Bush VHF64

[Refugee]

Some single ended circuits have an output transformer with a bucking coil on them.
You will need an output valve that matches the transformer and the consumption of the other stages of the set to get hum free operation.
The theory relies on there being some ripple on the anode of the output valve being cancelled out by ripple on the supply on the third primary terminal that carries the power to the remainder of the set.
Do you remember energised speakers?

[Murphyv310]

I bought a VHF64 last year. In terms of sensitivity and selectivity I'm well pleased with it however the hum level is higher than what one would expect from this quality of receiver.   The primary of the output transformer of the single EL84 output valve is supplied HT direct from the reservoir capacitor.  This would be in order with a push-pull output stage because  the ripple is cancelled out but not with a single valve output stage.  Your comments about this would be welcome.
It's worth mentioning the Mullard 3-3 amplifier uses a similar HT smoothing arrangement.

Geordie McBoyne.

Hi.
In the 3-3 the EZ80 output from the cathode is fed via a 560 ohm resistor then to the 50uf reservoir cap. Although 50uf is the valves maximum capacitance the 560 ohm has to be a belt and braces approach. with 50uf there would be very little ripple, the 3-3 doesn't have a hum bucking transformer though.

[boater sam]

There are other causes of hum besides ripple on the HT to the output valve from the feed through the transformer.
EL84 can and do cause hum all on their own!
Many makes and models of sets feed the output off the reservoir capacitor.

[Murphyv310]

Hi.
Yes I've had hummy EL84's & many other valves. The 3-3 is quite amazingly hum free even with a really efficient 15 inch speaker and good cabinet it is very hard to detect hum.

[Old Sparky]

A recent thread over on UKVRRRDF concerning the Revelation 'Silvertone AP15' record player, caused me to check out the circuit, especially as I've owned a 'Silvertone AP15 for many years. Despite my having replaced the reservoir & smoothing caps. some years ago, there is still a small amount of hum present, so I'm wondering if the ECL86 valves used could be responsible. Trouble is NOS ones seem to be around £50 or more a pair, so I won't be changing the valves in mine!!

[ppppenguin]

Remember that a small amount of hum was commonplace on many valve radios and grams. Better designs such as the 3-3 (and Quad/Leak/Radford of course) have very little hum.

[Murphyv310]

It would be an idea to up the smoothing capacitor value a bit after looking at the HT line on the scope if the ripple is excessive.
The reservoir capacitor could be increased too up to the maximum value quoted for the rectifier valve and could be above that value if a series resistor was added after the cathode and prior to the reservoir capacitor. Also choke HT filter is better than a resistor.

[ppppenguin]

Also choke HT filter is better than a resistor.

Absolutely true but chokes are big and expensive compared to Rs and Cs. That may not have been the case in the 1920s and early 1930s when caps were big and expensive but it's certainly been true ever since.

[Geordie McBoyne]

Reference to the attached circuit diagram of the output stage of the VHF64.
It's possible that the components R40 and C70 form a hum cancelling circuit. A small amount of 100Hz ripple is fed back to the cathode of the EL84. Because of the presence of the cathode bypass capacitor and the values of the feedback resistor and capacitor the amount of feedback will be very small.   The reactance of the cathode bypass capacitor C69 at 100Hz is 31ohms and the 0.2mfd feedback capacitor C70 is 8000ohms.

Geordie McBoyne.

[Refugee]

That is an example of a hum bucking capacitor. Well spotted.