[Lawrence Austin]

Anyone ever had a 6Q7 cause the audio to fade out over about 4 sec then fade back in over about 4 sec? Darn thing did it intermittently quite a few times before dying all together. 

Had this for a few decades but never looked into it because it played fine. Never knew there was the shadow meter due to the dead bulb. Re-capped, some resistors changed, new tuner bank rubber mounts. Thanks to Tom Oppat a new volume control. Never played like this before!

https://www.youtube.com/shorts/_KgJQChPr...ture=share

[RodB]

That's a four second amplitude modulated signal.

[morzh]

Who's Tom Oppat? I know Mark...

PS. It does play nice.

[Lawrence Austin]

Mark Oppat! My memory isn't as good as it used to be. I should know not to trust it!

[MrFixr55]

Lawrance, at 71 years, I hear you re memory! Very nice looking, very nice sounding radio. Great job!! Was that an over the air station or 100 mW home broadcast unit? I have 2 transmitters, A vellerman kit, nice and simple but good circuit and a Talking House unit which am a little disappointed in. Noo good AM music stations in the NY Metro Area, so I go to the computer or the Music Choice audio programs from Cable. Sometimes, I pipe directly into the radio for less distortion.

[RodB]

At 71 you can hear distortion? My tinnitus cancels all the distortion.

[Arran]

I have had the plate load resistor on 6Q7/6SQ7s fail causing this very issue, they either change in value, going high resistance, or go open entirely, a number of Canadian RCA and C.G.E sets have this problem. Another thing that can cause it is the cathode bias resistor on the audio output tube, if the tube uses cathode, or self bias, a lot of current flow through there and they can drift as it heats up, I will have to look up the 37-630 to see if it uses that or fixed bias.
Regards
Arran

[Lawrence Austin]

MrFixer55, that was a Talking House transmitter going through a Vectronics antenna tuner. Seems to work better than the automatic internal tuner for 1620kHz.

[MrFixr55]

My tinnitus causes intermodulation distortion. Earlier this summer, I went to a concert at St. Agnes Cathedral (Rockville Centre, NY). All volunteer orchestra and the all volunteer Cathedral Choir performed Beethoven's 9th Symphony. They did surprisingly well! I also got to sing with the Cathedral Choir and a pro brass sextet for the ordination of three priests. There is nothing like live music.

Music, especially classical music is hard to listen to on AM. I wonder if some of this is due to overmodulation, distortion due to the carrier frequency being low enough to still be imposed on the audio after demodulation. (All the textbooks show the carrier and a sine wave at audio frequency before detection, and only the sine wave but with "saw teeth" after demodulation.) Before Armstrong invent FM, there was, for a short period of time, the "Apex" band somewhere around about 40 some-odd MHz, where the separation between stations was much wider than the broadcast or shortwave bands. I wonder what the fidelity was, in comparison to FM. If all else was equal (bandwidth, etc.), does FM provide better fidelity and lower distortion due to FM vs AM or is FM's only advantage freedom from static, lightning and other atmospheric noise that can amplitude modulate a carrier?

[RodB]

FM is capable of modulation to 15kHz (beyond my range) where AM is good to about 5kHz. But there are so many differences, pointing to any one is difficult. I'm sure Armstrong was just as fascinated as we are.

BTW in my senior high school Radio TV class we used to identify a television chassis by how strong the horizontal oscillator sounded. At 15,750 CYCLES we would zero beat two chassis sitting next to each other. Goofy kids!

[MrFixr55]

AM Good to 5KHz in the Broadcast band. Remember that for TV, the channel bandwidth is 6MHz and the Video Amp bandwidth is at least 4MHz and TV Video is AM.

Re Zero beating several TV chassis, "When Nerds break bad".