RoyC

Yeah, which means the woofer moves more freely than it would in a sealed cabinet. This is obviously not a finely tuned design.

Interesting...perhaps someone tried to turn them into Amati's. It looks rather professionally done from here. The configuration of the early Amati crossover I saw many years ago was the same as the LST's, but had film capacitors instead of NPE for the tweeter and mid, and high end resistors (like those in the photo) in place of the contour transformer. Roy

Yup, KLH caps should always be replaced. The sealant will remain tacky, so it never really "dries". Roy

Norman...your photo shows your 33's to be a ported speaker, not aperiodic, in which certain low frequencies are emitted from the port.

In my opinion, your assumption could not be less true, Jab. What do you think dried out causing the original sound to be "lost to time", not having any "semblance to the original"? A competent re-foam job will bring them extremely close to original spec, and new woofers will not. Keep in mind that the crossover and tweeter must be compatible with any woofer you choose. Obviously, it is much easier to simply screw in new woofers, but you asked for an opinion. BTW, Dayton and Simply Speakers (and most other typical replacement drivers used to fill old cabinet holes) are all made in China, if that really is of concern to you. Roy PS I didn't know you were in Italy. 🙄

I just finished restoring another pair of KLH 17's. Although they were functional, measurement of the original caps showed them to be significantly out of spec. ESR was over 6 ohms for each of the 2uf caps! It is always prudent to replace KLH capacitors. Although caps won't have a bearing on the performance of the woofers, I would wait on treating the surrounds until after the woofers are installed. The "push cone" test can then help you determine the degree of any re-treatment they may need. There are some KLH woofer surrounds out there which have been later re-treated with stiffening material, resulting in compromised bass response. No amount of additional new sealant will be helpful in this case. Roy

The Cello Amati was essentially a clone of the LST. The original version used all AR drivers, so it is plausible. Roy

"The 1¼” cone tweeters look fine. If their resistance measures between 4.3 & 5.2 ohms, consider not replacing them until you have listened to them with the restored woofers installed." I agree with this statement made by AR55 above. Voice coils don't wear out with age. When AB Tech (AR's authorized repair service) went out of business, Ebay's "Vintage_AR"/Larry Lagace (for whom I do much work) ended up with the last of AB Tech's inventory...but those parts are long gone. Even those last replacement woofers, however, were not a match for the original. If you decide to go down the rabbit hole, I would be happy to take those old worn out woofers off you hands. Roy

I have not, Bill.

An aperiodic design allows a specific amount of air to move through a tuned, restrictive "vent" or valve in the cabinet. It is meant to allow the woofer cone to move a bit more freely than it would in an acoustic suspension cabinet. Its intended attributes are to enhance bass response at lower volumes, yet provide much of the cone control of an acoustic suspension speaker at higher volumes. Obviously the woofer and its cabinet have to be designed to work together to prevent bass response aberrations and to provide adequate power handling capability. It's anyone's guess how finely tuned the KLH 33 and 10 (which seems to be more of an actual ported design?) were, but it would make sense that the woofer surrounds of these models would need to be relatively well sealed to allow the vent/port to do the job properly. Btw, the most famous aperiodic speaker is the Dynaco A25, the woofer of which has a rubber surround. I don't have enough experience with these models to make any subjective comments about them. Roy

I use shims whenever possible. An added benefit is the ability to manipulate the height of the cone using the friction they provide. I've repaired more than a few woofers with rubbing voice coils due to botched foam replacements. Roy

It is actually not all that unusual to find this. The drivers were nearly glued in with putty in those days, so performance was not likely to have been affected, and the grille frames were hot glued in place, so nobody would likely see it. Pulling the woofer with all that gooey putty under it probably wasn't going to happen if the cabinet was sealed properly. The only other eyes on them would have been in a service center, given AR's very generous warranty. I'm sure AR never expected it to be a topic of conversation a half century later through something called the "internet." When reinstalling the T-nut dab some epoxy around the outside of it, tap it in, and tighten a screw through it while it dries. The T-nut hole itself is sometimes stripped, so the glue can be very helpful. When reinstalling the woofers don't use too much downward pressure when starting the screws, as it can dislodge the T-nuts. It is likely how it occurred most often in the factory. It sucks when it happens after most of the screws are already installed. I've been servicing AR, as well as many other speaker brands, for over 40 years, and have found some very interesting things along the way. Many years ago I spoke to a former AR factory tech (amplifier section) who recounted the story of a speaker returned due to an unpleasant odor, especially when it was driven at higher volumes. When it was opened to investigate, they found the remnants of a sandwich in it! Btw, KLH woofer screws are much worse. The cabinets don't have T-nuts, just lots of stripped screw holes in flaky particle board cabinets.

