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Stepped Attenuator to replace Potentiometer.

Hello: We have a set of Wolcott Monoblocks, for which I am researching replacing the volume pots with something like a DACT stepped attenuator for each mono block. The problem is that I can not seem to find a compatible attenuator that could replace the stock 25 meg ohm pots.

The circuitry is point-to-point wiring, powering eight 6ca7, two 6dw8, and two 12at7 tubes per side. It might be more of a hassle to mess with getting around the stock pots to substitute attenuators; however, I'm not an engineer and would therefore appreciate your informative advice and suggestions. Thank you.
somut
somut OP
82 posts
 
Good evening to all. Pardon the misperception: I just zoomed in on the gain pot of each monoblock and I saw a faint "." stamp followed by a space in "OHMITE TYPE-AB 0. 25 MEG OHM." Apparently, this means I could substitute a 250kOhm resistance attenuator. Please correct me if I'm wrong, as I was with the previous calculation above. Thanks gentlemen.
ngjockey
1,042 posts
 
Just to confuse you some more...

IAG used to carry Sowter based mono TVC attenuators. Input impedance was way off at 30K but offers galvanic isolation and conversion between balanced/SE. Impedance can be raised with series resistors, on each leg if balanced, but I'm not sure how that would affect the range.

Is it safe to assume, from the vintage, that it is single-ended?
somut OP
82 posts
 
The amps are not single-ended; they are push-pull at about 225W per side, each side using eight 6CA7's.
ngjockey
1,042 posts
 
You caught me. Not the single-ended I meant. Single-ended as opposed to balanced or differential circuitry. Neither RCA's or XLR's in and out are necessarily a true indication.

This kind of application stuff, is more critical than the printed rating.
somut OP
82 posts
 
Ngjockey: that's a pretty technical question, which will take me a day or so to research: for now, all I know is that the input on the amps are the two pronged banana plug type (I plug the the male RCA connector coming from the preamp. into a banana plug adapter, which then gets plugged into the banana inputs on the monoblock).

By the way, I contacted Marchand Electronics and they are making up a set of Elma attenuators. I briefed through IAG's attenuators -- they appear to make some cutting-edge Shallco attenuators; however, given the depth measurement on a Shallco attenuator, it would not fit inside these monoblocks without some surgery to the chassis and wiring infrastructure. Thanks again.
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