Re: [Amps] filament supply options discussion, follow up

[R.Measures <r@somis.org>]

On Apr 24, 2004, at 12:01 PM, Steve Jackson wrote:

> Lots of good feedback so far.  My responses, in no
> particular order ...
>
> 1.  Measuring grid current:  thought of that, and felt
> that I could use some kind of low-drop shunt to
> directly measure at the bottle ... would have great RF
> characteristics and also super accurate DC specs, not
> hard to fabricate, either
>
> 2.  Peril of low fil resistance at cold power-on:
> actually, I leave some juice running on the fils at
> all times.  The fils are run at a very low current
> when "off" which allows the minimum load necessary to
> keep the power supply happy, and never allows the fils
> to go to totally "ice" state.  There are two relays:
> one is an SPST that shorts out a series resistor when
> powering up the amp.  Besides, the power supply
> current-limiter is easily adjusted to flatten out the
> peak, anyway.  Much easier to do this with a DC supply
> having all the bells and whistles than to have to
> design either a ramp-start xfmr primary gizmo or a
> nicer (like the L7 fil xfmr) secondary, which is
> perfect in that it naturally limits inrush; if it was
> only available anymore.  The second relay is a big
> DPDT 100A teclco-surplus latching contactor that
> allows me to reverse the polarity of the fil voltage
> every 100 hours (I'll have a Hobbs meter on the PS
> deck).
>
> 3.  Very easy to adjust the switcher to make anywhere
> from 4.5V to 6.5 v with its trimpot or remote control,
> plus it has a remote sense line that can accomodate
> the small I2R loss of the 8 AWG and Anderson PowerPole
> jumper I am planning.  Actual fil voltage will be
> metered in my amp, with a dedicated meter.  I'd have
> to buy a $$ Variac to try the same stunt with a
> conventional design.

  A rheostat is more suitable for adjusting filament voltage since a 
10% adjustment range is enough and a Variac provides a 100% adjustment.
> 4.  The 100W supply will loaf at the 75W fil
> requirement of a 3-500z, and if I upgrade to the
> 3CX1500D7, I can simply put two of these in parallel
> as they are designed to do that (diode-sum the
> outputs).  $2 for a 100W fil power supply makes the
> whole thing seem cheaper.
>
> I am glad I haven't heard any showstoppers yet (not
> even from Rich!).  And, the savings here are adding
> up, so far, well into a three-figure number, not
> counting the hernia I am not going to have.
>
> Now, if I only had a real metal shop setup to build
> the cabinets ...

  There is no need for a sheet metal shop if one uses the construction 
technique used in the Ugly Amplifiers on my Web site.
http://www.somis.org/Building_ugly_amplifiers.html
PEM ® self clinching nuts and machine screws are used to fasten 
precision-cut 0.125" 6061-T6 Al plates together to form a box.  One 
trip to a metal dealer to have the plates accurately cut,  -  such as 
Industrial Metal Supplies in L.A. - and the rest can be done in one's 
garage with a vise and basic hand tools.  (The last time I did this, 
the cutting charge was under $2 per slice)  Another advantage of this 
construction method is that - in the Extremely Unlikely event of a 
meltdown - there's quick access to any area.   Yet another advantage is 
that the resulting box is the right size, so there's no wasted space.  
However, for those who build RF amplifiers to impress un-RF-savvy 
visitors, this construction technique obviously won't do.

cheerz, Steve

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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