Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on February 28, 2011, 04:56:35 pm
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Hello,
I'm building an attenuator for a small 5W Champ type amp.
I'd like to use a rotary switch but have trouble to find some sturdy ones (they have to be 2P3T at least).
The Alpha ones are common and cheap. They are rated at 300 mA @ 125V.
Would they handle the juice from an OT (a small one that is, the Champ won't exceed 10W at full tilt)?
Thanks fo helping!
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Install a Variable Voltage regulator instead.Works better than an attenuator.
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Would they handle the juice from an OT (a small one that is, the Champ won't exceed 10W at full tilt)?
No. 10w into an 8Ω speaker calculates to 1.1A (1100ma). Or 10w into a 4Ω speaker calculates to 1.6A (1600ma).
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Install a Variable Voltage regulator instead.Works better than an attenuator.
IMO, listen to phsyconoodler.
Easy, simple, at a very good $$, -plus- many more tone/volume options!
Think about it for a little...... 3 steps of attenuation -?-, or a full smooth sweep of the rotation through the whole sweep of the control pot? :wink:
Brad :smiley:
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The Alpha ones are common and cheap. They are rated at 300 mA @ 125V.
Would they handle the juice from an OT (a small one that is, the Champ won't exceed 10W at full tilt)?
No. 10w into an 8Ω speaker calculates to 1.1A (1100ma). Or 10w into a 4Ω speaker calculates to 1.6A (1600ma).
I've also been wondering about these Alpha switches. If it's rated at 300mA/125VAC is that equivalent to 600mA/62,5VAC or 1,5A/25VAC? Or is the 300mA maximum current rating at any voltage?
Could these be used as a impedance selector switch in a small amp?
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Thanks everybody for the comments.
Sluckey, thanks for the bit of math.
VMS, look at what Sluckey said. If I'm not mistaken, he used the P = IČ*R.
Replace P with 10W (a 5 clean W amp might put out 10W max, it's a guess and a ballpark value). R is the speaker load. Say you have an 8 ohm speaker, lets round it up to 10 ohm. In the formula above :
I = sqrt(10 / 10) = 1A
Therefore I guess he little 300mA switch won't handle it.
Strange thing : Vox uses this switch in the Vox AC4 as the attenuator selector switch.
They might use a 4P3T version as a dual 2P3T to double the ratings though. And the speaker might be a 16ohm to reduce the power dissipated through the switch. Just a thought.
Psyco and Willabe, I will definitely look into those VVR things.
Do you have any trick to implement them? Would you control the juice on the whole amp or on the power tube only?
Thanks for sharing!
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VMS, look at what Sluckey said. If I'm not mistaken, he used the P = IČ*R.
Replace P with 10W (a 5 clean W amp might put out 10W max, it's a guess and a ballpark value). R is the speaker load. Say you have an 8 ohm speaker, lets round it up to 10 ohm. In the formula above :
I = sqrt(10 / 10) = 1A
Therefore I guess he little 300mA switch won't handle it.
Yes, but the 300mA is when the voltage is 125VAC. I'm asking what happens if the voltage is only 25VAC. Does the current rating go up?
My math: P=U*I => 0,3A*125VAC=37,5W ==> I=P/U => 37,5W/25VAC=1,5A
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Yes, but the 300mA is when the voltage is 125VAC. I'm asking what happens if the voltage is only 25VAC. Does the current rating go up?
No. When the switch contacts are closed, there is (should be) zero volts across them and the resistance should be zero as well. Current is the only factor to consider when the contacts are closed and the current will be determined by the voltage applied to the load.
Voltage ratings on switches are useful when the switch contacts are open. IOW, how much voltage can exist across open contacts before you have an arc across the contacts or the insulating parts of the switch body break down?
Switches don't really have a power rating, except that for large switches and contactors you will often have a horse power rating (power), but that's just a convenient way for industry to match switches to motors that are usually rated in HP.
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Thanks sluckey for the explanation!
That makes perfect sense now.
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Consider a switch as a conductor. This means you can forget about power and only consider ampacity. It's all about surface metal, just like wire. If the switch is rated for 1 amp @ 300V, this does not mean it's good for 2 amps at 150V. It is still only rated for 1 amp. It'd probably be fine at 600V, but it is not certified as such.
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Look in here, for VVR.
http://www.el34world.com/Forum/index.php?topic=10977.0
http://www.el34world.com/Forum/index.php?topic=11007.0
Brad :smiley: