WAD 2A3 PSE Tweaks - Part 1

[FAQ Team]

The WAD 2A3 is a zero feedback parallel single ended triode amplifier. It has built up an enthusiastic following since its introduction in June 2002. A number of modifications have surfaced since its release that are sufficiently different and interesting from the normal component upgrades to warrant documenting. They are:

* Operating point change for the input stage
* Battery BIAS for the input and driver stage
* Application of global feedback
* 5U4 replacement with PSU capacitor change

Operating point change

The aim of this modification is to improve the linearity of the input stage by changing its operating point. Firstly the HT (B+) to the input stage is raised from 275vdc to approx. 300vdc. This can be done by paralleling a 2Watt 2K2 resistor with the existing 5K6 R14 resistor in the power supply or by replacing R14 with two 2 Watt 3K3 resistor in parallel. Setting R14 is dependant on the mains voltage in your area, it may prove that the above value does not achieve 300vdc, if so, keep adjusting R14 until you get to 300vdc. A high mains voltage will require R14 to be reduced in value from the above suggestions.

Next, the anode load resistors for the input stage and driver need to be changed. Parallel 4, 2 Watt 100K resistors with R5/6/11/12 respectively or replace all four with 2 Watt 36K resistors.

Lastly, parallel 4, 2 Watt 3K3 resistors with the cathode BIAS resistors R7/8/13/14, respectively or replace all four with 2 Watt 1K2 resistors.

The anodes of the 6SN7 should read between 140-160vdc and the BIAS voltage at the junction of C1/2/5/6 and R7/8/13/14 between 5.0-5.5vdc.

Current draw from each 6SN7 stage will have increased from 2.5mA to approx. 4mA.

Battery BIAS

The principle behind battery BIAS is to replace the cathode BIAS resistors and cathode bypass capacitors with a rechargeable battery of similar voltage to correctly BIAS the Valves. This can be applied to the input stage, driver stage or both.

A suitable battery for this application would be the Vartar high temperature Ni-MH 4.8V 110mAh unit available from <http://www.cpc.co.uk> part number: BT0233366 4 will be needed if applying this modification to the input and driver stage. A better sounding alternative would be to use continuos charge Ni-Cad batteries, these are a nominal 1.2v therefore 4 wired in series will be required, 16 in total. When charging 4 of these batteries is series should give around 5.2 - 5.4v. RS 378-3304 is suitable. Some creativity will be needed to fit these into the chassis.

Remove components C1/2/5/6 and R7/8/13/14 and electrically insert a battery between the 6SN7 cathode (pin 6) and earth/ground with the + terminal connected to Pin 6 for the input stage and another battery between pin3 and ground for the driver stage. That's all there is to it! The battery BIAS modification can be used in conjunction with the Operating point change above.

Global feedback

The 2A3 seems to be speaker dependant, being a low powered parallel single ended triode amplifier with no global feedback its performance may be directly related to the speaker load it drives. If this really is the case then it limits the choice of suitable speakers to a few when compared to a feedback push pull design.

This modification applies a small (6db) amount of global feedback to lower output impedance, distortion, noise and increase bandwidth. It should be noted that sensitivity will be reduced by the same 6dB amount from its quoted 200mV to 400mV.
Firstly, a 100R .6W resistor needs to be inserted between ground and R7/8 so that the ground path for R7/8 is now via this new 100R resistor. The negative leg of C1/2 needs to be connected to the junction of the new 100R resistor and R7/8. It is this junction that will be used to inject the feedback.

Next, a wire needs to be taken from the positive speaker terminal to any spare tag on the corresponding RH & LH tag boards. Using the spare tag, solder a .6 Watt 3K3 resistor and bypass (parallel wire) it with an 820pF high quality polystyrene capacitor. Take a wire from the other end of the feedback resistor and capacitor to the junction of the 100R resistor, C1/2 and R7/8. The feedback modification can be applied in conjunction with both the Battery BIAS and Operating point modifications.

The Battery BIAS and feedback modification can be made switchable to allow quick A/B comparisons, the diagram below details all three modifications when used together with switches for convenience.