Set the pot so as to give the gain that you require.
Then, measure the pot seting. Slider contact to high, and to low.
Then install a two resistor divider.
You can leave the pot in place, disconnected, in case you need to reverse the mod.
What pot? How old is this Wolcott? I've been involved with this company for many years and there is no gain control I'm aware of.
Are you referring to the feedback control? If yes, that does not serve the same function.
Hello Mr. Porter:
It is good hearing from you. I spoke with you some time ago from up here in New Hampshire. We are using the original CA-250 monos from 1972 -- pretty much renovated throughout -- that was listed as MR-250 (for Marine Research). As for the controls on the front of the monos, they are gain controls. We have a Threshold Stasis R5 controlling the main volume. I was just wondering if it be worth an upgrade on the MR-250s to upgrade the gain controls to something like DACT attenuators, known for their precise management of signal levels; however, given the 25 meg ohm impedance on the gain control pots, I am at a loss as to how to get around this without having a tech do some surgery on the innards. Any ideas? Thanks and hope all's well in TX.
Now I understand, that's why I ask how old. I knew anything in the last 20 years had no volume pot.
Good call on the impedance issue, that's mandatory to maintain. I'm not qualified to tell you what to change to make it right. Have you tried to contact Wolcott audio for advice? Hank is very bright.
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.
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.
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.
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.
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?
The amps are not single-ended; they are push-pull at about 225W per side, each side using eight 6CA7's.
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.
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.