determining an amplifiers current draw

[Toxophilite]

HI I want to figure out my stereo amplifiers current draw
Despite the two 7189 pushpull power amps and 7 preamp tubes the transformers seems to be holding up well
One of the reasons I'd like to know is because I want to put a 'sag ' resistor in series with the SS recitifier to drop the B+ down 40 or so volts. If there's amnother way to get a bit of a drop that would be cool too. I don't have to have a voltage drop. i jknow the Sherwood S-5000 ran 7189s at 455volts to get 20 watts a side, but I don't necessarily need to run them with that high a B+

Also it'd just be cool to know.


if I go here:
http://thesubjectmatter.com/calcptcurrent.html


and put in as much information as it lets me it gives me: (4 power tubes, 7 preamp tunes , SS rectifier etc.)
(However it doesn't have an option for a stereo amplifier and I'm not sure if it makes a difference)

Selected transformer voltage: 375-0-375
Calculated voltage at first capacitor (B+): 513.75V. Subtract 6V, if using choke.
Calculated filament current (typically the 6.3v secondary): 5.35A
Calculated current: 139.18mA at 21995R calculated load (10% plus factored in). Preamp valves current draw is estimated at typical 12AX7 max dissipation of 1.2W at 330V, i.e. 3.6mA per triode

[sluckey]

Set your meter to measure AC current. Connect the probes across the fuse holder lugs. Pull the fuse. Turn the power on and measure the current draw.

[Toxophilite]

Hmm I'm not having any luck with that?

[PRR]

> stereo amplifiers current draw

From the rectifier?

From the wall?

7189 data shows 400V 8K 24W needs up to 130mA per pair, so 260mA for four. That's probably the number you want for "sag". (I have no idea why that calc gets 139mA but for a 22k load.)

Total B+ current includes your little bottles but we rarely run them much over 1mA, so unless you have a large flock, ignore it.

A less-math way is to just throw 100 Ohms in and see how much it sags.

Wall-power: figure the plate side of a tube amp won't be much over 50% efficient (40% for self-bias). So 24W out needs 45W in. The big bottles eat 5W heater power, 50W. Twice for stereo, 100W. Round up 20% for small jugs and PT loss, call it 120W from the wall. You may find that a 1A fuse dies young and a 1.5A fuse lives forever. If you are on a generator: the 5W heaters will draw more currnet at cold-start, and this may amount to some over 120W for a split-second, but the plate power is zero at that moment. So a 120+W generator may do.

[Toxophilite]

Still not getting a reading off the fuse , despite having my electrical engineer brother in law on the phone.
However I was using the *&^*& calculator wrong and neglecting to input the R load in ohms
which gives me 285 ma. Much as you said.


Gosh! As S. Luckey said I hope my PT can handle it. Those hammond PTs are pretty robust looking , I guess I'll see what happens at rehearsal on wednesday eve.I'll bring a fire extinguisher and a back up amp.

[tubeswell]

Still not getting a reading off the fuse , despite having my electrical engineer brother in law on the phone.
However I was using the *&^*& calculator wrong and neglecting to input the R load in ohms
which gives me 285 ma. Much as you said.


Gosh! As S. Luckey said I hope my PT can handle it. Those hammond PTs are pretty robust looking , I guess I'll see what happens at rehearsal on wednesday eve.I'll bring a fire extinguisher and a back up amp.

Steve was saying to connect your ammeter in series with the  PT primary (mains side) winding by putting the meter in place of the mains fuse.


Another way would be to put a 5W 1k resistor in series with one of the PT primaries and then measure the voltage drop across the resistor, then divide that voltage drop by the exact resistance of the resistor to get the (average) current passing through the resistor.

[SILVERGUN]

Set your meter to measure AC current. Connect the probes across the fuse holder lugs. Pull the fuse. Turn the power on and measure the current draw.

This includes switching the meter leads over to the "A" or "mA (in your case, if you have this feature)" connectors

And then, don't forget to switch them back for any other readings

[jjasilli]

PRR> stereo amplifiers current draw From the rectifier? From the wall?


Exactly!  The question as originally stated is convoluted.  The total current draw of the amp, including heaters, is not directly relevant to the stated goal of dropping plate voltage -- Unless you want to use a bucking tranny, which is not a bad idea.  Just keep heater supply above 5.3VAC.


The total power consumption of the amp is probably stated on the back of the amp in Watts.  Since the amp when built probably expected a 115VAC, maybe even 110VAC if old enough, you can calculate the current draw with the Power Formula.


PET PEEVE RANT:  Post the schematic or at least identify the amp.

[Toxophilite]

This is a finished home build
Here's for your pet peeve!
This is the amp I've been building but it's sometimes a little hard to get answers on a specific topic amidst a build thread.
I think people knew what I was talking about.
I did have my meter in series as indicated, the leads on the meter moved and set to ac, but I'm going to check the fuse inside the meter.

