Hello, Let's say a radio is recapped correctly and plays well in all aspects. What might be some probably causes of a low level hum in an ac set- never had this in an ac/dc set. I know this is a vague question but maybe there are a few typical checks?

Hi Jim

It's usually electrolytic filter caps causing hum. Since you replaced caps check audio tubes for leakage between filament & cathode. Try other audio tubes. Is your 1st audio tube wires shielded. Some were, if replaced by regular wire will cause hum.

Does hum go away with volume turned down? If so the problem is before your audio stages.

Norm

:Hello, Let's say a radio is recapped correctly and plays well in all aspects. What might be some probably causes of a low level hum in an ac set- never had this in an ac/dc set. I know this is a vague question but maybe there are a few typical checks?

:Hi Jim
:
: It's usually electrolytic filter caps causing hum. Since you replaced caps check audio tubes for leakage between filament & cathode. Try other audio tubes. Is your 1st audio tube wires shielded. Some were, if replaced by regular wire will cause hum.
:
: Does hum go away with volume turned down? If so the problem is before your audio stages.

Hum does not go away when volume is turned down. It is a mild hum and it seems to be on sets that I bypassed a multisection e-can with several axial electrolytics. Maybe I am not aware of possible interference in respect to placement of components under the chassis. The hum is only noticable at low volume.

:Norm
:
::Hello, Let's say a radio is recapped correctly and plays well in all aspects. What might be some probably causes of a low level hum in an ac set- never had this in an ac/dc set. I know this is a vague question but maybe there are a few typical checks?

Is this an early 30's late 20's radio with directly heated tubes (without cathodes #26 and #45 tubes)? Many of those types of sets have a normal amount of hum (a little more than you will find on an ac/dc radio). There is a hum control variable resistor on the audio output stage on some of those radios. It is tied across the filaments with the center tap going to ground. If it has one adjust it for minimum hum.
MRO

Make sure, too, that you didn't miswire something. Be sure that the condensers you used are fresh, and that old ones are not still wired in. Be sure that cathode bypass condensers are fresh. A leaky old one can cause excessive hum. Replacement resistors of incorrect value can cause excessive hum. For instance, in an amplifier I built from an old schematic, there was a 50M by a resistor, but I couldn't tell whether it was an M or an ohm symbol. I put in a 50 ohm resistor. The resistor served as a filter of sorts for the pre-amplifier B+. The amplifier had rather excessive hum. Replacement of the resistor with a 50K resistor (50M in those days) cured the problem. The amplifier is now silent (Magnavox A-501B).

Also, be sure that pre-amplifier tubes are shielded properly. Sometimes replacing a glass one with a metal one can cure problems as long as pin 1 is connected to the chassis. Sometimes audio bypass condensers and wires can pick up hum, especially the one that usually feeds the grid of the 1st audio tube. This would pick up hum regardless of volume control position in most radios, as would a grid cap wire. You can wrap about 10 turns of enameled or insulated solid wire around such a condenser, or put a long spring around such a wire. Connect either to the chassis. This will shield the condenser or wire. Be sure that the spring does not contact any other wires or contacts, especially if mounted under the chassis. If used on a grid cap wire, be sure that it doesn't contact the grid contact. Crimp the spring so that it holds onto the wire's insulation just before the grid cap clip.

Be absolutely sure that if you are following the literal wiring, that someone else didn't mess it up before you. Be sure that the radio doesn't utilize a negative filtering system (field coil on the B- side). If it does, usually all of your electrolytic positives will common at the rectifier cathode, and will have the negatives tying off at various places along the field coil circuit. Reversing the procedure so that there is a common negative will ruin operation (may be impossible to do anyway).

If you did any work with the speaker, be sure that you didn't connect the hum bucking coil backwards (if one is present...field coil type speaker). Reversing the field coil leads can have the same effect as reversing the hum bucking coil leads.

Leaky and shorted tone condensers or non-electrolytic B current filter condensers can cause hum. Leaky or shorted audio bypass condensers can cause hum. Usually strong distortion will be evident in the audio.

I think that everyone gave a lot of the key areas to look at. However, if you still can't solve the problem, a model number will help.

Thomas