Kegger
R.I.P. 1/12/1966 - 6/1/2017
When it's "done" I will sticky it..
(more people respond to non sticked threads, so for now)
How does a tube amp work?
- Thread starter BmWr75
- Start date
When it's "done" I will sticky it..
(more people respond to non sticked threads, so for now)
Thanks for this line, safety with higher voltages is what should be first and foremost in everyones mind when dealing with tube amplification. Low wattage ain't low voltage. Sorry for the pollyanna.
Back to your thread, man - and a good one it is. Thanks!
To put it simply- a capacitor is a SHORT for AC (lets it through, unimpeded, for the most part), but an OPEN CIRCUIT for DC (it won't pass DC from one side to the other).
As a coupling cap- it's in-line, so the "AC short" is letting the signal through, while the "DC open" is blocking any DC from plates, bias supplies, and the like.
As a power supply cap- it's ACROSS the line- so any ripple (which is AC) is "shorted to ground", while the DC is unimpeded (since it doesn't "leak" any of the DC to ground). This means that the voltage across the cap will be mostly "averaged out" to a steady DC voltage.
Polarized caps are used when there is going to be a large amount of DC across a cap, most of the time, and the voltage is ALWAYS going to be the same polarity (i.e, the positive side of the cap will always be positive, relative to the other side). Also, it's easy to make a very large-capacity cap as a polarized cap (electrolytic, etc). This is why they are used in power supplies- the voltage is pretty much constant (and the same polarity) and you need lots of storage capacity.
OTOH, a coupling cap has lots of AC on it, and not so much constant DC. It is possible that the overall polarity (once you consider the DC and AC components of the signal, added to each other) MIGHT be inverted at some parts of the AC waveform across the cap. So, you want a non-polar cap- one that will act the same, no matter what polarity it sees in the input signal.
Regards,
Gordon.
New questions: How does a varistor work? Why is it beneficial to install one upstream of the power transformer? Is there a proper way to install a varistor?
These questions are based on a thread about restoring a Sherwood S5000 II tube integrated amp. One poster said "Add a CL-80 varistor to reduce surge and to lower B+".
These questions are based on a thread about restoring a Sherwood S5000 II tube integrated amp. One poster said "Add a CL-80 varistor to reduce surge and to lower B+".
A discussion on varistors and thermistors starts at post #3 in this thread:
http://www.audiokarma.org/forums/showthread.php?t=124211
Post #15 is especially helpful: http://www.audiokarma.org/forums/showpost.php?p=1327720&postcount=15
http://www.audiokarma.org/forums/showthread.php?t=124211
Post #15 is especially helpful: http://www.audiokarma.org/forums/showpost.php?p=1327720&postcount=15
Kegger
R.I.P. 1/12/1966 - 6/1/2017
CL-80 THERMISTOR.
It's a device that when when cold has a fair bit of resistance, as it gets hot the
resistance drops to just a few ohms, so installing 1 in each leg of the primary of
a power transformer will keep current flow down at start-up, also it provides an
bit of slow ramp-up voltage as that device heats.
An additional byproduct is since there is an few ohms of resistance left after hot
that it will drop off a few volts from the wall, with todays wall voltage an vintage
amps designed for less this can be a nice benefit, quite often ends up Very Close
to what the older pieces of equipment called for.
It's a device that when when cold has a fair bit of resistance, as it gets hot the
resistance drops to just a few ohms, so installing 1 in each leg of the primary of
a power transformer will keep current flow down at start-up, also it provides an
bit of slow ramp-up voltage as that device heats.
An additional byproduct is since there is an few ohms of resistance left after hot
that it will drop off a few volts from the wall, with todays wall voltage an vintage
amps designed for less this can be a nice benefit, quite often ends up Very Close
to what the older pieces of equipment called for.
Dandy
Addicted Member
Thanks for the link, and again really clear explanation of the place of thermistors for tube amps.
But as varistors seem to be used for current shunting, I'm not sure that varistors can be used for inrush protection in tube amps. The Wikipedia article says: 'A varistor provides no equipment protection from inrush current surges (during equipment startup), from overcurrent (created by a short circuit), or from voltage sags (also known as a brownout); it neither senses nor affects such events. ' Maybe the poster referred to in #45 should have said CL-80 thermistor?
But as varistors seem to be used for current shunting, I'm not sure that varistors can be used for inrush protection in tube amps. The Wikipedia article says: 'A varistor provides no equipment protection from inrush current surges (during equipment startup), from overcurrent (created by a short circuit), or from voltage sags (also known as a brownout); it neither senses nor affects such events. ' Maybe the poster referred to in #45 should have said CL-80 thermistor?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
A varistor is not what you want for current surge, there what you want for voltage surge.
