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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
 
Thanks for the responses. I have not contacted Henry Wolcott as of yet; he and Mr. Lomax are quite busy (great, knowledgeable gentlemen by the way!), so I might first try the technician who knows these amps well enough to wrestle with their intricacies and quirks. Eldartford, I went to the DACT website and they offer the reknown CT-2 Attenuator, with the greatest resistance rating at 500kOhm. Given the gain pot on these monos indicates 25 meg Ohm -- and if my calculations are on track -- do you mean I would need to increase resistance by 50 Ohm were I to get the 500kOhm Attenuator? I'm not an engineer, so I just want to clarify.

I called Shallco and await response on an attenuator with a closer if not equal impedance match. You guys sure seem to know your stuff: Thank you for your informative suggestions, and long live audio engineering in advancing the Arts & Sciences.
eldartford
4,262 posts
 
Somut...You better check on the resistance of that pot. 25 megohms is far higher than any audio gain pot I have seen. I could believe 25 Kohms, although that would be a bit low.
ngjockey
1,042 posts
 
Marchand can make custom attenuators. More expensive than DACT but at least worth an email. The DACT is series, as opposed to ladder, if I remember correctly. Size might be an issue as well. Goldpoint could be another option (goldpt.com). I have never seen anything over 100K but it might be possible with a shunt. What wattage are we talking about here? And what range? Is it possible to get a meter on it? Powered off, of course.
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?
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