Yes...even though the pots themselves could vary between 15 and 16 ohms. This variance is not very significant, however, when the parallel resistance of the pot is taken into account. The white dot was considered a starting point, but did have a specified resistance value for each model.

It wasn't the variation in the pots. The notches were model specific and were there to place the arrow of the pot shaft in a specific position relative to the white dot. It was meant to position the pot so it would provide the appropriate amount of attenuation from full increase to the dot in accordance with that model's specifications. I only mentioned them to illustrate the fact that AR never intended, nor implied, that the dot was the halfway mark of the pot's rotation. The comparison is always the interesting part when refurbishing these tweeters. Due to the nature of the construction of the old beasts, achieving consistent results is a challenge...regardless of the material used to suspend the dome.

The AR-5 dual cap values are 24uf (mid) and 72uf (woofer), and the small cap for the tweeter is 4uf. I recommend replacement. The crossover configuration, level controls/pots, and associated wire colors are the same as the 3a. Roy

Bill, AR never implied the white dot to be the 50% point of the rotation. See attached label and partial factory schematic of the AR-3a. It shows the "normal" white dot setting of the AR-3a's tweeter to be 1 3/4 ohms off full increase and the "normal" dot setting to 3 1/4 ohms off full increase for the mid. I have other documents that state that the fully increased setting is considered to be "flat". As i mentioned earlier, when the pot's anchor tabs are seated in their corresponding notches on the crossover board (see attached photo of an AR-4x I'm presently repairing), the pot is placed in the appropriate position to meet AR's specs for the dot. The normal/white dot settings and corresponding internal pot mounting positions varied with model (ie. the AR-3a is different than the AR-2ax or the AR-4x seen in the photo). If the white dot was meant to be halfway for every model and every driver, it would make all of this meaningless. If you haven't done so yet, take a peek at the forum Library. It will provide insight into AR speakers of the era.

"Cheat? Yes. On the tweeter pot, the white dot lines up with a 75-80% setting, from the factory. The mid is pretty close; at the white dot, you’re close to 50%." Cheat? No. AR set up the controls so the white dot was "normal" for the typical room. Full increase for the both the mids and tweeters varied with each model. It is why the pot's anchor notch is placed differently for each driver in each model. This is a common misconception for people new to AR. It had much more to do with design goals than whether or not a tweeter (or mid) with more output could be manufactured. The original schematics and drawings are quite specific in this regard. Roy

True, it can only be assumed that AR carried over the design goals from Bill's earlier era specimens, as they were sold interchangeably. Other than the foam blob suspension, the tweeters were certainly different. DCR of the early large orange dome tweeter is under 2 ohms and the later smaller black dome is around 6 ohms. Since they both were used with the same 4uf crossover capacitor cap and level controls, this alone would have made them behave much differently if their mechanical properties were the same. The huge magnet structure made the earlier tweeter perform as intended at higher frequencies with a more restricted/less compliant suspension. The later small dome was more compliant, and had a soft foam material under the dome instead of packed fiberglass. In other words, AR relied on mechanical properties more than electrical to achieve the same, or similar, design goal. Dispersion was really not all that different between the early and later versions, especially when the cabinet grille is taken into account. Roy

L-pads are constructed differently. They use two resistor coils vs one in the pots. The effect is that it eliminates parallel resistance at the fully on and fully off positions. AR pots still have 15 to 16 ohms (depending on the version) in parallel with the driver when it is fully on.

If the AR-11 tweeters you are using in your AR-3a's are not crossed over properly, they will augment the mid frequencies, most likely causing the issue you described. Those tweeters don't suffer the same type of degradation as the originals. Roy

Finally someone with actual experience with them. A lone voice (here) amidst all the speculation. Roy

It's never easy, Bill, as many subjective claims are made in audio forums. I really doubt anyone thinks you didn't do a good job. You were just asking for feedback... Below is a page taken from an AR document. It may be of some assistance to you if you are running response curves. Note the 2ax at the far right. I believe it said it was with the level controls at max. Roy

Agreed...Increased sensitivity is obvious, but quality is hard to determine in this way. I think RickB meant "apples to apples" as it relates to consistent results.

Nice recovery. Shims would have been helpful as well. Is the red and clear material around the dome a butyl rubber compound or rtv? I've seen a red rtv gasket material used for this purpose as well.

Nice work! Have you decided on your primary system yet? How are the AR-11 tweeters (per our discussion) doing in your 3a's? Roy