Please explain drift & dispersion, find cap brackets

[dick]

SInce my 3a's are 40+ years old, I've noted in reading in the resot ration guide that the caps have no doubt drifted. What does this mean? That they clip off more frequency or add more to the served speaker? Does the resulting sound of drifted caps result in lost frequencies or overlapping sounds? Also, in reading threads here, I've noted that the early ARs are dispersion type speakers. I no doubt incorrectly think of this something like spreading all sound about as opposed to pointing it? But if pointing, then to where? to a sit down sweet spot? So what is dispersion in the 3as and how does this differ from contemporary speakers?

Clearly still the beginner, I'm nevertheless enjoying reading what is said on CSP Forum and hopeful to learn more. Whatever the 3a has, I'd sure like more of it and will soon get to cleaning pots and changing caps. The deliciously articulated and rounded (my innocent sub-audiophile speak) sounds really seem to activiate air molecules in our high ceilinged wooded interior.

Lastly, for cap replacement, where does one find the cap mounting brackets shown on page 16 of the restoration guide? Madisound said they don't carry them.

[Carlspeak]

To answer your dispersion question, we speaker guys use the term "off-axis response". You may have seen it written here at CSP from time to time. You are correct in assuming it's the 'spreading' of the sound over a wide angle from the front of the speaker from side to side. Thus, one of vintage AR's unique qualities is it's broad angle of continued high frequency response. You don't have to worry as much about sitting in a sweet spot to hear the full audio spectrum. A "beaming" design will have poor off-axis response and a narrow sweet spot. This is the case with most modern speakers.

With regard to drifting, caps have two properties which can drift or change value as they age. One is the basic microfarad value and the other is equivalent series resistance (ESR). Generally, both go up with age. Particularly with electrolytic types due to their chemical make up. But other vintage designs do as well.

Drifting of the uF value can shift the crossover point beyond the designer's intent. IF this happens, a driver's frequency response 'window' can broaden and overlap another's creating a hump in the frequency response. Drifting ESR (usually upward) will diminish the driver's output by acting like a resistor in series with it. This is a particular malady with tweeter circuits; most of which have a capacitor in series with it.