Fetching ideas for the altec 436 circuit

[mich]

hi all,

Looking for a simple way of "fine tuning" the output level of the 436.

2 db range will suffice.

I have an output attenuator (600R balanced) following the output transformer.-

Cant think of a way of doing that in the circuit without changing the compression,and after the output txformer it will be dificult to do with a pot?

 

[PRR]

> after the output txformer it will be dificult to do with a pot?

Why? Use a 1K pot for 0dB-20+dB trim, or 100 ohm pot plus 500 ohm fixed for 0dB-2dB trim. Or just load with 60+60+60+420, and tap 0dB, -1dB, -2dB, -3dB as needed.

Who works to 2dB accuracy anyway?

 

[Schmilsson]

http://classicapi.com/catalog/product_info.php?cPath=48&products_id=106

 

[mich]

Why? Use a 1K pot for 0dB-20+dB trim, or 100 ohm pot plus 500 ohm fixed for 0dB-2dB trim. Or just load with 60+60+60+420, and tap 0dB, -1dB, -2dB, -3dB as needed.

- I prefer a pot than taps on a switch, so it will be from the positive phase of the transformer to a 500 ohm resistor to one side of the 100 ohm pot ,with the other side of the pot to the negative phase and the outpot taken from the wiper and positive?

||------------Out +
||      R
||      |
||      |
||      P-------Out -
||      |
||      |
||      |
||-----

Who works to 2dB accuracy anyway?

its for calibrating tube mismatches between two channels- i got only 1  "near" close match out of 30 6BC8's...Thanks for the help

 

[abbey road d enfer]

hi all,

Looking for a simple way of "fine tuning" the output level of the 436.

2 db range will suffice.

I have an output attenuator (600R balanced) following the output transformer.-

Cant think of a way of doing that in the circuit without changing the compression,and after the output txformer it will be dificult to do with a pot?

Have you tried varying the common cathode resistor of the final tube (R7, 220R)?

 

[mich]

Have you tried varying the common cathode resistor of the final tube (R7, 220R)?

- No, because i guess it will also alter the control voltage - even if just a little.but its something to try if the pot wont work.

 

[emrr]

1st, are we sure P3 is matched in adjustment here?  It will vary threshold, ratio, and output level all in one go.  If it's a pot, as original, it's impossible to truly calibrate stereo action as a practical matter.  But you didn't mention stereo.  Not sure, but I would guess any difference is threshold/ratio might not be noticed here, and it may work fine as a trim control in conjunction with the input pot; a balancing dance. 

 

[mich]

Indeed P3 is a pot - and i'm trying to calibrate for stereo operation.but i have both CV's linked so each P3 is afecting both(when feeding both channels).

and variations are most noticable when doing different gain reductions - e.g - 0.4db difference between channels with 2 db GR , then 1.5db difference with 8 db GR and back to 0.5 db difference with 15 db GR...

now,for me its fine...1.5db difference when playing music through it dosent bother me at all,but i gave the comp to a friend in the studio and he is passing signal generator tones and such through it, and the output is not balanced - so i thought of some kind of fine tuning the output to make over for those variations.

 

[emrr]

The joy of tube compressors!

I see your issue now.  I think the addition of a pair of those 600 ohm T attenuators (or a pair of pots) to the outputs is the simplest solution.  Or PRR's idea of a 100 ohm pot on top of a 500 ohm resistor.

I find myself lining up tube limiters with test tones and only mono centered tracks AFTER I've got it roughed in by ear.  Time consuming and imprecise.    Most of what I have already has either a T or a ladder on the output already.  

Nothing other than perfect tubes is a complete solution.  If you have a program that varies greatly in dynamic range, and crosses the threshold by a good amount during processing, the balance will also shift based on the tube curves.    Here we also have the issue of an output stage with no feedback loop, so gain is likely to vary there as well, being different 5 minutes after startup and then again 2 hours later. 

 

[mich]

The joy of tube compressors!

Yes!!......and the sorrow that comes with it........

Anyway, the pot worked out perfectly (Thank you again PRR!) - i mounted a pot with a built in switch so i can disconect the termination alltogether if i dont want it.

Nothing other than perfect tubes is a complete solution.

- Very True - i guess thats one reason why they "parralel" a few of them..