Hello All,
Been searching for information on the reason for the high filament voltages seen in vintage TV schematics.
Example are 25W4GT (Damper) and 19BG6 (HV Rectifier) as compared to a 6W4GT and 6BG6.
Best I can come up with is a series filament supply chain and believe this was an agreed methodology that was developed in the USA back in the 1950s(?).
Due to the differing filament current requirements, this was the means to maintain the correct supply current by raising the supply voltage for the filaments.
So the supply here would have had to have been a current supply and not voltage.
Any further information on this would be greatly appreciated.
Thankyou.

Smokey

I'm not into TVs and somebody who is may address your particular question about those two valves.

However, in radios designed to operate on AC and DC supplies, which of course have no transformer in the power supply, the valve filaments are wired in series across the mains. The sum of the filament voltages equals the voltage of the mains supply.

In order for that series scheme to work, the filament current of each valve involved must be same value, such as 300mA which is the case for the 24W4GT and 19BG6.

Not something that really applies to most Australian built sets as very few had the non standard heater voltage valves. The circuits you've been looking at sound like 1940's to early 1950's designs, very different to what was used here.

The reason valves such as the 25W4 exist, was to be used in a series string heater circuit. The series string design was done purely for cost reasons, as it eliminates the need for an expensive transformer. The American 110v AC supply when rectified was perfect for use as the HT voltage and by careful selection of valve types, the heater string voltages are arranged to add up to around 110v as well.