PHILIPS: Model EV4435A PA Amplifier

New to the bench, although I have worked on one of its larger siblings before.  This one has been sitting in a corner of The Guitar Repairers for a while, and hopefully will be back there soon, in working order.

These Philips amps are quite industrial-looking and built like tanks.  This one is missing the top cover and base plate.  It has had some modifications done on the inputs.  

The amp is rated at 30w, powered by a pair of 6CM5 valves (normally used in TVs for non-audio purposes).  These seem to be more of an Australian thing; I haven't really seen them pop up elsewhere.

The service manual can be found here, schematic below.

PHILIPS MODEL EV4435A PA AMPLIFIER

MIC TRANSFORMER - EV4428

20:1 ratio & very interesting construction...  basically it's a transformer wrapped in foam and stuffed in a can.

OT MEASUREMENTS

The service manual specifies three output impedances: 330 Ω, 160 Ω and 80 Ω.  These correspond to the nominal 100 V, 70 V and 50 V line outputs.

Primary DCR 150Ω 

Transformer presents approximately 2.4–2.5 kΩ plate-to-plate

Secondary measurements 

1 - 2.    9.47:1

1 - 3.    5.5:1

1 - 4.    2.75:1

2 - 3.    13.13:1    2.76k with 16 ohm speaker, but it's questionable whether it could handle 30w

5 - 6.    27.67:1.   (NFB winding)

2V monitor speaker winding: 127:1 

POWER SUPPLY

One unusual feature of the Philips EV4435A is how it obtains the lower screen-grid voltage required by the 6CM5 output valves.

The high-voltage secondary of the power transformer (L3) has a centre tap, while the two ends of the winding feed a full-wave bridge rectifier (W2–W5).

Rather than grounding the centre tap, Philips uses it to provide the screen-grid supply.

Because the negative output of the bridge rectifier is grounded, the centre tap develops a positive, full-wave rectified voltage relative to chassis. This is then filtered to provide approximately half the main HT voltage.

According to the original service manual, the expected voltages are:

6CM5 plates: 310–330V DC

6CM5 screens: 155–165V DC

It's a clever way to obtain two different HT voltages from a single transformer winding without requiring a substantial voltage-dropping resistor or a separate screen-supply winding.

SCHEMATIC

I think the 4435A is basically the 4435, with an input transformer for a low-impedance microphone.