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Project Specific Discussions => Dynamic Processors => Topic started by: Falk on November 20, 2019, 02:59:42 AM

Title: Altec 436 / RS124-ish Clone
Post by: Falk on November 20, 2019, 02:59:42 AM
Good Morning,

I am planing a Vari Mu build which is as close as possible to the named compressors and currently I am designing a front panel.
What would you guys go for? Any feedback is highly appreciated.

Link to three designs:
https://www.dropbox.com/s/dfyqlgxxuoudykc/Design_3colours_Zeichenfl%C3%A4che%201.jpg?dl=0

Thank you!
Falk
Title: Re: Altec 436 / RS124-ish Clone
Post by: weiss on November 20, 2019, 09:18:16 AM
Like the design. I'm also building two clones at the moment.
I would go for the black one. Will you use brushed metal? or powdercoating?
Title: Re: Altec 436 / RS124-ish Clone
Post by: kCal on November 26, 2019, 05:46:14 AM
Definitely green!
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winetree on November 26, 2019, 11:31:46 AM
Green is Altec.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on January 09, 2020, 06:56:41 AM
This is how it got in the end.
Title: Re: Altec 436 / RS124-ish Clone
Post by: ruffrecords on January 09, 2020, 09:47:30 AM
I would be wary about including company logos, especially EMI. They are known to be litigious.

Cheers

Ian
Title: Re: Altec 436 / RS124-ish Clone
Post by: RuudNL on January 09, 2020, 10:22:25 AM
Nice, but I would omit the  'EMI' and 'Altec', because they aren't...
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 09, 2020, 11:59:46 AM
Green is Altec.

Maybe but RS124 are cream.
Title: Re: Altec 436 / RS124-ish Clone
Post by: weiss on January 09, 2020, 06:13:23 PM
This is how it got in the end.
nice, i like the design!
Title: Re: Altec 436 / RS124-ish Clone
Post by: giohopper on July 05, 2020, 05:58:24 AM
This is how it got in the end.

Nice design Falk.
May I ask you which schematics did you use?

Giovanni
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on July 05, 2020, 06:25:11 PM
I used the one attached but would recommend to read the threads about the altec 436 and emi rs 124 in the forum. There are a couple of them providing all the information needed.

Best regards,
Falk

Title: Re: Altec 436 / RS124-ish Clone
Post by: giohopper on July 06, 2020, 11:43:42 AM
I used the one attached but would recommend to read the threads about the altec 436 and emi rs 124 in the forum. There are a couple of them providing all the information needed.

Best regards,
Falk

Thank you very much!

Giovanni
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 23, 2020, 12:11:37 PM
Hello good afternoon, I have a power problem in the section of the tube heaters.
my toroidal teansformer on the 6.3v winding has 1.8A to feed the 3 tubes on the 6AL5 pins 3 and 4 on the 6cg7 pins 4.5 and on the 68c8 4 and 5 as well.
My problem is that the filaments do not turn on, I do not get any sound.
With the 3 tubes I measure the alternating voltage and it does not decay, it stabilizes at 6.5v.
I did not tap on the toroidal winding.
I need help because I don't know what happens.
Thanks a lot
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on December 23, 2020, 12:38:26 PM
Have you wired the filaments in parallel?

Heater current:
6AL5  0.3A
6cg7 0.6A
6bc8 0.4A

Thats a total of 1.3A which equals 72% of the current your toroidal can deliver. When the filaments are cold, they act almost as a short circuit so the current is much higher. I would at least use a power tx that can deliver double the current needed.

Other than that: Disconnect the compressor from the power socket. Disconnect the heater wiring from the toroidal. Take out the tubes, connect a multimeter to the heater wiring and check for continuity of the wiring. When no tube is installed there should be no continuity. Install only one tube and you should read continuity. Check this for every tube solely. Check if the heater secondary of the toroidal is okay.

Double check if you counted the pins right. The pins are counted from the side of the pins and not from above.

Good luck!
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 26, 2020, 12:43:32 PM
Falk thank you very much, my lack of experience made me wire the socket of the tube looking at it from above, so as you said I did it the other way around.
now I have another problem, it only sounds a loud hum, I have followed the simple scheme of altec 436 as it is with input transformer escor 10k 10k and output transformer xpp1 600-15,
When I remove the 6bc8 tube the thunderous hum disappears, when I touch the pins of this valve it sounds the touch that I am but no sound when I put music through the input jack.
Thank you very much and greetings.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on December 26, 2020, 01:58:02 PM
Do you have an xlr connected to the input? The hot and cold pins must be connected. The input is very sensitive and the hum might come from an open input. What kind of power supply do you use? What kind of hum is it? 50Hz/60Hz or 100Hz/120Hz?
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 28, 2020, 03:51:03 AM
thanks again for answering, I finally undid the project because I was not able to make it sound or remove the buzz.
so I have followed this scheme that I took from another tutrial and now it sounds and the buzzing has gone, but it sounds very low level and distorted,
I have bypassed the transformer input potentiometer by connecting the two tube inputs directly to the jack plug and the same audio is experienced.
Now I don't know where to walk.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 28, 2020, 09:40:11 AM
thanks again for answering, I finally undid the project because I was not able to make it sound or remove the buzz.
so I have followed this scheme that I took from another tutrial and now it sounds and the buzzing has gone, but it sounds very low level and distorted,
I have bypassed the transformer input potentiometer by connecting the two tube inputs directly to the jack plug and the same audio is experienced.
Now I don't know where to walk.