A thermistor goes in series an drop voltage across it when there is current, varistor's what
you see in say power strips, they go in parallel from line to ground, and close when voltage
is excessive to shunt that "excessive" voltage to ground to try an not blow your equipment
like say during a lightning strike an such.
A thermistor goes in series an drop voltage across it when there is current, varistor's what
you see in say power strips, they go in parallel from line to ground, and close when voltage
is excessive to shunt that "excessive" voltage to ground to try an not blow your equipment
like say during a lightning strike an such.
Kegger,
In another thread on restoring a Sherwood S5000 II amp, a poster said "Add a CL-80 varistor to reduce surge and to lower B+". I am working on an amp like this now and have bought a single CL-80 varistor. Thought I was supposed to install it between the power cord and power transformer input. But now I'm not sure. This amp uses diode rectification.
Should I install it upstream of the PT or not? If yes, what is the proper way to do this?
Thanks,
Scott
In another thread on restoring a Sherwood S5000 II amp, a poster said "Add a CL-80 varistor to reduce surge and to lower B+". I am working on an amp like this now and have bought a single CL-80 varistor. Thought I was supposed to install it between the power cord and power transformer input. But now I'm not sure. This amp uses diode rectification.
Should I install it upstream of the PT or not? If yes, what is the proper way to do this?
Thanks,
Scott
Kegger
R.I.P. 1/12/1966 - 6/1/2017
A CL-80 is Not a Varistor, it is a Thermistor. (check my description just above your post)
"Thought I was supposed to install it between the power cord and power transformer input"
Yes, an I tend to use a pair an put 1 in each leg of the AC coming into the power transformer but
you can use just 1 as well, you'll just get Less surge protection an less wall voltage drop that way.
(so if only using 1 it would go in 1 leg of the ac coming into the pt from the wall cord)
"Thought I was supposed to install it between the power cord and power transformer input"
Yes, an I tend to use a pair an put 1 in each leg of the AC coming into the power transformer but
you can use just 1 as well, you'll just get Less surge protection an less wall voltage drop that way.
(so if only using 1 it would go in 1 leg of the ac coming into the pt from the wall cord)
I guess McShane was just humoring me when I asked him for a "CL-80 varistor". What I got was a "CL-80 GE/Thermometrics inrush limiter - 3 amp limit". Wish I had two now that I know the proper way to install them.
People using the wrong terminology is very confusing to tewb newb's like me. Thanks for the clarification and patience Kegger.
People using the wrong terminology is very confusing to tewb newb's like me. Thanks for the clarification and patience Kegger.
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I guess I would confuse people then. I just refer to them as 'NTC's... "negative temperature coefficient" devices. That is, the hotter they get, the less resistance they have, to a point.
We deal with both NTCs (negative) and PTCs (POSITIVE temperature coefficient) devices here at work. NTCs are great voltage-limiting inrush limiters, while PTCs are what are otherwise known as "polyswitches" or "self-resetting fuses" in speakers. Put too much through them, and they start to "open up" as they get hotter, limiting the current that's available to the load. They can save your bacon, for short term amp-clipping situations... they're oftentimes used to protect expensive compression horn drivers in PA speakers. Sometimes it works, sometimes the horn blows anyway- but even if they're only 50% effective, that's half as many horn driver element replacements, over time...
Regards,
Gordon.
We deal with both NTCs (negative) and PTCs (POSITIVE temperature coefficient) devices here at work. NTCs are great voltage-limiting inrush limiters, while PTCs are what are otherwise known as "polyswitches" or "self-resetting fuses" in speakers. Put too much through them, and they start to "open up" as they get hotter, limiting the current that's available to the load. They can save your bacon, for short term amp-clipping situations... they're oftentimes used to protect expensive compression horn drivers in PA speakers. Sometimes it works, sometimes the horn blows anyway- but even if they're only 50% effective, that's half as many horn driver element replacements, over time...
Regards,
Gordon.
Correct. They are non-polar, in essence.
Think of them as a variable heat sensed resistor- they can be used under the same install conditions a resistor can.
Regards,
Gordon.
This is a picture of an axial lead polar electrolytic capacitor. The positive end is marked with a "+". Pay close attention the polarity when replacing polar caps. Installing them backwards many times will explode them once power is applied.
This is also a holding spot for expansion of this thread as needed.
This is also a holding spot for expansion of this thread as needed.
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Now i'm a book and a half in, I have visions of formulas dancing in my head, I'm convinced my son is right vacum tubes work by invisible magic.
bobrown14
Super Member
This is what a capacitor looks like after it explodes:
Note: it was installed backwards and made a VERY loud bang just like a hand gun.
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