I have developed a new coating material for the bass driver surrounds, a flexible, thin layer yet not sticky, giving a lower Fs than with the original Altec coating, and it won´t run down. The surround is still fully protected in moist environments.

Damping of resonances (ringing) in Altec sectoral horns 511B and 811B. Below is a detailed working process describing needed materials for the job (moulding with beach sand and resin/hardener).
This is the first part of 2, on the moulding of the bowes in the front of the horns, part 2 will tell how to make the more difficult bowes on the backside of the horns, in text and with photo´s. See first part HERE!

There have been some discussions on the Joe Net about the Altec Bass drivers. I have made a couple of documents, released a few of years ago on my homepage, about the deteriorations of the performance of the drivers, due to the aging of the coating of the surrounds, and what happened with these drivers as the years went by.
The shown example is an Altec 416-8B sn. 39880. Fs is 27,27 Hz and the impedance peak is 117 Ohm before restoring, see photo. After restoring Fs is 22,43 Hz and the impedance peak is 213 Ohm. In other words: Fs is reduced by 17,75 %, the impedance peak is 213 Ohm = 84,96 % higher. Both achievements are absolutely in the direction that we want.

The higher Fs impedance means less resistance moving the piston (cone) and bigger fluctuations at the deepest frequencies, at the same current than the uncleaned, and with less distortion .
Result: Deeper bass capability, and with better dynamics in the range up to app. 250 Hz (listening impressions).

Furthermore the cleaned and restored surround is much better looking, giving the speaker a much better and newer appearance. Read more here.

My next 2 projects will be a balanced in/out true differential tube microphone amplifier (with some other stuff), and an Onken 360 litre cabinet with Altec 416-8A or 416-8B drivers. Drivers will be my restored versions and the sound response will be measured in the 20 Hz to 1000 Hz range, against a standard Altec bass driver version (same model) like it is, bought on Ebay in the typical condition they are in when received (in 2011).
The data of the bass drivers here are typical completely changed from their new state (when they left Altec factory). Free air resonance has gone up, the cone is much harder to move, moving mass has increased with 30 – 50 grams, all because of the stiff surrounds due to the impregnation getting hard over the years. So do not rely on original new Altec data, inserted in the Thiele/Small scheme, resulting data will be wrong.

Spectral data sheets for my monolithic Mike amplifier (dynamic mike, condenser mike, signal is swept from a balanced 100 Ohm source from 20 Hz to 20 KHz) and results are excellent, dynamic range, THD and SNR is in a class of its own. Second and third harmonic are below 100 dB of 1000 Hz fundamental (who can hear second and third harmonics below 100 dB of fundamental, they are at the least 100.000 times smaller than fundamental ?).
At 4 different settings of input level and active gain, you can watch the impressive 4 spectral curves with data below the curves here. The signal out from the amp has passed 3 transformers, one from the analyzer generator to the amp, and 2 transformers in the amp.
Software is SpectraPRO, sound card is Lynx L22.

In the March news I wrote about my Professionel Microphone Amplifier which is in development. The left side (channel 1) is now finished with minor changes. This one is the monolithic channel, with top world class integrated circuits, and regulated low noise power supplies for dynamic mikes, as well as regulated phantom power 48 volt (switchable in/out) for the condenser mikes.
Frequency response is 10 Hz -0,1 dB and 20 KHz -0,4 dB for the input signal passing 2 transformers, with 600 Ohm balanced output transformer loaded with 600 Ohm and balanced input transformer 600 Ohm.
This microphone preamp has been used for all measurements during the testing of my modified Altec A7 VOTT bass cabinet (mine is the road version 828, reinforced) sound frequency response from 20 Hz to 1100 Hz, with remarkable results. See the resulting curves and pictures from the session here.