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 Post subject: Crossover Parts For Tube Amplifier
PostPosted: Feb Tue 20, 2018 12:30 pm 
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Joined: Sep Sat 05, 2009 9:45 pm
Posts: 1131
Location: Sun City, Arizona
My 1960 Rock-Ola jukebox has a stereo-tube amp. I have recapped the amp a few years ago and it is working fine. When I got the machine, in many pieces, I discovered the amplifier crossover circuit was missing. While the sound seems OK, I would like to have the high frequency speaker doing its job too. The speaker crossover is external to the amplifier. I do have the schematic and it appears easy enough to build a crossover. The components are: one 3.0 MFD (no voltage value given) Capacitor and two 5 Ohm resistors (no wattage value given). The machine's two 16 Ohm (12") speakers are in the circuit. I do have the machines original high frequency speaker (horn). I could provide a picture of the crossover schematic if needed.

Since I am seriously lacking knowledge about crossovers and other stuff I wondered what a good voltage rating for the capacitor might be and a wattage rating for the resistors. The parts listing and schematic do not include this information. It seems they would be of minimal values but I would like to have more knowledgeable people's thoughts. I imagine the capacitor is a non-polarized capacitor but is not specified as such. Thank You All for your help and thoughts!


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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Tue 20, 2018 5:31 pm 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 2975
Location: USA
For capacitor something like 100v or higher is ok. I think modern ones are film, polyester, or polypropylene. Make sure it is non-polarized.

Ordinary 1/2w resistors are fine.

This is assuming the crossover is at the speaker where voltages are relatively low. If it's anywhere else, you should show the schematic for us to figure out. Don't let audiophiles talk you into buying the ultra-expensive stuff.

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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Wed 21, 2018 12:51 pm 
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Joined: Sep Sat 05, 2009 9:45 pm
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Location: Sun City, Arizona
I photographed the schematic for the crossover. I'll see if I can get the amp schematic in a readable form tomorrow morning. Thank You for the information!

https://imageshack.com/i/poHeCeswj


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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Wed 21, 2018 5:02 pm 
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That's a first order crossover. Just get the components as recommended.

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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Thu 22, 2018 11:23 am 
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Joined: Sep Sat 05, 2009 9:45 pm
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Location: Sun City, Arizona
Will Do! Thank You very much! We'll get on that little project!


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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Thu 22, 2018 2:24 pm 
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Joined: Aug Sat 01, 2009 2:23 pm
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[quote="AJJ"

Ordinary 1/2w resistors are fine.

[/quote]

Uh...no....they're not "fine". Resistors in passive crossovers like this are used for series attenuation and impedance trimming. They will dissipate considerable power and 1/2 watters are WAY too small for this purpose.

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 Post subject: Re: Crossover Parts For Tube Amplifier
PostPosted: Feb Sun 25, 2018 10:22 pm 
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Joined: Nov Fri 10, 2006 12:24 am
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Location: Thornhill, Ontario
jmsent wrote:
AJJ wrote:
Ordinary 1/2w resistors are fine.
Uh...no....they're not "fine". Resistors in passive crossovers like this are used for series attenuation and impedance trimming. They will dissipate considerable power and 1/2 watters are WAY too small for this purpose.
Agree. Calculate the maximum voltage, V, that can appear across the amplifier output, i.e. P=(V^2)/Z, where P is the maximum power amplifier output and Z is the nominal output impedance. Solve for V. Then look for the largest part of that voltage that can appear across any resistor in the X/O, call it Vx. Then the power rating of that resistor should be (Vx^2)/(resistor value.) However, In practice, they'll be subject to far lower power as the amplifier is generally idling well under max power when playing music... I suppose there may be exceptions with rock 'n roll!. :-)
Cheers,
Roger

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