Make sure you have your i/o connections properly made.  Are you connecting to it with a balanced or unbalanced  interface/desk  ?    If you are unbalanced you need a connection bewteen cold & ground
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 28, 2020, 11:23:21 AM

hi Rob how are you?
Well, I have passed sound in both ways and I get the same result, in unbalanced mode connecting pin 1 and 3 of the xlr in point and balanced I get the same result, I have also connected the xlr directly to the first tube, I have to say that I use a 6ES8 instead of 6BC8 (I read somewhere that it gave less problems).
I have connected only the volume potentiometer and the tube balance potentiometer as in the diagram.
Any support would be of great help to me since I don't know where to start.
thank you
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 28, 2020, 12:35:43 PM
hi Rob how are you?
Well, I have passed sound in both ways and I get the same result, in unbalanced mode connecting pin 1 and 3 of the xlr in point and balanced I get the same result,

If doing this you need in trs terms to connect tip to tip & then sleeve to sleeve & ring.          The point is that since the input & output of the rs124 are transformer balanced you must have connections to both sides of these transformers.  Make sure this is the case or you will have a low level signal that distorts.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 28, 2020, 12:46:26 PM
I have connected the trs as follows:
jack input
tip to 1 wsm 10k 10k
ring al 4 wsm 10k 10k
ground to rack chassis

output transformer (inputs)
pin 1 6gc7 to pin 1 xpp1 600-15
pin 6 6cg7 to pin 5 xpp1 600-15
40uf positive capacitor to pin 2 xpp1 600-15
  Departures
pin 7 xpp1 to pin tip jack
pin 9 xpp1 to pin ground jack with jumper to ring pin.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 28, 2020, 12:49:43 PM
I have connected the trs as follows:
jack input
tip to 1 wsm 10k 10k
ring al 4 wsm 10k 10k
ground to rack chassis

output transformer (inputs)
pin 1 6gc7 to pin 1 xpp1 600-15
pin 6 6cg7 to pin 5 xpp1 600-15
40uf positive capacitor to pin 2 xpp1 600-15
  Departures
pin 7 xpp1 to pin tip jack
pin 9 xpp1 to pin ground jack with jumper to ring pin.

Have you checked the voltages you are getting at different points with those on the diagram ?
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 28, 2020, 01:16:02 PM
I have measured from the union of the two diodes to the beginning of the 10k resistance and I get 207v in DC.
from the union of the two diodes to pin 6 of the 6bc8 tube and I get about 165v.
from the union of the two diodes to the union of the two 47k resistors and I get 182v.
on the 3 of the output transformer I get 130v.
on pin 6 of tube 6cg7 I get 130v.
All these values are taken with the unit turned on without passing audio through it, I do not know if it is correct to take the readings in this way.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 28, 2020, 01:32:16 PM
WHat do you get on pins 3 & 8 of the 6cg7 & the wiper of the wiper pin of the balance pot of the 6bc8 ?

Also from what you seem to be saying you haven't got exactly the RS124 circuit.   
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 28, 2020, 01:47:07 PM
in the 6cg7 tube I get
on pins 3 and 8 -160v.

6bc8 potentiometer with potentiometer turned counterclockwise:

pin 1 52v
pin2 52v
pin 3 52v

I have the altec 436 circuit that I think is simpler.
I have a toroidal power transformer with a 135v secondary with 120mA
and another secondary of 6.5v of 2 A
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 28, 2020, 02:48:39 PM
in the 6cg7 tube I get
on pins 3 and 8 -160v.

6bc8 potentiometer with potentiometer turned counterclockwise:

pin 1 52v
pin2 52v
pin 3 52v

I have the altec 436 circuit that I think is simpler.
I have a toroidal power transformer with a 135v secondary with 120mA
and another secondary of 6.5v of 2 A

Sounds like you have an Earth missing somewhere or you have done something wrong.  Check your work.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 29, 2020, 03:47:59 AM
Well, you were right SR Rob, I forgot to connect the 33k resistor, now the ditorsion has gone, the volume sounds high for a couple of seconds and falls again to a very low volume, after another seconds it rises again and so on, maybe a voltage issue?
Thanks
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 29, 2020, 07:03:37 AM
Check the main Power Supply voltage when this happens.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 29, 2020, 02:37:47 PM
I have measured the voltages while music was playing through the unit and there was no voltage drop, I have checked the connections and everything seems to be fine, however something makes me suspect the 1uf capacitor (according to the original scheme of altec 436) that goes to the tube 6cb8. When I cut the compressor current there are a few seconds that everything sounds as it should.
I'm going to try to draw a picture of my connections in case you can help me finish this project that is causing a headache, and they said it was simple.
thanks
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 30, 2020, 06:09:11 AM
It seems that I have solved the problem of the fall, it was a bad job of wiring in the circuit, I am learning little by little thanks to you, I do not have anyone close with this knowledge.
now I have a problem in volume potentiometer.
I have a double 50k potentiometer with 8 pins, I have never used one of these and it is giving me an oscillation, when I change to the configuration of the altec 436 diagram it sounds very loud and does not act.
any suggestions?
Thanks a lot.
Title: Re: Altec 436 / RS124-ish Clone
Post by: DerEber on December 30, 2020, 11:15:27 AM
It seems that I have solved the problem of the fall, it was a bad job of wiring in the circuit, I am learning little by little thanks to you, I do not have anyone close with this knowledge.
now I have a problem in volume potentiometer.
I have a double 50k potentiometer with 8 pins, I have never used one of these and it is giving me an oscillation, when I change to the configuration of the altec 436 diagram it sounds very loud and does not act.
any suggestions?
Thanks a lot.

grab a Multimeter, paper and pencil. Meassure: Ohm :D
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 30, 2020, 11:42:36 AM
In a nutshell it's probably a mistake that you need to find.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on December 30, 2020, 04:31:58 PM
In a nutshell it's probably a mistake that you need to find.

