PHILIPS: Model EV4437 PA amplifier

I picked this one up recently from a factory in an industrial area - apparently, it had been sitting there as long as anyone could remember.  They remember it being used at one stage to play music.

I know one thing for sure - they never opened it up to dust it.  At least a millimetre of dust, maybe two was sitting on this.

PHILIPS EV4437 PA AMPLIFIER

6 x Philips 6CM5 valves (it was missing two of them)

Fixed bias

12AX7 preamp & phase inverter

Solid-state rectifier

Negative feedback via a separate tap on the output transformer, back to the phase inverter

It has the standard three knobs - Mic, Pickup and tone control.  The Mic volume has a pull switch for pass cut, which just selects between a small and larger value coupling cap.

The 6CM5 valves are beam power tetrodes, which were actually designed for TV tubes - something about horizontal deflection, which I know nothing about...  

What I do know is that these appeared in quite a few Australian guitar amplifiers, likely due to price and availability.  You can still find these at very reasonable prices online.   I'm not a fan of the cap on the top of the valve - it has over 300v on it, and it's not insulated.  Ouch.  

Additional inputs had been added, all attached to one triode, and I have my doubts about the giant orange power cord.

This is a 100v line system, but it can be reconfigured for normal guitar speaker impedances.

I think they put the wrong plate on this amp, as the 4417 is a slave amp from this series.

It's a fixed bias amp, but for some reason, there are 22ohm resistors to ground from the cathodes of the 6CM5 pentodes.  People sometimes put 1ohm resistors here to measure bias, but to find this in an amp like this is unusual, cause the bias is pre-set using resistors.  

PHILIPS EV4437 SCHEMATIC

I haven't completely checked mine against this schematic yet - but there are definitely some differences.  Some I know are mods, some may be factory. 

The power supply is interesting, as it has a number of different feeds, for the B+, screen voltages and bias.

FURTHER READING

Full evaluation by Tim Robbins - Dalmura site, although this is for the 4437a, which has some additional features

Aussie Guitar Gear Heads

Amplifiers with Valves - EV44 series

Don't know why I even took this photo - this is the shed in an industrial area of Brisbane where the amp was found.