[Boatanchors] SWEEP TUBES AND FPM 300S

[Jim Liles]

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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