Alan-san’s Modified Model 7 and PX25A (DA30) Mono SET with Hashimoto Transformers
I had spent almost a year from 2016 to 2017 looking for the very rare vintage tubes to use for the PX25A mono SET requested from Alan-san, on the Japanese auction site and certain vintage shops, and fortunately enough, I was able to manage to get all of them, just we looked for.
These vacuum tubes are a pair of WE310A mesh and small punch for driver tubes, new dome-type GEC PX25A for power tubes, and Marconi U52, STC 5R4GY for rectifier tubes. And also, Hashimoto transformers, including custom ordered power transformers, were delivered to me in the beginning of the summer in 2017. And the new, several-featured PX25A mono SET completed wiring work by the end of this summer. You can see it in detail in the Part 2 link HERE.
But at that time, I got another request from Alan-san that he wanted to have a modified Model 7 like mine. Of course, Alan-san’s' request was in perfect agreement with my own idea that the Model 7 might be inevitable to maximize the true potential of the PX25A mono SET, just I had been assembling.
As you may know, it would be extremely rare in the world to have many auctioned Model 7 of various types, “Original, 7 K, and Replica” except on Japanese auction sites. Of course, it was still expensive, but luckily, I was able to bid off the Model 7 original for Alan-san, which was relatively reasonably priced among the rare originals, but the serial 15318 is close to the initial lot and said to be particularly valuable.
However, since the Model 7 original was a product from more than half a century ago, it could not be completely original, and also the Model 7 we had been looking for was intended to be modified by replacing many of its parts, so the most important point in the selection was that it have a beautiful appearance, and, in fact, even the original wooden case was fully restored by my familiar woodworking company so beautifully that it could look like a new one before shipping to Alan-san.
Well, I sent this email to Alan-san after his Model 7 original was delivered safely to me on 3rd September 2017:
“Well, I'm now studying how to modify, but am so impressed with the beautiful and careful work of wiring and soldering of your Model 7. I once heard that the early production serial of Model 7 had been produced with the excellent craftsmanship. I'm truly convinced that was true. Of course, I'm so delighted to have the chance to see such a fine artistic work, thank you very much, Alan-san, and so I will do my best for the modification and try to make a few new challenges.”
This modification was already described specifically in detail in Part 7 (HERE), so if you interested, please refer to it.
In early November of this year, three large, reasonably heavy custom-packed cartons, contained the PX25A mono SETs and the modified Model 7 crossed the Pacific Ocean and arrived safely in Alan's hands.
His first communication about them, which arrived on November 28, is as follows, and the scene of Alan-san and Paula-san enjoying the mystical glow floating on the wall of the PX25A when the lights were turned off just came to my mind.
“Hello Yazaki-san, Okay, lights out now ... WOW, WOW, WOW, what a surreal glow, a wonderful soft cloud! I think that I might be up all night ... the Altec Coronas are so HAPPY, like Paula-San and myself! As soon as the first tune rolled through this remarkable system, Paula-san immediately commented, "This is so real, so alive, I feel that the singer is sitting right here in our room. Thank You Yazaki-san!”
Of course, I was so delighted and fully satisfied to hear this from Alan-san. Yes, it could be said that Altec Corona has finally come to its full potential.
However, it really had taken almost two years for both Alan-san and I to get his system into the state that he listened to that day.
JBL 2405 Add-on Super Tweeter System
As I've described here, I'm a self-confessed follower of Altec vintage speakers, and I have been actually a long-time user, but when compared to the modern designed wide-range speakers, I had to feel the shortage in the highest frequency range, which is also close to the audible limit about this 2 way speaker system.
To be honest, sorry that I have to conclude that there was a fatal strategic error in Altec's product development plan at that time. I think the long-term success of the 604 series as a studio monitor from the 50s to 60s may have cast a shadow over it on the contrary.
In this field, and in the general consumer market field, Altec had fallen behind JBL from the end of 60s or the early 70s, probably because a good quality and full-fledged tweeter had not yet been developed.
This movement seems to be in sync with the widening of the music source range itself and also the high fidelity of improved electronics. Anyways, I offered my idea about the measures to expand the playback range of high frequencies to Alan-san as our next step looking for “Real-Sound” on 19th June 2018.
“Dear Alan-san, Today I’d like to offer you my idea for improving the sound quality of your system. Simply stated, it is the addition of super tweeter into your Corona. Yes, the Corona is something of a masterpiece, and worthy to be called so, I believe."
"First of all, the design and the structure of Corona's box is superb. There is only one parallel plane, top and bottom, and it would reduce the harmful effect of inner standing waves, and the slim legs could bring out highly the resolution of the low range, I suppose. And the woofer of vintage Altec, 803A, 416A and 414A, would be the best one for reproducing very natural and beautiful mid to low range among an awful lot of type’s woofer in all ages and countries, I believe."
"Well, I have regarded two-way speaker systems as the ideal balance between the range and the coherent characteristic of the total sound. We could feel the good coherent or phase characteristic as the feeling of comfort."
"The multi-way system could not easily bring out such a comfortable feeling, you might have already experienced. About the Altec system, the sufficient range and its good phase characteristics would be realized by the outstanding woofer and horn system, a two-way with simple network."
"But the one negative point would be the rapid roll off or the decline from 10 KHz of horn driver. Although, I believe the range of over 10 kHz would be rather important for getting the airy sound stage and the definite outline of the sound. Especially, such signal information is much more comprised in present music source."
It was fortunate, that the roll off curve of over 10 KHz is so smooth, and so the addition of a super tweeter with a simple high-pass filter could be successful, I think."
