Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: John on June 04, 2018, 02:00:30 pm
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So, after getting tired of plug/unplug during each phase of building or repair, I built a switched bulb limiter (against Sluckey's excellent advice some time ago). In an effort to keep me from making mistakes, I added LED ind. lights to it. Picture of schematic is attached. Now, for my question:
I have test points for inserting an ammeter. Could I put 1 ohm resistor across those TP's and use a volt meter instead, OR will AC current burn up that resistor? Or will it give a false reading due to the sine waveform instead of nice smooth DC?
I just realized after typing all this out that it should go in the TE forum, if a mod wants to move it that's fine. Thanks!
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You need a hobby! :icon_biggrin:
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Well, I did install those jacks!
http://loafinshed.com/PRRI/PRRI.html
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120v, let alone AC, isn't suitable for any LED I know of. There might be LED modules meant to run off 120v mains supplies... or you could use more neon (or similar) lamps suitable for mains voltage... just don't stick a 3v LED straight across the wires. The indicator's current will also affect the reading if you place them after the resistor, but you should be able to take this into account.
The resistor should work. Multimeters will read AC voltage just as well as they'll read AC current... some better than others.
Power in the resistor is proportional to the square of current. Mostly a problem for non-limited, power hungry amps. Easy enough up to 2a (240w), a 5w resistor will handle the 4w dissipation, if you want to handle more than that I'd move down to 0.1ohms.
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> 1 ohm resistor across those TP's and use a volt meter instead, OR will AC current burn up that resistor?
Same as DC current. 1 Amp(AC) on 1 Ohm makes 1 Volt (AC) and 1 Watt. Hoffman's 1r cathode resistor is rated 1W for many months, so would be ample for small amplifiers; may burn-up on large amps. 1 Ohm 10 Watts is good for 3+ Amperes and covers a lot of amplifiers. Obviously your meter must be switched to _AC_.
However, you need busy-work? Consider the low-price V/A meters. I use two to monitor my house power.
(https://i.ebayimg.com/images/g/QVoAAOSwIgNXn108/s-l300.jpg)
The selection is overwhelming. You *must* have "AC". (This will have a doughnut, onboard or separate. Ones with a big-bolt slab are DC and no good for you.) It must cover ~~120V (most go to 300V but some start at 200V and are not sure to work on 120V). Most read to the nearest Volt; we could wish for 0.1V but I can't think why it matters. You don't really need over 10 Amps but choices are mostly 50A and 100A, but with a 0.1A readout, and mine reads the 1.7Amps of my idle house pretty dang close.
Advice: buy extra. The first two I got, one had a bad backlight. Not essential, except I have two side by side so it looks "wrong". I fixed the backlight ($0.13/hour solder work) but went back to get two more for future spares; the price is stupid-low so why not?
Watch shipping. A few ship from the US. Most are booked slow-boat from China so at least 10 days and sometimes 30+ days. Even if you get and can decipher the tracking you won't know. I once got real-time tracking through China Air over the pole, landed at LaGuardia, and sat in Customs an unknown time before appearing in my box a week+ after landing.
https://www.ebay.com/itm/Digital-AC100-300V-0-100A-BLUE-LCD-DISPLAY-PANEL-VOLT-AMP-meter-With-CT/222613935522
https://www.ebay.com/itm/Digital-AC-100-300V-50A-Ammeter-Voltmeter-Blue-LCD-Display-Volt-Amp-Panel-Meter/222369627262?epid=581939448
https://www.ebay.com/itm/Digital-AC-300V-100A-Blue-Lcd-Dual-Panel-Volt-Amp-Combo-Meter-CT-110v-220v-240v/400569495312?hash=item5d43cd6f10:m:mlcUP5cureFzvMN3OMf7R9g
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120v, let alone AC, isn't suitable for any LED I know of. There might be LED modules meant to run off 120v mains supplies.
Yes, these are rated for 120 vac. The long barrel on them houses the resistor needed, I guess. This is what I bought:
https://www.ebay.com/itm/PL8B-120-Alpinetech-8mm-120V-AC-DC-LED-Metal-Indicator-Pilot-Light-Lamp-Wire/222104975015?ssPageName=STRK%3AMEBIDX%3AIT&var=520960376593&_trksid=p2057872.m2749.l2649
However, you need busy-work?
Nah, not that much :icon_biggrin: So far I haven't tested anything with more than 2 power tubes, so I'll just go with the resistor. If it burns, they're cheap.
Thanks guys!