[AMRadio] Tube Tester Question (TV7D/U)
My mutual conductance tester TV7D/U has a gas test function. The way the manual explains its function is that the tube is set up with a fixed plate voltage, and when you press the Gas 1 button, a negative bias is applied to the grid, and you manually use the bias pot to adjust this voltage until the plate current deflects the meter to the first large division (10 on the 0-120 scale). Then, while holding down the Gas 1 button, you simultaneously press the Gas 2 button. This inserts 80,000Ω in series with the grid. If the tube is gassy, it will draw a small amount of grid current which will cause the bias voltage to decrease, due to voltage drop in the series resistor. This reduction in negative bias voltage causes the plate current to increase. The manual recommends that a tube be discarded as gassy if the plate current increases by more than 10%, that is to say, if the meter increases more than one small division. My question is, what if the pointer deflects downwards when the Gas 2 button is pressed? I see it deflect downwards far more often than upwards, and with some tubes the decrease is several small divisions. If the tube is free of gas, shouldn't it remain still? Nothing in the manual mentions the possibility that the plate current might decrease when the resistor is inserted in the grid circuit. There is no such thing as "negative gas" in a tube, but it would seem that if inserting resistance into the grid circuit causes the plate current to change in either direction, this could be a problem, since it is very common to have 80k or more resistance in the grid circuit of a tube type amplifier, particularly with R-C coupling. What could be the cause of a decrease in plate current, other than a small contact potential bias, which should be thoroughly swamped out by the external fixed bias in this test circuit? I am aware that sometimes a triode amplifier that is designed to handle a very low signal level, such as a microphone pre-amp, may have no cathode resistor, and depends on contact potential to generate a fraction of a volt negative bias to operate the tube. But in these circuit, there is usually several megohms of grid resistance. I always thought of this as a rather flaky way to generate the operating bias for a tube, but I am not even sure this has anything to do with the above described phenomenon. Don k4kyv __ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [AMRadio] BC-348-R HFO Cover Needed
Jim, You are correct about the grid cap on the 6K7. The person that changed the 6K7 to a 6AC7 was looking for higher front-end gain. To get it, he installed a shielded wire from the coil selector box through a hole in the small chassis plate next to the first RF socket, and from there to the G1 pin on the socket. Ray - Original Message - From: "Jim Tonne" To: "Discussion of AM Radio in the Amateur Service" ; Sent: Tuesday, February 09, 2010 7:42 AM Subject: Re: [AMRadio] BC-348-R HFO Cover Needed > > > >> the 6AC7 (9000 umhos) instead. >> Either one far outshines the 6K7 with a >> transconductance of 1650 umhos. I >> think I have a 6K7 around, and it would >> be interesting to compare > > I think the 6K7 has a grid cap on top of > the tube but the other tubes mentioned > don't. > > - Jim W4ENE > > > __ > Our Main Website: http://www.amfone.net > AMRadio mailing list > Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Post: AMRadio@mailman.qth.net > To unsubscribe, send an email to amradio-requ...@mailman.qth.net with > the word unsubscribe in the message body. > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > __ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
[AMRadio] The late Mike Hardie, VE7MMH/VA7MH - SK
The late Mike Hardie, VE7MMH/VA7MH - SK Dear Friends, I am Mike Hardie's daughter, Elizabeth, and I am writing to inform you of his recent passing. Please excuse this email if you have received it in error. I am trying to find and let his friends from his beloved "radio community" know. Should you have any questions, please don't hesitate to contact me at elizabethhar...@hotmail.com Sincerely, Elizabeth HARDIE, Michael Moran June 18, 1946 Feb. 7, 2010 On February 7, 2010 the world lost one of the good guys. Major Mike Hardie passed away suddenly while working in the British Virgin Islands. Born to Elizabeth Rose and John Alexander Hardie, Mike left his North Vancouver home to start a career as a fighter pilot with the Canadian Air Force. After thirteen years of beating the sound barrier on bases all over Canada, Mike left the Air Force and returned to his roots on the North Shore with his growing family. He spent the remainder of his career with the federal government as a civil aviation inspector. Mike's greatest joys were his wife, family and friends. He is survived by his wife and constant companion of forty years, Therese (Mayea), and children John (Jessie), Peter (Kirsti), Patrick (Lisa), Elizabeth (Dustin), Michael (Renee) and Lorraine. Mike could be found with a spring in his step taking his three grandchildren: Nathan, Noah, and Madeleine on walks to the park. A generous man who shared his talents for all things mechanical, Mike was often asked by family and friends to help repair cars, appliances, electronic equipment and home improvements. Whenever asked if he could help fix something, his answer was always, "Bring it over and we'll have a look". For the last decade, Mike was a Ham Radio and Morse Code operator. He enjoyed refurbishing radios from the 1940's and 1950's. His absence will leave a void in the airwaves. Mike exemplified integrity, wisdom, loyalty and hard work. He leaves a legacy to be treasured by all who knew him. A memorial service will be held in Mike's honour on February 20th, 2010 at 11:00am at The Church of Jesus Christ of Latter Day Saints 941 Lynn Valley Road. __ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