"Well, I had used the JBL 2405 before I managed to get the Onken 5000T Esprit tweeter, so long time ago now. The JBL 2405 tweeter's sound was so musical for me, and furthermore, it covered horizontal wider area compared to usual horn tweeters. Also, the early model has 16 ohm voice coil and the efficiency is around 105 dB / w, m, just as same as your driver."
"I suppose we don't need to change your present Corona and its network, the N-800E. It would be only the add-on tweeter system, featuring the 2405, including the filter and the divided resisters. I believe your Model 7 and PX25A mono SET could fully bring out the true potential with such new system! Warmest regards, Shirokazu Yazaki”.
Here, I would like to describe my consideration for N800-E, the network of Altec Corona. I was not able to find out the circuit diagram of type E, but come across the diagram of N800-D on some web-site.
Note from Jeff: Great Plains Audio graciously provided this N800-E crossover schematic for my use back when I was considering building a replica of it with premium components (a project I would still like to do one day). Thank you, GPA! I thought it might be of interest to Yazaki-san and those reading this article to see the Altec N800-E schematic.
I suppose, if the basic circuit design of E type would be the same as D type, it was a very classic or textbook-like network design with -12 dB/octave curve and -3 dB at cross over frequency, so-called constant resistance type.
In addition, a minimum resistance division circuit as attenuator, made of two resistors for adjusting sound pressure to the low-range was inserted into the high-pass side, that is, on the mid-high horn and compression driver side.
I think it was certainly a typical network circuit design for that time. It seems that the network design of many speakers available in the current market are still based on -12 dB/octave curve, which is determined by a pair of LCs.
However, since the 1970s, the classic -3 dB at crossover point was changed or improved to -6 dB crossover design. This would be mainly because the combined sum of acoustic outputs with low range and high range in classic -3 dB cross over setting, there will be 3 dB increase at this cross over frequency.
However, regarding the mid-high range of the Altec Corona, the relatively steep cutoff characteristics of the H-811B horn, are added, and it is necessary to attenuate the level by about 6 dB according to the efficiency of the woofer, 803A.
Therefore, this 3 dB rise near the crossover frequency of this classic -12 dB / octave network design could be effectively reduced to be more flat, and the sound of the woofer and the compression driver could be smoothly connected, both characteristically and audibly, I supposed.
Above all, Paula-san, Alan-san’s wife, who is also one of the users, gave a straight and profound feedback of this Corona as “so real, so alive”, and should be noted. In some cases, the mention or the opinion of a woman's perception of the sound quality might be essential to judge, good or bad, the performance, don’t you think so?
In any case, I came to the following conclusion about enhancing the high range of Altec Corona.
At first, the network called N800-E, which would had been completed successfully by the Altec engineers during that time, who were familiar with the characteristics and the sound of the 803A woofer and 802D compression driver, after large numbers of listening sessions, should be adopted as it is.
The next concept was that the band above 10 KHz, which is the highest frequency range, should be supplemented by JBL 2405, horn tweeter. In other words, it may be easy to understand as an add-on super tweeter system featuring the JBL 2405.
Before I started designing, I wanted to incorporate Alan-san’s freedom of adjustment or sound options into the design specifications of the system as much as possible. So, specifically, the cutoff frequency of the high pass filter was set to be selectable from 10 KHz, 12 KHz and 14 KHz, and a continuously variable attenuator was adopted so that the level adjustment could be finely adjusted.
In addition, an impedance correction circuit was incorporated near the input terminal of the 2405, which also made it possible to be changeable ON / OFF. Alan-san and I called this attempt the "Tweeter Adventure", and at the same time as I proceeded with the design, I started looking for a good quality used JBL 2405 on the auction site in Japan as usual.
Soon after, I was fortunate enough to find a beautiful looking 2405, and I also checked the 16 ohm printed sticker on the back and was able to win the bid. However, what wasn't straightforward was that according to my measurement, its nominal impedance was actually an 8 ohm version. I couldn't believe it and confirmed this to the seller.
The seller was an audio shop, but he said that he only removed this 2405 from the JBL 4343 speaker and had not replaced the diaphragm at all. The JBL 4343 / 4343 WX was a pretty large 4-way speaker that dominated the audio market in the 1980s, just in the middle of the Japanese bubble economy, and truly a best seller product at that time, but the nominal impedance of this speaker was certainly 8 ohms in the specification.
So, I was understood that the 2405 I got was actually 8 ohm tweeter. Anyway, I asked to replace the 8 ohm diaphragm with a 16 ohm version at a specialized shop, and at the same time the gap of the magnetic circuit was cleaned, and so Alan-san's JBL 2405 was reborn as a tweeter in mint condition.
Well, I would like to transfer the first impression of this JBL2405 add-on super tweeter system given by Alan-san on 20th September 2018 here, and finish this section.
“With kind help and encouragement from Yazaki-san, I am now using a wonderful cartridge, Shure Type 3 with MR stylus cartridge and WE24GA cartridge shell leads. It is simply a wonderful fit with Marantz Model 7 preamplifier, immediate, rich, and a very musical performer that assists in the presentation of Real Sound! The tweeters have nicely extended the range of the Altec Corona speakers. There is a very natural sense of more air and the notes hang with a little more gestalt, more of what I hear when listening to live music - a suspended belief of being in the room with the performers. At this point, I’m still enjoying the flexibility of moving between the settings at 10, 12, and 14 kHz. The presentations are different-in-kind; however, I do find it to be fun and engaging, and am loving the ability to adjust the settings, sort of like a little tube rolling directly with the transducer! Yazaki-san, I did find your feelings to be right on, the impedance compensator is glorious and is a wonderful fit with the new tweeters!”
