> 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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|