[Elecraft] Use of Heathkit SB200 amp

[N2EY at aol.com]

In a message dated 9/12/07 11:12:05 PM Eastern Daylight Time, 
ithink1 at charter.net writes:


> the Heathkit SB-200 has several
> modifications that can be made to it, one of which is the reduction of drive
> power to excite it.

What is modified to do this?

> 
> Does anybody have any idea what the lowest power required to excite an
> SB-200 amp is?
> 

I presume you meant "what the lowest power required to excite an SB-200 amp 
to full rated output is"

The SB-200 uses a pair of 572B triodes in grounded-grid. The data sheets list 
the 572B as needing 50 watts of drive in that service, so a pair will need 
100 watts of RF to drive them. 


> Without mods, it is around 100 watts, but I use to have one with the 
> reduced
> drive mods, and I drove it with 30 to 50 watts for around 300 watts out. 

The tube ratings (for a pair) say that 100 W of drive will give 600 W output, 
so 50 W of drive would give 300 W output. 

I don't see what could be modified to reduce the drive requirement for full 
output, except perhaps changing to grounded-cathode operation instead of 
grounded grid. But such a mod would be extensive, requiring neutralization and other 
changes.

 I
> 
> never tried testing the lower end with that mod. Though.
> 
> Bottom line is:  if an SB-200 had the reduced drive mod, could a QRP K2 at
> 10 to 15 watts with an amp key interface safely/adequately drive an SB-200
> to say, 50 to 60 watts out?
> 

Since the SB-200 is a linear amplifier, I don't see why not. Of course it's 
using a sledgehammer to drive a 3d finishing nail.... 

The problem is, how do you tune up the SB-200 with such low drive power?

---

I'm curious to know what is changed by the "low drive mod" for the SB-200.

Thanks

73 de Jim, N2EY


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