UPGRADE YOUR SINGLE ENDED 300B AMPLIFIER TO FIXED BIAS
WITH REAL RESULTS

One of Rupert Robertsons AN Kit Ones

 

ARTICLE BY RUPERT ROBERTSON,

21 BARONS COURT ROAD,
WEST KENSINGTON,
LONDON W14 9DY.

 

Hundreds of enthusiasts started building valve amplification with an Audio Note Kit One or similar. Such designs offer a musical performance for the low cost and provide a platform to build upon. A number of well proven ‘upgrades’ can be obtained from reputable suppliers and if you change enough components you will eventually end up with an amplifier which will more than justify the amount spent in both time and money on the project.

Most of these designs use a simple circuit with up to 3 stages to form a simple power amplifier. The Kit One is particularly well designed and uses the 300B valve in single ended mode with the usual cathode resistor biasing arrangement .

Outlined here is an upgrade suggested by SJS Electroacoustics to convert such an amplifier to ‘fixed bias’ with improved coupling capacitors.

HOW DOES IT IMPROVE THE SOUND?

The amplifier delivers a more definite and satisfying musical experience with better bass. Provides more control when the amplifier is driven hard.

I am not a reviewer but suffice to say that, although subtle in some ways like many upgrades, I have two amplifiers which were identical. One has now been upgraded to ‘fixed bias’ and therefore A/B comparisons using the same valves have been easy. I shall shortly be modifying the second amplifier.

It would be easy to ‘backtrack’ and replace the original components if I decided that after all I preferred the original sound! The joy about most upgrades is that they are effective, needn’t cost a fortune and are reversible: if you don’t like the result you can remove it although you probably won’t want to remove this one. .

 Inside Rupert's AN Kit One

 

WHAT DOES IT INVOLVE?

To achieve this improvement you need to carry out two simple steps:

a) If you are upgrading an Audio Note Kit One then replace the tagstrip in the middle of the chassis with a larger one and fit the necessary components to provide the 300B valves with the necessary ‘bias’ voltage.

b) In addition you will almost certainly benefit from upgrading the signal capacitors and dispensing with the volume control if you can drive the amplifier with a decent preamp. My own preference is for top grade AN capacitors or more recently Hovlands of a larger value. You will no doubt have your own preferences!

If you have already built such an amplifier and are able to relate at least some of the components on the circuit diagram to the components in your amplifier then you won’t experience problems. One or more decent multimeters and a basic knowledge of Ohm’s Law will help in setting up.

 

COST

The circuit was originally supplied to me by SJS Electroacoustics. They are also able to supply the components for approximately 120 (Sterling) and are well versed with the overall circuit and it’s operation. Telephone or send a fax to SJS in the UK on 0161 439 0727. Prices are reasonable and the technical service is excellent: this is far more important than making a small saving by ordering from a cheap supplier.’ In return you may not receive the same quality nor any advice which can be invaluable.

Specialist dealers in your own country can be a goldmine so use them.

 

THE NEW CIRCUIT

The following schematic is based upon the AN Kit One but you should be able to apply it to your particular circuit.

Figure 1: the new circuit

 

INTERNAL LAYOUT.

Again the following layout is based on the AN Kit One chassis but you should be able to build it into a wide variety of chassis without too much of a problem.

Figure 2: now to wire up your AN kit 1

Figure 3: interior photo of converted amplifier

rrint1.GIF (67366 bytes)

Figure 4: another interior photo of the converted amplifer

rrint2.GIF (68667 bytes)

Note: Only the bare minimum is shown here as the rest should already be in place. The relay and 10K Ohm resistor can be mounted on a separate tagstrip. If you are upgrading the signal capacitors then try and wire them ‘point to point’ where possible, thereby isolating the pcb or unecessary wiring. Larger capacitors can probably be ‘tie-wrapped’ in position.

 

SETTING UP.

1) Before you switch on, check your circuit.

2) If you have two or more multimeters then place one in series between each output transformer primary and 300B anode on the valve base. Switch to read up to approx 300mA.

3) With all valves in place, switch on and watch the ammeters.

4) The measured currents should stabilize as the valves warm up. Adjust the two potentiometers as necessary so that each channel passes approx. 75mA. The actual amount of current depends upon your preference so once you have the amplifier working properly you can conduct your own tests.

5) If you are not able to adjust the circuit satisfactorily, switch off and refer to

‘Additional Information.’

6) Leave switched on for at least half an hour to allow the components to stabilize and readjust.

7) The amplifier is now ready for use.

 

ADDITIONAL INFORMATION.

1) If the initial ‘switch on’ does not produce satisfactory results, use a voltmeter to check the voltages at the various stages. If the voltage before the diode is 425V AC then subsequent voltages should be the same if the current drawn is the same.

Check also that the HT DC supply to the output transformer is as indicated.

If the current flowing through the Tx and valve is too high then the negative bias voltage is too low and vice versa. The two potentiometers should ideally be somewhere in the middle of their ‘range’ once the correct current ‘Ia’ has been reached.

2) If you are experiencing problems and the various voltages are not corresponding , then adjust the value(s) of the first two resistors as necessary.

3) Rather than insert an ammeter into the circuit every time you want to measure Ia, you can measure the Tx primary resistance and measure the DC voltage across the primary when the circuit is switched on. The current flowing can fairly reliably be determined by using the equation Ia = V/R.

Alternatively you can insert a high quality 1 Ohm resistor between the cathode and 0V and measure the voltage drop across it using the same equation.

 

HOW IT WORKS.

The cathode resistor determined the DC current flowing through Tx and 300B, thereby biasing the valve.

Removing this allows virtually unlimited current to pass so a negative DC bias voltage needs to be applied to the grid to control the current passing from anode to cathode.

The negative bias voltage is also used to energise the 2880 Ohm relay. The normally open relay contacts are used in series with the DC HT supply to both output Tx’s. If the negative bias fails for any reason, the relay de-energises and cuts the HT voltage to the precious valves. This provides sensible protection for the valves and DC supply. The relay coil and 10K Ohm resistor combine to act as a ‘bleed resistor’ so don’t change their value or leave them out!

The 425V AC is half wave rectified by the diode and then smoothed and reduced in voltage by the RC network. This is a negative voltage with respect to 0V.

The’wiper’ feeds the 300B grid with a low current and allows voltage adjustment which in turn affects the anode current ‘Ia.’

Recommended power dissipation for the 300B is generally given as

30-32 watts. Increasing this will provide more power but may shorten the valve’s life and adversely affect sound quality. It’s up to you to experiment and settle on a current and HT voltage which you are satisfied with.

W = Ia x (Voltage between anode and 0V)

Example: 32.25W = 0.075 A x 430 V DC

The value of the signal coupling capacitors have been increased to produce a lower ‘corner’ and compensate for the new value of following resistors connected to 0V.   f = 1/(2 x 3.14 x R x C)

 

SUPPLIER

SJS Electroacoustics
PO Box 21
Bramhall
Cheshire
SK7 2FF
England

Telephone/Fax +44 (0)161 439 0727
Email: valveamp@sjselectroacoustics.co.uk