SB-102 mic level

[Kenneth G. Gordon]

On 25 Feb 2009 at 19:58, Emil Dular wrote:

> Clear up my thinking for me if I don't have it right. If the problem
> were low meter reading rather than low audio drive wouldn't a
> wattmeter show me with unusually high output rather than about the 105
> watts I expect to see?

Well, on 80 meters, about all you can possibly get out the the SB-102 
with the stock (HP-23) power supply is 110 watts on CW or Tune, even if 
the mA meter was reading incorrectly. So, the meter could be reading low 
and you would still get 105 to 110 watts output, but not substantially more 
than that.

> 
> If instead of the 210 milliamps the meter says are going to the finals
> it is really 250-275 or more that I'd expect to plates in the finals
> to light up furnace red.

Well, only if you held the key down too long. At 250 to 275 mA, your 
power supply voltage would drop quite a bit...unless you are using a 
REALLY robust home-brew power supply. At 210 mA and 800 VDC, that 
is about 180 watts input, which is exactly what is specified.

> 
> Not to say the shunt resistors might not off, but wouldn't you see
> other symptoms.?

Not really. In my experience, the first thing that shows up if those 
resistors are off is low plate current readings. Remember, the bias 
voltage will be off, then, because the plate current will be reading low, 
and if the rig draws "50 mA" of idling current under those conditions, the 
final amp tubes will get a bit hotter than normal, since they would be 
probably be operating at class AB1...or maybe, if it was off far enough, 
even class A, or close to it.

Ken Gordon W7EKB

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