I don’t mind saying that I love the sound of tube amps, but the volts inside them terrify me. I have a rudimentary understanding of how they work, but I am not DIY when it comes to construction. I prefer to leave that to the experts. But I recently bought some 'upgraded design' Chi-Fi mono 300B SET amps and something has thrown me…on the back of the units is a pot for regulating the filament voltage. All the manual has to say about this knob, translated from simplified Chinese, is: “Note: Filament voltage balance adjustment knob. Tube parameter discrepancy can be adjusted to improve signal-to-noise ratio.” I assume this pot adjusts the AC voltage on the 300B heater, and a bit of research suggests for the 300B this should be ~5V. It has been suggested that hum can be suppressed by adjusting the heater voltage. Am I on the right track? Should I play with these knobs (one on each channel) and let my ears do the adjustment? Or is this more about compensating for the mains supply voltage variation from 220VAC – the manual says “The power supply voltage must be within 5% of the specified voltage.”
I’ve got a DVM and was thinking about how I should measure this voltage. Should I power up the amp with the 5U4C rectifier and 6H8C input stage installed, but the 300B removed, and measure the voltage across pins 1 and 4, or is this dangerous? Alternatively, to measure under load (and fully up to operating temperature) I have considered wrapping some thin wire around pins 1 and 4 of the 300B before inserting them, and measuring the voltage in situ.
I don’t want to try any of this until someone tells me I’m a bloody idiot and should leave well alone, or otherwise…
Thanks, Rowan
I’ve got a DVM and was thinking about how I should measure this voltage. Should I power up the amp with the 5U4C rectifier and 6H8C input stage installed, but the 300B removed, and measure the voltage across pins 1 and 4, or is this dangerous? Alternatively, to measure under load (and fully up to operating temperature) I have considered wrapping some thin wire around pins 1 and 4 of the 300B before inserting them, and measuring the voltage in situ.
I don’t want to try any of this until someone tells me I’m a bloody idiot and should leave well alone, or otherwise…
Thanks, Rowan
