All good points you make here Jim.
When it comes to the small desktop amps like the SB220 and TL922 having
separate filament transformers I find that inserting a pair of GE CL-60 NTC
thermistors in each leg of the filament transformer primary winding provides
a simple remedy for implementing a soft-start. The observed glow of the two
3-500Z filaments comes up nice and slow as the thermistors warm up...no big
surge in brightness at initial turn on as it was with the stock design.
This is in addition to a simple short-duration resistor / AC relay coil
style step-start to both the HV and filament xfmrs timed predominantly from
the HV electrolytic charge up time constant. This relieves the inrush
current surge stress on the front panel ON/OFF switch contacts and the
electrolytic capacitor bank.
BTW Roger, the 4CX3000A / 8169 is a nice tube in the tetrode class; just a
pity the damn breechblock sockets are not so easy to source and cost a lot
of dBs.
Cheers,
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Jim Thomson
Sent: Friday, 15 June 2012 11:59 PM
To: amps@contesting.com
Subject: [Amps] filament instant heating query
Date: Fri, 15 Jun 2012 00:45:25 -0400
From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
Subject: Re: [Amps] filament instant heating query
To: amps@contesting.com
On 6/14/2012 11:59 PM, Charles Harpole wrote:
> to check on a supposition............
>
> It appears possible that the sudden application of voltage to a tube
> filament causes degradation of the filament.
>
>
There are others on here who can probably explain this better than I, but...
This is typically a problem with filaments that draw a lot of current.
At turn on the resistance is a fraction of that once the filament is up
to temperature. If the current is not limited the sudden rise in
*current* well beyond the operating current produces a very strong
magnetic field that interacts with the current flowing in the filament
which can cause the filament to move mechanically.
Take a 4CX-3000. The filament runs 9V at 41.5 Amps which is just shy of
375 watts, but the cold, inrush current can be several times higher if
the transformer can provide it. That would cause the filament to
"jump". A number of the larger QRO amps run much higher current than
the 4CX3000. This is the reason for "step start" or ramping up the
filament voltage so there is no sudden very high current pulse at turn on.
73
Roger (K8RI)
## real simple. Cold resistance of a thoriated tungsten tube is one
twelth of its hot resistance. If u believe a fil xfmr will limit the inrush
current to no more than double the normal operating current, then go for
it...and suffer the problems.
## a real simple step start with just ONE resistor in EITHER leg of the 240
line is all you need. Then you can kill 2 x birds with one stone...and step
start both the HV supply and the fil. Just set it for 2-6 secs, b4 you
short out the resistor, no brainer, and no rocket science.
## as regards tube life....when the fil is lit... the clock begins ticking.
Turn on a sb-220...with no HV...and no drive applied...and no cables going
in-out....and the tubes will be soft in the same number of hrs as a working
sb-220 with hv and drive applied.
## In the telco I worked at for 34 yrs...everything is powered up and
running full bore 24-7-365..never shut off...and its all on UPS since day 1,
including microwave and cell sites. Equipment runs just fine.
## running xcvrs at half power or less is another wasted effort. Typ xcvr
operates at only 50% eff..when run at full power output. Run it at half
power output....and eff drops to 35%. Run it at qtr power output....and eff
is in the basement. You save nothing.
Jim VE7RF
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