Always.
Measure your pot and use common sense as DerEber and Rob have suggested, you'll get there in the end :) 
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 31, 2020, 07:21:45 AM
Good afternoon, resolved that I have found out, the volume up and down works well, the vumeter has been easy to solve, as Winston said in one of the forums, remove the diode bridge and connect directly with the 34r resistor between the two pins of the meter.
I just have a doubt about the volume potentiometer, when it is at low volume it sounds an annoying hummm, typical of guitar amplifiers, I don't know if this is normal in this unit, and when I raise and lower the volume for a few moments it sounds like a slight hiss, I don't know if this is also normal ?.
Thank you very much for all your guidelines
and happy new year to everyone too.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 31, 2020, 07:33:54 AM
Good afternoon, resolved that I have found out, the volume up and down works well, the vumeter has been easy to solve, as Winston said in one of the forums, remove the diode bridge and connect directly with the 34r resistor between the two pins of the meter.
I just have a doubt about the volume potentiometer, when it is at low volume it sounds an annoying hummm, typical of guitar amplifiers, I don't know if this is normal in this unit, and when I raise and lower the volume for a few moments it sounds like a slight hiss, I don't know if this is also normal ?.
Thank you very much for all your guidelines
and happy new year to everyone too.

I have found the ALPs blue velvet dual logs to be quite good in this position.   I have used them on a lot of different vari mu limiters I have built over the years, & always had good results.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on December 31, 2020, 08:06:39 AM
Regarding your hum, if you have DC on your heaters then it shouldn't be an issue.   

I've never personally built an RS124 from scratch with AC heaters and, if that's what you're doing, the layout will be more critical to getting low hum:  such as heater wires twisted and bent towards chassis away from other wiring etc.  You may find there's a benefit to a having a hum-bucking pot across the LT, typical was a 250 ohm linear of adequate wattage with the wiper grounded.

Having said that, I've converted lots of original Altecs to RS124 and they used AC heaters.  Just good layout practice kept hum low enough to not matter.

As for hiss when you turn the pot up, no you shouldn't get that happening.  The noisiest position on a pot will be at the -6dB position because that position presents the highest source impedance to the valves.  But with 25K pots that shouldn't be an issue vs using 250K pots.  We're looking at pot source resistances of approx 6K25 and 62K5 respectively with 25K and 250K pots.

If you've built just a stock Altec 436 circuit then, once you iron out the bugs, I'd look at adding the RS.124 modifications to be fair.  I think it'd be a more useful tool.  Start with the re-biasing/balance pot stuff, the lower value time constant cap with release time switch,  and the 100K anodes to anodes resistor mods for best bang for your bucks.  Add the other stuff as and when.

Happy New Year to all  :)   

 
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on December 31, 2020, 08:31:44 AM
P.S. although I haven't tried the Alps pot, I'll second Rob's suggestion that a decent pot with good tracking is worth it in this position.  And without the balance pot from the RS.124, probably even more worth it. 

I used a TKD in the past but since they're crazy expensive I'll probably go with Rob's Alps Blue Velvet in future.

Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 31, 2020, 10:34:38 AM
Regarding your hum, if you have DC on your heaters then it shouldn't be an issue.   
 You may find there's a benefit to a having a hum-bucking pot across the LT, typical was a 250 ohm linear of adequate wattage with the wiper grounded.

I almost always find it a problem not to have the L.T connected to ground in some way.   You can use a humbucking pot as John describes, or just a pair of 100R fixed resistors, one from either side of the L.T winding to ground.   I have even noticed an improvement just shorting one side of the L.T to ground, although I would always use one of the previous 2 methods in practise.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on December 31, 2020, 02:17:19 PM
I apologize for the question, but what is LT?
the original altec 436c circuit has 2 potentometers, one with a 25k threshold and the other with a 1M which I think is the ratio.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on December 31, 2020, 06:21:21 PM
I apologize for the question, but what is LT?
the original altec 436c circuit has 2 potentometers, one with a 25k threshold and the other with a 1M which I think is the ratio.

L.T stands for low tension which is the heater supply.  H.T is high tension which is the 235v part of the supply.

The original 436c has the 25k as the threshold I believe & the 1M is the release.   If you change to an RS124 circuit the 25k isn't used.    The RS124 works MUCH better than the 436.  In my view the 436c is pretty much just a quirky effects box whereas the RS124 is a very useable compressor.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on January 02, 2021, 03:22:02 PM
I have finished the 436c, from my humble opinion it has disappointed me, I do not see it more useful than the amplification one.
It is true that I have no experience with varimu compressors but I imagine more of an optical type compression, but I see that this compressor is even more subtle, so I started by completely rectifying the 120v winding and adding the two 100k resistors , All the ground noise has gone, now it sounds without noise, I have left the 1M potentiometer, which I did not notice any use.
According to the scheme that Mr. Winston published, a hold is added (I think it's a release), an output attenuator is added, and a 3.3m potentiometer. I don't know what it is for?
I don't see that it has an attack, I don't see the threshold either.
suokngo that will be automatic like the optician la2a?
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 02, 2021, 08:38:57 PM
I'm not really sure why you would expect it to sound like an optical compressor when it isn't one.  If that's what you wanted maybe you should have built  an La2a.

The control you say is the hold control is actually the release control. In between the release positions it has a hold function so that you can get the unit to be compressing before the track starts so there are no surprises on the first transient after which point you switch the release control to the desired release constant.

The attack is fixed, although it is possible to add a limited amount of control.  Basically the sidechain isn't capable of driving very fast attacks, so maybe of limited use.

I can't see any 3M3 pot of the diagram, can you explain where this is in your build.  Did you use it instead of the release switch ?

It is true there is no threshold control but this isn't a big problem because you just wind up the input pot and it compresses more which has the same effect as lowering the threshold.  There are many compressors like this, including most Vari mu's & 1176's to name but a few.

