Vactrol Based T4B. Absolute Value Circuit

I was checking out some of the old RO T4B building threads and saw a vactrol based T4B. Im wondering how I use a vactrol based GR circuit in an LA3A AC sidechain. I know that LEDs can run off AC but seems like it would need a full wave rectifier.

Ive been checking out something in Art of Electronics called the "Absolute Value Circuit" 1981 Ed. Page 120. Its a dual opamp based full wave rectifier that looks like it might be a perfect solution for this issue.

Any thoughts?

Helsing

Details escape right now but I do recall some subtle pin issues with the LA3.

It revolved around the use of ground pins in the octal connector. The Dual Vactrol unit has a shared ground and two HOT pins.

As I said my memory is vague but I'm sure if you study the schematic you will see the that a link is needed .... or is it a link that need to be cut.

:roll:

Kev,

What I most curious about is if AC triggering of the vactrol led is a compromise since the LED would be running on only the positive half the wave.

Helsing

Punk is the Vactrol king. Drop him a PM.
Kev had a page on this. Is it still alive?

I don't think I have removed it.

looking .... looking
:shock:
http://www.recording.org/users/kev/T4B.htm

and cj's masterpiece
http://www.vacuumbrain.com/docs/t5.html

... did we use a single diode on each leg ?
???
:?
does the LED actually get reverse voltage ?
It may not need a diode.

"....compromised "

perhaps :roll:
we were looking for a simple solution to use a test device which ended up being useful in normal operation even though it was not a clone of the original.

Kev, just a 1N4148 or 4001 across the led ought to do it. reverse polarity of how the led is orientated.
:guinness:

> a vactrol based T4B

The green-glow cell is capacitive and frequency sensitive. An LED will not work the same.

Since the green-cell works at very high voltage, and the LED is floating, no clever rectifier is needed. Use a simple full-wave bridge rectifier. Then add a big resistor from that to the LED, maybe 5K. That will limit, but you have scret the frequency response. You can either take all the EQ out of the LA2 sidechain, or try replacing the LED resistor with a cap, maybe 5,000pFd. It may take a little of both approaches.

I think you should use well-choosen Vactrols in modern gear designed for flat LED response, but to get an LA2 and the unique green-cell response you need to use a green-cell.

[quote author="PRR"] ... An LED will not work the same. [/quote]
no it didn't

but it did provide a adequate test unit and/or temp fix.

Byron and Kenetek ( I think it was ) did provide a few graphs and drawings to show the differences.

Ounce cj had done some work with the more original pieces the development of the Vactrol based iseas did fall by the wayside.

Kenetek Pro Audio.
http://www.kenetek.com/
...
we haven't heard from him for a long while.

[quote author="Kev"]does the LED actually get reverse voltage ?
It may not need a diode[/quote]
Yes it does.

-Simple test:

Wire up the circuit that you're thinking of using, but replace the LED leg of the vactrol with two diodes in opposed-parallel (i.e. nose-to-tail parallel.)

Feed signal. If one LED lights, you have inidirectional current, but if they both light, you have bi-directional current.

[quote author="PRR"]The green-glow cell is capacitive and frequency sensitive. An LED will not work the same. [/quote]

Recent testing of the T4 which I ran to check that the Bloo T4s behaved as the Teletronix versions did has indicated that the EFL panel is largely flat across the band. The feed circuit for the first set of these tests was the output of a crown amplifier, but with the series capacitance inside the T4B still present.

However, transferring the T4B to an LA-2a (which feeds a higher impedance source from the plate of a tube, via a DC-blocking capacitor) indicate that the response is largely flat across the audio band, so long as the drive voltage does not vary (The LA-2a has a pre-emphasis to the GR driver tube)

That being said, Vactrols do not have the time constant 'memory' which the CdS photoresistors do, and all the ones which I've played with need 'slugging' capacitance to avoid becoming variable clippers. THis means attack and release controls are now a reality, but totally change the circuit topology which is needed.

Vactrols are essentially unusable in applications which currently use the T4, unless you in-build a time-constant and rectification circuit into the T4 replacement module... all this has to be passive of course. This means thar the behaviour will be fixed instead of variable...

It's possible, but it will be so unlike the T4 in character, I abandoned the idea as unworkable.

Keith

I thought up a way to provide a variable compression curve for the T4: Use the clear relay package that theprototypes were made from, stick them inside the chassis, and put a light bulb or el panel on a pot so you can vary the ambient light that the cells see. This shoulfd change the character of the compressor. I will have to give it a try.

Funny you should mention it... we tried that this week. (accidentally!)

-Lousy result... very little compression range remaining , since the attenuation range of the cell is unfortunately finite. -The slope isn't altered from the rotation point (i.e. the deviation point from the 1:1 line) instead it's altered from the destination point, but the rotation point isn't moved downwards... the threshold of the EFL panel isn't actually moved anywhere... it still takes about 20V before it starts to glow.

As I mentioned, it was an accident (the tape that I'd used to cover the clear relay turned out to be transluscent instead of fully opaque) but the result was rather bad. We ended up wrapping a black T-shirt around the can and matters improved a great deal.

Keith

Cool! That's one less thing I have on my plate.

[quote author="cjenrick"]Cool! That's one less thing I have on my plate.[/quote]

..how many volts do you usually have on your plate, CJ...?

:wink:

Keef

The plate in my head or the plate on the 6V6?
:razz:
Man, I heard about a guy who had a plate in his head due to a water skiing accident. He lived in Detroit. He said he couldn't shovel his driveway off for very long because the plate in his head would freeze! Yukka!
:shock: