Hi! There is no p2p schematics, i followed partially the EMI original schematics you can find on this thread. I mixed it with some caps values of the others 6al5 varimu like ua175, etc.... for highpass detection and release cap.
EMI RS124
- Thread starter ed rees
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rock soderstrom
Tour de France
Hi guys, I'm planning my first RS124 build and I'm wondering, how often do I have to use the balance pot, does it really have to be on the front panel or is it enough to position it on the back? What do you think?
Rob Flinn
Well-known member
On the front is useful.
rock soderstrom
Tour de France
Thanks Rob
rock soderstrom
Tour de France
Amazing ! I was away in the countryside and spent a day redrawing it on paper haha. I didn’t know of such a redraw !
In the end, what’s the general opinion regarding the power transformer ? The Altec one seems to deliver 117V, but the RS124 one leans toward 255/1.44 so around 180V. From what I read in this thread the lower the HT, the smoother the compression?
The other weird thing for me is the heater wiring : grounded on one side, looks like the best way to mess up the ground. But clearly indicates that aside the primary being different in US / UK, the power transformer is not the stock Altec.
In the end, what’s the general opinion regarding the power transformer ? The Altec one seems to deliver 117V, but the RS124 one leans toward 255/1.44 so around 180V. From what I read in this thread the lower the HT, the smoother the compression?
The other weird thing for me is the heater wiring : grounded on one side, looks like the best way to mess up the ground. But clearly indicates that aside the primary being different in US / UK, the power transformer is not the stock Altec.
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Rob Flinn
Well-known member
I used Rcore transformers for my builds, I can't remember exactly what the spec was but I got them from a French company and they were only about 25euros each. Unfortunately I can't open the boxes up and find out the spec, because my RS124's are at my house in in Aude & I'm in the U.K for the moment. It might have been something like the link below with an appropriate dropper resistor.. I found when grounding the heater winding that the fact that is is grounded made a lot more difference than whether it was grounded by a centre tap or resistor divider. Although obviously the resistor divider or centre tap are preferential.
https://www.ebay.co.uk/itm/27324541...oLY0CCj0C3z/Cvem/YXd2uCWmxoR|tkp:BFBMrpOx6-Jj
rock soderstrom
Tour de France
I was also about to redraw the schematic, then I thought I should search for it using the forum search function. Voilà, direct hit!
One schematic is drawn for 200 Ohm output impedance, the other for 600R. I haven't checked both properly yet, but it looks good so far...
Where did you install the dropper resistor, directly after the rectifier or simply increased R12 and R13 accordingly? Did you give the power supply a little more capacity or did you proceed exactly as planned?
The R124 has really little ripple filtering! It's very unusual for me, but it's also my first symmetrical circuit project...
Rob Flinn
Well-known member
The dropper would have been after the rectifier. I don't know what value it was because I built these boxes over lockdown & I'm not good at keeping notes of what I did. I am not going to France now until July so won't be able to look at what I did unfortunately, but it's not exactly rocket science.
rock soderstrom
Tour de France
That's true, but I wonder (as a trainee rocket scientist ;-) if a series resistor after the rectifier doesn't affect the voltage divider that feeds the “balance” circuit?
Anyway, I'll include a series resistor of sufficient wattage in my PCB design so that different transformers can be voltage matched.
Rob Flinn
Well-known member
Not sure why you would think it does. The voltage divider would be after the dropper too..
rock soderstrom
Tour de France
I'm not sure if I'm making a mistake, but the additional series resistance would add to the upper part of the voltage divider, right? But the corresponding value would probably have little effect on the ratio of the two 100k resistors. I think we can file this under brain fart
Rob Flinn
Well-known member
You just select a dropper that provides 265v to junction r13, r14, r18 & c7.I'm not sure if I'm making a mistake, but the additional series resistance would add to the upper part of the voltage divider, right? But the corresponding value would probably have little effect on the ratio of the two 100k resistors. I think we can file this under brain fart
rock soderstrom
Tour de France
I'm tempted again to make a bunch of mods from scratch. Canceled - I'm building the compressor as originally intended. What was good enough for the Beatles should be good enough for me as well.
rock soderstrom
Tour de France
Which neon lamp do you use for the balance oscillator? It seems to be a 110V type. Is there anything else to consider apart from the voltage? Source?
Haha Rock ! We are having the same questions and reflections one day after another. Seems like the modern equivalent would be the A1A / ne2 model. I have bought a 65/95V version from banzai, I’ll see if it blows up or not. It looks like it is responsible for the oscillation freq clicking because as soon as the voltage reaches its operating point it lights up and leaks to ground ?
rock soderstrom
Tour de France
Yes, it seems we are on the same path!I also have two NE2 with (I think) 60V left over from my LA2A project. I have not yet been able to find a 110 volt type among the usual suspects.
It could be like that. I'll be honest, I'm not entirely sure how it works. We'll see when we build it! Good luck!
Edit: What would a 95V type change, the frequency of the click?
Edit : I bought a pair of A9A from Banzai. 65-95V ... so they will probably blow up haha. The original calls for 110V indeed
EDIT : scratch my previous comments ! It's is DC that's being fed to the neon ... so the facts that it lights one & off is creating the ticking noise / low freq oscillation I suppose.
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Rob Flinn
Well-known member
I managed to find some 110v bayonet fitting neons on ebay which work great & are easy to replace if one dies on me.
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I think this one was the transformer you once linked to:
https://www.audiophonics.fr/en/r-co...Cr2-NRgF4h9Ugi11VJVDFSJlAhfjTFL7qKPZRLgUml6t8
But this one also seems even more appropiate (less voltage to drop) when you're ok with using AC heaters:
https://www.tubetown.net/ttstore/de/ringkern-netztrafo-30va-8808.html
