I have a very faint hum. I really like quiet radios. Filter Caps are fairly new 50 - 50 600V Can.

Are there any tips on reducing Hum? Component placement or the like. I have a Grundig and want to try and make it humless.. How can you remove hum almost totally?

THX

Not sure of the schematic for your radio, but if it uses a half wave rectifier, you could swap it for a full wave solid state rectifier (plus resistors to match tube resistance, and capacitors across rectifier to reduce switching noise). If the set is AC/DC, this would lift the chassis from ground reference at all times, but if insulated properly, this shouldn't be a problem.

You could add a choke in the power supply.

You could also swap the output transformer for a hum bucking transformer, if you can find one to suit your radio. A hum bucking transformer has a tapped primary. The tap is closer to one end than the other (not center tapped). The tap connects to the rectifier, the end closest to it connects to the B+ supply that feeds the rest of the radio, and the end farthest connects to the output plate.

I am not very familiar with the following suggestion, but perhaps someone more familiar with this can offer ideas. At any rate, perhaps a solid state voltage regulator capable of the voltages encountered in your radio could further reduce hum by maintaining voltage just below the point of ripple.

T.

I believe your Gundig has a full wave bridge rectifier. And it does have the taped primary on the output transformer. The Gundig is one of the most quiet in the way of hum. Being that the voltage select was to high, it may be now the other tubes may have some leakage now.
Try replacing the detector AF tube. You could add an in-line filter choke in the power supply, but don't think that's necessary. Your filter capacitors may also be starting to fail. Even if they are not that old.

If you have a capacitor analyzer that has a power factor measurement, that will indicate if your electrolytics have begun to develop internal resistance, which lowers their filtering capabilities.

T.

In one of my radios I have seen the use of a resistor as a buffer after the rectifier, with no filter capacitor prior. This works fairly well, and is sort-of on par to 'input choke filtering,' but is more wasteful, but with the trade-off of more compact size. Select a resistor between 50 and 200 ohms that reduces hum enough without dropping voltage too much. Place the resistor at the rectifier, with no capacitive filtering prior. If space allows, use a choke instead.

If there is a capacitor across the output cathode bias resistor, removing this will reduce hum at the expense of reduced output, reduced bass response, and reduced overall clarity.

If you remove all tubes except the output and hum goes away, then the B+ feeding the other tubes may need more filtering. This is especially critical to the 1st audio, and there is quite a bit of room for additional resistance at the expense of voltage reduction, without appreciably reducing gain.

Negative feedback, if not already present, can reduce hum and greatly improve frequency response of the amplifier, and also greatly reduce distortion, though it also reduces gain a bit. .1 MFD through 100-200K from the output plate to the 1st audio plate does the trick. Reduce capacitance down to as low as .01 MFD if bass suffers greatly. Use two resistors in series instead of one, and connect a .001 MFD (or there-abouts) capacitor from their junction to B- to brighten up the high notes.

Another method calls for grounding one side of the output transformer secondary, and connecting the ungrounded side to the low leg of the volume control. Be sure that the secondary is connected in such a way as to not produce squealing. Feedback must be negative. This set-up affords negative feedback and low hum at low listening levels, and full gain at higher listening levels. If this arrangement develops parasitic high frequency oscillations, even when connected properly, shunt with the smallest capacitance necessary to B-.

T.