Converting old sweep tube amp to 6M

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Larry J. Rolewic:
The other remarks are worthy of consideration.  But to put it a bit more simplisticly (from a simple mind) the sweep tubes are:
1.  VERY expensive now
2.  Never designed to operate at 50 MHz+, so VERY inefficient.
3.  Reason #2 not withstanding, using them in parallel makes matters worse, whether it's three or seven (or any other number) of tubes.

    the sweep tubes were quite inexpensive when the circuits using them were designed; an old (1978) ARRL Handbook had a project using four.  But it was designed up to 20 Meters, (14 MHz+/-) and noted that the efficiency dropped off at higher frequencies.

    If you are an inveterate homebrewer, you can probably convert such an amp to 6M, using such tubes as a 6146, but that obviously will require a fair amount of work.  The most useful part of such an amp will mkost likely be the power supply.  

BTW, don't assume the amp (as is true of most CB/11M amps) was designed for Class A (almost NEVER!) or even Class AB operation.  If you are in luck, it MIGHT be biased for Class B operation, which would still lead to higher levels of distortion than desired (or possibly exceed FCC standards) but might even be operating Class C, which isn't linear at all, but that meant (means) little to those that sell CB amps.

    In short, sweep tubes, especially in parallel, are not well suited for 6M operations, and the efficiency will be poor.

Steve Katz:
Couple problems with the "CB" amps:

As Dale mentioned, parallel capacitance from using multiple sweep tubes can be a huge problem, prohibiting getting the tank circuit to resonate as high as 50 MHz.  It gets to the point where *no* amount of "reduced inductance" in the plate tank coil is enough, you'd need to put the coil inside the tubes!  The "minimum" plate TUNE capacitance of the pi-nets in most of these amps isn't small enough to work at 50 MHz, at least not when you're using paralleled tubes.

Another issue is that these amps were all designed to work with about 4W RF drive, so you're right: You could easily overdrive them to destruction with 12W PEP, unless you "pad" that 12W down to 4W permanently.   And of course "RF sense" keying for SSB work isn't very good.  That circuitry depends on a constant carrier to work properly, and the amps were mostly designed for AM use back then (with sweep tubes).  Might work okay for FM on 52+ MHz, but for SSB or CW, RF sense keying is a loser; you'd at least want to change that to some "hard key" method.

Add to this the fact that nowadays sweep tubes are *very* expensive -- usually much more expensive than the value of the amplifiers they're in.  I see most "sweep tubes" sold are NOS (new old stock) and go for $25 to $35 per tube -- about ten times what they originally sold for back in the 60s or 70s when the amps were designed.  If you have to buy 3-4-5-6 of them, it's a pretty major investment.

It'd be easier, cheaper, more efficient and better overall to just build an amp from scratch using a single 4X150A or something.


Larry J. Rolewic:
Steve and I are pretty much in agreement.  You would probably be much better off rebuilding the "amp" around something like a 4X150/4CX250 tube, than the sweep tubes.  You would get better efficiency and less circuit loss (with more realistic circuit values for the output components) than with parallel connected sweep tubes, as well as significantly lower filiment power going up in heat!  Even with the lower plate voltage compared to the max for the 4X150, you would still be ahead of the game.
    But in a nutshell, the sweep tubes just are not all that good a choice esp. on 6M, and if you have to rebuild, or re-design ("MOD" to some people) the amppp to work on a different band, it's best to use the best possible components (and output devices) rather than try to kludge something together.
    Good luck.

Gene Brewer:
Thanks for all the inputs and ideas. I decided that given the amount of work and uncertainity in even being successful at the conversion, that I would go the solid state route. I have found a T E Systems 0510G 6M amp with 10W in 170W out and preamp. So when that comes in I'll just have to wire it into the shack 12V battery bank I run.

Thanks again for the helpful insights.

Gene KI6LO

Larry J. Rolewic:
Probably a wise decision; I doubt you will regret the choice,and it will get you on the air a lot faster!  Good luck, and hope to see you on 6 M.
WA9SVD (or as some insist, WA9SVD/6)  Have fun on 6 M!


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