SB-200???

[Kenneth G. Gordon]

On 19 Aug 2009 at 21:39, Paul Kraemer wrote:

> Working on a SB-200 for a friend (really)

Good for you, Paul.

> I only have the download copy of the manual

Buy a real one off eBay. They are cheap there and well worth having.

> and note the following 
> discrepencies:
> The grid resistors are 22 ohm 1w instead of 33 ohm 1 watt. One of them 
> appears original, one was changed.

As long as they are close to equal in value, that isn't too important. They 
provide a small amount of negative feedback making the amp more stable.

> The grid choke rfc2 looks "funny" to me. Too new and a 3 pi style. Measures 
> 2.5milli henry not 28 micro henry.
> There is no 33 ohm 1 watt in the antenna relay line and doesn't appear there 
> ever was
> There are a new matched pair RF Parts 572-B installed but I am taking this 
> carefully repairing the obvious first, like the HV meter resistor already 
> replaced once before and open alc bias resistor. The filter caps all esr 
> test perfectly ---surprise---but I'll be cautious with a variac first and no 
> tubes.
> Questions:
> Was there a early or late version for the 22 ohm 33 ohm grid resistor?

That wouldn't be surprising. Heathkit often made small improvements over 
the life of their kits.

> Is the bias provided by the 33 ohm in the relay coil circuit beneficial?

Yes.

> Is the choke in the grid bias critical?

No.

In addition to the other sites mentioned by a previous poster, I highly 
recommend the following site:

http://www.xs4all.nl/~pa0fri/Lineairs/SB200/sb200eng.htm

I think it is one of the best, although Measures' site gets into some of the 
technical aspects in greater details. And Rich is one heck of a good guy to 
boot.

Also, Rich's modification to enable full QSK for the SB-220 can be very easily 
installed in the SB-200, once you get your mind around what looks at first 
glance to be strange. It isn't, by the way.

Lastly, I have done some experimentation with various brands of 572Bs 
lately, and have discovered some significant things about them.

If you decide to use a set of Svetlana's, you will have to add a neutralization 
circuit. The Svetlana's are far more sensitive to that sort of thing than any of 
the others. Also, the Svetlana's LOOK much smaller than any of the other 
brands,and really should only be used in pairs. They are definitely, however, 
fully capable of the same, or more, power output as any of the larger tubes.

Secondly, I have had extremely good luck with the Chinese-made versions. 
The company that makes them is Shuguang, and the eBay seller of those is 
very reliable.

All of the original Cetron's (OEM) now available are at least 20 years old and, 
IMHO, their condition is questionable. Any you find should be checked 
thoroughly before buying them.

Carl Heuter, KM1H, who buys 572Bs regularly for the SB-200s he modifies 
for 6 meters has had good luck with both the Penta Labs versions, the 
Shuguangs, and the Svetlanas.

Svetlanas have not been made for a number of years, although there is a 
seller on eBay who has about 400 NIB left to sell. I have forgotten his call, but 
he is in Florida and has a K4 call. He is also very reliable.

One more note on the Shuguangs: I recently bought an SB-200 off eBay for a 
very reasonable price. The amp had been "modified" by a CBer. The finals 
were a pair of Shuguangs which had obviously been hammered pretty hard. 

The final tank coil had been scorched on that side nearest one of the tubes.

Yet, both tubes still show full power output in my other SB-200.

Amazing.

Ken Gordon W7EKB

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