Check my mathsssss

[Amie]

Hi all.

Quick Q 

I've a little idea I want to try involving using a P series valve.

I have a measured 13 volt AC supply available in the equipment this is to be fitted to, am I right in thinking using a diode dropper I'll get the ≈9volts (equivalent power) that the heater in the valve is rated for? (13•0.707)

Read about this recently but not sure of the conversion factor

TA!

[ppppenguin]

From first principles.

With a diode dropper the power is switched on for half the time. So the heater or heater chain is run at half power. Power = V*I and V = I * R (ohms law) So for constant R, power is proportional to square of V. So we'll need to divide the incoming voltage by SQRT(2). Approx 1.414. Which is the same as multiplying by 0.707.

At these low voltages the diode drop will also be significant. So reckon on losing another 0.7V or so. You'll end up with about 8.6V on the valve heater. This is fine for a nominal 9V valve heater.

If you have a true RMS meter you can measure the voltage. Otherwise it's possible to calculate what an ordinary averaging meter would read but I really can't be bothered.

[Amie]

Thanks Jeffrey

This isn't going to be anything more than a short term experiment so should be fine.

Amie.

[ppppenguin]

Since P series valves are usually cheap and readily available it makes sense to arrange heater supplies for them. There are a few that are false friends. ISTR that PL84 is not the same basic valve as EL84.

A thought. If you wanted to use a PL84 instead of a UL84 you would need to ensure that any other valves in the 100mA chain were not overrun. I reckon a 4uF motor run capacitor would supply about 300mA to the PL84 heater. All sorts of other possibilities with transformer windings, rectifiers with smoothing, diode droppers or cap droppers.

[Amie]

Thanks. I had considered the PL 84 once before but was warned there wete differences. But then it is possible to buy reasonable quality new ones of the EL84 for around £10. P valves are a bit of a challenge but some are worth the trouble powering the heater. I've a guitar amp for instance that has a pair of PL36s for output. (25v heaters ?)Magic....if you remember not to buzz test the top caps with your finger

The reason I asked in this instance is that I was just sorting through some junk and found a vhf tuner adapter, I was investigating its insides and realised it was a little super regenerative circuit, something that I always find interesting, but it was sans valve. The connections seem to match up with the ECF80, which would be period correct @1955 by the date stamp on the "tin enclosure" though I'd only got the 30C1 here to hand, (I'm at a friend place...) It works! Sounds a bit muddy but its quite sensitive...

[Kalee20]

If you have a true RMS meter you can measure the voltage. Otherwise it's possible to calculate what an ordinary averaging meter would read but I really can't be bothered.

Cautious yes here!

The answer should be an emphatic Yes. But I have seen a few anomalies with so-called 'true RMS' meters, surprisingly by Fluke, where the AC ranges actually filter off any standing DC. If you stick across a 9V battery, for instance, the display flicks up and then settles at 0V. Whereas, a true-RMS meter should read 9V when connected to 9V DC.

Such a meter won't show the true RMS of a diode-rectified AC supply.

I do intend to check, and then challenge Fluke. But I need to get a round tuit.

[Amie]

I'd do that for you!

Trouble is my Fluke is two counties away ATM , this guy here only has an old Robin that must be 40 years old... its not true anything least RMS I don't think... My Escort one is though, I'll fish it out of the packing box try that and report ...

[Mark Hennessy]

In my experience, just about every RMS multimeter I've come across is AC coupled by default, so measures the AC component of the signal. Which is usually what you want IME. You can measure the DC component in DC mode, and sum them together as appropriate.

A handful of multimeters - usually quite posh ones - have separate AC+DC modes that do the maths for you. This is reserved for the more expensive models in a range for obvious marketing reasons.

That said, some of the very cheap Chinese multimeters that I've reviewed are not AC coupled when in AC modes. As a result, they get confused by DC offsets on the signal - something I noted in my reviews.

I think that if a multimeter does anything other than AC coupling in its AC modes, it should (a) make that very clear, and (b) give you the option of measuring just the AC component, and at the upper end of the price end, that's exactly what they do. If no such provisos exist, then assume AC coupling is present. That stood me well until those cheap Chinese multimeters arrived on the scene...

In my experience, if you approach Fluke, you'll be lucky to get a sensible answer at first, though if you persist, you might eventually get to speak to someone more technical. If they really don't want to answer the question, they eventually send you free stuff to make you shut up and go away. I got a baseball cap, a notebook and a memory stick, all with Fluke branding. I should probably put them on eBay one day...

[Kalee20]

In my experience, just about every RMS multimeter I've come across is AC coupled by default, so measures the AC component of the signal. Which is usually what you want IME. You can measure the DC component in DC mode, and sum them together as appropriate.

Thanks - it's not my imagination then!

The trouble with doing the summing is the DC component has to be added in quadrature to the AC component, the square root of the sum of the squares, so needs calculator or pencil-and-paper. Which is messy.

A moving-iron or hot-wire analogue meter will read true-RMS of course- if the AC component only is wanted, it is simple to connect a blocking capacitor in series. But a meter which claims to read true RMS, yet doesn't, is going to catch somebody out.

[Amie]

Well. The Escort says zilch!

In my experience, if you approach Fluke, you'll be lucky to get a sensible answer at first, though if you persist, you might eventually get to speak to someone more technical. If they really don't want to answer the question, they eventually send you free stuff to make you shut up and go away. I got a baseball cap, a notebook and a memory stick, all with Fluke branding. I should probably put them on eBay one day...

Not direct from Fluke but how about getting a free Fluke multimeter
The one I have is "only" an electricians 110 but was free!
It was left by a smart meter installation tech. at my great grandmother's house, the company was called and told but they said it wasn't worth the trouble getting it back.
I think that that model is about £150... If they had asked nicely we could have dropped it into a carriers agent and they could have had it returned for a couple of squid their loss!

A moving-iron or hot-wire analogue meter will read true-RMS of course- if the AC component only is wanted, it is simple to connect a blocking capacitor in series. But a meter which claims to read true RMS, yet doesn't, is going to catch somebody out.

After I had finished with the experiment I did try my "measure the space charge current" method against powering the valve with a regulated dc supply... The answer it gave was 8.6Volts for equivalent space charge, measured across cathode/grid,

Could have done that from the start ...