Altec 436 / RS124-ish Clone

[Bernardo]

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.

 

[Rob Flinn]

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 ?

 

[Bernardo]

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.

 

[Rob Flinn]

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. 

 

[Bernardo]

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

 

[Rob Flinn]

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.

 

[Bernardo]

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

 

[Rob Flinn]

Check the main Power Supply voltage when this happens.

 

[Bernardo]

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

 

[Bernardo]

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.

 

[DerEber]

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

 

[Rob Flinn]

In a nutshell it's probably a mistake that you need to find.

 

[Winston OBoogie]

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  

 

[Bernardo]

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.

 

[Rob Flinn]

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.

 

[Winston OBoogie]

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   

 

 

[Winston OBoogie]

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.

 

[Rob Flinn]

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.

 

[Bernardo]

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.

 

[Rob Flinn]

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.