Electronics > Repair

Valve filament overload

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HMartins:
Hi.

I am trying to recover an old valve based equipment using RCA 6AQ5, 6U8A and 6X4. The equipment has a 110V main transformer primary but my country is 230V. Placing a proper capacitor in series with that primary I can reduce the primary voltage to 110V.

After heating, the equipment has a stable consumption but until the valve cathodes start driving, the voltages in the valve filaments tend to rise above 6.3V.

With the present in series capacitor the filament voltage rises to 6.5V them as valves start to drive, the overall voltage drops and the filament voltage falls to 5.3V.

My question is: is it dangerous for the valves if the filament voltages rise to 7.5 .. 8V for, say 10 seconds (the time the valves start to drive)?

Valve specifications state that voltage should not go above 10% but they may be talking about continuous operation.

It is the first time I touch valves since the 70’s :) As much as I remember old B/W TV valves used to be much more ‘overloaded’ during warm up period specially in series based heating circuits, but I am not sure.

Thanks for any help
H. Martins

TimFox:
It is always dangerous to go beyond the manufacturer's maximum values.
Typically, tubes were designed with a maximum heater voltage spec, although some were specified for current in a series string.
If you apply 6.3 VAC to a cold tube heater, the initial current (and therefore the power) will be much higher than the hot value after warmup.
Therefore, it can be useful to include a current limiting resistor or thermistor in the circuit to reduce that effect.
To simplify your circuit to one 6.3V/0.3A tube, assuming 60 Hz, it would be equivalent to applying
(230/110)x(6.3V)= (13.17 V) through a series capacitor to the heater, which when hot is (6.3 V/0.3 A) = 21 ohms resistive.
Doing a "complex" calculation, that would require 69 uF, for a total circuit impedance of 43.9 ohms (absolute value).
When the tube is cold, we can assume its resistance is very low, so the total impedance becomes 38.5 ohms (capacitive).
Feel free to audit my calculations, remembering that the total impedance is complex, and to scale it to your actual heater circuit at 0.45 A for three tubes.
Therefore, the voltage across the tube heater is very low, and the current is only 14% high, and the initial power is actually low.
With a "straight" connection to a filament transformer, rated at 6.3V/@>0.3A secondary, the initial power would be far higher with 6.3 V and a high current (>0.3 A).
The problem with your test circuit is that the heater voltage after warm-up is too low for proper tube operation:  it should be above 5.7 V.

Ian.M:
*DON'T* do it!   The heaters are being underun after the startup surge, which depending on circuit conditions increases the risk of cathode poisoning or stripping, and also results in a large shift in the tube operating characteristics.  Also the startup overvoltage drastically increases the odds of filament failure at switch-on - always the most stressful event for any filament. Heater voltages should be maintained within +/-5% of nominal for normal tube life and performance.

Get an autotransformer to feed the equipment with 115V (the nominal US mains voltage for most of the 'tube' era').  If your mains voltage is higher than nominal 230V, you may need to adjust taps on the equipment transformer primary if it has that facility, or buy an autotransformer with voltage adjustment taps.

Wallace Gasiewicz:
Is it possible that the transformer mains has a center tap?
If so you could change the primary input to accommodate the 230 volts.

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