Ah the classic Altec 436C compressor (see here for previous coverage on PS dot com).
Here’s a fresh high-res scan of the original product-sheet (2pp):
DOWNLOAD: Altec_436C
I built one of these some years ago and it really sounds great. I used UTC ouncer -series transformers. Don’t be fooled by the tiny size – these are very good units. In fact, Ouncers are used in the early Urei 1176 as well as UA-175 and 176 compressors.
You can see how super-simple the circuit is. Aside from using a conventional power-supply circuit (rather than the voltage doubler that the original unit uses), i built mine pretty much exactly the same as the schematic.
Now, if you look at the schem, you can see that there is no provision for an output control. This is a problem because these things add a lot (like 20 db or more) level to whatever you put into them if you have the input control high enough such that the unit is actually compressing. In the past I have gotten around this by using an outboard Daven H-pad attenuator that I mounted in a little box. This is not ideal for ergonomic reasons. Anyhow… while studying the (very similar) Gates Sta-Level schematic the other day, I was intrigued by the very simple, very inexpensive variable balanced output pad that the Sta-Level uses.
It’s like $5 of parts. Five 1/2 watt resistors and one pot. Based on information in the Sta-Level manual, adding this circuit after the output transformer of the 436C will provide a minimum 10db and a maximum 16db attenuation. Perfect. Now, it’s true that using this control will vary the effective output impedance of the unit slightly; but according to Gates, “This pad has been carefully tested to assure that the small impedance mismatch resulting from this range adjust-ment will not affect frequency response or other characteristics. ” And Gates was writing this back in the day when the Sat-Level would almost certainly be seeing a 600ohm load. Considering that nowadays it will more likely see a bridging load of 1500 – 10k ohms, I think it’s safe so say that this circuit should be (at least as) sonically-transparent (as a vacuum-tube vari-mu limiter can be).
Gonna dig up some matched 160 ohm resistors and give it a shot… more to come…
5 replies on “Altec 436 Compressor: Taming the Output Level”
Hello, I have an Altec 436C. I found it nice in some ways, but almost unusable. I wanted to know, is the frequency response quite flat on yours ? Because on mine it isn’t at all.
Chris
I don’t have an original. but i’ve serviced them. it should be fairly flat 40-15K. if its not, it needs servicing.
Hello!
I was wondering if you could help me on how is working the Altec 436c.
My specific question is:
how big is the gain made by the 6bc8 tubes ? I calculated approximately 20-25 (no units) . But it wouldn’t make sens beacause the 6cg7 tubes are biased with -3V at the cathodes.
How i am seing this is: if I put 1V peak sin wave as an input, it gets amplified by 25 first and then goes to the push-pull stage. but if the cathode are at -3V the input signal of the push-pull stage will easily be more than 3v (in this example it would be 1×25=25V)
And I know I’m not supposed to insert signals larger than the cathode bias.
What am I getting wrong?
Thank you very much for your help! It would be a lot appreciated.
Cordially,
Étienne
Could you tell me please the utc transformers you used?
Hello: I am building a 436 clone from the attached diagram. I figured with your experience you could explain one thing: There are two stepped switches at the bottom of schematic from the 6AL5 back to the input stage. I understand input gain, output gain and ratio, but what to these other switches do ? Can they be potentiometers as well ? Thanks !!
http://i1110.photobucket.com/albums/h448/DBPendrill/436ControlAmp.jpg