Tap Cap?


2016-03-14 4:30 pm
Ok, so I have anther project on the go... well it is pretty much finished apart from smoothing around the edges.

It's a UY85 / UL84 combo, and is fed by a typical tap off the AC motor. The tap itself is running with a little over-voltage which once both heaters are warmed it is not much a problem, but when the amp is first powered on... it takes a few seconds for the UL84 to heat. In this time, the UY85 glows much brighter than I think it should but reduces it's intensity to normal once the UL85 is on.

My question is this... could I put in series a capacitor to try and take the edge off the initial UY85 heating? should this have the desired effect? and if so, what value would be ideal... higher than 130v of course, but with regards to the charge stored?

The aim again really is to take the strain off the UY85 heater for the initial UL84 heat up and assist with it's longevity.

Thanks in advance.
Replace the vacuum diode with a SS diode (say a UF4007), a resistor, and a NTC inrush current limiting thermistor. As you are dealing with a 100 mA. series heater string, replace the vacuum rectifier's heater drop with a 10 W. 390 Ω resistor and slightly account for today's higher average AC mains voltages.

FWIW, an Ohmite brand TUW10J390E is under $1, in the U.S. Perhaps Farnell or some other convenient U.K. distributor has an equally good value.
Yes you can. Remember:

Z= R + jX

Once you know R, the rest follows. It was done lots of times, the only thing is it causes deterioration of your power factor since only resistance does any useful work. Reactance doesn't.

No you can't, it is the different case, where when filaments are hot their resistances are higher. He needs to speed up heating of a thicker filament, not just voltage change.


2016-03-14 4:30 pm
Hi everyone,

Thanks for all the replies.

I did further reading and it seems a capacitor dropper is initially not possible on it's own due to a 90 degree phase shift, but may be suitable with an applied discharge resistor and surge limit resistor network, a link is below; -

UK Vintage Radio Repair and Restoration - Dropper Calculations

Also, in theory should you have a little over voltage from your mains transformer to a heater a similar dropper can be applied without using the capacitor but instead just a dropper resistor... is this correct?

Dropper resistor calculation - UK Vintage Radio Repair and Restoration Discussion Forum

Thanks again,
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