Practical Television magazine July 1960. TV sound amplifier.

The SUG26 radio chassis would be ideal for the TRG24 TV. No problems as long as the radio dial can fit in the existing aperture in the cabinet.

I think the metal work is common with the AC11/RG11 chassis anyway (which is what's fitted to the TRG24), and from what I remember the dial seems a little undersized on the TRG24? It wouldn't be difficult to adapt the dial drive to fit; it's remotely mounted from the chassis so it would almost be a question of using the TRG24's radio dial drive drum in place of the SUG26's if needed to get the pointer travel correct.

Hi Rich MPT4
Thanks for letting us have a peak at the K70 output stage, it's interesting that they designed it with an output transformer with a feedback winding. I'm not familiar with the arrangement of R985 feeding into the pentode cathode resistor though.

Eddie

Hi Rich MPT4
Thanks for letting us have a peak at the K70 output stage, it's interesting that they designed it with an output transformer with a feedback winding. I'm not familiar with the arrangement of R985 feeding into the pentode cathode resistor though.

Eddie

It's a pretty common arrangement in many a radio set...

What's the advantage over just using a 270R cathode resistor to generate the required 5V DC bias ?

VB

Maintaining a relatively low distortion operating point as the valve ages and loses emission? At the expense of around 10mA of HT current and a resistor needing a rating of 4-5W, though.

R985 is connected to HT supply [1] which also feeds the anode circuit. Could this complication be serving more than one purpose in that a small portion of ripple on the supply (100Hz in this set), injected into the cathode is enough for a type of hum cancellation?
Rich

More that thirty years before the K70 was made Philips introduced in 1939 the model 855X radio receiver. Like the K70 the negative feedback loop was supplied from a separate winding on the output transformer. Whereas the negative feedback circuit in the K70 is relatively simple the NFB network in the 855X was unbelievably complex. The feedback winding has a tap so that a degree of positive feedback is introduced to counter the negative feedback. I'll post up the circuit later today.

Till Eulenspiegel.

R985 is connected to HT supply [1] which also feeds the anode circuit. Could this complication be serving more than one purpose in that a small portion of ripple on the supply (100Hz in this set), injected into the cathode is enough for a type of hum cancellation?
Rich

Yes, I did wonder about that, but there's a 100uF cap across the cathode resistor, so precious little of the ripple wil appear there.

VB

Compare the simplicity of the K70 negative feedback circuit of that of the Philips 855X radio receiver. Note the strange EFM1 combined tuning indicator and audio amplifier valve V4.
The pentode section of this valve is controlled by the AGC system thus creating automatic volume control of the AF amplifier. Forward AGC so to speak.
There are two feedback paths, the main negative feedback loop and a degree of controlling positive feedback. It's complex but it works well.

Till Eulenspiegel.

There is a big market in devices to upgrade the sound of flat panel tv's. of coarse the are external (sound bars).
some of these devices cost as much as the tv .
In a 1970,s P.TV there is an article by LLJ for a PP amp using 2X ecl80 this was fitted with a B9A socket so could take the place of the sets output valve.
Who remembers the Celestion Telefi box?
Rob T

In a 1970,s P.TV there is an article by LLJ for a PP amp using 2X ecl80 this was fitted with a B9A socket so could take the place of the sets output valve.
Who remembers the Celestion Telefi box?
Rob T

The LLJ 2 X ECL80 push-pull amplifier was a copy of the audio amplifier employed in certain early 1950s Ferguson TV sets. See attached circuit diagram.

The Telefi box. Was that the one where threre was no elctrical connection to the set? A signal pick-up probe was positioned close to the intercarrier amplifier. There was evidently sufficient radiation of the 6Mhz FM signal for the Telefi box to process the signals.

Till Eulenspiegel.

Thanks Till
can you remember how LLJ overcame the common cathode of the ECL80?
you are correct about how the telefi worked.
Rob T

Hi Rob,
The cathodes bypass capacitor C43 is only 50 microfarads, however, if you think about the fact that this is essentially a balanced circuit so the out of phase signals across the cathodes will cancell out. The push-pull sound amplifier was employed in the models 984T and 991T and many other Ferguson models including some dual band models.

Hi Trevor,
there is an Ebay seller offering the dual pentode PLL80, the 0.3amp version of the rare ELL80. The characteristics are similar to the EL95. The PLL80 might well be worth considering for a high quality push-push audio amplifier in TV sets.

Till Eulenspiegel.

it was the common cathode between the triode and pentode of the ECL80 that I cant remember how he got round.
Rob T

I'm currently "de-hunting" my recently acquired Pye FV1C. It's missing most of it's valves but the sound output valve a PL33 is still present and I'm looking forward to getting this up and running as original. I'm not planning on chucking any money at it though, so if the valve is a duffer it might have to have a downgrade upgrade.

Eddie

it was the common cathode between the triode and pentode of the ECL80 that I cant remember how he got round ...

Rob, have you not looked at the circuit diagram in an earlier post?

It clearly shows a tapped cathode bias resistor network with the volume control, which doubles as the triode grid leak, having its earthy end returned to the tap so that both pentode and triode sections have different, but correct, bias voltages ...

sorry for the confusion terry
it was the LLJ circuit I was referring to.
Rob T

The ECL80 triode-pentode was a popular valve with set manufactures. It was employed in many functions including the sync separator with triode section used as part of a line or frame oscillator. It has even been employed as the mixer-oscillator in many five channel TVs including the Pye FV1 series. Decca used it as the line flywheel sync discriminator. Of course the most popular function was as an audio amplifier and output valve and in this application the common cathode could be a problem because of feedback if the cathode bypass capacitor was not up to the job. The bypass capacitor was often 500 to 1000microfarads, a big deal in the olden days
Perhaps you can remember some other functions the valve was used for?
The successor valve was the PCL83 which had separate cathodes for the two sections.

Till Eulenspiegel.

sorry for the confusion terry
it was the LLJ circuit I was referring to.
Rob T

Now you've got me really confused, Rob!

In the earlier post I referred to it says:

... The LLJ 2 X ECL80 push-pull amplifier was a copy of the audio amplifier employed in certain early 1950s Ferguson TV sets. See attached circuit diagram ...

Now, in my book, a copy is a copy is a copy ...

Anything else would be different ...

sorry for the confusion terry
it was the LLJ circuit I was referring to.
Rob T

Now you've got me really confused, Rob!

In the earlier post I referred to it says:

... The LLJ 2 X ECL80 push-pull amplifier was a copy of the audio amplifier employed in certain early 1950s Ferguson TV sets. See attached circuit diagram ...

Now, in my book, a copy is a copy is a copy ...

Anything else would be different ...

the circuit that till posted does not use ECL80's .
that is the circuit that the LLJ one was based on (I think)
Rob T