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 Post subject: Eico 147A / 6X4 filter cap sizes
PostPosted: Feb Mon 05, 2018 6:35 am 
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Joined: Oct Thu 02, 2014 5:57 am
Posts: 602
Location: Memphis, TN
http://bama.edebris.com/download/eico/1 ... Tracer.pdf

I'm recapping one of these, and I've run into a little discrepancy on the schematic as opposed to what an RCA tube manual recommends.

The schematic uses 2 20 mfd electrolytics and 2 10 mfd electrolytics and 3 resistors for an RC filter. No biggie there. The thing that's got me curious though is that the tube manuals say no bigger than a 10mfd cap unless the plate resistor size is increased to limit the current. The first cap is 20 mfd on it's own...

So what am I missing here?


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 Post subject: Re: Eico 147A / 6X4 filter cap sizes
PostPosted: Feb Mon 05, 2018 6:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
I would use schematic value. The transformer has high resistance windings (low current) and adds series resistance.

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Norm


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 Post subject: Re: Eico 147A / 6X4 filter cap sizes
PostPosted: Feb Mon 05, 2018 12:48 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7699
Location: Long Island
Norm already has it covered, but if you read the fine print in the tube manual, what they are talking about is the maximum filter capacitor size assuming there is no current limiting elsewhere in the circuit except the 6X4 itself--and you are drawing the maximum allowable DC load current at the same time. The signal tracer pulls far less B+ current than the 6X4 is rated for, and the power transformer HV winding puts some impedance in series which helps limit the current. Some of the 147A signal tracers I've seen still had their original Eico branded tubes in them so it does not appear that they are being abused. Nevertheless, I would stick to the original design values of the caps and not go any higher (22-uF in place of 20-uF would be okay though).

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"Hell, there are no rules here--we're trying to accomplish something!"

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 Post subject: Re: Eico 147A / 6X4 filter cap sizes
PostPosted: Feb Wed 07, 2018 6:32 am 
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Joined: Oct Thu 02, 2014 5:57 am
Posts: 602
Location: Memphis, TN
Thanks guys! I didn't even think about the transformer windings factoring into it... It's funny you mention original tubes, because this one has them. So, I'll recap it with the schematic values and hopefully it'll stay that way. :D


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