I'm quite surprsied you are disappointed by the EMI version of this compressor.   I was always disappointed by the Altec versions, but the RS124 is to my ears completely much better.   Last year I built two RS124's at the same time I finished a Fairchild 670.   I'm not saying they sound anything like the same, but I like them both equally, and I built the pair of RS124's for less than 1/4 of the cost of the Fairchild.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on January 03, 2021, 01:57:57 PM
Hi Rob, sorry maybe I explained wrong, I use goolge for my translations because my level of English is not fluent.
I didn't mean to say that altec 436 sounds bad, I wanted to say that it has little play due to the limited controls, it actually sounds good. regarding the emi rs124 I have now started the modifications, I just finished with the Hold and it seems that it works well, the attenuator of I don't understand the output, I don't know how it works. Could you add some stereo potentiometer as in the input?
Thank you very much and sorry for the misunderstanding.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 03, 2021, 02:39:24 PM
Hi Rob, sorry maybe I explained wrong, I use goolge for my translations because my level of English is not fluent.
I didn't mean to say that altec 436 sounds bad, I wanted to say that it has little play due to the limited controls, it actually sounds good. regarding the emi rs124 I have now started the modifications, I just finished with the Hold and it seems that it works well, the attenuator of I don't understand the output, I don't know how it works. Could you add some stereo potentiometer as in the input?
Thank you very much and sorry for the misunderstanding.

You can simplify the output attenuator like the one I use.  See pic.  When the pot is turned fully down the xlr pins 2 & 3 are shorted so there is no output, but the transformers still drives into 600R.   It works really well and is simple.  It's actually one the Winston recommended to me 10+ years ago !
Title: Re: Altec 436 / RS124-ish Clone
Post by: Juanaca on January 06, 2021, 07:21:48 AM
Thank you very much Rob, this solution that you have published has worked very well, now it seems that everything works as it should, now I see this device as a good tool.
I have read in an external thread to this forum that if the input volume potentiometer of 50k is changed for another of 250k it works better?
is this true?
a greeting.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 06, 2021, 10:50:13 AM
I have read in an external thread to this forum that if the input volume potentiometer of 50k is changed for another of 250k it works better?
is this true?
a greeting.

I haven't tried this, so I don't know.  It would change the input impedance of the unit due to the 250k being reflected back through the input transformer.  As with a lot of internet advice you need to try it for yourself.  Can you linl to the place you read this ?
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 07, 2021, 11:20:19 AM
It's actually one the Winston recommended to me 10+ years ago !

I remember that Rob   :)

On the input, I doubt using a dual 250K attenuator over a 50K will do anything much, other than possibly under-terminate the input transformer.  This might (or might not) give frequency response changes or peaking from the transformer.   It will add noise. 
Also, you're generally looking to keep the R grid 1 value as low as possible with vari-bias comps, although a 250K pot with a single valve isn't too radical.   

What specific parameter is it that you want to change about the RS.124 Juanaco?


edited for clarity.
 
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 14, 2021, 08:19:04 PM
...  ... Last year I built two RS124's at the same time I finished a Fairchild 670.   I'm not saying they sound anything like the same, but I like them both equally, and I built the pair of RS124's for less than 1/4 of the cost of the Fairchild.

Hey Rob,
from distant memory: was it you who had a serious, pretty elaborate & expensive chassis made up for a Fairchild type clone unit a good few years back?   I have a memory of reading that on here but?

Regardless, I'm interested in your opinion on building one.  Is it a worthwhile use of funds in your opinion? 
I've been thinking about messing around and coming up with my own Fairchildy type unit over this year, but, to do it justice, the cost can get a little daunting for sure  :o

I've had some ideas for expanding the RS.124 type comp to be able to do what a Fairchild can do but, if the F.Child type circuit kills, then so be it.  I'd go that route.

Thanks man,  D.J.H.   

Edit: Just to add, I am familiar with what 660's and 670's can do, I've used a fair few  and refurbished/rebuilt 3 X original 670's thus far.  I guess I'm more interested in your opinion of value per £ spent.   Should I bite the bullet etc   :D   

Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 15, 2021, 03:50:26 PM
Hi John, 

Yes I was doing a p2p Fairchild, & had a chassis made, but it kind of got shelved.  I had all the parts and a stack of 6386's but always seemed to get too busy to get a big block of time to finish it.  In the end I bought a used drip pcb and I finished that build about the same time as I finished the RS124's last year.     It works welland sounds good, I can't help feeling a bit ashamed of myself not completing the p2p build. 

In terms of value for money I don't think I would build one now unless I won the lottery.   It took about 5 years to get all the parts starting around 2000. The audio transformers were well over £1k.   I was getting the 6386's for around £40 a pop at the time.   I had a Parmeko vintage oil filled line up for the mains transformers & Davens etc.   It was a lot of money, I think I was about £2-3k into it & have 3 sets of 6386.    I thin a lot of people start this project & probably get 1/2 way through it to realise that it's a bit of a money pit.   Just the cost of the 6386's is eye watering.             

I did experiment with a gain cell that used 4 x 6bc8 in parallel a la Fairchild which worked pretty well. I think I tried to use an solid amp for a sidechain amp, but never really got it to work.  I never really got it further much than that, and am doing a big ATMOS install in London for the next couple of months so have little time for anything.    Funny how one seems to get busy doing everyone elses projects, giving one little time for ones own.  Maybe one day I will get some time, grow some balls & complete my p2p version ...
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 16, 2021, 06:40:41 AM
Hey Rob,
thanks for the reply and your insights.
I don't think there's any shame at all in diverting the project to a pcb version, you got it done and it works well which is what counts.  I've never once got beyond thinking about it.

Like you, I did mess with a breadboard gain cell of 4 X 6BC8 (actually, I think it was 6ES8) to see what was what.  I got it working well enough that it's certainly a viable route.

I've never fancied paying the going rate of 6386's (old or new) myself and did think about Larry Janus' (Larrchild on here) version with triode strapped 6BA6 pairs. 
Still, 6ES8 requires much less voltage from the side-chain amplifier for a given gain reduction so something along the lines of Tim de Paravicini's EAR 660 side-chain is fairly inexpensive.

