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Output Transistors


Quad 303 Service transistor availability

Our 110.00 service charge does not includes the cost of blown homotaxial output transistors. Genuine NOS homotaxial transistors are now becoming very scarce.   The cost of these will be added at 7.00 per transistor to a maximum of 28.00.  The cost is due to their scarcity and the need to manually select high Vceo devices from a batch of transistors.  Please don't be put off by this. The majority of 3O3 repairs we receive are completed at our standard price of 110.00. On some occasions the output devices are blown or are below specification and replacements are then necessary. 

What are homotaxial power transistors?

When Quad designed the 3O3, they chose RCA transistors for the power output, voltage regulator and driver transistors. This was a no-brainer for the Quad engineers because RCA in those days was one of the leading power semiconductor manufacturers.  Several years before the Quad 3O3 was launched RCA had invented their homotaxial  process, one that dramatically increased  the power performance of silicon  transistors at low cost.  The devices used for the main output transistors of the Quad 3O3  (the high current end of the output triples)  are the RCA 38494. These are robust, homotaxial power devices in a TO3 case, similar to the popular homotaxial 2N3055 but selected for  higher collector-emitter breakdown voltage (Vceo) and current gain. 

Why are homotaxial types so important for Quad 3O3 repairs?

The power transistors used on the 3O3 are homotaxial types. They have a large chip area and a thin substrate. The inter-electrode capacitance, is therefore relatively high. For example, the collector-base capacitance (Cob) of the 38494, at low voltages (e.g.10V), is as high as 1500pF. Similarly the collector emitter capacitance is around 500pF. This meant that these robust power transistors were unsuitable for high frequency (HF) applications. At audio frequencies, this was not such an issue. The high capacitance and poor HF performance was a blessing in disguise for the Quad 3O3.

The positioning of the 3O3 PCBs relative to the heatsink meant that long wires were necessary to couple the boards to the main output transistors. Normally the long wires would create instability and ringing on fast rising/falling signals but the sluggish performance of the output transistors at  HF, along with other designed in factors, keeps the system stable.

The problem started when  the homotaxial process was superseded by epitaxial fabrication. In this process, even more powerful transistors could be manufactured on smaller sized dies by diffusing deep into the die rather than across it. This meant smaller capacitance and  superior HF performance. Unfortunately, the homotaxial process soon fell into obsolescence. Well meaning DIY enthusiasts and non-knowledgeable repair shops,  fit epitaxial types in place of the correct homotaxial variety. We often see new (epitaxial) 2N3055s, 2N3773 and MJ150 series transistors fitted or sold as DIY kits. The result often goes unnoticed as the instability is parasitic and above audio frequencies.  The resulting sound quality though is mediocre at best. Another indicator in some cases is an elevated heatsink temperature.

What we do

 In addition to the driver and pre-driver transistors, we check the four output  and regulator transistors of every 3O3 we receive for service. We measure  the collector-emitter breakdown voltage (Vceo),  the base-collector current gain (Hfe) at 1amp and the all important 'Cob' check' to confirm they are homotaxial.  If any of these parameters are low, the device/s are replaced with genuine Homotaxial devices specially selected for high Vceo and current gain*. If your 3O3 has epitaxial devices fitted we will inform you prior to carrying out further work, as we will then have to add a surcharge. Please bear in mind, even if the whole set have to be replaced (unlikely), the full standard service charge, including the full output set, will only be 137.00


Fake homotaxial transistor Cob reading 480pF

Inserted into a 3O3 the circuit would go unstable.

Genuine homotaxial transistor reading 1381pF

Vceo 110V and HFE 95 @1amp.


Output waveform into 8 ohms of a 3O3 incorrectly fitted with epitaxial output power transistors, (similar to the type shown above). The dark area shows supersonic HF parasitic oscillation caused by instability in the system.   


1kHz sinewave signal on a correctly serviced 3O3 amplifier fitted with homotaxial output transistors. 56W rms into 8 ohms one channel driven. 


*A few years ago we were fortunate enough to purchase a quantity of military spec RCA Homotaxial power transistors. These were then tested for collector/emitter breakdown voltage (Vceo) greater than 100V@25oC. These are exclusively used to replace faulty or below spec devices on the Quad 3O3.

We have used genuine Motorola MJ15003 transistors on some 3O3s with limited success. (The Cob of this robust transistor is high for an epitaxial device.) Some 3O3s fitted with these show the characteristic parasitic oscillation and others show a stable display on a sinewave output even at full power. Applying a test square wave however, or an awkward load often brings back the problem. The current gain and Ft of the combined upper and lower triple output sections also seems, not surprisingly, (Nyquist S.C.), to govern whether it remains stable or not.  

When our homotaxial transistor stock runs out we will be forced to use transistors like the MJ15003. To ensure stability, extra HF damping capacitors may then be necessary and we will add them as necessary.   


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Last modified: Monday, 27. April 2015 12:57