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Old 19th Nov 2015, 3:05 pm   #1
regentone001
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Default Plug in push pull amps for TV's.

Many years ago I see to remember reading,in I think Practical Television, of how to build a push pull amp to replace the single ended amp used in most tv's. It was designed to plug into the existing socket for the output valve so needed no alteration to the existing circuit and could just be unplugged and a valve put back in to return it to original. I think the amp used 2 x ECL80s.
Does anyone remember this and did anyone ever build one? If so was it worth it?
Thanks
Steve
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Old 19th Nov 2015, 3:16 pm   #2
BassoonBloke
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Default Re: plug in push pull amps for tvs

Hi Steve,

I've never seen or heard of this but I can't see why it couldn't be done. One of the triodes as a driver, the other as phase splitter to set up push-pull of the two remaining pentodes. The only real issue I can think of is where you are getting the LT (6.3v) and HT (200+V) from. I imagine that might be a bit tricky without using an extra PSU.
It would obviously give more volume, but extra sound quality would depend on the level of bias of the output stage, components and circuit.

Alan.
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Old 19th Nov 2015, 3:23 pm   #3
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Default Re: plug in push pull amps for tvs

The push pull ECL80 amp is in this thread.

https://www.vintage-radio.net/forum/...ad.php?t=33649

Keith
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Old 19th Nov 2015, 3:27 pm   #4
julie_m
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Default Re: plug in push pull amps for tvs

ECL80 heater is 300 mA, so you could just stick the two valves in series and adjust the heater dropper resistance. There presumably is already a screen grid supply that can also be used for the triode anodes; the pentode anodes will go to the centre-tapped primary of the new output transformer, and the tap will need to be connected separately to HT+.

I'm sure I've seen the circuit mentioned on here somewhere.

EDIT: Keith beat me to it
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Old 19th Nov 2015, 3:36 pm   #5
regentone001
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Default Re: plug in push pull amps for tvs

that is exactly it. Thanks Keith
Has anyone built and used one?
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Old 19th Nov 2015, 7:46 pm   #6
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Default Re: Plug in push pull amps for TV's.

No I haven't built it yet but it's on my 'to do' list. This was published some time ago in Practical Television and it's always intrigued me. I think you have to be careful with decoupling and layout otherwise you end up with a cathode coupled oscillator!
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Old 19th Nov 2015, 9:09 pm   #7
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Default Re: Plug in push pull amps for TV's.

Baird Television supplied a plug in push-pull audio amplifier for certain models.
The amplifier was intended for the schools model 638, this model received a number of modifications in order to conform with certain Education Authority requirements such as the fitment of a mains isolation transformer.
No puny ECL80s in this amplifier, this one uses two PCL82 triode-pentodes.

From the service manual: This is mounted on a block in the cabinet by two 4BA nuts. It is near the sound output stage of the main chassis.
Connection into the circuit is done by removing the sound output valve (PCL82) from the main chassis and replacing it with the noval plug from the amplifier. The socket on the amplifier HT lead is pushed onto the plug pin which has been added to the vision/sound panel of the main chassis
.

Because the set has an isolated chassis it is possible to fit an external loudspeaker.

DFWB.
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Old 19th Nov 2015, 10:41 pm   #8
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Default Re: Plug in push pull amps for TV's.

Circuit diagram of the Baird push-pull audio amplifier.

DFWB.
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