Altec 436 Mods

[NewYorkDave]

Ha! He added an attack control, balance control and a zero trim for the meter. No big deal... this plan is very similar to the mods I did on Soundguy's 436. And he even added a U-pad on the input, just like I did! I guess obvious problems often have obvious solutions.

But unlike this guy, I wasn't savvy enough to sell my mod on eBay for hundreds of dollars. Like a chump, I just gave it away :roll:

 

[rascalseven]

I doubt the guy who reverse-engineered that mod was the guy selling it. The date on that cover letter is July 1991, and it does look typed, typos and all.

My guess is, whoever "Richard" is may be the guy who decided to sell it all these years later, or maybe someone Richard shared it with.

Bit of a disappointment for $200, I'd say.

JC

 

[SUPERMAGOO]

436 can work w/ pcc189 in place of 6bc8?

 

[Viitalahde]

[quote author="NewYorkDave"]Like a chump, I just gave it away :roll:[/quote]

No more Mr. Nice Guy :twisted: :twisted: :twisted:

 

[leigh]

[quote author="leigh"]I'm gathering parts to overhaul my 436.

For P2 and P3 (release and threshold controls), did you use linear pots or log? (You mentioned that the 100K pot you added for attack was linear.)[/quote]
To answer my own question: I measured both these pots, and they are linear.

Leigh

 

[SUPERMAGOO]

i have mp3 here compresed by 436 clon .not work right by now but i like!!!

http://groupdiy.twin-x.com/displayimage.php?album=40&pos=40

 

[leigh]

[quote author="PRR"]Wait.... no regulated supply??? The voltage at the top of C8B needs to be steady: this could be your instability. Toss 100uFd in there and see if stabilty changes. A fat-cap may solve the dynamic issue, but for pro-quality output regulation against line variation it should also be regulated with a Zener or gas-tube. ~200V seems a suitable value. This will however harden the soft-knee curve a bit.[/quote]

For the record: I just tried adding another 100uF to the top of C8b, and unfortunately it didn't help the recovery lag issue (as I describe here: http://www.groupdiy.com/index.php?topic=10173 ).

I tried it both parallelling the extra 100uF with the existing cap, and removing the original cap as well for a full replacement.

I'm still baffled by what is causing this to happen.

Leigh

 

[leigh]

I found this explanation in a thread at rec.audio.tubes, full text is here:Release time mods for Altec 438A tube compressor

"The more complicated answer is that you really should reduce the size of the cap if you want a faster release time, since the cap is hanging off the plates of the output stage and can directly cause slew limiting. This is because the current that is drawn to charge the GR cap is signal current, drawn only at the signal peaks. Not very hi fi... The smaller the gain reduction cap, the less slew limiting and thus less distortion."

This might account for the dropouts I'm hearing. They become more severe the shorter the release time. This is all for stock values in the RC circuit. I describe the weird dropouts in detail here: http://www.groupdiy.com/index.php?topic=10173

Another way to put this behavior: the meter is lying! To wit: I'm sustaining a note, and it's holding it at 10 dB gain reduction, and then I release the note. The meter starts swinging back, and when it's passing 5 dB gain reduction, I play another note. However, I can't hear the note right away! So it's not hitting it with 5 dB of reduction right then, it's hitting it with 30 dB or more... and it's lying to me about it! In the next half second or so, the note fades in quickly, and the needle returns to its previous spot at 10 dB gain reduction.

Ah, this is turning into quite a love/hate relationship with this little green box.

Leigh

 

[craigmorris74]

Leigh,

Are you planning on trying the smaller gain reduction cap? Would love to hear your results.

Crai

 

[NewYorkDave]

Note: the photos linked at the top of the thread are still there, but the schematics have been removed.

 

[pucho812]

everything o.k. dave?

 

[leigh]

[quote author="craigmorris74"]Are you planning on trying the smaller gain reduction cap? Would love to hear your results.[/quote]

In a perfect world, yes.... however, I had to get the 436 off the bench and back in the rack, so I could use it for some projects. I don't know when I'll pull it out again.

