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Old 19th Mar 2021, 8:30 pm   #36
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: AVO 163 amp board ~ transformers

In the VCM163, that transformer isn't really working as a current transformer. R1 is the shunt resistor converting anode current into voltage, so it's a voltage transformer.

The shunt resistor could have been scaled for the turns ratio squared and stuck on the secondary, and that would have made it a current transformer by the usual definition. But in this case there would be no DC path for the anode current. So that 10 Ohm resistor is where it is for DC continuity to the valve anode, and the resonator capacitor C1 blocks DC to the transformer primary, so there is no DC current in the transformer primary and they don't need to worry about saturation.

As far as analysing that sort of thing, it isn't difficult, it's just a matter of building up a database in your head of circuit configurations and design tricks so that you recognise them when you see them. Learning how to do it deliberately doesn't work. Exposing yourself to circuit design ideas and looking at a lot of stuff allows it to sneak up on you. This way works, and though slow, it's painless. If I can do it, anyone can. Just a matter of time.

In this amplifier, TR2 is interesting. Its base is being moved up and down by the signal which has been buffered by TR1. Its emitter is being moved up and down by the feedback signal extracted from the meter rectifier. The difference in base-emitter voltage of VT2 sets the collector current VT2 will flow and that gets further amplified to drive the meter rectifier.

So the idea of the transistor bridging two voltages and comparing them. Remember the film with Charlton Heston with one foot on the croups of two separate horses attached to two separate chariots? VT2 must have a similar feeling.

This floating transistor comparator is used in the input stage of just about every transistor amplifier which doesn't have a long-tailed pair at its input. It's a snippet of circuit worth knowing so you can spot them and understand them

VT1, the emitter follower is a very standard circuit, another one for the database.

With a circuit and an explanation, you can read through it and pick up a good number of the points made. Have a break, have a beer or whatever and come back the next day and go through it again. You'll pick up things you missed the day before and other things will snap into clarity. I used to read articles several times picking up new layers of understanding each time. I still do when I come across something I can't follow immediately.

Glad to have helped a bit.

David
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