Is a 25VA steps really enough for a Millett

[regal]

People are building Dynahi's with 80VA transformers. Granted they run at a higher voltage, but the Millett has two tube heaters.

With diamond buffers the case gets hot, more than a few watts being dissapated. Also the regulator in the steps is taking at least 5 watts.

I venture to guess that a Millet with a 50VA transformer would sound better than these little 25VA steps.

 

[Nisbeth]

Quote:

Originally Posted by regal I venture to guess that a Millet with a 50VA transformer would sound better than these little 25VA steps.

I think that this is an entirely separate issue from the title of your thread.

If you use a larger transformer you will get lower load regulation/PSU impedance which may quite plausibly result in better sound, but that does not mean that 25VA isn't enough for correct operation. IIRC, the Millet draws around 350mA, which means that the power consumption is around 8.5W (or something in that ballpark. Even with a 50% total efficiency on the STEPS, 25VA is still plenty of power.


/U.

EDIT: Since the STEPS is a regulated supply, I would think the the effect of adding more transformer power would be smaller than if you are using an unregulated supply, i.e. for a power amp.

 

[tomb]

Tube Heaters:.2 @ 0.15 A ea = 0.30 A
Tube LEDs:.....2 @ 0.02 A ea = 0.04 A
Panel LED:......1 @ 0.02 A ea = 0.02 A
DB's:............4 @ 0.025 A ea = 0.10 A
............................................---------
Subtotal .................................0.46 A

Typical voltage for Millett = 27VDC @ 0.46 A = 12.46 VA

Regulator Loss, typical ~ 32V - 27V = 5VDC, 5VDC * 0.46 A = 2.3 VA.
Rectifier Loss, typical ~ 1.4V * 0.46 A = 0.65 VA

Total VA used ~ 2.3 + 0.65 + 12.46 = 15.41 VA.

Most recommend no more than 80% load or transformer saturation occurs, some say 50%:

15.41VA/0.8 = 19.27 VA absolute minimum,
15.41VA/0.5 = 30.82 VA max safety factor.

Based on these calculations, 25VA is perhaps adequate, but 30VA would give the ultimate in flexibility and reserve with a bit of penalty perhaps in rejected heat. The less loaded the transformer, the higher voltage (above its rating) it will maintain, thus burning more W/VA through the regulator/rectifier. So, there is little advantage to going with more VA.

One other thing this shows - a 500ma walwart is touch-and-go for a Millett with DB's.

EDIT: It's always nice when you do all the work and someone points out a stupid, silly mistake.

 

[AndrewFischer]

460mA, not .46mA

 

[regal]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Tube Heaters:.2 @ 0.15 ma ea = 0.30 ma Tube LEDs:.....2 @ 0.02 ma ea = 0.04 ma Panel LED:......1 @ 0.02 ma ea = 0.02 ma DB's:............4 @ 0.025 ma ea = 0.10 ma ............................................--------- Subtotal .................................0.46 ma Typical voltage for Millett = 27VDC @ 0.46 ma = 12.46 VA Regulator Loss, typical ~ 32V - 27V = 5VDC, 5VDC * 0.46 ma = 2.3 VA. Rectifier Loss, typical ~ 1.4V * 0.46 ma = 0.65 VA Total VA used ~ 2.3 + 0.65 + 12.46 = 15.41 VA. Most recommend no more than 80% load or transformer saturation occurs, some say 50%: 15.41VA/0.8 = 19.27 VA absolute minimum, 15.41VA/0.5 = 30.82 VA max safety factor. Based on these calculations, 25VA is perhaps adequate, but 30VA would give the ultimate in flexibility and reserve with a bit of penalty perhaps in rejected heat. The less loaded the transformer, the higher voltage (above its rating) it will maintain, thus burning more W/VA through the regulator/rectifier. So, there is little advantage to going with more VA. One other thing this shows - a 500ma walwart is touch-and-go for a Millett with DB's.

Love stats like this.

 

[tomb]

Quote:

Originally Posted by AndrewFischer /img/forum/go_quote.gif 460mA, not .46mA

No 0.46A, not 0.46ma.

 

[steinchen]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Tube Heaters:.2 @ 0.15 A ea = 0.30 A

with the heaters wired in series you need only 1x 0.15A for both tubes

to sum it up the Millett hybrid draws about 300mA @ 24V ( =~ 8W )

 

[amb]

My own measurement was 180mA DC current draw @27V at idle. With 33Ω load at maximum output (both channels driven), it increases to 240mA. This is with twin 12AE7A tubes, and including two tube-illuminating LEDs at 10mA each, and one BUF634P output buffer per channel with 221Ω BW resistors. Granted, this is not with the dDBs, but I can't imagine the dDB causing the current draw to almost double without also overheating its output transistors.

So, a 25VA transformer is more than enough.

 

[tomb]

Boy, that's what I get for trying to do a calculation.

 

[tomb]

OK - I did some measurements of my own tonight.

The Millett Hybrid Max at 27V has an inrush of about 600ma and idles at 430ma.

Unless I'm making another series-parallel addition mistake, the DB's on my Max are running less amperage (4 x 45 = 180ma) than a typical bias with the revMH DB's - 200ma (8 x 25ma). So, the only addition over and above a basic revMH with DB's is perhaps the coil load in the onboard e12 relay. Regardless, if we plug my measured 0.43A instead of my mistakenly calculated 0.46A into my original calcs, that still yields a VA load of 14.36.

Assuming the same loading range options, then:
14.36VA/0.8 = 17.95 VA absolute minimum,
14.36VA/0.5 = 28.72 VA max safety factor.

 

[blueworm]

I cant remember all the values.
My millet with discrete DB ran for almost a year @24v using a steps with 15VA transformer. No problems whatsoever.

I have since switched the steps to power a m^3, and replaced the millet with a SOHA.