[Amps] Amps Digest, Vol 130, Issue 10
David Lisney
g0fvt at hotmail.com
Sat Oct 5 17:40:51 EDT 2013
Hi, regarding the travelling wave tube amp, you mention 9Kv at only 5ma, it occurs to me that in that instance you could generate your ht from a tiny inverter, possibly followed by a tripler if necessary. With an inverter running at perhaps 50Khz the smoothing caps could be very small.
Just a thought, regards David G0FVT.
Sent from my iPad
> On 5 Oct 2013, at 17:01, amps-request at contesting.com wrote:
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> 1. Re: Plate modulation from power supply ripple? (peter chadwick)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sat, 05 Oct 2013 17:51:18 +0200
> From: peter chadwick <g8on at fsmail.net>
> To: TexasRF at aol.com, jim.thom at telus.net, amps at contesting.com
> Subject: Re: [Amps] Plate modulation from power supply ripple?
> Message-ID: <17401056.29821380988278665.JavaMail.www at wwinf3717>
> Content-Type: text/plain; charset=UTF-8
>
> Gerald,
>
> Ripple on the helix will produce phase modulation - a well known function of the TWT used in the Serrodyne. (An electronic warfare approach, where you receive the radar signal, amplify it in a TWT with a sawtooth modulation of the helix voltage and send it back through another antenna (or circulator and the receiver antenna) to make the radar think you're going in a different direction!)
>
> Now if you can get hold of a copy of 'Second thoughts on Radio theory' by M.G.Scroggie, pages 191 - 200 deal with R-C smoothing, and he shows that by splitting and using a number of smaller capacitors and resistors, you can get better ripple reduction than one big R and C. So if you have say a 10 mFd filter cap and a 12 k glitch resistor using two 5mfd Caps and two 6k resistors, you'll get a lot more ripple reduction. That should allow a reduction in glitch energy.
>
> BTW, do you run depressed collector?
>
> If you can't get the book, I guess I could scan the necessary pages for you.
>
> 73
>
> Peter G3RZP
>
>
> ========================================
> Message Received: Oct 05 2013, 12:44 PM
> From: TexasRF at aol.com
> To: jim.thom at telus.net, amps at contesting.com
> Cc:
> Subject: Re: [Amps] Plate modulation from power supply ripple?
>
> Jim, it is not that simple. The there is about 9000vdc and normal helix
> current is less than 5mA.
>
> Right now there is a 12.5K ohm glitch resistor to limit any fault current
> but that is not enough to fully protect the tube in case of a flashover.
> More R could be added but then the voltage drop during normal operation
> becomes too much and changes in the helix current screws up the already marginal
> voltage regulation.
>
> Reducing the amount of filter C would reduce the stored energy but at the
> expense of higher ripple voltage. I need to spend some time experimenting
> with the filter C value to see just how low it can be for acceptable 120 Hz
> hum on the carrier.
>
> The thought of less hum level because of linear operation class was new to
> me. The filter C design was based on the tube spec showing .08 dB per volt
> change in gain. In that scenario 10v of ripple would cause a gain change of
> .8 dB. .8 dB is a power difference of 20% which would be very noticeable
> if it was present in the form of hum modulation.
>
> The tube runs class A and is fairly linear but not perfect by any means. It
> is run at maximum power output, around 300 watts at 10.4 GHz . Running the
> tube this way is considered to be in power saturation. That does no damage
> to the tube but is not that great for linearity. We mostly use cw or one
> of the digital modes so linearity is not an issue.
>
> A suggestion was made to use three phase 400Hz voltage derived from three
> audio amplifiers to reduce the needed C and that would certainly work..
> 9000v at 5mA is only 45 watts Before taking on a major rework like that I will
> do some homework with existing power supply.
>
> I thought this was all figured out. Wrong!
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 10/5/2013 4:12:17 A.M. Central Daylight Time,
> jim.thom at telus.net writes:
>
> Date: Fri, 4 Oct 2013 08:26:13 -0400 (EDT)
> From: TexasRF at aol.com
> To: ww1c at outlook.com, garyschafer at comcast.net, amps at contesting.com
> Subject: Re: [Amps] Plate modulation from power supply ripple?
>
>
> My current interest is mainly in microwave eme operation. Traveling wave
> tubes are used to generate power. The homebrew power supplies have a lot
> of
> filter C in the helix supply to reduce hum on the transmitted signal.
> After
> this discussion I am wondering if there may be more C than necessary.
> Normally it would not matter but if there is a flashover in the twt the
> helix
> structure could be damaged. There are trip circuits to shut the power
> supply
> down but most of the stored energy would be dissipated in the helix.
>
> 73,
> Gerald K5GW
>
> ## How much B+ ?? How much plate current ?? Just put a 50 ohm
> glitch R
> in series with the B+.... and wire a fast hv fuse just in front of the 50
> ohm glitch.
> Anything arcs, the glitch limits the fault current to a safer value. The
> HV fuse interupts
> the fault current..and will easily do that in less than 2 msecs. But
> you have to size the
> fusing wire correctly. Add the glitch + HV fuse on top of your
> existing protection.
>
> Jim VE7RF
>
>
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