Going with an RS.124 type circuit, even if we add a low output impedance side-chain amp with adjustable gain and DC threshold to get fast limiting options, we have the problem of the 6ES8 to output valve coupling caps "blocking" and recovery time.
Others (UA176) used an interstage transformer there to avoid that problem.

One other experiment I tried though was direct coupling of the gain valve into the output valve.  The big voltage swing that you get with a resistor loaded 6ES8 caused common-mode issues (actually, it played havoc) with the 6SN7 output valve so, my experiment tried a pair of low DCR chokes on the plates to hold them at a fairly constant voltage.  Still, good chokes are also expensive so I've since thought about trying a solid state alternative to chokes.

Anyway, I'm waffling.  Again, I appreciate your thoughts which pretty much confirm what I suspected.   
Thanks again, good luck with your ATMOS install, hope it goes smoothly :)


Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 16, 2021, 11:28:26 AM
John

The 4x 6bc8 gain stage I made was driving straight into an output transformer.   I was only using Edcors for i/o so I'm not sure if in the long run it would have killed the o/p due to d.c but it was sounding very nice.   I was able to control it with a bench PSU for testing.  Not wanting to reinvent the wheel, I was just using a generic solid state amp I bought as a kit off ebay when I was trying to get a sidechain going. However I seem to remember it wasn't liking driving a bridge rectifier.   I had a job come in which diverted me a way from the project.
Title: Re: Altec 436 / RS124-ish Clone
Post by: jazzcrisis on January 23, 2021, 10:26:57 AM
I've had some ideas for expanding the RS.124 type comp to be able to do what a Fairchild can do but, if the F.Child type circuit kills, then so be it.  I'd go that route.

I built a souped-up stereo RS-124 recently that I think comes close to a Fairchild's capabilities. It's definitely as fast, but does sound a little different. It's essentially a stock RS-124 circuit with the following additions:

1. (2) 6BC8 in parallel per channel. (still requires careful matching, but makes it much less sensitive to tube variations)
2. Regulated B+
3. Cathode Follower feeding the sidechain.
4. True stereo sidechain.
5. Sidechain HPF.

It's a monster, and I can get it faster than I'd ever need it without any hint of thumping or motorboating. Very, very good master bus compressor.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 23, 2021, 03:11:29 PM
I built a souped-up stereo RS-124 recently that I think comes close to a Fairchild's capabilities. It's definitely as fast, but does sound a little different. It's essentially a stock RS-124 circuit with the following additions:

1. (2) 6BC8 in parallel per channel. (still requires careful matching, but makes it much less sensitive to tube variations)
2. Regulated B+
3. Cathode Follower feeding the sidechain.
4. True stereo sidechain.
5. Sidechain HPF.

It's a monster, and I can get it faster than I'd ever need it without any hint of thumping or motorboating. Very, very good master bus compressor.

Nice one!   :)
With a lower impedance drive from your side-chain cathode follower then the attack time should be able to be pretty fast.
If I understand you correctly, then the ratio is still about the same as an RS-124 yes? About say 4:1 ? 
   
 
 
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 25, 2021, 06:45:30 AM
Yet Another RS124-ish Clone PCB Project

Hello all. I have been lurking here for a bit, and finally have had some time to work on a bucket list project of an RS124 clone that can easily be implemented into stereo builds. Here is the current schematic based on the one Winston O'Boogie posted, and a 3D PCB Mockup. Part of the plan is to have the BOM printed entirely on the PCB for ease of use. Pretty sure how it is set up currently one would be able to use the original attack/release switches as well as implement the original output attenuator.

(https://i.imgur.com/dxXawyd.png)
(https://i.imgur.com/k6ohF5I.png)
(https://i.imgur.com/NWrkHcM.png)

Changes Implemented:

- Adjusted some cap values to closest easily available
- Added Power LED instead of lamp
- Input and Output Pots are now 50K Dual Logs, will probably implement the shorting pin 2/3 method listed above
- Attack is a 100K Linear Pot
- Release is a 5M Linear Pot
- Hold switch is a standalone switch for whatever the release was left at (see question about this)
- “Superfuse” mode is a 500K trimmer resistor and switch in parallel with the release (see question below)
- Input/Output designed for use with original transformers, Sowter transformers, and Edcor PC series.

Possible additions:

- True Bypass
- Original Power Lamp wire terminal


Questions for the community:

- Hold Circuit: I think I have implemented it correctly in the schematic by converting it to a switch that kills the release pot to the cap. If someone could confirm this that would be awesome.

- Superfuse: With my understanding of how it works from this thread: https://groupdiy.com/index.php?topic=68787.0 , from what valveandsound said, I think I implemented the 500k trimmer resistor correctly in the schematic. If this could be confirmed that would also be very awesome.

- I did my best to try to recreate the original schematic of the altec + the emi RS124 (minus the attack release switches) so that it was a bit easier for me to understand. I may have mucked up parts so having another set of eyes on it would be helpful.

- If someone has a bit more clear schematic/explanation of the original pot switch wiring I can add it to this schematic for reference when building.


Cheers!
Title: Re: Altec 436 / RS124-ish Clone
Post by: rock soderstrom on January 26, 2021, 04:26:50 AM
I built a souped-up stereo RS-124 recently that I think comes close to a Fairchild's capabilities. It's definitely as fast, but does sound a little different. It's essentially a stock RS-124 circuit with the following additions:

1. (2) 6BC8 in parallel per channel. (still requires careful matching, but makes it much less sensitive to tube variations)
2. Regulated B+
3. Cathode Follower feeding the sidechain.
4. True stereo sidechain.
5. Sidechain HPF.

It's a monster, and I can get it faster than I'd ever need it without any hint of thumping or motorboating. Very, very good master bus compressor.

