Interesting TV audio amplifier.
 

The audio output stage of the Philips 21TD144A employs two PL84 pentodes in an interesting circuit. Try and figure it out how it works.

DFWB.

Re: Interesting TV audio amplifier.
 

It's the single ended stacked push pull circuit that was discussed last week but with 2 PL84 instead of an EL84 and an EL86.
See post 11 https://www.vintage-radio.net/forum/...nded+push+pull


The Mullard info sheet lists this configuration but with an EL84 and either a PL84 or a UL84, the EL84 being higher voltage than the other valve used.

EL86 info shows this use.

Re: Interesting TV audio amplifier.
 

From Piet's Home-built Television website:
A description of the operation of the transformerless audio output stage:
Output TransformerLess (OTL) audio power amplifier. This was an entirely new concept, where the bulky output transfer could be deleted. Although the first publication that triggered this development was not from Philips - by Petersen and Sinclair in 1952, although their publications was still using transformers - it was picked up and further optimized by the company. The basic idea is to make a push-pull output stage but without the availability of opposite polarity devices (like today an NPN and PNP transistor). There are two ways to do this. The first is to use two power pentode output valves (here PL84) which are connected in series, and drive the two gates in anti-phase. The point between the anode of the lower valve and the cathode of the upper valve is then much lower ohmic than a classical anode output. This "full-OTL" would come later in the early sixties, but the first implementation was a "light-OTL" where the upper valve was not driven and effectively acted as a constant current source load of the lower valve. The common node between the two output valves was AC connected to g2 of the upper valve, which drove the series connected loudspeakers. The output impedance was much lower than in a classical amplifier, but still 800Ohm. The breakthrough was that Philips developed 400Ohm speakers, of which two were serially connected. One was a large bass speaker mounted on the sidewall of the cabinet, one was a smaller treble speaker mounted on the front panel below the picture tube. As can be seen from the circuit diagram, the voltage on the cathode of the upper valve is 107V, and this requires a valve that can withstand such a high cathode-to-filament voltage. Later Philips would introduce the EL86/PL86 specially designed for this parameter. This concept provided excellent sound volume and OTL would remain a Philips high performance differentiator for many years to come. OTL was almost simultaneously introduced in the Philips radios and audio amplifiers from 1954 onwards.
Visit: https://www.maximus-randd.com/piets-...ision-pt2.html
And page one: https://www.maximus-randd.com/piets-...ision-pt1.html

DFWB.

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.

Re: Interesting TV audio amplifier.
 

In the schematic posted in the first post, can I ask what the third connection on the Loudspeaker is?
R's 193 and 194 connected to them.

Thanks,

James.

Re: Interesting TV audio amplifier.
 

Static discharge R for the chassis/basket?

Re: Interesting TV audio amplifier.
 

Is this like the Philips radios that used the unusual 800 ohm speakers?

Re: Interesting TV audio amplifier.
 

R193 and R194 will be static bleeds on the speaker frames and I think that 800 ohm speakers is very plausible, certainly not low impedance. Previous threads confirm this.

A search on "EL86" will bring up many posts in several threads about this OTL (output transformer less) circuit

Re: Interesting TV audio amplifier.
 

Is it a variation of the 'Bi-Ampli' circuit found in some of their continental radio's?

Re: Interesting TV audio amplifier.
 

Not bi-ampli but a similar output stage to that which some bi-ampli sets use.

The last OTL output stage in a Philips set was probably in the F6 TV chassis produced up to 1972. That one employed a PCL86 as the lower valve and a PCL805 as the upper valve.