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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
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PAS3 to volume pot

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ronton3

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Joined 2010
#1
Long ago when I was collecting, and trying to build tube audio gear I modified a PAS3(with the help of a Tube mentor who is no longer available). I bypassed the tone controls and filters and put in Alps 250k Volume and 100K Balance controls. I now want to use it as a Volume Pot to control volume from a new Oppo BDP-83(which is arriving tomorrow) when I am using it as a music player for Tube amps. I think I can rewire from an input to an output and bypass the switch. If I wanted to use the PAS3 as an on/off switch for the Fisher 80AZs, with EL37 tubes that I intend to use first, can I just disconnect the Circuit boards and remove the tubes? should the Alps and Jacks be grounded to the chassis? I would appreciate a simple diagram of where to put the in and out wires on the 250k pot. Does this seem like a reasonable thing to do? I want to leave the PAS3 so I can reconfigure it in the future if need be. Thanks for any advice and help. ron
 
#2
Not really all that reasonable. A 250K pot does not a good passive attenuator make. A 250K pot may have a worse case source impedance of almost 63K which makes it terrible at driving any cable at all. For an application such as this it would be smart to use a pot with a resistance of 20K or less IMHO.

The power switch in a PAS-3 is not particularly good at switching the kind of currents needed to handle those Fisher amplifiers. (It's current rating is just inadequate based on experience.)

I'd set aside that PAS-3 and put together a lightspeed attenuator or similar for what you want to do.
 
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#4
I am now looking into the lightspeed which sounds great, but for tomorrow is the 250K pot okay in the preamp to be used as a preamp.
Click to expand...

Kevin has already explained why a 250 KOhm control will NEVER be satisfactory, in a passive setup.

Before you spend $$ on a good passive setup, try an economical solution first. Your new disk player must be capable of driving a 10 KOhm load, as that is the IHF "standard". Build resistive voltage dividers from metal film parts that total at least 10 KOhms. If you "shed" approx. 70% of the disk players O/P, connecting to 1 of your PAS' line level I/Ps should work tolerably (perhaps quite) well. If you don't already have suitable parts on hand and are unwilling to wait for a delivery, "Rat Shack" Catalog # 271-309 might contain parts that can be used.

IMO, the best way to construct voltage dividers of this sort is by installing 1/8 W. Vishay/Dale RN55 parts inside the RCA plugs, at the preamp end. Mouser has those superior, tiny, metal film resistors.
 
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