Lowering HT voltage on RS124 clone, or not?

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Winston OBoogie

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It's been ages since I played with the threshold voltage and Doug has messed with it more recently than me, but I remember going through the range (resistor voltage divider and pot like on the 436C) and then ending up right back where it was on the original 436B and RS124.
Just as my own preference of course, I just liked it where it was which is with a quite gentle knee.

Whichever way you like it is the correct way 👍
 

MaxDM

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It's been ages since I played with the threshold voltage and Doug has messed with it more recently than me, but I remember going through the range (resistor voltage divider and pot like on the 436C) and then ending up right back where it was on the original 436B and RS124.
Just as my own preference of course, I just liked it where it was which is with a quite gentle knee.

Whichever way you like it is the correct way 👍

I like it where it is, but EMRR was mentioning that a higher voltage will change the threshold, even though it's such a small change that perhaps it's inaudible?
 

Winston OBoogie

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I like it where it is, but EMRR was mentioning that a higher voltage will change the threshold, even though it's such a small change that perhaps it's inaudible?

Yes that's what I was meaning really. EMRR (Doug) has played around with threshold more recently than I have so he might know if the change in voltage you have, however small, is audible.
My recollection is that the change of a volt or so upwards was audible. But it's been 10+ years so I might be wrong.
 

MaxDM

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I’ll post some examples with gain reduction deactivated, so that threshold will not be an issue.
 

emrr

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That bottom zero to a couple is the most audible part of the curve.

I’ve changed a lot of limiters to have lower ratios, and the same effect has been noted in all.

an RS124 with a higher threshold/ratio becomes just another vari-mu. FWIW. The ‘thing’ being chased with an RS124 IS that ratio/threshold position.

my personal conversion is a 436C, and I sometimes raise the threshold slightly for that harder sound.
 

mjrippe

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I like it where it is, but EMRR was mentioning that a higher voltage will change the threshold, even though it's such a small change that perhaps it's inaudible?
Going from 3.33v to 4.12v is a small voltage change, but a LARGE percentage change (almost +25%) so perhaps it makes more difference than you expect?
 

MaxDM

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Let’s see.

Right now I’m enjoying the weather, but I will record the same loop with no GR tomorrow.

My instincts say its the higher current/voltage/slew rate, but I’m interested to know!
 

Winston OBoogie

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Not really related to your tests Max, but I wouldn't mind knowing the voltage either side of your 6BC8's 47K anode resistors at each of your two H.T. voltages.

Just for my own curiosity. If you have time that is.

Cheers.
 

MaxDM

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Here is the drum loop again, with an electric guitar on top.

GR is completely inactive, so it's working as an amplifier, and yet, to my ears, there is a sonic difference, in terms of punch and hardness.


Voltages on B+ were 264 and 325 (low/high)
Voltages on output transformer, center tap, 151 and 186
Voltages on top of 47K anode resistors, 208 and 256
Voltages on the bottom of the 47 K resistors 79 and 98
Cathode voltage on second stage 3.44 and 4.27


 
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emrr

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Yes, I hear what sounds like a headroom difference, simplistically speaking.

When I went over DaveP's Gates SA-39 tribute compressor, I set his regulated B+ control at it's highest point, as it sounded most like an original SA-39, and softer at lower settings. Similar to this.
 

Winston OBoogie

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For sure there is a difference here listening to the straight amp sans compression but. at least as far as how I'm able to monitor, there seems a bigger subjective difference when listening to the compressed files.

Max, when you're @ a higher H.T. voltage and compressing, does the varying current of the input 6BC8 change the H.T. supplied to the output valve, or is that output valve supply independent and pretty solid @ 180V?

Regardless, it's interesting to hear the differences, much appreciated :)
 

Winston OBoogie

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One thing to keep in mind is that the RS.124 (as originally set up) was inserted in the signal chain (pre output channel group fader) at a relatively low level - at least some 10 dB lower than we might use it now.

There probably is some benefit to increasing headroom, although I don't think this is the RS.124's biggest folly.
 

MaxDM

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For sure there is a difference here listening to the straight amp sans compression but. at least as far as how I'm able to monitor, there seems a bigger subjective difference when listening to the compressed files.

Max, when you're @ a higher H.T. voltage and compressing, does the varying current of the input 6BC8 change the H.T. supplied to the output valve, or is that output valve supply independent and pretty solid @ 180V?

Regardless, it's interesting to hear the differences, much appreciated :)

One thing to keep in mind is that the RS.124 (as originally set up) was inserted in the signal chain (pre output channel group fader) at a relatively low level - at least some 10 dB lower than we might use it now.

There probably is some benefit to increasing headroom, although I don't think this is the RS.124's biggest folly.

The main voltage, feeding both tubes might sway a bit with the lower voltage, but with more compression, it should go higher, so less soft, because of less current being drawn by the first tube. This might explain why they used it at high compression, perhaps.

My feeling is that the value of 150v on the plates of the output section was chosen primarily to limit current consumption overall on the Altec design.

The choice of a 117v transformer with a voltage doubler also seems to point towards that. As a guess, I would associate that as being a cost limitation imposed by Altec when they hired the engineer to design it, as well as prolonging tube life.

To Beatle or not to Beatle...
 

MaxDM

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Yes, I’ve noticed that it seems to be set up for -10, and to get it louder you need to compress quite a bit, making the hold function a necessity!
 

Winston OBoogie

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Good points for sure.
As far as I've been able to tell, based on conversations with dudes who who were there at the time using the RS.124, it wasn't uncommon to be seeing -10dB + on the meter when compressing individual instruments. Ken Scott has said in print at one point that "Revolution" was -20dB all the way for instance...

My own "experiments" with higher H.T. were quite positive, as you seem to be leaning towards. It was certainly beneficial for the gain reduction valve. However, one thing I personally found to be a positive, was when the output valve's cathode (DC threshold source) was fairly static.

Anyway, in the big picture, whatever floats ya boat is OK with me.

Thanks again Max.

'Beatles' Schmeatles 😉
 
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emrr

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Mine generally lives in the -15 to -20 range, right before it’s obviously overloading. 12dB pad on the output. Usually I’m offended by the concept of so much compression, but some devices are made for it.
 
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