BluegrassDan

Altec 436A - Recovery time selection switch
« on: July 12, 2020, 07:47:48 PM »
Trying to integrate a recovery time selction switch into an old Altec 436a. Merging the EMI RS124 notes, it seems pretty straightforward. Does this look correct?


BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #1 on: July 12, 2020, 07:51:31 PM »
Here is the original Altec schematic.

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #2 on: July 12, 2020, 07:53:14 PM »
And the Abbey Road mod.

Re: Altec 436A - Recovery time selection switch
« Reply #3 on: July 12, 2020, 08:46:50 PM »
Hey Dan,
Your grafting of the release control from the RS124 to the 436 looks OK to me.   
Having built quite a few of these (whether converted 436's or as scratch builds). I found that the slower settings were mostly useless for govmt. work to be honest.   
Personally, I would half the value of the release TC cap to the RS124's value of 0.5uF  for a kick off.    It would be nice to be able to  lower that value for faster release options,, but it's not really practical without a re-jig of things with a lower impedance drive from the  side  chain circuitry.
To keep it simple, 1/ make sure you attach the release circuit stuff to the 'cold' side of the 33K resistor for lower distortion.  2/change the release cap to nominal 0. 5uF (.47uF). and  3/. Try out the 100K  output anodes to input anodes feedback resistors.  It's easy. and costs  next to nothing to compare.

As always, IMHO.  YMMV.     :)   
« Last Edit: July 12, 2020, 08:58:42 PM by Winston O'Boogie »
Note to self:  Don't let c**ts wind you up

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #4 on: July 12, 2020, 10:18:16 PM »
Thanks for the input, Winston. Those 100k resistors...do they clean things up a bit?

The "cold" side, you mean the tube grid side?

Re: Altec 436A - Recovery time selection switch
« Reply #5 on: July 12, 2020, 10:42:55 PM »
Hey Dan,
Sorry.  Yep,  by the "cold" side, I meant the side (end) of the 33K resistor on the RS124 schem  that's furthest away from the 6AL5 diode.  In  other words, just as it is on that schematic. 

With the 100K anode to anode resistors you should  see a tighter control on the balancing and response of the variable-mu input valve so, yes, they clean things up a bit.  But since it's easy to chuck them in and compare, see what works for you.  I also think shifting the bias point up on the curves as it's done via the RS124's  increased in value cathode resistor is good but, depends how far you want to mess around with the original 436 circuit.

You've got good ears, see what works for what you want to do. 
Note to self:  Don't let c**ts wind you up

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #6 on: July 12, 2020, 10:50:28 PM »
Thanks a billion!

Re: Altec 436A - Recovery time selection switch
« Reply #7 on: July 12, 2020, 11:02:37 PM »
Thanks a billion!

No big deal.  Let us know what you settle on.   
Note to self:  Don't let c**ts wind you up

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #8 on: July 13, 2020, 05:05:00 PM »
Winston, do you have any experience throwing a dual-gang pot on the front to control the input gain?

Re: Altec 436A - Recovery time selection switch
« Reply #9 on: July 13, 2020, 05:31:48 PM »
Winston, do you have any experience throwing a dual-gang pot on the front to control the input gain?


Do you mean in lieu of the dual pot across the IP trans. secondary?

If so, I haven't done it on this particular circuit.  But I don't see why a pot feeding the primary wouldn't work.   In the RS.124 thread we talked about trying a 2K5 (or thereabouts)  mono pot.
The reasoning was that there really aren't any decently affordable dual gang pots that will track well which, in theory, would upset the balance of the push-pull circuit.
In practice though,  it hasn't really caused a problem using the 25K dual in the stock position on the units folks have built. 

Having said that, why do you ask?  Is there an issue you're having?
Note to self:  Don't let c**ts wind you up


BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #10 on: July 13, 2020, 07:32:47 PM »
Just looking for modification options.

Re: Altec 436A - Recovery time selection switch
« Reply #11 on: July 13, 2020, 09:24:43 PM »
Just looking for modification options.


OK yep, I  hear you.
Well,  personally, I really do think the RS.124  exactly as per the schem I posted years ago is a worthwhile route and upgrade path over the stock Altec 436.
 


Note to self:  Don't let c**ts wind you up

jazzcrisis

Re: Altec 436A - Recovery time selection switch
« Reply #12 on: July 14, 2020, 10:47:12 AM »
Hi Dan,

Just going to echo Winston's thoughts on those 100k feedback resistors. They really, really make the unit more stable at fast release times. I actually ended up having to lower mine just a touch for whatever reason (something particular to my build I believe.)  Also, I just bought your latest record and it's killer! "West of West Virginia" is such a great song.

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #13 on: July 15, 2020, 06:38:29 PM »
Thanks for the purchase!!!

