[Collins] 6146 testing in KWM-2

[Robert & Linda McGraw K4TAX]

Jerry's answer is absolutely correct.  A tetrode with no plate voltage but 
with screen voltage will draw excessive screen current with failure imamate. 
The suggested test procedure is not suggested.  As he said  "don't do it".

A transconductance tube tester is a better choice.  If this is not 
available, then operate the transmitter in tune or CW mode with only one 
tube in the PA box and note the output vs. input.  Cycle through the batch 
of tubes and select the closest matched ones for a "pair".

73
Bob, K4TAX


----- Original Message ----- 
From: "Dr Gerald N Johnson" <geraldj at ispwest.com>
To: <collins at mailman.qth.net>
Sent: Wednesday, July 12, 2006 10:43 AM
Subject: Re: [Collins] 6146 testing in KWM-2


> In the latest Collins Journal, there is a suggestion to pull the plate
> connection of one 6146 at a time to compare the tube currents for
> matching. That is a BAD idea because it doesn't isolate tube current and
> its abusive to the tube with the plate cap disconnected.
>
> When a pentode or tetrode is operated without plate voltage, the current
> that would have been drawn by the plate is drawn by the screen which
> doesn't have the power dissipation capacity. The cathode current may
> remain the same and the screen may well glow red. That's not its normal
> operating condition and isn't healthy for the screen. So the current
> reading is from both tubes, not a true indication of balance. Then
> reading 20 milliamps on a 300 milliamp full scale meter is fraught with
> large errors from pivot friction.
>
> Don't do it.
>
>
> -- 
> 73, Jerry, K0CQ, Technical Advisor to the CRA
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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