• 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.

Heathkit w7-M Power Supply Sidebands

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A friend brought his Heathkit W7-M a while back for a look at D%, Etc. While we had the Spec'A hooked up I figured, why not a look at the PS sidebands on the signal. Refer to the attached.

I was surprised at the size of the 120 Hz, Etc sidebands on the One KHz test tone. The PS is a SS FWDoubler running into a pair of 150 microF Caps. No choke, but they have used a simple trick cct we resorted too at times, another 100 microF across the result.

The PT utilization is good in a FWD, so no problem there. But the other surprise was a rather large 60 Hz (power frequency), component greater than the 120 Hz. One would think symmetry would hold the 60 Hz down. Must be a problem with one cap in the doubler I think. But we had to cut it short, so don't know.

This all done using a PicoScope ADC-216 & Differential Probe set to x1000.

Information only.:)


  • Heathkit W7_M Power Supply Sidebands.jpg
    Heathkit W7_M Power Supply Sidebands.jpg
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:)I avoided ground loops in the test setup by using a diff probe. Always something to be aware of.

FWDoublers are seen by both source & load as FW. But half-wave doublers are seen as full wave to the source & half-wave at the load. This one is a FWD.

The PS sidebands in this one are easily the largest I've seen in a common hifi amp. A small choke of say 2H would provide a substantial improvement. But I guess the bean counters were busy doing there job.

Time permitting I will ask the proud owner to bring it back for another look. But unlikely before Winter.
The sideband result is not particularly surprising. To get a very rough estimate of the likely sideband level just add 20-30dB to the hum specification. This assumes a PP circuit, with simple PSU, so much of the hum comes from the output stage. The PP OPT will tend to cancel supply rail ripple, but it can't cancel second-order IM between supply ripple and signal. Feedback will reduce both hum and IM by the same ratio.

With a very low ripple PSU you get much less IM, but hum stays as it will still get in earlier in the circuit.
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