• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Blues

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Having successfully built a number of Mullard 5-20 amplifiers, I had sufficient parts in the junk-box to build a Mullard 5-10 amplifier as well, which I did, simply for old times sake.

The chassis was duly punched, powder-coated and assembled. I even used a couple of tag boards which I had. My pride & joy was taking shape. Had no idea where I was going to use it when fininshed - cross that bridge later.

Amplifier finished, hiss from speakers, not good. Check all voltages, all good. Replace EF86, hiss gone. A valve producing hiss? Philips valve, at that. Oh well, hiss gone now. But wait - now I hear a strange pulsating noise, weird...

Scope everywhere, all clean, but a regular low-level 10mS pulse on the secondary of the output transformer. Obviouysly mains related (we have 50Hz mains here). After hours of fruitless checking, I am no closer to a solution. Amplifier works 100%, but the low-level pulsing is still evident.

Steps taken: replaced all valves, all other components brand new, but checked them for specs anyhow. Checked all wiring, again & again. Re-routed some components in case layout at fault. Switched off Flourescent lights, soldering iron, cell phone. Pulsing still there.

Today, after another session more or less repeating all the steps above, I found the fault:

The power supply of my cordless phone, plugged into the same power-strip, but completely forgotten about, obviously a switch-mode design. Tiny little phone, tiny little power-supply built into the transformer which plugs directly into the AC mains. Unplug transformer. Pulsing gone!!

Poured myself a stiff drink to celebrate while listening to the sweet, now clean, sounds of the amplifier.

It is just that type of thing that is so hard to track down and probably one reason I have less hair now. ;) It is great you discovered the source of the problem!

I learned the hard way that I can't have my soldering station turned on when testing amps on the same circuit (not even plugged into the same outlet). I finally put in a separate circuit for my bench stuff (easy because it is located 4 feet from the circuit breaker box).
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