EMI RS124

[rock soderstrom]

I'm not sure.
The Thermionic Culture Phoenix that is also based on the 436 has a 2W potentiometer shared between the two cathodes of the output tube and it's used to fine tune the balance, along with another trimpot in the plate circuit of the input vari mu tube.
Perhaps the original 436 /RS124 balance arrangement actually has room for improvement but you probably wouldn't care about that if you are looking to build an exact copy of the original.

It's true, for my planned build I'm strongly oriented towards the original. Nevertheless, I'm also looking at what other solutions and mods are available, and I might even build a heavily modified version afterwards. The Nu vario mu does the balanceing in a similar way as the Culture Phoenix. Are the schematics available for the Phoenix online?

 

[beatnik]

I've found here on the forum schematics for the Chiswick Reach VK-1 which is basically the predecessor to the Phoenix. If you can't find them let me know and I will post them.

 

[rock soderstrom]

I've found here on the forum schematics for the Chiswick Reach VK-1 which is basically the predecessor to the Phoenix. If you can't find them let me know and I will post them.

I found the schematic, thanks for the tip. It's actually very similar to the Nu Varimu.

 

[Rob Flinn]

It's true, for my planned build I'm strongly oriented towards the original. Nevertheless, I'm also looking at what other solutions and mods are available, and I might even build a heavily modified version afterwards. The Nu vario mu does the balanceing in a similar way as the Culture Phoenix. Are the schematics available for the Phoenix online?

I built two RS124. & converted a pair of Altec 438's for a friend. In my view they sound great & I do not have any issues with balance etc The trimmer that is in the EMI design is sufficient to stop them thumping. I didn't find the need to fix something that wasn't broken in the first place.

When you press the trim button to remvove the click from the oscillator it never completely gets rid of the click on my boxes, I just turn it to minimum level, but it's not a problem

The only thing I would recommend is to buy a few 6bc8 or ECC189 to select from.

 

[rock soderstrom]

I built two RS124. & converted a pair of Altec 438's for a friend. In my view they sound great & I do not have any issues with balance etc The trimmer that is in the EMI design is sufficient to stop them thumping. I didn't find the need to fix something that wasn't broken in the first place.

You are one of the voices that make me build according to the original circuit diagram first. The two variants mentioned use different tubes, maybe that's why the use additional trimmers.

The only thing I would recommend is to buy a few 6bc8 or ECC189 to select from.

I bought 2 sets of 4x ECC189. NOS NIB. Plus a few tasty 6CG7s, that should do the trick.

 

[Rob Flinn]

You are one of the voices that make me build according to the original circuit diagram first. The two variants mentioned use different tubes, maybe that's why the use additional trimmers.

That is one reason to build point to point. You can build as standard & then modify if you decide you need to without having to hack the pcb to bits.

 

[rock soderstrom]

That is one reason to build point to point. You can build as standard & then modify if you decide you need to without having to hack the pcb to bits.

That's true, P2P is a bit more flexible, but since I make my own boards at home, that's not a real handicap. I can change everything very quickly the way I want it.

My idea is to build original first, then maybe a modified version if I feel like it, just to see what it does.

I'm collecting a few ideas for this.

 

[Murdock]

I’ve read several post about adding interstage transformers to this design which seems to cure stability “problems” and allow for faster timings. And indeed in my simulations it worked.
But can somebody please enlighten me about what kind of ratio would be useful? I saw one with 1:1 another one recommended a 1:2. As far as I understand the UA175 compressor which has some similarities uses a step down interstage of 2:1 ratio.
Is it “better” to boost the signal a bit with a step up? Or is it about better impedance bridging?

On another note, some recommend regulating the B+ to the 6bc8/ecc189.
For what voltage should one aim for? In the original design the B+ varies from 145v to 245v depending on amount of compression. So would one regulate to 245v?
UA175, which uses the same input tube regulates with an OB3 tubes (“only” 90v).

 

[Rob Flinn]

I don't have any experience adding interstage transformers to this box, but I have never experienced instability problems with RS124 builds either. What problems are you having with your build that you are considering these measures ?

 

[Murdock]

I don't have any experience adding interstage transformers to this box, but I have never experienced instability problems with RS124 builds either. What problems are you having with your build that you are considering these measures ?

I've not build it yet but I'm gathering informations and possible mods and would like to understand some of them better.
I think stability problems when going for shorter timings which seems to be only possible with an interstage transformer or adding another stage as Dave did.