I also suspect that it wouldn't be such a simple mod. If you change the attack/release circuit's cap size, you of course would need to change the resistors (inversely, proportionally) to maintain the same time constant (t = RC, for seconds / ohms / farads). However, when you change the resistors there, my understanding is that it's going to change the current draw through the tubes, and I don't understand that well enough to just start haphazardly swapping in parts. The 436 is a simple circuit, but still just a bit beyond my complete grasp, and the different sections interact in ways I can't predict. I'm all for trial and error, except when it comes to expensive parts and high voltages!

Cheers,
Leigh

 

[leigh]

Just stumbled upon this Eddie Ciletti article, on bringing an Altec 438 up to spec:

http://mixonline.com/recording/applications/audio_dynamic_resurrection/

438A schematic:
http://www.purpleaudio.com/pdflib/altec438Acomp.pdf

This is the same circuit as the 436, plus a mic pre. He's working on an 438A model, so no thresh/speed controls, but he adds these controls, along with a fast/slow switch.

The bummer is, Mix online doesn't include the diagrams! So I don't get to see what his RC circuit looks like. Anybody got this issue (Mix, Jan 2002) who'd like to scan in the diagrams?

Thanks,
Leigh

 

[solder_city]

dont worry, if its an e.c. mod its either obvious or crap anyway

 

[skipwave]

[quote author="leigh"]This is the same circuit as the 436, plus a mic pre.[/quote]

Holy Crap! 90db gain!:shock:

I need to build that for my ribbon mike.

 

[solder_city]

do you have any idea how much 90db gain really is?

thats 31662 times. thats enough to amplify a 10 *microvolt* (-78dbu) signal to +12dbu! and i can assure you that unless your ribbon mic doesnt have a transformer in it, its output is quite a bit more than 10 microvolts

nobody outside of radio astronomy needs 90db gain.

i admit that ive never used a 438 but i have heard others bitchin about the excess gain making it hard to use. but i have used a 436 and that has so much gain on its own you can plug a mike right into it.,

 

[soundguy]

I will attest to that.

A stock 436 is really hard to use as is, I use to have 4 438's and can promise you that they are totally useless as a result of the gain present on the front end. Its present ALL the time, it might be overkill for your ribbon mic but itt is guaranteed TOO much for every single other mic you have.

Makes MUCH more sense to build these things out to standard +4 line level specs. If you have a mic that needs crazy gain you could always follow your mic pre that doesnt have enough with a line amp and put that in series with the limiter. I have always wondered both what altec was thinking when they made those things and also how people use them in modern recording studios that arent PA systems in grocery stores. I had a bank of -34dB pads that I built just so I could interface 438's with the modern world of +4dbu standard. If you are building one from scratch the only thing you stand to gain from keeping their gain structure is a pain in the neck. This is a design that absolutely can be improved if you know the only place you are going to use it is in your recording studio doing "normal" things.

If the mod just mentioned doesnt feature intense padding you can file it under solder city's suggestion.

dave

 

[leigh]

Ah, well to clarify, I've got a 436C that I'm still trying to improve. I've got no intention of cloning a 438.

My interest in the Eddie Ciletti/Mix article was that he took an "A" model 438, which has no compression controls, and added a variable RC circuit to give control over speed and threshold. My work on the 436C (which, being a "C" model, does have a threshold and a release control) has lately focused on playing with the parameters of that RC sidechain. So I was interested in seeing what Eddie came up with "from scratch" for an RC circuit for his model A, and compare it to what's stock in a model C.

But at least solder_city expresses skepticism in Eddie's work... is that the consensus around these parts? I've not looked at Eddie's stuff before in detail.

Leigh

 

[solder_city]

what i said before was a little mean. i should clarify. i have no personal beef with eddie. but what ive seen from him doesnt justify the rep he seems to have (with some people) as being some kind of super tech. i think that having your name in a magazine gets you instant credibility with certain segments of the population.

 

[skipwave]

[quote author="solder_city"]do you have any idea how much 90db gain really is?[/quote]

No, not a clue. Never had a pre that could do more than 60, and most would get noisy at that point.

This was just my being reactionary, after having done some singing into my ribbon and being surprised that 60db was just barely enough to get a usable signal. A line amp and limiter, as dave suggests, would be a more flexible solution. I could also just sing louder. :razz:

[quote author="solder_city"]nobody outside of radio astronomy needs 90db gain. [/quote] :green: