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From: John Williamson <johnwilliamson@btinternet.com>
Newsgroups: alt.comp.os.windows-11,alt.comp.os.windows-10,comp.os.linux.advocacy,comp.os.linux.misc,uk.tech.broadcast
Subject: Re: OT - TV
Date: Wed, 30 Sep 2026 11:19:27 +0100
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On 30/09/2026 11:13, John Williamson wrote:
> On 30/09/2026 10:27, Carlos E.R. wrote:
>> On 2026-09-29 17:54, Jan Panteltje wrote:
>
>>> Yes some tube TVs had 800 Ohm speakers without transformers.
>>>
>>> Example of a tube output stage without transformer to drive a 800 Ohm
>>> speaker:
>>>
>>> https://2.bp.blogspot.com/-GaJkZ9klfLw/VOecHjVgPuI/AAAAAAAAA4g/H2qcMWJ3tuA/s1600/Schema.png
>>>
>>>
>>
>> I'm not familiar with valves EZ80 and 6CA4, I don't understand the
>> drawings. :-?
>>
> The EZZ80 and BCA4 valves are full wave rectifiers in the power supply.
> (Equivalent to two diodes in each  valve, with a tapped transformer
> secondary. Back when semiconductors were expensive, I've done similar
> with a pair of germanium diodes, as it was cheaper than a bridge
> rectifier.)i
>
> The relevant stage for this thread is the output stage, which is two
> EL504 pentodes being driven in a sort of push-pull configuration by the
> input triode. The EL504s each have an independent DC supply.
>
> The circuit specifies an 800 ohm load, but I first expected to see a
> more standard 600 ohm load...
>
Sorry, my mistake, the EL504s use the bottom one being driven to conduct 
more which pulls the cathode voltage of the other down, making it a 
class A output stage.

<Note to self. Visit the optician>:-)

The other supply is for the input triode.

-- 
Tciao for Now!

John.