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Re: [Amps] Amps Digest, Vol 130, Issue 10

To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Amps Digest, Vol 130, Issue 10
From: David Lisney <g0fvt@hotmail.com>
Date: Sat, 5 Oct 2013 22:40:51 +0100
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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@contesting.com wrote:
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> Today's Topics:
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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@fsmail.net>
> To: TexasRF@aol.com, jim.thom@telus.net, amps@contesting.com
> Subject: Re: [Amps] Plate modulation from power supply ripple?
> Message-ID: <17401056.29821380988278665.JavaMail.www@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@aol.com
> To: jim.thom@telus.net, amps@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@telus.net writes:
> 
> Date:  Fri, 4 Oct 2013 08:26:13 -0400 (EDT)
> From: TexasRF@aol.com
> To:  ww1c@outlook.com, garyschafer@comcast.net, amps@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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