McMichael Duplex Transportable 1934

[Alan Jones]

I have just completed a mains operated battery eliminator for this set and the set is now working but the audio is quite 'hummy' and 'tinny'. It sounds a bit as if it is a single sideband set.
The eliminator is purely a transformer with 3 output windings for HT, LT and Grid Bias. Each output is fed into a separate full wave diode arrangement with suitable smoothing capacitors, and the GB output is connected as negative and is variable to give -6vdc.
The voltages on-load are 110 volts, 2 volts and -6 volts. Currents are as expected.
Any help on why the audio is so noisy would be greatly appreciated.
I was surprised the set worked at all so it would be a shame to give up on it now.

[ukcol]

Hello Alan

Although you talk of "suitable smoothing capacitors" the smoothing for the grid bias in particular will need to be very good to give a low hum performance.

Perhaps you could try a 6 volt lantern battery or similar as a temporary test for the grid bias supply?

The "tinny" sound is not likely to be a power supply problem but perhaps a loudspeaker problem for example,

[McMurdo]

have you done any restoration work on the radio itself? Sometimes tying the 0V of a power supply to mains earth will stop hum

[Herald1360]

Have the bridges got suitable RF suppression capacitors across their diodes? The reverse recovery spike on slow diodes can generate a lot of RF hash modulated at mains frequency and its harmonics.

[PJL]

The grid supply does seem to already have some filtering so if a battery here does not help, try taking the LT supply from a battery. Also try changing the orientation of the transformer just in case it is inductive coupling.

[Alan Jones]

Hi folks

I will certainly try all these suggestions.

I have not done any restoration work to date on this set, but will try tying the 0V to mains earth and check all earthing arrangements on the chassis.

I have now inserted a resistor in the HT+ feed to reduce this to about 100 volts.

Is a smoothing choke still necessary in the HT section or is that 'old hat' with silicon diodes and modern circuitry?

The reactance control seems to be either on or off during its rotation at various points but has no 'smooth' control. Is that how it should behave or could it be faulty?

Thanks a lot.

Alan

[Herald1360]

The benefit of a smoothing choke is its higher ac impedance than its dc resistance, so you don't "lose" as much dc voltage as with a resistor for the same amount of ripple reduction. Historically, large capacitors were more expensive than a choke plus a smallish capacitor, nowadays the boot is on the other foot, so to speak so in principle at least you can just add capacitance to get the ripple as low as you like. In practice it's not quite that simple due to ESR and ESL and other shortcomings of real world capacitors. Wiring layout is also important- avoid any layout where a part of the wiring is common to both the feed and load connections to the capacitor- this effectively increases the ESR of the capacitor and reduces its ability to "short out" the ripple.


As an aside, output devices which are effectively current sources (like tetrodes or pentodes) are much more tolerant of ripply HT than ones which look like variable resistors (like triodes). The most obvious illustration of this is in radios with pentode output stages where the output valve HT feed is taken directly from the reservoir capacitor which may have several volts of ripple on it leaving the extra RC smoothing to get down to millivolts to supply only the low current front end circuits.

[Alan Jones]

Hi folks

I originally built this battery eliminator when I was still in full-time employment and too busy to perfect it. I am now retired.
I have now obtained all the components to rebuild it to an updated spec.(I have only retained the transformer which was OK), using the very useful information you have all sent me.
I did a quick check on the set itself and tightened many of the chassis screws. I also found a wire trapped (probably during manufacture - and in time dampening the input circuitry) between the chassis and the frame aerial woodwork, and since releasing it have found that the reactance control is now working better.

Thanks very much and I will keep you posted as to how it all turns out.

Alan

[Alan Jones]

I have now built the new battery eliminator including all the mods you very helpful people suggested and the radio is virtually hum-free and I can now select stations on both LW and MW at a reasonable volume and quality.
The speaker is slightly faulty (the moving coil is still slightly touching the surrounding magnet assembly - but I can live with that). There may also be issues with the valve biassing (faulty capacitors and/or resistors) which are preventing the radio being as good as it could be, but I do want to keep it original with no modern components so do not intend to do any more work on it.

Thanks for all your help.

Alan