 

[Rob Flinn]

I've not build it yet but I'm gathering informations and possible mods and would like to understand some of them better.
I think stability problems when going for shorter timings which seems to be only possible with an interstage transformer or adding another stage as Dave did.

I normally try to build the thing as standard before even considering modifications. I found this box works well & sounds very good as it is. YMMV.

 

[heatwalk]

I assume you regulate the voltage to what the unit is designed to run at before compression so when compression kicks in and current drops the plate voltage holds. Part of the regulation if you saw on the 175 feeds the threshold voltage. On the 436/124 this would swing with compression therefore becomes part of its sound. Regulation I believe allows for harder knee/ higher ratio.

I think I saw a drip schematic for their 124 that used two OB2s in series to regulate the plates?

The 175 plate resistors (plus bal pot) are ~1/10 the value of the rs124 so the stages while same compression tube are different in design.

 

[freshtapescent]

I've not build it yet but I'm gathering informations and possible mods and would like to understand some of them better.
I think stability problems when going for shorter timings which seems to be only possible with an interstage transformer or adding another stage as Dave did.

On my pair, I did stepped log ladder inputs for great CV matching across the whole range, cathode AND plate balancing pots, bought reasonably well matched 6es8s (in pairs for, and with the plate to plate feedback switched on I can go screaming fast. Also haven’t had to adjust either balance pot since finishing them last year…which I attribute to the precision of my input switches.

I also considered interstage transformers, but once I got it all up and running with the above HW, I realized it wasn’t necessary for my purposes. I can still get click/thump but only with my attack or release switches in their ‘short’ positions…and generally only during the transition out of no gain reduction. So the first kick or snare might click as the meter moves off 0, but as long as the meter doesn’t make it back to 0, the next one won’t click. So super fast attack is ok, as long as release is a little longer, and super fast release is ok as long as attack is a little longer. I also have a foot pedal operated HOLD, and holding at -3dB gain reduction for whatever reason eases that transition.

Not that they were super hummy/noisy to begin with, but I also did split the first psu filter resistor into two parts and put half in the ground return path to make a balanced filter and isolate that nasty reservoir cap current…and that may or may not have helped in any way, but I can’t measure any power frequency noise so whatever worked.

 

[thakava]

I built two RS124. & converted a pair of Altec 438's for a friend. In my view they sound great & I do not have any issues with balance etc The trimmer that is in the EMI design is sufficient to stop them thumping. I didn't find the need to fix something that wasn't broken in the first place.

When you press the trim button to remvove the click from the oscillator it never completely gets rid of the click on my boxes, I just turn it to minimum level, but it's not a problem

The only thing I would recommend is to buy a few 6bc8 or ECC189 to select from.

Are the 6bc8 and ecc189 interchangable?

 

[Rob Flinn]

Are the 6bc8 and ecc189 interchangable?

yes.

 

[thomasdf]

It’s beginning ! 3U Hammond old stock steel chassis (LA2A style), it is hard as hell ... I am very glad I bought some Greenlee punches, they do a great job ! I have bought a 24mm punch from Amazon and it's cheaper ... but so is the result. Anyway !
I'll follow the stock RS124 schematic first, and see where it goes from there. Power transformer aims at 180VAC secondary
Input TX is UTC A18, output is custom wound by a very nice forum member ... to be continued !

 

[Rob Flinn]

It’s beginning ! 3U Hammond old stock steel chassis (LA2A style), it is hard as hell ... I am very glad I bought some Greenlee punches, they do a great job ! I have bought a 24mm punch from Amazon and it's cheaper ... but so is the result. Anyway !

if you use some oil on the thread & the cutting edges of the punches they work much better !

 

[beatnik]

Input TX is UTC A18, output is custom wound by a very nice forum member ... to be continued !

I would be interested to know how the A-18 performs in this application, with regards to noise pickup from the power transformer.

When I used it as an interstage it really benefited from the additional A-33 shield, but they have become very rare to find.

 

[thomasdf]

if you use some oil on the thread & the cutting edges of the punches they work much better !

I used oil for the drilling and punching (hole & tool). Some sweat and blood too ! DIY for the win !

 

[thomasdf]

I would be interested to know how the A-18 performs in this application, with regards to noise pickup from the power transformer.

When I used it as an interstage it really benefited from the additional A-33 shield, but they have become very rare to find.

I’ll report back ! Fingers crossed. Power transformer is a C Core Sengterbelle from Ali Express …