Hi jazzcrisis, are you willing to post the schematic of your modifications? Sounds very interesting. Cheers
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on January 26, 2021, 04:14:16 PM
Yet Another RS124-ish Clone PCB Project

Hello all. I have been lurking here for a bit, and finally have had some time to work on a bucket list project of an RS124 clone that can easily be implemented into stereo builds. Here is the current schematic based on the one Winston O'Boogie posted, and a 3D PCB Mockup. Part of the plan is to have the BOM printed entirely on the PCB for ease of use. Pretty sure how it is set up currently one would be able to use the original attack/release switches as well as implement the original output attenuator.


Hi, the PCB looks great. I built this compressor a while ago and use it for every(!) work I do. Its simply a great sounding device. So easy and amazing. However. I like your approach of implementing everything on the pcb with easily available components. I am wondering if the trace width and spacing is already finalized. Because the traces appear really thin. Especially those for mains and HV.  I can not comment on spacing and clearance but for my humble knowledge of PCB design this is definitely to be considered. Excuse me if I am wrong. My question only arises from my visual impression of this sweet 3d graphic.

Best,
Falk
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 26, 2021, 09:39:45 PM
Hi, the PCB looks great. I built this compressor a while ago and use it for every(!) work I do. Its simply a great sounding device. So easy and amazing. However. I like your approach of implementing everything on the pcb with easily available components. I am wondering if the trace width and spacing is already finalized. Because the traces appear really thin. Especially those for mains and HV.  I can not comment on spacing and clearance but for my humble knowledge of PCB design this is definitely to be considered. Excuse me if I am wrong. My question only arises from my visual impression of this sweet 3d graphic.

Best,
Falk

Rad! I own a Chandler RS-124 and use it on almost all my projects at work. Have always wanted a more mastering oriented version that wont break the bank  ;D
The PCB is not final at all! I should have mentioned that in my post. 8) It is in the very very early stages, I just like designing the PCB as I go along figuring out the schematic. I am more of a technician and a visual learner than a traditional E-engineer so its how I am fully learning how this thing works. I just have kicad autoroute stuff for temporary visuals so not to be staring at rats nests the entire time. I will do much thicker traces and ground fills when the layout is closer to done, and do more exact footprints on some of the components. Very glad you like the idea of being able to easily switch out options and have everything placed on the board with minimal floating parts.

I would love to add the mods described by Jazzcrisis to the project, all with the options to make it "stock" or to have it be a modded beast.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 27, 2021, 05:30:34 AM
The pictures are a little big to be able to view them as a whole on my device but...

A few things that jump out:

To keep hum down you'll need to ground the centre-tap of your heater winding rather than let it just float.  Elevating an AC heater supply by about 25 volts with respect to cathode can help further if needed.  Do that with a divider from the clean H.T. and the  cap bypassed 25V injected at the 6V3 AC C.T.

Looking at the specs of your specified Hammond power transformer (269BX), the HV winding isn't 117VAC but 300C C.T.,  so copying the original Altec voltage doubler will be a no-no.
If you already have that transformer then use a full wave bridge across the whole winding and plan on losing over 150V DC in wasted heat with larger series resistors in the filter banks.
However, if you can, buy the Hammond 269AX instead  with 250V AC C.T.  and you won't need to lose so much DC voltage. 

The dual pot at the input is supposed to be sized to appropriately terminate the input transformer so I wouldn't assume a dual 50K is correct for all input types. 
On a particular transformer I used, a dual 25K  was more appropriate.   Just giving a heads up here, not pointing out an error. 


However, a dual 50K is WAY too high a value for the output - in terms of load on the output transformer, but more importantly in terms of presenting a high source impedance to the outside world at most settings. 
The simplest output level control would be a single 1K log and would be a decent all round load for the output transformer and be a low source impedance (approx 250r at worst case) to send down the cable.

I don't know what the ratio is of the Sowter output transformer but, last I looked, the ratio they were specifying for an Altec was higher than original.
You're ideally looking for about a 7:1 turns/voltage ratio.   So a 30K:600.   Others have used slightly lower ratios so there is a little leeway.  But 7:1 was what worked for me.

The meter wouldn't generally be your lower specified  1mA, never mind a 100mA.
Just use a decent VU meter with the internal diode rectifier removed.

When laying out your actual PCB, use the same good practices you would if it were a point-to-point layout.  This has been posted before, but Merlin's page here:

http://www.valvewizard.co.uk/Grounding.html

is a good guide to follow.   Not only in terms of grounding, but with respect to keeping  the current of each stage tightly contained within its own small loop.   Doesn't matter if it's point-to-point or a pcb, the same rules apply.


   

 

 
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on January 27, 2021, 07:22:51 AM
@ Winston. Awesome post! Thank you.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 27, 2021, 09:08:23 AM
I found a cheap R core mains transformer on the net that was a LOT cheaper than the hammond & worked very well.  It was about 30 euros.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on January 27, 2021, 10:29:40 AM
I found a cheap R core mains transformer on the net that was a LOT cheaper than the hammond & worked very well.  It was about 30 euros.

Yep, I'd be looking at something like that too.  Nice price.   
I can't say I've ever considered using Hammond myself.  I only mentioned them because it's the brand Teambanzai had specified.

Actually, I'm on the hunt for a good and reasonably priced isolation type (240:240) to use for H.T. supplies myself.  I want electrostatic shields though (primary to secondary & possibly another from the core) which narrows my options.

L.T. from a separate supply, maybe switching?  If not, then something also with a shield and high enough AC to allow for DC regulated.  Bipolar +/- 3.15V   
 
Whichever way, it won't be torroids as I find they're just too wide-band and easily pass hi freq. mains hash into your unit.  Likewise, they can pass junk from the loop of the secondary, rectifier, and first cap back into the mains. 
 

