Hello, experienced,
I have this question because the stancor C1412 rated at 4H, 250mA, gives me a big hum at 220-230mA. It is from the choke itself, not from the speakers. I can hear the annoying hum from the other side of the room in a quiet night.
To make this amp quieter, and to plan for the next amp, I need to know what choke to order that will not hum. What is the head room do I need? for 230mA, do I need choke rated as 500mA? 400mA? 300mA? The only current product I know is Hammond. With what percentage of the rated current will the Hammonds start to hum? I need your experienced answer. Thank you!
yours frustrated,
vax, 9000
I have this question because the stancor C1412 rated at 4H, 250mA, gives me a big hum at 220-230mA. It is from the choke itself, not from the speakers. I can hear the annoying hum from the other side of the room in a quiet night.
To make this amp quieter, and to plan for the next amp, I need to know what choke to order that will not hum. What is the head room do I need? for 230mA, do I need choke rated as 500mA? 400mA? 300mA? The only current product I know is Hammond. With what percentage of the rated current will the Hammonds start to hum? I need your experienced answer. Thank you!
yours frustrated,
vax, 9000
Chokes often make a racket in a choke-input filter. This unwanted side-effect can be reduced by adding a low value cap., typically 0.68uF or less, before the coke (where a smoothing cap would go in a C-L-C filter). Its value needs to be small, to retain the properties of a choke-input filter.
If your L/C is resonant at the line frequency, it'll make a fuss too. But such a resonance is usually accompanied by smoke.
Remember that the choke passes not just the wanted DC load current, but also an (unwanted) AC ripple current due to the ripple voltage at the input of the filter and the AC short circuit after the choke (the capacitor). If the choke was of infinite inductance, it would have infinite reactance, making the AC current zero, but it isn't, and that can cause two possible problems.
(1). If you have a choke input filter with a choke of less than about 15H, you will find that the AC current tends to be quite large. This can make the sum of the peak positive AC current and the DC load current exceed the choke's current rating. Exceeding a choke's current rating saturates the core, causing inductance to fall, increasing the AC current. It's a vicious circle. PSUD2 is a good way of determining the peak current through the choke.
(2). If the choke is old (>20 years), its iron core may have deteriorated, reducing its flux handling capability and hence maximum current rating, taking you directly to the first problem.
(1). If you have a choke input filter with a choke of less than about 15H, you will find that the AC current tends to be quite large. This can make the sum of the peak positive AC current and the DC load current exceed the choke's current rating. Exceeding a choke's current rating saturates the core, causing inductance to fall, increasing the AC current. It's a vicious circle. PSUD2 is a good way of determining the peak current through the choke.
(2). If the choke is old (>20 years), its iron core may have deteriorated, reducing its flux handling capability and hence maximum current rating, taking you directly to the first problem.
In hte book Valve amplifiers (available from amazon) Morgan Jones provides the equations to calculate the AC current through the choke.
Erik
Erik
Thank you guys for the help. I forgot to calculate the AC current when I designed the power supply. This is my first tube Amp and I surely learned a lot from it.
It does indeed appear that your choke may be a bit undersized...
That aside, transformers and inductors are generally impregnated with varnish or epoxy to quiet them down, also to avoid insulation breakdown from constant rubbibg of conductors against each other. Oddly, it seems that impregnation is not always done these days.
Is your inductor 'pregged?
🙂
That aside, transformers and inductors are generally impregnated with varnish or epoxy to quiet them down, also to avoid insulation breakdown from constant rubbibg of conductors against each other. Oddly, it seems that impregnation is not always done these days.
Is your inductor 'pregged?
🙂
It is the EI type choke with two end bells. There is kind of "paint" on the laminations. I don't know whether this makes it "pregged"? I decide to reduce the DC from 220mA to 170mA to see whether the buzz is acceptable. This means less power from the two output tubes (85mA each).
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