Thursday, October 31, 2024

PHILIPS: 976B Valve PA Amplifier

eBay purchase - it listed a bit on the high side, but after a quick chat the seller revised the price down to something very reasonable, so I couldn't pass it by.

External references: Tim Robbins blog. Radio Museum

PHILIPS MODEL 976B, VALVE PA AMPLIFIER

Made in Australia in the mid to late 1950s, this is a compact valve PA head is rated at 8 watts.

I wasn't super excited when I cracked the lid open - I think this is referred to in the USA as a 'barn find'. Fortunately it mostly superficially and it cleaned up OK. The transformer rust came off without much effort. The electrolytic filter caps don't look so good, but will see how they test after bit of time on the variac.

VALVE LINE-UP

2 x Brimar UK 12AX7 dual triodes
2 x Miniwatt 6M5 pentodes, cathode biased (EL80)
1 x Miniwatt 6V4 full wave rectifier (EZ80). 9 pin miniature type

Expected output, 8w

INPUTS / OUTPUTS / CONTROLS

INPUTS: PU (phono input for crystal cartridges) & Microphone
CONTROLS: PU Volume, Mic Volume, Tone (music / speech)
OUTPUT: Speaker binding posts, fixed impedance, currently set to 1200 ohms!

TRANSFORMERS

POWER:
CZ344217
300-CT-300 for B+
6.3-CT-6.3 for heaters

OUTPUT:
OT 903
0, 300, 600, 1200 + 2 ohm FB winding

An input transformer has been wired in, which is connected to both inputs (PU and Mic in parallel) - the PU volume control has been disconnected. I doubt this is a factory mod, but who knows.

I am happy to see a lot of poly caps, as I was expecting mostly the wax paper type that is always leaky. I have a feeling that these may have been replaced at some stage, but not recently.