On 6/15/2012 9:08 PM, Leigh Turner wrote:
>
>
> 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.
>
Pricey aren't they? <:-))
I have 3. One is a full socket complete with the brass mounting ring and
chimney. The other two are honest sockets but without the mounting ring
or chimney. The mounting ring is fairly easy to make. Other than time
consuming (Lots of individual parts), the sockets would be relatively
easy to make except as they are still available it probably would not be
a good idea to make some knockoffs to put up on e-bay.
The prices cover a wide range from a couple hundred USD to as much as a
rebuilt tube. New they are, or were in the neighborhood of $1,600 USD.
I see the prices of the 3CX3000 sockets has been creeping up too, but
the highest I've seen them has been less than $400. I think Henry had
some brand new ones for $395 and they had new tubes as well. I don't
remember the price of the tubes.
73
Roger (K8RI)
>
> Cheers,
>
> Leigh
> VK5KLT
>
> -----Original Message-----
> From: amps-bounces@contesting.com
> <mailto:amps-bounces%40contesting.com>
> [mailto:amps-bounces@contesting.com
> <mailto:amps-bounces%40contesting.com>] On
> Behalf Of Jim Thomson
> Sent: Friday, 15 June 2012 11:59 PM
> To: amps@contesting.com <mailto:amps%40contesting.com>
> Subject: [Amps] filament instant heating query
>
> Date: Fri, 15 Jun 2012 00:45:25 -0400
> From: "Roger (K8RI)" <k8ri@rogerhalstead.com
> <mailto:k8ri%40rogerhalstead.com>>
> Subject: Re: [Amps] filament instant heating query
> To: amps@contesting.com <mailto:amps%40contesting.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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