|
Title: Johnson Valiant Buffer (6CL6) Post by: AB3FL on December 06, 2010, 10:14:37 PM I have what may be a problem with my new toy. I have a valiant that has somewhat low buffer/grid current. With the drive control down all the way, grid current is about 1/2 what it should be. Also the 6CL6 tune has the blue glow inside and run HOT. I have to crank up the drive and this causes the 5763 to get a little blue and run blue. The current shunts are good and the voltages are good. I have 2 5763s and the both behave the same way. I ordered some 6CL6s and they should be here soon. Both tubes test good on my emissions tester. All of the voltages on the pins are good. Any ideas? Original PS with 5V4 and 866As.
Tom - AB3FL Title: Re: Johnson Valiant Buffer (6CL6) Post by: WQ9E on December 06, 2010, 10:40:12 PM Tom,
Have you tried multiple bands or just one? Can you try it with a crystal? Has the problematic VR tube dropping resistor been replaced in the VFO? If the VR tube dropping resistor has gone up in value your VFO output is probably down. Do the voltages at V3 match fairly closely what is in the manual (these are measured with the transmitter in transmit mode)? The 6CL6 does run hot but it shouldn't be blue; that usually means a gassy tube or one or more element voltages high. C96/97 (grid coupling caps) have been a problem in a couple of Valiants I have repaired. Title: Re: Johnson Valiant Buffer (6CL6) Post by: The Slab Bacon on December 07, 2010, 08:33:28 AM Definately beware of the postage stamp mica coupling caps between the vfo and buffer and the buffer and driver. I had the one between the 6CL6 and the 5763 go shorted on mine. This put the full buffer plate voltage onto the grid of the 5763. the 5763 turned red, then blue, then left for the happy hunting ground!! It was pretty kool to watch it happen. This is a rather common problem with Valiants and Rangers.
Also, be sure to check the plate and screen dropping resistors. Also check the 25k drive pot, they are known for going bad as well. Title: Re: Johnson Valiant Buffer (6CL6) Post by: AB3FL on December 07, 2010, 01:50:17 PM EVen in the crystal position, it glows blue. All caps checked out OK. The only voltage off on V3 is pin 3. It should be 130 and it's only 90. This is the screen grid. All resistors checked OK. I think the tube is just bad. Too bad I don't have a hicock tester. Oh well. I have 2 NOS 6CL6s comming in later this week to test with. The Chernobyl resistor has been replaced and moved outside the VFO compartment. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Tom - AB3FL |