40-100W bulbs are good to have for general testing. We start with a likely one, working our way up. 40W is good for source equipment. 60W is midway.
100W is more generally useful including mid size amplifiers, will still glow bright enough and limit if there is a hard short even in a phono.
But it may not save a fuse or will allow more thermal stress in non heavy loading equipment. The bigger the bulb the more nominal current it passes.
Half Voltage mains creates double the current in a transformer's primary for the same VA power. Primary fuses are stronger in USA spec amplifiers for that.
100W is more generally useful including mid size amplifiers, will still glow bright enough and limit if there is a hard short even in a phono.
But it may not save a fuse or will allow more thermal stress in non heavy loading equipment. The bigger the bulb the more nominal current it passes.
Half Voltage mains creates double the current in a transformer's primary for the same VA power. Primary fuses are stronger in USA spec amplifiers for that.
Really, the use of a dim bulb only adds uncertainty. The proper device is a variac (variable AC transformer). That way you actually know what voltage is being applied despite mains variations - and it's steady. It should have an ammeter so you can read current. Circuit conditions change current draw, which then varies the applied AC voltage.
There is but one reason people use dim bulb testers. They are cheap. That isn't a good reason to do things incorrectly. I have never once seen a tech who knows what they are doing use a dim bulb tester. Tried it, too many variables and questions.
With a variac you can slowly increase your applied AC voltage (knowing what it is) while monitoring current. This is better for equipment in case you have an internal regulator failure. A dim bulb tester in that circumstance creates damage. With a variac you can monitor regulator outputs and stop before you cause additional circuit failures.
Example. I just had a device with a power transformer clearly marked "120-240 VAC". Voltages were too high in the equipment. When I reduced the input voltage with the variac to the proper internal voltages, guess how much I was applying? 100 VAC, so a Japanese mains transformer marked as North American 120 VAC. It was then possible to run the equipment and check for further faults. This equipment had cleared other shops with a clean bill of health. It wasn't possible to change the primary except 240 - 120 VAC.
Same for setting bias currents. With a dim bulb tester, that will change the applied voltage which will in turn vary the bias current.
So using a dim bulb tester is not recommended practice by any equipment manufacturer. Try and get a warranty contract doing things that way!
There is but one reason people use dim bulb testers. They are cheap. That isn't a good reason to do things incorrectly. I have never once seen a tech who knows what they are doing use a dim bulb tester. Tried it, too many variables and questions.
With a variac you can slowly increase your applied AC voltage (knowing what it is) while monitoring current. This is better for equipment in case you have an internal regulator failure. A dim bulb tester in that circumstance creates damage. With a variac you can monitor regulator outputs and stop before you cause additional circuit failures.
Example. I just had a device with a power transformer clearly marked "120-240 VAC". Voltages were too high in the equipment. When I reduced the input voltage with the variac to the proper internal voltages, guess how much I was applying? 100 VAC, so a Japanese mains transformer marked as North American 120 VAC. It was then possible to run the equipment and check for further faults. This equipment had cleared other shops with a clean bill of health. It wasn't possible to change the primary except 240 - 120 VAC.
Same for setting bias currents. With a dim bulb tester, that will change the applied voltage which will in turn vary the bias current.
So using a dim bulb tester is not recommended practice by any equipment manufacturer. Try and get a warranty contract doing things that way!
Nice! I have an e27 socket on a plank 🙂
Dual button On/off thru bulb and bypass bulb. I also added banana females in series later to plug an amp meter to.
I mostly use it for quick testing of unknown trannys and faulty duts. Quick tell if smoothing cap has a short or so.
Yea, inrush current from big donuts can kill bulbs.
Cold bulbs has quite an inrush themselves, abt 15 times steady state. Wakes up sleeping electrons 🙂
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Hi Salas,
I'm torn on that.
A DBT can be used by someone skilled because they know what the characteristics are. Hobbyists often generalize far too much and will use a technique where it isn't appropriate. Plus they tend to deviate, thinking everything is fine as long as it functions.
I think, and it's up to you of course, initial power up with the DBT only. If it works without problems, connect direct and adjust from there. It would be very nice to know what the actual input voltages are, and you can infer that from the DC rails. I think if I recall you do have that check. Check while alignment is under way. All you need is someone using a lower power lamp, causing the voltages to vary. Then they connect it direct for use and it runs too hot.
I'm torn on that.
A DBT can be used by someone skilled because they know what the characteristics are. Hobbyists often generalize far too much and will use a technique where it isn't appropriate. Plus they tend to deviate, thinking everything is fine as long as it functions.
I think, and it's up to you of course, initial power up with the DBT only. If it works without problems, connect direct and adjust from there. It would be very nice to know what the actual input voltages are, and you can infer that from the DC rails. I think if I recall you do have that check. Check while alignment is under way. All you need is someone using a lower power lamp, causing the voltages to vary. Then they connect it direct for use and it runs too hot.
Hi everyone,
I am working on this phono amp enclosure; I want to make the enclosure in 3D print. 4 sides and a top cover and keep the bottom plate in aluminium—is it a good choice, or should the phono pre be in metal enclosures?
I am working on this phono amp enclosure; I want to make the enclosure in 3D print. 4 sides and a top cover and keep the bottom plate in aluminium—is it a good choice, or should the phono pre be in metal enclosures?
UFSP has strong on board shielding that can be friendlier to non all ferrometal boxing. But keep the power amp especially and the UFSP's own external PSU at a distance.
Ha! Salas is SO FAST. What he said—
but IMO:
ideally you want a metal enclosure—making a faraday cage...even more critical in a phono pre I'd imagine. I know from personal experience listening to the circuit with headphones directly connected to the amp—you can hear the white "noise" of the amp attenuate taking the lid on and off, and also moving the PSU closer or farther away from the amp.
but IMO:
ideally you want a metal enclosure—making a faraday cage...even more critical in a phono pre I'd imagine. I know from personal experience listening to the circuit with headphones directly connected to the amp—you can hear the white "noise" of the amp attenuate taking the lid on and off, and also moving the PSU closer or farther away from the amp.
Someone can also combine 3D print and inside surface copper foil tape dressing
My plan is to make PSU and UFSP in the same enclosure, something like this below.
and cover with the 3D-printed enclosure.
Will it be for MM or MC? MM is 20dB less susceptible to hum fields. MM and magnetically shielded small transformer can be viable.
Hi. Should this be taken the other way also, that it is unfriendly to ferrometal boxing? Ferrometal can block what alu and copper can’t, EMI from cables, transformers etc. Grounded alu, and better copper is effective against RF that tends to be everywhere in nowadays environments
Yes, for a phono preamp with so much low frequency gain I would definitely go for metal. The thicker the ferrous metal enclosure, the better the shielding will be. Thickness does matter, so heavier gauge metal is your friend.
Good test equipment usually has more than one shield barrier for low level circuits. Your build determines how good your circuit will perform no matter how well designed the power supply is.
Good test equipment usually has more than one shield barrier for low level circuits. Your build determines how good your circuit will perform no matter how well designed the power supply is.
”The thicker the ferrous metal enclosure”
Yes, and better even multilayer ferrous metal, like a tranny core with an oxide layer in between the sheets. Kills the magnetic flux effectively when grounded.
But yea, now we are in the higher leagues, but I would definitely metal box it.
Yes, and better even multilayer ferrous metal, like a tranny core with an oxide layer in between the sheets. Kills the magnetic flux effectively when grounded.
But yea, now we are in the higher leagues, but I would definitely metal box it.