A Chinese tube never does from filament issues no matter??what amp they are in and what the filament voltage is. ?It will die from a G-F short or some other issue long before it dies from any filament issue of tenths of a volt. ?When operating tubes 24/7 key down in commercial service you could extend filament life in a 3-500 by watching the filament voltage.
On Thursday, October 19, 2023, 3:02 PM, Adrian Fewster <vk4tux@...> wrote:
On 20/10/23 04:37, Dave w6de wrote:
In
this article Richard Measures
AG6K (SK) suggested using a smaller wire to feed the 3-500Z tube
to knock down the filament voltage.
?.. was one of a few methods, and with measurement being a
priority ;
"FILAMENT
VOLTAGE
The
filament-voltage, measured at the sockets, in my stock TL-922
was approx. 5.31v RMS @120V/240V line input.[2] This voltage
exceeds the manufacturer's maximum allowable filament-voltage
for the 3-500Z.
The
filament-voltage of low-operating-time 3-500Zs can be lowered
to approx. 4.8v for much longer tube life with no reduction in
RF power output. This approx. 9% decrease may not sound like
much, but according to one 3-500Z manufacturer, Eimac?, every
3% decrease in thoriated-tungsten filament-voltage doubles the
useful emission life of the cathode, provided that the
filament-voltage is kept slightly above the level that causes
a decrease in output power. A 9% decrease in filament-voltage
can increase the useful emission life by 2-cubed or 8-times.
In other words, one pair of tubes will last as long as 8 pairs
of tubes.
Reducing
filament voltage to achieve maximum power-grid tube life is a
considered to be good engineering practice in commercial
transmitters.
The
filament-voltage can be lowered to the desired level by
connecting (2) approx. 16milli-Ohm, 5W resistors in series
with the filament-leads on the filament-transformer. An easier
way to lower the filament-voltage is to replace the #14 wires
from the filament-transformer to the filament- choke with #22
high temperature insulated hook-up wire. Each wire will
dissipate about 4W [14.7A rms X .25v] over its approx. 40cm
length.[3] This raises the wire temperature only slightly to
the touch. The new wires can be loosely attached to the cable
harness, but they should not be buried in the cable harness;
they need to breathe. Although 200 degree C Teflon? insulation
would be nice, 105 degree C vinyl insulation is satisfactory.
Because
of regional variation in line-voltage /electric-mains voltage,
the actual filament-voltage should be measured, before and
after modification, at the sockets, with the amplifier
upsidedown and the bottom cover removed.
To
perform this measurement, the amplifier is switched on and the
standby/operate switch is set to standby.
If
a mains-voltage of 108V/216V is used with a TL-922 whose
filament-transformer taps are set for 120V/240V, the
filament-voltage probably does not need to be lowered.
Caution:
Bodily contact with the 120V/240V primary circuits, the +2000V
/ 3200V, or the +110V power-supplies can be fatal. The
built-in "safety interlocks" do NOT protect the operator from
all of these dangerous voltages - even if the amplifier is
switched off. To be foolproof, the amplifier must be
disconnected from the electric-mains. "
73
vk4tux