[SILVERGUN]

Since you're taking care of pet peeves, I got 2 more dots for you to remove:

[jjasilli]

Nobody knew what you were talking about, even if they thought they did.  By stating " I want to figure out my stereo amplifiers current draw"  and  mentioning "Sherwood", the inference is that  you were re-building a vintage hi-fi amp for the purpose of hi-fi.  There is no indication that your are building a new guitar amp.  A picture (schematic) is worth a thousand words.


BTW: to measure current you need to set your meter to AC Current, not AC Voltage.  Many ordinary DMM's can only read in the mA range.  The whole amp, with heaters, etc., is likely to draw at least 1A or a lot more, which can damage the meter or at least blow its fuse. 

[Toxophilite]

Thanks jjasili
Yes my initial post was misguided, misleading and generally a tissue of misdirection.
My profound apologies for any confusion it might have engendered in you.

Yes, Ac current/ My meter has a 10 amp jack for the red lead for measuring current as well as .25 A jack for measuring ma


Thanks Silvergun I will correct my schematic


Thanks everyone!
I have a better idea now.
Off to play my nice stereo guitar amp!

[jjasilli]

 

[92Volts]

PRR's on the right track with the datasheet-- how much current tubes pull at full (or overdriven) volume depends on the tube and load impedance. Technically you'd want to draw loadlines to evaluate this. Good thing most manufacturers directly tell you.

If 260ma is a full-power spec for 2 pair of 7189s, 300ma sounds reasonable for overdrive volume, plus preamp tubes, plus a safety margin.

In PP class AB, full power current can be way more than idle so you'd need to play at high volume while measuring... if you wanted to actually measure.

... but you're unlikely to play full overdrive all the time, so you can also estimate and not worry much if it slightly exceeds the rating of your transformer. It will usually be much lower.

Which Hammond transformer are you using?

[jjasilli]

I'm not aware that anyone measures this stuff under full signal or overdrive conditions.  SOP is what PRR said. Just measure at idle. You can drop up to 50V B+ at idle with a power resistor w/o screwing up your amp's operation under full current conditions. 


I use an inductive power resistor to further emulate the reactance of a tube rectifier.  PRR's guesstimate of 100 Ohms is probably close enough for 4X tube current draw.  (If not, you'll then have enough info to use Ohm's Law to calculate a more precise value to your liking.)

If you drop 40V across a 100 Ohm R, that's .4A.  W = 40V X .4A = 16W.  So use at least a 25W R.  This is meant to be illustrative.  Do the W calculation with your actual values.

[92Volts]

I'm not aware that anyone measures this stuff under full signal or overdrive conditions.  SOP is what PRR said. Just measure at idle. You can drop up to 50V B+ at idle with a power resistor w/o screwing up your amp's operation under full current conditions. 

Except true class A (mostly single-ended) amps, idle current/power are lower and not predictably related to full signal. You could bias any (push pull) amp near 0mA, and measure almost 0 idle power... let's say 5x less than normal. With the downside of crossover distortion, it will output and consume similar power at full signal despite the low idle current.

There are ways to estimate full-signal power based on idle measurements, but I'm not sure that's easier or more accurate than estimating with datasheets, loadlines, or ballpark efficiency.

[Toxophilite]

My meter had a blown fuse, ...because I initially moved the com line (not through ignorance just a momentary thoughtless mistake..that's why there are fuses!)
Anyway with a new fuse in it it's measuring 444 ma which then goes up to around 600+ma when the amp warms up
This seems a little #*%^#*$^%#$(^& crazy to me


Oh and to clarify as to usage. I generally don't want to drive my amps into overdrive at all.

[PRR]

> 444 ma which then goes up to around 600+ma when the amp warms up
This seems a little #*%^#*$^%#$(^& crazy to me

WHERE?? (I'm still convoluted.)

If the DC B+, it does seem huge for a 7189 quartet.

If the 120V AC line, 53 to 72 Watts, which is not a shock for what amounts to a 48W amplifier working idle.

[Toxophilite]

Sorry .
Current measured at the fuse lugs with the fuse removed as S. Luckey suggested


the amp is biased to be running at 18watts on one amp (becasuse one tube is weaker) and 20 watts on the other based upon the Webber online bias calculator


Annnd more evil unintentional misdirection on my part, the amp is running Russian 6P14P-EV tubes which are considered by many to be the closest modern 7189 equivalent.  they are considered to be very rugged and allegedly handle high plate voltages. Endorsed by Vladimir Putin himself for sheer masculine KGB ruggedness!


Data sheets, both of which list it as potentially producing 14 watts a piece
I should've asked this question on my build thread but it had seemed to become somewhat of a ghost town.


http://www.tubes.ru/techinfo/HiFiAudio/6p14pev.html
or here:
http://www.thetubestore.com/lib/thetubestore/Preferred-Series-7189-Data-Sheet.pdf
both list

[shooter]

The math to get from here;

If the 120V AC line

to here;

If the DC B+,

Is ~~~~~ 444 / PT step up factor?
say 4:1 would yield 111ma