On 10/6/2010 10:40 PM, TexasRF@aol.com wrote:
> The problem with switchers or using any regulated supply is the cold
> resistance of the heater/filament is typically 10% of the hot resistance.
Isn't it worst than that for the tubes like the 4CX3000A7 and 3CX3000A7
or the imported GS35B
> That
> issue is automatically handled by the current limitations of a typical
> filament transformer. The transformer becomes pretty much a constant current
> source until the heater temperature rises enough to get the transformer out
> of
> saturation.
>
> On big tubes requiring many amps of filament current this can be a real
> problem. If the transformer has to supply say 30 amps, then it will supply
> many times more current than that for short periods of time. A cold tube can
> be subjected to very high stress under these circumstances and can even have
> the filament burned open, just like a 100w light bulb blows out. No
> problem with the $1 light bulb but loosing a big tube that way is heart
> breaking.
> (Been there, done that, have multiple tee shirts).
>
> The solution to the over current problem is reducing the startup voltage
> enough that the 10% filament resistance still draws rated current. That takes
> 10% voltage according to ohm's law.
Hence the step start or using a triac to ramp up the voltage. I really
like the idea of ramping up the voltage, but with the big tubes I'd use
a separate ramp for the tube and high voltage.
>
> So, for any regulated and current limited power supply, there needs to be a
> method of starting at low voltage and ramp it up as the tube warms up.
>
> Being the concept guy, I will bow out at this point and leave the answer to
> others. I have the answer for transformer filament power sources but no
> clue how to do it with a switcher or any other regulated/current limited
> supply.
I really don't like the idea of using a switching PS for the filaments
on "big tubes". I don't think DC on the filament is a good idea and
putting the square wave through leads and chokes to the tube(s) opens up
a whole new can of worms with noise that can be very difficult to
eliminate. Depending on the circuit you can end up with the old cathode
modulation.
Although not trivial, I doubt creating a "dual voltage" switching supply
would be much of a problem. Computer power supplies now run well over a
KW capability (those aren't the $15 dollar ones) and have multiple 30A
"rails" at 12V and other voltages as well. Those typically use the
"push, pull", high capacity, low RPM, 140mm fans. Since those came out
I can actually carry on a conversation here in the den between the two
full size towers that have 5 of those fans...each! That and I only have
to clean them about once a season instead of monthly or even weekly.
> If it was actually a current limited supply, not a current shutdown
> design, all would be well and the tube could warm up with a constant rated
> current source. A regulated current design instead of voltage would be
> wonderful.
Back in the "old days" of computers I had one system so heavily loaded
that just plugging in the small fan in the side would cause the PS to
shut down.
Then there was the "big stuff" (not computers) We used regulated current
AC supplies with a pair of LARGE (water cooled) SCRs. The controlling
sample was across a length of about # 12 nichrome wire about an inch
long. This received its current of 5A from a series of very large
current transformers on the secondaries of the main transformer. The
SCRs were running something like 1200A(or more) @ 480 VAC single phase.
That's substantially over half a megawatt. I watched an attempt to lift
one of the main transformers put a fork life right up on its nose and we
had rows of those things. Of course that was over 23 years ago so I can
only imagine what is there now.
But getting back to powering up one of those "big tubes" you are looking
at as much iron in the filament transformer as there is in the plate
transformer for some of the "economy amps". The tube requires CCS
ratings while the amp can get away with IAS with about the same KVA ratings.
Kind of a sobering thought as I have several of those so called "instant
on" tubes.
73
Roger (K8RI)
>
> By the way, that 10% resistance figure applies to light bulbs as well. The
> hot resistance can be estimated by r equals voltage squared divided by
> wattage rating. That is the well known power = e squared divided by
> resistance formula turned around backwards. The cold resistance can be
> measured with
> a VOM or DVM.
>
> 73,
> Gerald K5GW
>
>
>
> In a message dated 10/5/2010 11:01:14 P.M. Central Daylight Time,
> jim.thom@telus.net writes:
>
> Date: Tue, 5 Oct 2010 07:34:41 -0700
> From: Patrick Barthelow<apolloeme@live.com>
> Subject: [Amps] Switching Supplies for High Current Filaments
>
>
>
> Are there any sensibly priced switching supplies (even surplus) that are
> small and light, that could power some of the QRO tubes
> with high current filaments? Would, say, a 5v, 30A switcher which could
> power a pair of 3-500Zs, be economically
> competitive with a 5V 30A transformer, and even if it is not directly, it
> might still be worthy of consideration
> if it was small enough, and light enough to significantly lower size and
> weight in an RF deck?
> Would cold start surge current be a problem for switcher implementation
> for filaments?
> Best Regards,
> 73, de Pat Barthelow AA6EG apolloeme@live.com
>
> ## This was 1st tried on a GS35B 6m amp. The inrush current was so
> great that the
> switcher thought it was being cro-barred... and went into shut down mode !
> A bigger
> switcher was used..and that worked, but the 2nd one was almost double the
> capacity
> of the 1st one. Also, I believe my buddy had to use a Dc-dc isolation
> converter in their some where, to
> solve yet another problem.
>
> ## In the end, with numerous other problems, like heating of the air
> variable load cap, the RF deck was scrapped,
> and a new one designed from scratch. 2nd version used 2 x GS35B's... and
> this time, 2 x separate conventional
> fil xfmr's used, one per tube. The problem with a pair of 3-500Z's
> is.... the B- return is via the CT of the conventional
> fil xfmr. We gave up on the notion of using switcher's for fil
> supplies on oxide tubes. Some have had better success,
> so it can be done with oxide tubes. Then you have an adjustable,
> regulated supply. They will not work on a directly
> heated thoriated tungsten tube. On some of the switcher's... hash and
> noise was across some of the HF bands...and
> a bitch to filter out completely.
>
> ## hammond makes a 5V @ 30A [with CT] conventional fil xfmr...for 2 x
> 3-500Z's. Runs barely luke warm. It's priced
> way below what a dahl is.... and readily available in the usa, through the
> usual distributor's. Comes in 2 x version's ,
> with or without bell end housings, IE: bell housings or open frame. They
> measure 5.5 V no load.. and 5.0 V with a 30A load.
> Last I checked a few years ago, it was aprx $45.00 . Their is only one
> primary, 115 vac. You can also get it in 5v @ 15A .
> They also make a 7.5 vac @ 21 A. A friend in W6 land got his YC-156
> fil xfmr from hammond. The YC-156 fil xfmr was a
> torroidal type.
>
> later... Jim VE7RF.
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