[1000mp] HEATHKIT SB-200 vs YAESU MUSEN FL-2100B

[Tom Rauch]

> I have both the SB 200 and SB 220 here, and they both work well with
> my Mark V...Do not exceed about 500 mils grid drive on the SB 200, and

Ouch! That is far far too much grid current!

The grid current should be 100mA or less on a pair of 572's, and 
250mA or less on the 3-500Z's. 

having more grid current than that is a sign of light loading, which is 
by far the largest cause of arcs in tank components.

> within mine...and it is stock...I can not say the same for the SB 220
> with a pair of high mu Chinese 3-500ZG tubes...I will publish an
> article about my experience with these higher gain tubes, in the
> original circuit...and what it takes to calm it down..

The reason you probably though you had high gain and instability 
is the Chinese 3-500Z tubes are not pumped down well. 

I see hundreds and hundreds of tubes, and virtually 100% of the 
Chinese tubes have internal voltage breakdown problems because 
they lack a full vacuum. They also have poor metal-to-glass seals, 
and tend to have an abnormal amount of "leakers".

The slightest amount of gas in a tube greatly reduces voltage 
breakdown. A typical Eimac 3-500Z used to hold off nearly 15kV of 
dc potential, while most Chinese tubes I've seen were lucky to hold 
off six to eight kV.

Since peak anode voltage reaches nearly twice the dc anode 
voltage, the tubes live right on the edge of failure. If you underload 
or overdrive the PA, the peak anode voltage can climb much higher.

The result is an arc or a "big bang" often wrongly attributed to a 
"parasitic".

Russian 572B's were similar, they failed at about 5-6kV. 

I'm working now with a Chinese manufacturer of 572B's, and they 
are testing the 572's at 1600 volts and at 120 watts output! 
Approximately 90% of the tubes fail the same testing a Cetron tube 
passes. Put them in your FL2100, and "bang"!

The problem is not the "gain", but rather the lack of proper 
manufacturing and testing at the source.  
73, Tom W8JI
W8JI@contesting.com