[TenTec] Centurion 422 Bias Circuit Solved

Bob McGraw - K4TAX RMcGraw at Blomand.net
Thu Feb 5 20:28:18 EST 2009

I took a look at mine again this evening in the CW mode.  Actually the amp 
doesn't care what mode is used.  I slowed the QSK Delay from the transceiver 
to better observe the condition of the amp during the "non character" mode. 
Otherwise what would be the normal RX period between characters if in full 
QSK.   During the non-character mode with the extended QSK delay time, as 
long as the transceiver is in transmit mode, although not producing RF, the 
amp in also in the transmit mode and idle current of about 150 ma exists. 
This equates to about 420 watts plate dissipation plus filament dissipation. 
The remainder of the time the dissipation is limited to the filament heat. 
I consider 100 to 150 ma of plate current normal for the mode, being SSB or 
CW, for an amp of this design.

 I do recall that some older amps, both Tentec and non-Tentec,  had idle 
current at any time they were turned on.  Yes, this produces heat and not 

The SN on my amp is 12A10065 and was purchased by me from Tentec.  My notes 
show was received 3/1/96.

Bob, K4TAX

----- Original Message ----- 
From: "K0BX" <k0bx at arrl.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Thursday, February 05, 2009 6:44 PM
Subject: [TenTec] Centurion 422 Bias Circuit Solved

> 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
> SSB.
> 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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