DIY T4B, matching EL Panels and Cells

[joe-electro]

Excellent info, Dave.  Thanks!

PS - You're right about the avatar. Schmalien was getting a little old

Hi Joe,

Really good Youtube video, well done.

You need a new Avatar, you're really a regular guy, not weird at all!

Regarding the EL panels:  I can fill in a bit of history here.  The development work was done in the early 60's by many companies, mainly in the US.  They even talked about making giant wall panels for general lighting.  Apart from the child's nightlight application, they also hoped to use them for car instrument backlighting; the problem is that they need ~100Vac to work so that was a problem with only a 12V supply.  They lapsed into obscurity until the mobile phone industry required a backlight for their displays in the 90's, electronics had solved the high voltage problem by then.

Sylvania was the only company still making the stuff, but by then they had developed a waterproof coating which extended the life considerably.  No-one else was able to achieve the same level of brightness as the Sylvania phosphor, I know because my company tried and failed!  I think all the panels available now use Sylvania phosphor and they have cornered the market, (min order used to be $2000, about a pallet load) and their material is certainly better than that available in the 60's due to the continuous development over the last 50 years.

Although they start working at ~100Vac they increase in brightness up to 400Hz and start to tail off again past 1kHz, I think this is why there is a frequency shaping circuit between the 12AX7 and the 6AQ5A, the frequency response is thus the reciprocal or opposite of the EL panels response to frequency.  I left it in on my Green LED circuit but maybe that was a mistake thinking about it now.  I'd be interested to hear what others think on the subject.
best
DaveP

 

[Ptownkid]

Good lord...that's some serious t4b dedication. Nice rig!!!

 

[dmp]

I took some more data. First, compared a Drip EL panel to the panels being sold in the BM by 12volts. Very similar response. Drip's have a little less intensity and a slightly quicker release. These tests comparing EL panels were done with a single Silonex LDR. For this LDR, the 50% release times (50% in dB) were 11.7ms (Drip) and 19.4 ms (12volts). the specification in the manual for release is: "Appx. .06 sec for 50% release, 0.5 to 5 secs for complete release depending on amount of prev. reduction"






Next I compared the dynamic response of 10 new Silonex cells I got from Allied (I had previously compared 10, and this is a new set of 10) with a Drip EL panel. These have a gray element. I also compared 4 Silonex cells with a 'rust' colored element, as described above by Joe. Although the four rust colored LDRs were closer in response than the 10 gray cells, they showed significant variation. The rust color cells showed more sensitivity (GR) to the light source.

Gray:


Rust:


And finally, I selected 4 gray cells that had similar dynamic response. The 50% release point (dB) is at ~23ms using a Drip EL panel. Presumably, switching to a 12volts panel would lengthen the release.
I put these in my La3a and it sounds great, very fast. The meters of the two units respond identically when sent the same material.

 

[joe-electro]

Great test data!  However, the pulse length is not long enough to show one other significant difference between the rust and gray colored cells, and that is attack time.  The gray cells have a significantly slower attack, so much so that I consider them unusable, or at least definitely not true to the original sound of the classic T4B's.  I have been wondering whether other people (who might not be as familiar with the sound of T4B's as I am) consider them acceptable. Can we set up a poll to find out what people think?  I could set one up on the Kenetek site if there is enough interest.

Thanks!

Joe

 

[dmp]

interesting point! It would change my interpretation of the results if that were the case. While I have things set up I might be able to run another test with a longer trigger.

 

[dmp]

Here is the test of gray vs. rust LDRs with a longer trigger. Great insight Joe - the Rust colored cells have a steeper slope (faster attack).
For comparison, the 50% of max GR point from this graph, for the gray is at 105ms, while it is 67 ms for the rust.
The LA3A manual states 1.5ms or less for the attack time...

 

[dmp]

Anyone have an original LDR I could test?

 

[joe-electro]

I'll test an original vs. a rust colored vs. a gray cell and post screen shots of the results as soon as I can get to it. My T4B tester can test up to 8 cells simultaneously using the exact same test conditions.

Joe

Anyone have an original LDR I could test?

 

[joe-electro]

The difference in attack times in something I heard (and saw) immediately.  Even though it looks subtle on the graph it is signficantly different in sound. I have a test fixture using my "Frank" CLA-2A that allows me to switch between two T4B's, and it was immediately obvious which one had the gray cells.

I'm still looking for a better match for the originals, so if anybody has any ideas please post them here.

Joe

Here is the test of gray vs. rust LDRs with a longer trigger. Great insight Joe - the Rust colored cells have a steeper slope (faster attack).
For comparison, the 50% of max GR point from this graph, for the gray is at 105ms, while it is 67 ms for the rust.
The LA3A manual states 1.5ms or less for the attack time...