Later today my new $30 power supply is scheduled to land at my front door. The supply, a switcher, is rated at a wide voltage range input, and supposedly good for 6.3 Volts at 5 amps and 300 VDC at 300 milliamps. Targeted at vacuum tube audio circuits, it is touted as low noise.
Before I place all eggs in one basket, I thought it best to do some lab testing and this led me to thinking about test criteria and the actual conduct of the tests. I also generated a concerns list.
The first is the price and to put it in crass terms what would you expect from a $2.50 hooker? The price of $30 for the supply raises that same level of concerns.
Basically, we heard some good things about this switcher, but I do wonder if the glowing reports may belie that there have been known component failures in the supply. Gasp were those glowing reports AI generated?
If it fails at plug in, then we know the awful truth thus there should not be a mushroom cloud of smoke at the 1st application of power. That is followed by a 15 Minute test where the supply is idling (no load connected) where nothing gets really hot, or no burning odors are observed.
Now the big issue is how does this supply hold up under load. The current vision is that most of the load will be seen in the high voltage section and so we should look at some math.
There is also a power duty cycle factor to consider in that in receive we likely would see a constant load (9 tubes total) but it is in receive where only six are drawing current of less than 70ma. The max current draw is the PEP of the 12GE5 final on voice peaks and there we could see 200 milliamps or better total current draw on peaks.
Essentially 300 VDC at 0.3 amps is 90VA. (90 watts). If the load is a 1000 Ohm resistor, then the current is 0.3 amps. That resistor better be 100-watt rated! If the specs hold, then we should see no sag in the voltage and the current supplied should hold at up to 0.3 amps. I don't have such a resistor.
Now if we would theorize that even at 0.06 amp the supply would hold that would give us a really "good indicator". If it flunks the 0.06-amp test with a voltage sag or failure to supply that current level that is a sure sign it will flunk the max power test.
Two 10K resistors (20W) in parallel is 5K yielding 60ma with 300VDC applied and only needing 18 watts of power rating. The 60-milliamp current is close to the load that will be seen on receive. The 2nd part is holding that voltage and current over a 10 Minute period without sagging or smoking itself. This is a critical test as if fails this part ... it fails totally. The duty cycle with SSB voice is easily within the 0.3-amp range and transitory. It is the constant load on receive that is the most demanding.
Other parts of the testing would be to look at the ripple content and noise generation. A DSO will measure the ripple across a series of dropping resistors as you don't want to smoke the scope probes. For the noise I can use my Hermes Lite 2 SDR to look at the noise and evidence of switching transients across the HF spectrum
Now a concern that came up after the unit was purchased. The filament voltage is stated at 6.3 Volts at 5 amps. As a victim of habit, I just assumed that was 6.3 Volts AC, but that could be DC. I could not find that info from the product advertisement.
Perhaps a hidden bonus is to change over to all 6.3 VDC tubes (the 12Ax7 and 12AU7 can be wired for 6.3VDC) and this would mean I would power the filaments from this single supply and a smaller 12VDC supply could be used for the solid-state circuits.
So, DC would not work with my idea of back feeding a filament transformer for the Bias supply. I might have to add to the mix a small 115VAC to 115VAC transformer for a bias supply. eBay has them!
Things are certainly in a churn, and it is interesting of how other countries view the US. If you have any friends outside the USA, you might ask them. A majority of you voted for this option and was this your expectation.
Reduce your digital footprint as you are being watched.
Them that know can make things go.
73's
Pete N6QW