Harry Bloomfield wrote:

If (as is possible giving it's rating), it is part of a series heater
chain, the current rating is more important than the voltage. So 0.15A
or more. A 6v 0.3A would work in those circumstances but would be rather
dim.

The voltage rating is equally important if it is in series with
something, as it will only pass the rated current, if the voltage is
correct.

Personally, if it is in series with an heater chain and a replacement
cannot be found - I would fit a resistor of equivalent value to get it
working again and then add a white LED in parallel to provide the light
output. Even if it is not in series, then an LED will be appropriate.

I can't imagine what they would be heating, but their obvious purpose is
illumination - one for the dial, the other for the signal meter.

This is the model:

http://www.hifimuseum.info/tn_ST-88.jpg

Daniele