Some large power tetrodes and triodes require DC due to the risk of
vibration of the large filaments at 120 Hz. AC filaments are pleasant to
deal with as usually a center tap on the filament transformer is all
that is required to cancel hum in the output. However, with cavity
circuits at VHF, one side of the thoriated tungsten filament may be
considered cathode and connect to earth/chassis. In this case a CT
transformer isn't acceptable. This may or not scale to hamateur
amplifiers, but i post it here as information that it can be done either
way, AC regulated or DC regulated, with Thoriated Tungsten filaments. I
have operated tubes both ways in work equipment.
For 4CW250,000 tetrodes we use a CT transformer only. On the primary,
there is a variac for adjustment, another variac with motor drive for 2
minute rampup, and a Sola CV transformer for regulation (before the
variacs). Seems to work but a lot of iron and moving parts.
For TH555A 250 kW HF tetrodes, I built a saturable reactor AC
controller, on the 460 VAC input single phase, that can remotely ramp
the AC filament voltage to a CT transformer and also remote adjustment
without variacs via a small DC pot. This is good as the push-pull
amplifier is on another floor in a radiation area.
For Burle 7835 triode cavity amplifier, use 12 pulse transformer (delta
and wye) to get low ripple DC, this is applied to the filament. 7000
Amps at 5 volts!
The incoming AC is ramped via a stack of 12 variacs on a common chain
driven shaft, off the 480 VAC 3 phase circuit. Staco makes that as a
purchased package, costs about $10K unfortunately and has moving parts
that require maintenance (brushes).
For new THALES TH781 tetrode cavity amplifier, was using AC transformer
during initial testing years back, but it couldn't have CT (due to
cavity ground on one filament lead, with grounded grid operation). So
there was hum on the RF output. I switched to a switchmode supply, Power
Ten model, 10 VDC at 350 Amps. It works very well, ramps up over 8
minutes, etc.
For new THALES TH628 Diacrode cavity amplifier, using a pair of
Sorensen/Ametek "SGA" switchmode supplies, strapped in parallel,
master/slave controlled. The filament is 950 amps at 19 VDC, so using
11000 MCM cables the size of your arm. It ramps up and down over 8 minutes.
On 6/16/12 12:32 PM, amps-request@contesting.com wrote:
> Message: 9
> Date: Sat, 16 Jun 2012 20:27:33 +0200
> From: Dave White<mausoptik@btinternet.com>
>
> I've known plenty of people use ex-computer 5V DC switch mode supplies for
> the heaters on oxide cathode tubes like 4CX250Bs
(G6ABU used them for years in his 2 metre contest amps) - probably
because they're stable, cheaply available and easy to set-and-forget.
> I've not heard of anyone using DC with tungsten filaments though. It's a
> good question.
>
> Dave DL/G0OIL
>
> ----- Original Message -----
> From: Jerry Kaidor<jerry@tr2.com>
> Sent: 16 June 2012 18:10
> To:amps@contesting.com
> Subject: Re: [Amps] tube fils..again
>
> I was thinking about this last night. AC is hard to regulate. Has anybody
> thought of running filaments off DC? DC is easy to regulate. Cheap
> switching power supplies can deliver large amounts of efficiently
> generated, tightly regulated DC. Turn-on and turn off ramps can be
> precisely controlled.
>
> Regulating AC OTOH is hard. Techniques I can think of off the top of
> my head include - special transformers with IIRC air gaps that are
> controlled
> with DC windings that partially saturate them - transformers with multiple
> primaries or secondaries chosen by Triacs or SCR's - small motor-controlled
> variacs - powerful AC amplifiers with gain feedback...
>
> - Jerry Kaidor, KF6VB
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