[Boatanchors] SWEEP TUBES AND FPM 300S

[Bill Cromwell]

Hi John,

I use smaller sweep tubes and I use them at 20 to 30 watts. I attempt to 
run them in class AB1 (linear) for the lower driving power requirement 
(and the linearity helps, too). I use them for CW. Mostly they are 
loafing along.

Some time ago there was a "Quarter Kilowatt" (250 watts *input*) 
transmitter project in the ARRL Handbook(s) that uses a pair of 6KD6s.

I also use 6146s including the B model. Don't waste a lot of time tuning 
them up either. Go for a plate dip *immediately* and continue tuning 
from there. Otherwise invite the little crinkle noise followed by smoke.

73,

Bill  KU8H


On 01/10/2016 06:28 PM, Jim Liles wrote:
> John,
>
> Sweep tubes are beam power Tetrodes just like the 6146.  Neither has a 
> suppressor grid just side director plates.  As for rugged, don't know 
> of any 6146 pair running 550 watts SSB and 350 CW with 275 out SSB and 
> 175 CW.
>
> IMD will be -30db and they are not as delicate as most think.  The 
> tube is a 6KD6.
>
> If you ran a pair of 6KD6's at the same power level that 6146B' run, 
> you would not need a blower.  The SR-400A uses a pair of 6KD6's but 
> added a blower because it runs 500 watts in SSB.
>
> The notion that sweep tubes are inferior to the 6146 is simply 
> misguided. They are actually superior
>
> If you look at the grid structure in the 6146 and sweep tubes you will 
> find the same precision regarding the precise alignment of the control 
> and screen grid that make beam power tubes the great performers they are.
>
> I'll upload images of the 6146, 8122, and 6KD6 construction so you can 
> see for yourself what they are.  The control grid and screen are 
> precisely aligned.  If you look toward the cathode from the screen you 
> will see only control grid and from the control grid to plate you will 
> see nothing but screen.
>
> The other sweep tubes are also Beam power tetrodes.  The tube layout 
> in the data sheets would have you think they have a suppressor grid 
> attached to the directors but it is not so --- none do.  Beam power 
> tubes need no suppressor grid.  Also a Beam power tube like the 8122 
> is a tetrode also needing no suppressor or director plates because of 
> tube geometry.
>
> A super day to you Kindest Regards Jim K9AXN
>
> -----Original Message----- From: John via Boatanchors
> Sent: Sunday, January 10, 2016 2:41 PM
> To: boatanchors at mailman.qth.net
> Subject: [Boatanchors] SWEEP TUBES AND FPM 300S
>
>
> The industry switched to sweep tubes largely as an economic measure. They
> were very cheap as compared to a heavy duty beam power tube such as the
> 6146.  Some operated at lower plate current which made transformers 
> cheaper.
> They  would last much longer if they were air cooled and tune up 
> periods were
> kept  short which many ignored.  The best way I found to tune a sweep 
> rig,
> especially when tune up periods were extensive due to an antenna 
> tuner, was
> to  keep the drive low until the final seconds of the tune up period.
>
> I have three FPMs in the collection, The main weak link in the rig, in
> addition to sweep tubes and slightly unstable VFO was the drive belt.
> Fortunately, a repro belt is readily available.
>
> 73,
>
> John, W4AWM
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