Altec 436C Vu-meter

[beatbelieve]

Hello to everyone
I am realizing a clone of the Altec 436C Compressor and I'd like to know the specs of the original Vu-meter.
Any help?

 

[beatbelieve]

No one can help me?

 

[emrr]

I don't remember for sure, but I remember the info is here somewhere.  NewYorkDave post, I think.    Maybe 250 microamps? 

 

[PRR]

> the specs of the original Vu-meter

"VU Meter".

The actual specs fill a dozen pages, and are actually still Copyright IRE (whoever owns their publishing rights now).

It is NOT like any commodity meter.

The "VU-type" meters found on old TEACs etc are often not close enough to work well, except with TEAC's non-VU circuitry.

 

[emrr]

oh yeah; the Altec does not use a VU meter.  Wrong terminology.  It only has a gain reduction meter. 

 

[Winston OBoogie]

Hello to everyone
I am realizing a clone of the Altec 436C Compressor and I'd like to know the specs of the original Vu-meter.
Any help?

Take a standard "real" VU meter and remove the diode bridge.  This will give you a 200 uA (microAmp) meter which is about what you need.  That's also what the Fairchild meter was.  

In the 436, the meter is in // with a 34 ohm resistor.  Make the 34R a rheostat (say a 50R pot) and you'll be able to trim it for the zero.

I've been inside lots of 436's and fixed quite a few of the meters.  I've used a new 100uA movement inside the case with no problem and adjusted the // resistor/rheostat/pot accordingly.  

The Altec meter doesn't follow the real amount of dB compression absolutely accurately (neither did/does the Fairchild) but, so what!  You can make it follow more precisely but it requires a different topology and is beyond your question.


Ciao.

 

[beatbelieve]

Thank you

 

[diyfanatic]

how bout these...

I've bought 2 of these, brand new...beautifull blue color.
not tried yet

(MTR) 821863
Jewell Millitorr meter has "0 - 1000" scale.
Actual full scale = 178 microamps DC, 9 mVDC.
2-3/4" square. 2-1/4" hole required.


http://www.surplussales.com/Meters/MtrArbit_0-1ma.html

cheers 
Jr.

 

[Winston OBoogie]

A 178uA (weird value but whatever  ) will work fine.  Adjust the rheostat accordingly.

 

[PRR]

> the Altec does not use a VU meter.

I see.

It isn't even calibrated as a VU meter. Many limiters used a plain VU meter, and (as WoB says) didn't worry that the dB marks were a bit off. But a real VU reads 6dB below max at half-scale; the 436 meter shows about -12dB. This is (supposedly) correctly calibrated to the 6BC8's GR curve.

> 100uA movement

It actually wants a 0.2V sensitivity. An older high-quality 100uA movement might be 0.2V naked (2K internal resistance). I was startled to find that modern high-price plastic mA meters have sensitivities above 1V.... I guess magnets have got more expensive over the decades? Or do we just not care any more?

You could trim the "34" resistor higher, but go as high as 1V and you are starting to upset the sharpness of threshold.... oh, heck, the 436's low 2:1-4:1 compression ratio is so un-sharp that it hardly matters. And I have seen reputable mods using larger resistors.

Put in a modern cheezy 1mA meter, shunt with 220 ohms, diddle so the needle sits near full scale. 1/3rd scale is ~~~~-9dB, half-scale is ~~~-12dB, 1/8th scale is ~~~-20dB.

It won't sound right without the green paint. Many people won't know the difference if you use the OLD (2-banger) John Deere Green. (There's a song about it...)

> how bout these...

An amazing cosmetic match. Probably the same case-mold.

Leave it marked "milliTorr", confuse your friends. The current is near enough. "9mV" is suspiciously low, but as WoB says, you'll trim the shunt so idle is near full scale. Then 300mT is lightly trimmed, 20mT is brutally crushed, etc.

.... it is odd that this meter is shown with needle at full scale, while others are shown reading zero (as you'd expect if the photographer didn't have meter-test gear on stage). Maybe it is right-zero? Maybe you can fiddle the springs to bring it to the left side, but the springs are very easy to tangle. Maybe best to leave it as is, let the "normal" position be zero-left, and the "more squash" range be to the right.

 

[beatbelieve]

Many thanks to everybody!

 

[Holger]

diyfanatic, thanks for the link. These guys have cool looking meters.

 

[diyfanatic]

yep!!

I found the link in a thread here!! (the magic place...)

I can say that they look very, very nice.

there is some nice vu meter too...but pretty expensive.
lots of power trafo, input output trafo, case...

 

[bobschwenkler]

Any thoughts on the suitability of a 50uA meter? I've found a couple at a good price (this is for an original unit) and may just go ahead and give a try, see what happens.