Clip, clip, solder solder and we have a speaker lead. I found a vintage 1/4" jack in one of my junk...er...parts bins. Just the thing for a bit more vintage vibe.
On the right is my old Weston voltmeter that's connected to my variac.
This method seems to have its followers and its detractors. I like it for a couple of reasons: first, I can monitor the voltages and current draw in the amp as it comes up to full line voltage. If there's a problem, I'll see it at a lower voltage and have a better chance of shutting it off before there is damage.
Second, this method slowly charges the filter caps instead of hitting them with full voltage. While it is true that filter caps are rated for a specific voltage and it won't harm them to hit them all at once, they are a storage device, not unlike a battery. This method isn't about being safe - it's about charging. Charging them slowly helps extend their life. Also, I've found that in amps with caps charged this way seem to sound 'beefier' than amps I've just brought up right to full voltage.
The two arguments against this method that I hear are that the rectifier is not up to full voltage and therefore isn't producing B+ until the amp is running on about 70 AC line volts. This is patently not true - in fact, this specific amp had about 56 volts on the B+ line with the AC set on 40 volts, and continued to go up from there. Anyone who thinks there is no B+ until the line voltage is higher should try this method and see for themselves. Even on such a low AC voltage and filament voltage, a rectifier tube will produce DC.
The second argument is that this is "hard on the filaments" since they are running at lower voltage. This also is not correct. The Radiotron Designer's Handbook (4th edition), as well as Electronic Communication (the 1959 edition) state that filament and tube life is lengthened if the filaments are run on lower voltage. What is damaging is running filament voltage higher than the recommended value.
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