You're probably quite right Bob; I guess I'm a believer in an ounce of
prevention is worth a pound of cure :-)
Besides, I quite like the way the initial turn-on filament glow now rises up
in a nice slow and controlled manner....eliminating the inrush current surge
just seems so intuitively a good thing to do.
Yes the idea of a separate contactor relay, rather than a big switch for the
toggling the AC mains service to the amp ON/OFF is good.
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Rob Atkinson
Sent: Saturday, 16 June 2012 10:57 AM
To: amps@contesting.com
Subject: Re: [Amps] filament instant heating query / simple remedy
I'm having a hard time believing 3-500 filaments are stressed all that
much. I'd be more interested in metering the fil. voltage and being
able to adjust it. I know of no commercially manufactured ham amps
that meter the filament v. To me that is the amp/transmitter
equivalent of an oil pressure gauge in a motor vehicle, also something
you never see in mass market consumer vehicles.
I would not put the all the AC service through an on/off switch for an
amp. I'd use the switch for the AC to a 28 v. relay coil DC supply
and use relays for the higher current on/off to the h.v. etc.
primaries.
73
Rob
K5UJ
On Fri, Jun 15, 2012 at 8:08 PM, Leigh Turner
<invertech@frontierisp.net.au> 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.
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