Sb-220 power output at about half..

Thu May 24 12:29:24 EDT 2007

On 24 May 2007 at 22:33, John wrote:

> I will double check the plate caps on the tubes. These 3-400z's are
> testing full output for me in another amp. In this amp I was getting
> 800 out with 100 in until the condition which I described took place
> and the power is reduced to half it seems.

It sounds to me as though ONE tube is NOT working. Something small 
which you have overlooked is no longer connected. Possibly the 
cathode connection to one tube. Or perhaps there is something screwy 
with the grid connection of one tube. Perhaps the 117 VDC bias is 
staying on that one when it is supposed to be 5 VDC. 

Having only one tube working in a circuit designed for two would make 
major changes in the impedance, causing the change in the position of 
the plate tuning cap you have noticed.

I'll bet if you metered the cathode current of both tubes you would find 
one drawing rated current and the other drawing nothing.

Find what is causing that and fix it, and you will fix your amp.

Ken Gordon W7EKB

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