Classic Hifi Care

Here is an interesting and informative film from the horde of films recently acquired by Periscope Films. It does a good job of explaining tubes and their applications.

https://www.youtube.com/watch?v=3lDEQNQdvB4

Thanks, Rex. If I live to be 100, I do not think I will understand how tubes work or any of our electronic hobby. I should have learned all of this 60 years ago.

Bill

Mr. Carlson's Lab on YT also gives pretty easy to understand explanations on them as well ...

Don't forget "Uncle Doug", also on U tube.

I get how tubes work but have uncertain understanding of the why and wherefore of circuits they're used in. I never got an education on the reasons why a particular circuit was designed to do what it does or why the exact values of components used in it were chosen. And at pushing 73 I find it harder to absorb new info, though I keep trying my attention span and memory are increasingly lacking. Decreasing motor skills from PMR and lousy eye function aren't helping much either.

I know enough theory to get by and no more. I'll always remember something that my high school refrigeration teacher told us: "you don't need to be an engineer to make repairs".

Ken Doyle wrote: ↑Sat Mar 11, 2023 4:54 pm I know enough theory to get by and no more. I'll always remember something that my high school refrigeration teacher told us: "you don't need to be an engineer to make repairs".

Me too, Ken, at least with tubes. Later solid-state stuff turns me into a simple parts-replacer. But for tubes I have a rough sense of what's supposed to happen, and when the schematic includes voltage readings, I can stumble through the diagnosis. Usually.

Last night I was reading the latest issue of the Michigan Antique Radio Chronicle. It has a regular feature involving unusual problems, repairs, circuits, etc. set in a fictional story about a repair shop in a small town. The problem-diagnosis stories always leave me humbled about my lack of knowledge. This issue addressed repairs from a 1930s British HMV "pre-selector" TRF set to a 1970s Yamaha receiver and a Sansui integrated amp. It's worth joining the Michigan Antique Radio Club to get the publication.

Chris Campbell

My step-dad, my first elmer, taught me to read a schematic. Then, he taught me that I didn't always need to know HOW something worked, just how to troubleshoot it enough to figure out why it didn't work. I struggle with oscillators and with the converter circuit in a radio. I watched an Uncle Doug video that straightened most if it out for me. I detest oscillator coils, messing with them and making repairs on them, but I do understand how they function a bit better. Most of the time, if you can read a schematic, you can figure out what the tube elements are doing by how they are wired in the circuit. Uncle Doug has good videos on phase inverters and tube biasing as well. I find Uncle Doug's videos more to my way of thinking. I find Mr. Carlson long-winded and redundant in many cases. I also like Shango and Bob Andersen's videos.

A good schematic folder will also have the working voltages noted. Measuring them will give indicators as to where a problem lies. It doesn't hurt to have an idea of the operation of the components you are troubleshooting. The novice can select the one or ones that show the way.

For making sure your tube (or SS) amplifier is doing its job, use more than your ears. I have used tone generators to isolate issues to speakers or the 2 or more amplification stages. Most of the time, I have luck just replacing caps and resistors.

A dual trace oscilloscope helps you compare what is going in compared to what is coming out. If you're lucky enough to have a distortion analyzer, it does the same thing with a meter. An 8 ohm / 20 watt resistor as dummy load is used in place of a speaker, and you measure/view the voltage across it. Fisher service manuals have a procedure to do this. Nobody wants to hear 800 Hertz at 20 watts 5 feet away

Push-pull amps (class AB) sometimes have bias adjustments for the output tubes, requiring only a voltmeter and screwdriver plus a good schematic procedure. As the tubes age, bias will need readjusted. Class A, single ended tube amps usually do not have any such adjustment. Any PP Zenith, Fisher, Pilot, H-K, etc will have them, as they drive their tubes a bit harder than RCA Motorola or Magnavox, who likely valued reliability more.

Tube oscillators and mixers are simple to check with a scope but as frequency increases, become very finicky and require a procedure be followed to make them track the AM SW or FM dials. This is always included with schematics, all mfrs did it slightly different, any Riders comparison will bear this out. IF amplifiers (class C) are THE place to start alignment to make significant gains in selectivity and tonal range. A mis-aligned radio can sound awful. Invest in both an AM signal generator and frequency counter if you're going to do this at all. FM and mpx decoder can also be aligned with an AM generator and live signal. FM stereo signal generators are very rare.

Its no accident transistors were used in audio circuits several years before replacing tubes for RF-IF, as transistors with very low input impedance required an interstage transformer to match impedances and also split phase to feed push-pull output transistors which in turn, had no trouble driving a low-impedance speaker. Car radios used them as output amps first! Look up Delco DS-501

Once tubes replaced RF and IF tubes, Philco was first to do this in a TV, even beating RCA by 2-3 years until their tuners/IFs went all SS. I have a 67 Philco 19" color TV and in the analog days, it was a DX wonder. Magnavox beat everyone using them for outputs as early as 1963.

I like both but my HS Vocational technical instructor taught us only transistors but he also covered all the possible circuits too, including voltage regulators. I actually learned tubes by accidents and fireworks then solidified my theory in post HS tech school where we had a few old timers teaching them, we built an all-tube, 20-watt HF 7.3 MC transmitter using a frequency doubler stage and TV color crystal. Sorry that there was no field testing unless you got a FCC license, which I did at age 18

Thanks, guys. I just wish I had the smarts to understand it all. I'm late coming to this hobby and my old brain has trouble understanding all of this. It has taken me lots of baby steps just to get down reading a schematic, understanding values, and trying to diagnose what's going on. Someday, with enough time, I hope to get further along with this hobby.

Bill

Remember to start at the rectifier cathode then follow the B+. If that's not right, you won't get far until you find the problem and fix it...

Tubes = high input impedance, high plate voltage and high plate load resistance. Requires a transformer to match output to a speaker.

Transistors = low input impedance, low collector voltage and low enough output impedance to drive speaker directly.

Hybrid = Interstage audio transformers must be used to match a tube's high impedance to a low input impedance of output transistors

A look at schematics on the Fisher Consoles for the A69 and W59 show how an SS amp is driven by tube preamp. (Thanks Jon!)

That was an interesting video, about vacuum tube theory. I learned all of that a long time ago and it was nice to get this refresher.