40-50V
components will probably be sufficient as in normal operation the grid voltage will be -20 Volts.


The power supply is built with a single 6N606 toroid mains transformer (also from Amplimo).
This transformer has three secondary windings: a 6.3 Vac filament winding, a 300 Vac HV winding and an additional 40 Vac winding.
The filament winding is used for the EL84 penthodes. The preamplifier is more susceptible for hum, therefore the
ECC81 filaments are put in series and heated with 12.6 Volt from a separate supply using a 7812 stabiliser with
one diode in it's middle sense leg for the additiona 0.6 Volt (this is not in the schematic below)


The high voltage supply is switched on after a delay of 45 seconds, this to prevent a cold start of the valves which may damage the filaments
The switchon delay is fed by the 40 Vac winding. The switch "paraat" (means "standby" in Dutch) is the relay of the delay circuit.
The delay circuit is optional, it will extend the life of the valves.

Power supply schematic:


Delay schematic: (optional)

Download specifications of key components at the  Component specs page

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