[TenTec] Centurion 422 Bias Circuit Solved

K0BX k0bx at arrl.net
Thu Feb 5 19:44:52 EST 2009

Last night I wired up the cable for my Icom IC-756PROIII to do break-in 
CW.  CW keying to the 422, 422 to the rig.

So let me try to explain what I was looking for.

1.  In SSB, if you key the mike with no audio, you get a plate current  
of about 100 to 150 ma, 1/2 way between 0 and .2 ma.  This is normal for 

2.  In CW, using breakin, the older models of the 422 would put the amp 
in transmit and draw the 100 ~ 150 ma when it was in between keying.  
Like you are waiting for the DX station to say QRZ again.
This would cause a lot of heat from the tubes.  See the QST Review on 
the 422 from 1991.

3.  What I found is the later,  models of the 422 that are greater than 
the one in the QST Review does not do this.  Instead there is no idle 
current in breakin during the pause for QRZ.
I guess I should have waiting to ask the question until I had the wiring 
completed.   I had this heat problem with my Drake L-7 and Ameritron 
QSK-5.  It would lock the amp in xmit and just go around it for receive 
but you
could see the 150ma on the plate meter.

The last question I had was about the serial number and the date it was 
built.  From what I have found is that S/N with 'A' in it was before 
year 2000.  The last number of the S/N is the year.
So mine would have been built in 1992 and was the 004th on of that 
production run.  Any ideas if I am close or not??

Thanks for all the inputs

Joe K0BX

Carter wrote:

>k0bx at arrl.net wrote:
>> I just received a 422 Centurion. My question is: How do I determine
>> if the newer Bias circuit is installed?? This circuit cuts off the
>> bias when the rig is in transmit but is in idle.
>I think you sort of answered your own question.... if it's in transmit 
>but idle, plate current should be zero. Going back 10 or 12 years ago, 
>my bias board failed and then there was about 300 ma of plate current in 
>idle. You could take it out of its case and check the revision suffix on 
>the board (see below). Otherwise, I don't believe there is an easy way 
>to tell.
>I had the original bias board that failed; as of ten years ago, I 
>believe TT  had a -B and then a -C rev of the board.
>Note: all of the above is subject to the vagaries of my memory.   <grin>
>> QSL review from 1994 S/N 10A10091 had the old bias circuit and they
>> indicated that the newer one had the modifiy circuit. My S/N is
>> 80A10042. But no date of purchase was indicated in the manual. Any
>> ideas? Any ideas on when the 80A10042 was built?
>Probably ask TT.
>It's a nice amp! Enjoy it.
>Carter   K8VT
>TenTec mailing list
>TenTec at contesting.com

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