SB-200 users: a question.

[Kenneth G. Gordon]

On 19 Apr 2010 at 5:32, G. Lofstead wrote:

> How are you loading the sb200???

Normally. 

I.e., load to maximum output, then reduce the loading control a bit, 
keeping drive to a minumum, which in my case appears to be about 
90 watts. Grid current is below 100 mA, as it should be, according 
to the manual. In fact, you can adjust loading to the proper point by 
adjusting for MINIMUM grid current with all else remaining the 
same. I like 50 mA best.

> I have my Original 572's in my SB200 since the mid 1960's when they
> first came out!!

You must have gotten a good pair to start with: apparently I have 
NEVER had any good ones.

> I load to MAX power output and then the grid current is around the
> middle of the white "Grid" band...

Which would be about 50 mA if your metering resistors are still 
accurate.

> They are the original Certron tubes
> and still make around 650-700 W out..

As they should.

> I have been very careful not to overdrive the grids.

Good.

> If the plates are
> not showing color, you may be letting them get gassy... When the
> plates turn red, they absorb any gas in the tube.

That is not necessarily true: it depends on the KIND of gas, where it 
came from, and whether or not the "gettering" of the tube was done 
properly to begin with.

If the tube was not "gettered" properly, then, yes, running the plates 
red hot for a bit can help that.

Normally, 572Bs should show no color at full output.

At least that is what the tube manuals say...

Ken W7EKB

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