 
Title: Re: Altec 436 / RS124-ish Clone
Post by: Rob Flinn on January 27, 2021, 02:37:01 PM
I think this one the one I used

https://www.audiophonics.fr/en/r-core-transformers/r-core-transformer-30va-9v-0-9v-02a-9v-02a-63v-2a-240v-005a-p-8542.html?fbclid=IwAR04wvk3K2YgD0Cr2-NRgF4h9Ugi11VJVDFSJlAhfjTFL7qKPZRLgUml6t8
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 27, 2021, 03:07:39 PM
The pictures are a little big to be able to view them as a whole on my device but...

A few things that jump out:

To keep hum down you'll need to ground the centre-tap of your heater winding rather than let it just float.  Elevating an AC heater supply by about 25 volts with respect to cathode can help further if needed.  Do that with a divider from the clean H.T. and the  cap bypassed 25V injected at the 6V3 AC C.T.

Looking at the specs of your specified Hammond power transformer (269BX), the HV winding isn't 117VAC but 300C C.T.,  so copying the original Altec voltage doubler will be a no-no.
If you already have that transformer then use a full wave bridge across the whole winding and plan on losing over 150V DC in wasted heat with larger series resistors in the filter banks.
However, if you can, buy the Hammond 269AX instead  with 250V AC C.T.  and you won't need to lose so much DC voltage. 

Right on! Input from the man himself. Thank you for the input, the pcb image I posted is still very very early stages, since then I have added much more stuff to the schematic, fixed a few spots. Good catch on the transformer labeling on the schematic. The Hammond I planned to use is the 269AX, think the schematic auto labeled the symbol to a B in the picture, if you notice on the PCB silkscreen it is labeled as the 269AX. Everything in the PCB pic is what would be the alpha build, hence why there are no regular traces and layout is not as thought out as could be.  :D

Later today I will post an updated schematic of what I have down now that I think I fixed some of the problems you pointed out.

Cheers!

Title: Re: Altec 436 mk2 modification
Post by: vari-mu on January 27, 2021, 04:59:11 PM
 (http://moemesto.ru/vari-mu/file/15168148/Altec 436 int20-2.JPG)
I am mocking up the scheme of my modification Altec 436 mk2, this is an intermediate variation.
   1) Made a special output transformer with winding for the side chain detector
  2) An interstage transformer was manufactured and applied, which solves many problems.
  3) The filament voltage of the first stage is rectified to a constant one, also used for
adjusting the bias of the lamps of the first stage.
You can discuss, please, what someone doesn't like
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 28, 2021, 01:15:30 PM
Here is an updated version of my schematic. Added in switches to select between variable and fixed attack and release pots. I think I got the stepped output attenuator down correctly.
Title: Re: Altec 436 / RS124-ish Clone
Post by: peterc on January 28, 2021, 01:33:45 PM
OK looking good. I have always wanted to build one (or two) of these!

Teambanzai, on your schematic, there is a sort between pins 3 and 4 of the 6AL5.

Peter
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 28, 2021, 03:17:07 PM
OK looking good. I have always wanted to build one (or two) of these!

Teambanzai, on your schematic, there is a sort between pins 3 and 4 of the 6AL5.

Peter

Good Catch! I am updating it at the moment with a few more changes and fixes I caught.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on January 29, 2021, 03:41:41 PM

Actually, I didn't realize you were using your RS.124 type unit all the time.  I guess I assumed that, with your big adventure on the U23 project, you'd found that the U23 was more useful day-to-day in what you do.   
It's nice to have read your opinions sir  :) 
 

That RS124 type comp I built is so straight forward. The circuit is also much more easy going than the U23 circuit and simply sounds rock (n roll) solid. The U23 is a different animal and easily replaces the Stamchild 670 I own. It sounds noble and fine. Btw. I am using matched ECC189 (MAZDA NOS) and 6CG7 (new EH) in the RS124 and couldnt be happier. I really did not like the comp as much with 6bc8. The unit compresses within 0.1dB difference between left and right without any more matching than cathode voltage.

I did the last masterings with both the RS124 and U23 clones engaged and my clients are super happy. They are a great match.

(http://moemesto.ru/vari-mu/file/15168148/Altec 436 int20-2.JPG)
I am mocking up the scheme of my modification Altec 436 mk2, this is an intermediate variation.
   1) Made a special output transformer with winding for the side chain detector
  2) An interstage transformer was manufactured and applied, which solves many problems.
  3) The filament voltage of the first stage is rectified to a constant one, also used for
adjusting the bias of the lamps of the first stage.
You can discuss, please, what someone doesn't like

This circuit looks great. I think I would want to build it sooner or later. Would you keep us updated on this one?

Good Catch! I am updating it at the moment with a few more changes and fixes I caught.

BTW. I really like the Edcor stuff. Its great sounding trannies for almost no money. The downside is. They are super sensitive to magnetic fields. Its no a big deal to get the device they are built in quiet but once you rack the unit up with other gear it becomes a pain in the arse. I really hope to find some mu metal cans for the wsm series sooner or later to get rid of this really tiring process of combining diy rack gear that does not make a big problem together.

Best,
Falk
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on January 31, 2021, 02:05:44 PM
Okay! So after some after work late night grind, I think I have everything down from the original schematic + some extras. Attached is the PDF of the schematic.

Did a lil more laying out of the pcb as well, still in very very early stage:
(https://i.imgur.com/pPb49Mo.png)

-TB
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on February 01, 2021, 12:54:28 PM
teambanzai:

You have a full-wave bridge rectifier now, but there are issues with it.

First thing is to not ground the centre-tap of the high voltage winding, so lift that connection.

Secondly, it looks like you copied the scheme of the EMI version.  However, your transformer has a higher voltage secondary than they used (I estimate their HV winding as being around 190 volts unloaded), so your B+ will be close to 100 volts higher than required.
Take your secondary 250 AC and multiply by root mean square to get your approx DC voltage at the first cap.

