[Amps] SB220 filament

[Tom W8JI]

> hello!.. i bought a new digital multimeter to be shure... 
> it is 4.1 volts on the filament.. i will be resoldering 
> everything this evning.
>
> //SM7TVJ


It is VERY unusual to have a bad connection cause 4.1 volts 
without serious smoke. The reason is simple.

The tubes draw 30 amps at 5 volts. That's 5/30= .17 ohms 
when fully hot

The nominal voltage at the transformer is 5.4 volts. The ESR 
of the system is .4/30=.013 ohms plus .17 = .183 ohms

This means a solder joint to reduce voltage .9 volts would 
have to have about .038 ohms. At that point current would be 
about 24 amps, voltage at pins about 4.1 volts

The solder joint would dissipate .9*24 = 21.6 watts

How do we make a small solder joint that dissipates 21 watts 
without smoking? Pure luck allowing several joints to share 
the voltage drop and heat?

Maybe, but it doesn't seem likely in most cases.

Maybe someone changed something in the amplifier?