The Ubiquitous Radio Hum
 

I’ve become more discriminating about sound quality, and when I demonstrate radios for visitors they often ask, “What’s that hum?” So, I’m going back to four or five different sets to tackle the hum.

First question is, “What’s that hum?” 60-cycle or 120-cycle? I’ve tried to compare it with audio samples online, but I can’t trust my ears. Here are links to samples of hums from three of my radios. They all sound the same to me.

https://flic.kr/p/RP4HK9

https://flic.kr/p/RD4AT9

https://flic.kr/p/QCGqyn


This is the filter block for a 1931 Airline 62-141. I restored it to the original specs and configuration. An 8-mfd electrolytic as shown here. The filter block was untouched, and the schematic shows how it was wired. But I’m not sure about that blue disconnection--it was in some, but not all, of my notes on the restoration.

https://c1.staticflickr.com/3/2925/3...0076e896_c.jpg

Thanks for looking. If I can get this set fixed, then I’ll move on to the others.

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Hum is one of those mysteries that befuddle the best repairmen. My friend Walter, who's one of the best imho has had problems even he couldn't figure out. It it isn't really noticeable, I live with it.

Restored? Are your filter capacitors actually new?

However - mind you, I'm just beginning to learn about radios - I think old radios just hum. Especially AM.

There are several causes of hum in radios such as poorly filtered B+, capacitive / inducive pickup of the power wiring by adjacent parts on the chassis, coupling of hum from heater/filament to the signal elements of a tube, pickup of ambient hum (ever touched a phono input of and amp and heard the amp hum...Your body was acting as an antenna for 60Hz).

Isolating the main source is useful to fix it. pulling the plug to see if it is clear for the second or 5 that it runs off stored filament heat and cap charge will tell you if it is coming off the line or from the air. Replacing one on the power supplies (A or B) with a known pure DC source (like a battery) will help diagnose which supply is the issue.

If you ever decide to build a tube phono pre-amp from scratch you will be forced to learn these techniques...I helped a friend, who is a fellow engineer, make one. I gave him all the basic parts to make the circuit he found and a basic power supply and helped assemble the prototype...Some months later he gave it back because no matter what he did he could not get the hum down to an acceptable level...It was fun/educational whipping that thing into shape.

In vintage equipment some hum may be part of the design (especially cheap stuff and pre-octal based sets)...I look at it this way: if it is not easy to notice at normal listening levels then it is probably close to it's design standards.

My Zenith 11S474 was known to hum until it warmed up. Nature of the beast. Philco is free of hum. Obviously, I use it more.

I never seen a radio of that vintage that didn't have some degree of hum, particularly ones using direct-filament tubes (e.g., 26, 71A etc) with the filaments run on AC. Some had a 'hum balance' control but the hum was always there anyway.

Also check the DC resistance of those chokes - one with shorted windings will degrade performance greatly.

The winding just above the rectifier filament winding has a hum filter - the center tapped 15-ohm resistor. Check it as well - bad, and it could introduce more hum.

I wonder what would happen if you just rectified the filament current to DC? Assuming all else was working perfectly, would it reduce or eliminate the hum?

It doesn't matter. Anytime you have an oscillator you'll get hum. My friend Walter nearly pulled his hair out trouble-shooting a hum on an RCA stereo console. He could never find it: just mitigated it.

Dressing leads properly, 'specially in the preamp section, can help but I think a certain amount was non-mitigable.

By 'oscillator' don't you mean AC mains power?

What's funny is that I have an old Grundig Console Radio from the late 1950s that even though I replaced the old Filter Caps for the power supply the thing still has a hum to it which is weird because I always thought that the 60hz hum you heard on these old radios was due to failed power supply filter caps but apparently not so in the case of my old Grundig...:scratch2:

Looking at the sch, (assuming the 2 red "X"s are connected, elswise you would get nothing.. The "red" 8uF is in parallel with C16, and the input filter C14 is open. If you moved the minus of that 8uF from ground to the bottom blue "X", I think you will find the hum much reduced. C15 is likely to somewhat resonate the filter choke, also reducing hum. I'm guessing the 600 Ohm inductor is the speaker field, and the 70k Ohm heads to the output tube grid resistor for bias.

Half-wave rectifiers will exhibit 60Hz hum; full waves, 120Hz. Hum pickup from other sources in the radio, especially around the audio section, will be 60Hz. Sometimes lead dress has to be changed, getting sensitive audio wires tight against the chassis and away from AC lines. Sometimes replacing such longer lines with shielded cable, shield grounded, will help. First audio tubes often need to be shielded or should be a metal tube with pin 1 grounded, if an octal. Best to replace power supply filters with the same as spec'd. If adding capacitance to try to reduce hum, don't add it to the first cap right at the rectifier, but add it after the speaker field or the power supply resistor (assuming a capacitor input filter.) Adding too large a cap at the rectifier can look like a short to the tube on startup.