[Amps] How best to reduce filament voltage?

[Tom W8JI]

> I'm building a single tube 3-500  6m amplifier around an 
> OEM SB1000
> transformer. The  Heath transformer is dual input 
> 110/220VAC with HV,

It's actually spec'ed at 120/240 unless you got the 
Japanese/Eu transformer wich is 110/220.


> The problem is that the filament secondary appears to be 
> overly high
> and measures 6.5VAC no load and 6.09vac with the tube 
> installed.

The spec is 5.5 volts with a hot transformer at rated line 
voltage. The filament choke and wiring in the SB1000 limit 
inrush current and drop voltage to 5.0 at the tube pins.

>Since
> there's an exponential loss of tube life with only minor 
> filament
> overvoltage, I'm caught between a rock and a hard place.

Here we go again! :-)

That expotential life drop only occurs when a tube is a good 
tube and is operated well below ratings in a service that 
does not cycle the tube thermally. While it is a good idea 
to run the tube within the specified range and you should 
alter filament wiring resistance to meet that goal **AT THE 
TUBE PINS** I wouldn't get all worried about things that 
only really apply to other services. If we lose a 3-500 in 
amateur service it is almost always a poorly made tube or 
abuse in other areas, not low emission.

> instance, the obvious answer is to install a dedicated 
> filament
> transformer correctly designed for the application; 
> however, that
> involves an additional (redundant) component. The question 
> is whether
> it's somehow practical to reduce the filament voltage 
> without getting
> involved with an odd ball low ohmage, high wattage 
> resistor series
> installed on the secondary. Any comments would be 
> appreciated.

I don't know what your filament choke is, but the best idea 
is to eat up that voltage in distributed wire losses. This 
also helps limit inrush current.

Heath transformers tended to have insulation problems with 
the HV secondary, and omitted the buck-boost winding, so 
they are not as good a choice as Ameritrons transformer from 
Schumacher Electric. I'm not sure about the voltage 
tolerances of the Heath transformer, I only know what the 
drawings given to them said.

73 Tom