2A3 filament voltage...

[Ivan303]

 

Getting a reading of 2.375V or 2.37 on two different meters reading EH gold grids in circuit and AT THE TUBE PINS, not the bottom of the socket at recommended in the links below:

http://www.emissionlabs.com/Articles/TECH-BULLETIN/TB-05-Heater-Voltage/TB-05-Heater-Voltage.html

http://www.emissionlabs.com/datasheets/EML2A3-mesh.html#Notes

Now normally I'm not gonna get to bent out of shape over a 5% low heater voltage as I have a couple pair (can't find the third pair I bought over a decade ago) of EH Gold Grid tubes and they aren't THAT expensive anyway.

As I am thinking on a pair of EML Mesh Plates at $800 a pair, maybe, based on the links above I should be a bit concerned?

AC line voltage here is 119 VAC and it does not vary more than a volt or two either way ever.  We have our own transformer at the street serving only our home.  Newer homes around here have to share but we have one just us. 

The Transformers in the MonAmours is strapped properly for 115-130 VAC so that should not be changed I'm guessing?

I see an 8R10? resistor in series with the filaments(heater)?  Any idea what I could change that value to in order to get 2.5V +/- a percent or two at the pins, in circuit, based on the fact that I'm currently reading 2.37V?

Thanks.

[Paul Birkeland]

What's the AC ripple riding on the DC voltage?  When I setup the filaments voltage for the Monamour, I took the baseline DC then measured the AC ripple with a scope to account for the extra voltage that delivers (that your meter can't pick up). 

[Ivan303]

Uncalibrated scope OK?  Ebay special and not very special!

Assuming we're talking mv here?  125mv and I got the equivalent of 2.5V DC+AC?

Have no idea how a digital DC meter works, let alone how it deals with AC riding on the DC level (averages?  RMS? Ignores it altogether?).

[Paul Joppa]

Unless the meter claims "true RMS", it most likely uses the average voltage of a DC+ripple voltage. Some oscilloscopes will calculate the rms value of a waveform.

[Paul Birkeland]

Have no idea how a digital DC meter works, let alone how it deals with AC riding on the DC level (averages?  RMS? Ignores it altogether?).

Yeah, I confess I'm in the same boat.  This is less of an issue with something super duper filtered and regulated like the BeePre/Kaiju filament board, or even the Stereomour DC setup.  With 2.5V filaments, the requirements for how clean the DC needs to be for the amp to be very quiet are pretty low, so we kept it simple for the MonAmour.

[Ivan303]

Unless the meter claims "true RMS", it most likely uses the average voltage of a DC+ripple voltage. Some oscilloscopes will calculate the rms value of a waveform.

Pretty easy as a DC volt meter 'should' ignore pure AC or report it as average which, for an AC wave form that spends equal time above and below the DC reference, should be zero.

Calculating RMS from Peak to Peak I can do in my head (*.7), if my ebay scope is any good.

If I can find about .125VAC (RMS) riding on that 2.375VDC filament voltage in circuit and at the pins, I'm think I am golden.

Unless I've forgotten something and that happens often when you last waved a scope probe in anger way back in the 70's!

[Ivan303]

And we're good.

Turns out I'v got 122VAC this morning at 11:00am (usually listen/work on things in the evening so doubt I've ever checked mains voltage in the morning.  Always read 120VAC and usually one lower or one higher.  Maybe just high when the sun is hitting the solar panels?

But...

In circuit, on the pins of the 2A3, I'm now getting 2.42VDC ( instead of 2.37) and am measuring .051AC (RMS?), and that's on two different tubes, one Russian and on Chinese, for a total of 2.471 volts across the heater pins on the tube and I am at +/- 1% and all is good.
 

[Paul Joppa]

I hate to say this, but the rms value of (DC + AC) is the square root of (DC squared plus AC squared).

[Paul Birkeland]

In circuit, on the pins of the 2A3, I'm now getting 2.42VDC ( instead of 2.37) and am measuring .051AC (RMS?), and that's on two different tubes, one Russian and on Chinese, for a total of 2.471 volts across the heater pins on the tube and I am at +/- 1% and all is good.

