|
Title: Viking Valiant and upward creeping plate current Post by: radionut on January 09, 2011, 05:30:50 PM I have noticed on my Valiant that my initial adjustments for the plate current (usually 300 mA) creeps up by 30-40 mA as I get into QSO'sre any thoughts? I typically retune as time progresses to bring it back down to 300 mA.
Thanks, Mike, KD8MVH Title: Re: Viking Valiant and upward creeping plate current Post by: Pete, WA2CWA on January 09, 2011, 05:49:08 PM Could be a tube going soft; could be a resistor heating up, breaking down, and changing value. If you're using a coax feedline, if could be there's moisture in the cable. Capillary action of moisture in the cable can cause plate current to move.
Title: Re: Viking Valiant and upward creeping plate current Post by: WQ9E on January 09, 2011, 06:26:39 PM Mike,
It could be a gassy tube, watch the finals in a darkened room and see if you see a blue or purple glow in one or more of the tubes. You will sometimes see a small blue spot on the glass itself with beam power tubes, this is not a sign of a bad tube. A good tube checker will also catch a gassy tube. This is also one of the symptoms of a 6146 that has been driven too hard and had the control grid damaged. 8 mils drive is all that is needed. The original Valiant manual called for 12 mils and a note was sent out to reduce this to 8 after RCA got hit with a large number of warranty returns. If it is a single tube you can narrow it down by removing one final tube at a time (make sure the plate connection is not touching anything and do NOT try this by leaving the tube in place and just removing the plate connection since it will take out the tube screen grid). In the CW position load the Valiant to about 1/2 its rated CW input and hold the key down for 20 seconds or so and see if you still get current creep. Title: Re: Viking Valiant and upward creeping plate current Post by: K5IIA on January 09, 2011, 07:14:53 PM mine was doing something similar and after a small fire i found out it was the padding capacitors for the plate tune cap heating up.
Title: Re: Viking Valiant and upward creeping plate current Post by: ke7trp on January 09, 2011, 09:26:04 PM Put it straight into a dummy load and test again to make sure that the antenna, Tuner, ect ect is not changing. I just watched a friend spent 2 days working on his valiant for this reason, Only to find out that a Door knob on his vert was changing value.
If it still happens, check to make sure the Grid is not changing. If not, Then its likely one of load caps like suggested. C Title: Re: Viking Valiant and upward creeping plate current Post by: kc2hgc on January 09, 2011, 10:15:39 PM Also check the bias to the finals, maybe the caps are on there way out... ???
Title: Re: Viking Valiant and upward creeping plate current Post by: W2VW on January 10, 2011, 09:44:45 AM BINGO AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
mine was doing something similar and after a small fire i found out it was the padding capacitors for the plate tune cap heating up. |