You'll need to lower it so put a bank of RC filters to drop your DC to the required value. 

With this design, it's a little tricky to come up with values as the current draw isn't constant.  If it were, we wouldn't have a compressor.

The best way is to have a 265V regulator right before the junction of the 10K and 6K8 resistors that feed the 2 stages.   
If you're not doing that, then about 3K5 of resistance divided up between 3 or 4 caps before that junction will drop the required voltage in full current draw, no compressing mode (about 25mA)
This is a compromise because, although the B+ for the input valve will rise as compression happens (which  helps with headroom), it also rises for the output valve which we don't really want as power dissipation could be a problem etc. as well as a changing operating point for that valve.

You can see why EMI used a power transformer that was wound with just the right secondary voltage to alleviate this issue.

Also, I would never run AC heater supply on the pcb.  Of course, DC would be best, but you don't have enough voltage for that.  You need about 7.5 - 8 V AC to allow for a decent drop across the regulator.
So, with AC:  run twisted wire right up to the tube sockets' heater pins.  Put two holes in the pcb right at the heater pins where you can solder the twisted pairs.  These twisted pairs need to stay away from any other circuitry.  They should hug the edges of the chassis bottom, so I would experiment with placement for the least amount of hum.

   
Edit:  I looked again at the EMI scheme and the B+ voltage at the 10K/6K8 junction rises by only 10 volts between no compression and 20dB compression.  Without a regulator, yours will be rising more that that.
You may be better using separate filter banks for input and output valves to better isolate any interaction on the B+ line.   

Another edit:  That Hammond is a bit on the large side, even for a stereo unit, and it may be difficult to stop it humming away like a mutha into your audio transformers.
An external box for it might be best.



Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on February 01, 2021, 03:29:33 PM
P.S.  I probably sound like I'm being obsessive about your power supply. Power supplies aren't even that sexy to talk about!   ::).      :D
However, everything hangs on the power supply and, ultimately, it's what we're listening to.
There is some leeway with tubes,  but you need to know when, and by how much.

I guess I'm a little concerned that you're dealing with some lethal voltage and current here, and yet
you might be approaching it a little haphazardly? Are you doing the calculations, all that simple but necessary ohms's law stuff etc?
 
Anyway, that's probably enough from me  :)

 

 

 


 
Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on February 01, 2021, 04:23:59 PM
teambanzai:
   
Edit:  I looked again at the EMI scheme and the B+ voltage at the 10K/6K8 junction rises by only 10 volts between no compression and 20dB compression.  Without a regulator, yours will be rising more that that.
You may be better using separate filter banks for input and output valves to better isolate any interaction on the B+ line.   

Another edit:  That Hammond is a bit on the large side, even for a stereo unit, and it may be difficult to stop it humming away like a mutha into your audio transformers.
An external box for it might be best.

Awesome! Thank you!! I was feeling the same way after looking at the Hammond's specs, and thinking it may be better to go with external power supply box. If the original transformer that EMI used was around 190v on the secondary, do you think using something like this: https://www.edcorusa.com/xpwr058 (https://www.edcorusa.com/xpwr058) would work in place of the 269AX with less filtering needed? 180V on the High, 6.3V on the other.

I haven't purchased or built anything yet so it's not like I am stuck using the Hammond, and I have definitely not been messing with any high voltages yet in the real world with this, I am just trying to get a working schematic down that my pleeb brain can grasp + find solid parts that wont be a major headache (aka all the noise and other problems you have described) :)   ;D 



Title: Re: Altec 436 / RS124-ish Clone
Post by: teambanzai on February 01, 2021, 08:30:08 PM
Amazing! No rush, this is my after studio sessions project to clean my brain after usually 6-8 hours of hiphop/trap/rap tracking and mixing. Designing this unit to fit the needs of my more 50s-70s inspired artists I mix for.

I had planned on designing some custom RF shields for the Edcors, have a buddy who is a whiz with metalworking. If I was located in Europe I may snag that transformer, need to check if shipping costs more than the transformer itself to the States  ;D
Title: Re: Altec 436 / RS124-ish Clone
Post by: Falk on February 02, 2021, 01:30:33 AM
I used a 2 x 180V and 2 x 6,3V toroidal power tx. Works great. I am not sure if I get the exact 255V from the EMI schematic. It can be that I changed the drop resistor but I cant really remember. I am in contact with a manufacturer for mu-metal cans regarding shielding for the the Edcore WSM series. Lets see what the price tag is going to be. I ll keep you posted. I put a mu-metal shield around my power tx and have a rms noise of -82dB(FS) which equals -63dB(U) in my setup. Professional tube gear I bought for several thousand euro per unit is about 6dB better regarding SNR.
Title: Re: Altec 436 / RS124-ish Clone
Post by: Winston O'Boogie on February 02, 2021, 06:56:08 AM
I used a 2 x 180V and 2 x 6,3V toroidal power tx. Works great. I am not sure if I get the exact 255V from the EMI schematic. It can be that I changed the drop resistor but I cant really remember. I am in contact with a manufacturer for mu-metal cans regarding shielding for the the Edcore WSM series. Lets see what the price tag is going to be. I ll keep you posted. I put a mu-metal shield around my power tx and have a rms noise of -82dB(FS) which equals -63dB(U) in my setup. Professional tube gear I bought for several thousand euro per unit is about 6dB better regarding SNR.

Thanks Falk. 
It isn't that necessary to be exactly at 255V as such.  It's that this voltage should be pretty stiff and not deviate too much as the current changes. 
That way, the voltage dropped across the 10K resistor feeding the input valve is allowed to rise by up to 50 volts when compressing, but the voltage dropped across the 6K8 that feeds the output valve is very much mostly just a function of the approx 14mA  standing current of the 6CG7 and, therefore, doesn't fluctuate.

Good stuff on the mu-metal cans, keep us posted.