PHILIPS: Model 976B PA Amplifier

Philips 976B - I have had one previously that I documented here.   

PHILIPS MODEL 976B VALVE PA AMPLIFIER 

This is a small Australian-made valve PA amplifier from the mid to late fifties - about 8 watts, intended for 100v line systems running multiple speakers.  These were also sold as Steanes Sounds Systems using the same model number.

It's mostly original; it has had the filter caps bypassed replaced and a repaint at some stage. 

I had to add a power switch, and input and output jacks.  The original power cord was really short and not in the best shape, so that went as well.   

2 x 6M5 in the power stage, for about 8watts.

I replaced the original output transformer, as I was not keen on the configuration options.  It's sounding great with a M1120.  

The original filter caps have been left in place, with new ones added in the chassis.  I don't love the mounting method (or lack of), but at least the caps they used look the part.

PHILIPS 976B SCHEMATIC & TECHNICAL MANUAL

MODS / CHANGES

I've left it close to stock in terms of the circuit, as I thought it sounded pretty good. 

• Added input & output jacks
• Replaced the dodgy power lead and added a power switch & a jewel light 
• Unused PU level pot now a Vox style cut control 
• Swapped the output transformer due to limitations with secondary impedances 
• Rebuilt the preamp and phase inverter, as the carbon comp hiss and crackle was just too much
• Moved the filter caps to some tag stips 

Interestingly, there’s a small amount of bleed around the volume pot. It’s not a major issue—more of a curiosity. The two 12AX7 triodes share a cathode resistor, and there’s plate-to-grid negative feedback on the second stage, which creates an AC path around the pot. That allows a small amount of signal to reach the phase inverter even with the volume turned down.

6V4 RECTIFIER

6M5 POWER PENTODE

6M5 (EL80) pentodes appear to be unique to Australia - they're similar to an EL84, but with a slightly different pinout and a little less power.  It's often referred to as the noval version of the EL41.  Either way, I quite like the sound of these being driven.