SB-220

[Kenneth G. Gordon]

On 30 Mar 2010 at 17:29, mtippin wrote:

> First one I've had like this, Shows Plate current about 300ma, 0 idle
> current?? Ideas?? Soft Tubes??

Hmmm....I am having a similar problem with my SB-200. In 
my case, it turns out that the grid circuit has too much 
series resistance, which reduces the idle current to almost 
nothing, and reduces the high final plate current at the 
same time.

The SB-200 uses a high negative bias to cut-off the final 
plate current when the amp is NOT keyed, and this bias is 
used to operate the antenna relay when keyed. That bias is 
removed from the grids when the amp is keyed, dropping  
bias to the correct -2 VDC.

In MY case, the high bias is, apparently, caused by high 
resistance in the MOSFET switch used in my Harback SK-
201 soft-key device, which should show zero ohms when 
"closed" but in this case does not.

I have not yet checked the grid and/or antenna relay circuit 
in the SB-220, but if it is of a similar design, something like 
that may be your problem.

I would check the grid bias voltage when the amp is keyed, 
but with no drive. If that is not at the rated level, then I 
would suspect high resistance somewhere where it 
shouldn't be.

BTW, this too-high bias also forces the amp out of the 
proper linear Class (AB1, AB2, B) into something far less 
linear and may drastically increase distortion products.

Ken Gordon W7EKB

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