So, I got it assembled last night...stock....identical to the schematic. I was assuming this thing would be line level input, but it is totally mic level and ready for a ribbon or dynamic. Surprise, surprise.

Even with the stock 1uF release it is thumping a bit. 0.5uF is poping and bumping beyond usability. I'm gonna go through and see where the push/pull is out of balance. I'll try out the plate resistors too.

I fiddled around with a 100k pot in parallel with the 33k attack resistor. Interesting possibilities there!

Re: Altec 436A - Recovery time selection switch
« Reply #14 on: July 16, 2020, 02:01:08 PM »
 The balance control along with the neon bulb oscillator from the RS124 will go a long way in helping get rid of thumps.
The 100K anode to anode resistors will get the level down somewhat, and a control on the output to trim it helps.

In other words, convert that thing to an RS.124   :D
Note to self:  Don't let c**ts wind you up

scott2000

Re: Altec 436A - Recovery time selection switch
« Reply #15 on: July 16, 2020, 11:15:51 PM »
Anyone ever mess around with this any?

> You can't reduce them much below stock values without causing instability

Try C5 C6 much larger, like 1uFd, R10 R11 smaller like 33K.

These R-C networks interact with the R12 C4 network in complicated ways. My suggestion may make thigs worse, but I know you can handle trial-and-failure and maybe gain some insight from it.

Keeping C5 C6 smaller than C4 does help reduce bass distortion- it becomes a charge-pump instead of a simple peak catcher. You might try C5 C6 in the 0.1-0.2uFd ballpark (then R10 R11 need to be like 100K to maintain limiting down to lowest frequency).



Re: Altec 436A - Recovery time selection switch
« Reply #16 on: July 17, 2020, 12:01:44 AM »
Anyone ever mess around with this any?

To some extent, yes. 
It's been a long time so anything I say would be from memory.    Broadly - increasing C5, C6 alone  increased low freq distortion.   I didn't go as far as 1uF and am not sure where I took it to - maybe 4 X stock value?

A buffer before the diode would open up more possibilities.  You could graft on the one from the E.A.R 660 type compressor which is squalid-state and would be fine here too since, like the EAR,  we don't need big voltages.   

Note to self:  Don't let c**ts wind you up

BluegrassDan

Re: Altec 436A - Recovery time selection switch
« Reply #17 on: July 17, 2020, 12:16:59 AM »
This is a job for a friend and their budget is a bit limited (pun intended). So, here's what I did and didn't do. Obviously, I did the basics like replacing filter caps and resistors that had drifted in value.

MODS
Added 100k resistors between plates of V1 and V2.
Added a -20dB input pad switch
Added an H-pad on the output to drop the signal about 10 dB.
Added a two position attack/release switch. I like to call them "transparent" and "aggressive."

THINGS I DIDN'T DO
Didn't fool around with adding a balance pot on the cathodes.

ATTACK/RELEASE
Attemping to speed things up too much just made the thing thump, particularly at lower volumes. What I settled on was increasing R12 from 33k to 75k, which makes the attack slower.

I found that increasing the value of C4 BOTH slowed down the release and the attack. Lowering the value, coincidently, made both the attack and release faster.

Settled on making it switchable between 2.2uF (slow) and 0.68uF (less slow, but certainly not "fast"). Simply added a 1uF across a DPDT switch to put it in series with the 2.2uF for 0.68uF.

It's definitely more useful than before. Here's a video of it in action: https://youtu.be/V0HVOM5z2sI
« Last Edit: July 17, 2020, 02:37:56 AM by BluegrassDan »

Re: Altec 436A - Recovery time selection switch
« Reply #18 on: July 17, 2020, 02:26:09 AM »
Hey, hey, cool!

I didn't realise you were dealing with a unit without even an input level control.
Given the limitations you had to work within I think you did a great job.

Great singing too :)
Note to self:  Don't let c**ts wind you up

jensenmann

Re: Altec 436A - Recovery time selection switch
« Reply #19 on: July 17, 2020, 06:32:42 AM »
In one of my builds I made those plate-to-plate feedback resistors switchable between open and 100k in six steps. Without feedback the compression behaviour is wild. It´s more a joke than a usable compressor, good for destroying signals, but that´s not what we want all the time. Too much of a one-trick pony. With 100k feedback resistors it behaves very controlled. Being able to switch between these two extremes is a great tool. It should be called wildness-control.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort


 

Related Topics

  Subject / Started by Replies Last post
30 Replies
13259 Views
Last post August 26, 2006, 12:47:20 PM
by MikoKensington
8 Replies
3040 Views
Last post July 18, 2020, 04:15:35 PM
by BluegrassDan
27 Replies
13564 Views
Last post August 10, 2012, 10:13:27 PM
by rafafredd
2 Replies
2507 Views
Last post September 22, 2005, 10:51:22 AM
by NewYorkDave