Is that AC measured with a meter or a scope?  This is definitely a tricky spot to measure, and that dropping resistor value and the actual cap we provide were changed a few times to get things dialed in pretty well.

[Ivan303]

I hate to say this, but the rms value of (DC + AC) is the square root of (DC squared plus AC squared).

If that's the case, 51mv RMS of ripple riding on the 2.41VDC adds nearly nothing or at most 10 mv?

[Ivan303]

Is that AC measured with a meter or a scope?  This is definitely a tricky spot to measure, and that dropping resistor value and the actual cap we provide were changed a few times to get things dialed in pretty well.

Both AC and DC with a meter as my scope is uncalibrated.  On the pins of the tube with tiny probes while under power.

But, as the voltage range of the transformer strapping I have mine set to is 'more than 115VAC and less than 130VAC', and there is no real regulation in the 2.5 VDC supply, some serious variation should be expected. 

So, if one is really serious about 2.5VDC +/- just a couple percent, not 5% as was the rule in olden times and if one can keep their OWN AC line voltage at a reasonable +/- from a given number (say, in my case 120VAC) than it looks like maybe custom selection of the value of the current 8R10 3W resistor might do the trick?

Looks like its a high quality part (Mills?) and maybe even a high precision part?

I'd be more than happy to acquire a few and willing to futz around with it if you could give me some guidance as to where I might start to pick up just .1VDC or an increase in DC voltage of about  3%?

Thanks

[Paul Joppa]

That's the way I read it. But as PB says, it gets tricky when you try to get precision measurements at these voltages and currents!

One thing to check is the parts connections - mechanical as well as solder. The capacitor DCR plays a role, as do diode and resistor lead resistances, so connections with as much physical contact before soldering are desireable.

Did I mention temperature? As the  amp warms up, the power transformer wire resistance will increase (reducing voltage) but the rectifier voltage drop will reduce (raising voltage). Hard to predict which one dominates, but make sure the amp is at operating temperature when making these measurements.

 By the way, the series resistor is 0.1 ohms, not 8.1 - there's a slash through the zero to distinguish it from the letter O. Sort of an engineering-geek font. You could get some more and parallel the one that's there and measure the change.

[Paul Birkeland]

I do remember starting with 0.08 ohms and that being too low for what I measured.

[Ivan303]

That's the way I read it. But as PB says, it gets tricky when you try to get precision measurements at these voltages and currents!

One thing to check is the parts connections - mechanical as well as solder. The capacitor DCR plays a role, as do diode and resistor lead resistances, so connections with as much physical contact before soldering are desireable.

Did I mention temperature? As the  amp warms up, the power transformer wire resistance will increase (reducing voltage) but the rectifier voltage drop will reduce (raising voltage). Hard to predict which one dominates, but make sure the amp is at operating temperature when making these measurements.

 By the way, the series resistor is 0.1 ohms, not 8.1 - there's a slash through the zero to distinguish it from the letter O. Sort of an engineering-geek font. You could get some more and parallel the one that's there and measure the change.

Yep, eyes are getting old.  0R10 3 Watt it is. And looks like Mills sells 0.1 5W that looks pretty close to what is in there.  Plus lots of sizes like 0R12., 0R15 and 0R18 which can be paralleled up for an endless combination of resistances, on of which might get us closer to 2.5VDC.

And yes, first measurement was 2.37VDC in the evening after on all day and the other measurements(more like 2.42VDC) were in the morning with only 15-20 minutes of warm-up BUT then the line voltage was different in the evening vs. the morning by about 1-2 percent as well so... Fortunately the range of line voltage in that room is 'only' from 119VAC to 122VAC but I'll start keeping better track of that as well

Just got to get it closer to 2.5VDC on average and if it goes a couple percent over to 2.55VDC or a couple percent below to 2.45VDC from time to time we can call it good!

[Ivan303]

I do remember starting with 0.08 ohms and that being too low for what I measured.

So I plan to buy about 4 each of the 5W Mills in 0R10, 0R12, 0R15 and 0R18 and fiddle around a bit, paralleling them to obtain various resistances close to 0R10 but a bit less and try to get the voltage to 2.5VDC or thereabouts and see if it holds. They are 'relatively' cheap at Prats Connextion.  That's as long as I can keep my nose out of the Teflon Cap department.