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ukcol · 5th Mar 2018, 11:09 am

The 21TD144A circuit looks quite a lot different to the circuit in the snip below that I have seen before.

1] The speakers are AC coupled to the mid-point but in addition they are DC coupled to the screen grid of the upper valve.

2] The triode of the PCF80 is added to provide a negative feedback path between the output mid-point and the screen grid of the lower valve. This must be DC negative feedback only because the -3dB point of R188/C173 is well below the audio range at about 3Hz.

It is certainly is an odd one, good old Philips.

5th Mar 2018, 11:09 am	#4
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Interesting TV audio amplifier. The 21TD144A circuit looks quite a lot different to the circuit in the snip below that I have seen before. 1] The speakers are AC coupled to the mid-point but in addition they are DC coupled to the screen grid of the upper valve. 2] The triode of the PCF80 is added to provide a negative feedback path between the output mid-point and the screen grid of the lower valve. This must be DC negative feedback only because the -3dB point of R188/C173 is well below the audio range at about 3Hz. It is certainly is an odd one, good old Philips. Attached Thumbnails