EMI RS124

[Rob Flinn]

You have a problem either with the sidechain, or the sidechain signal isn't getting to the 6bc8.  This is a very simple compressor so it should be quite easy for you to check for mistakes.

 

[rickc]

what rob said.  you have a sidechain problem. Ignore the meter for now.

using a DMM or other high-impedance meter, measure the voltage at the PLATES of the 6AL5, which should be connected together.  that voltage should go negative, and proportional to signal level, AFTER some amount of initial drive. That offset is the threshold level. In the 436C, there's a pot for adjusting that. In earlier units, the threshold is fixed, I believe at the cathode voltage of the 6CG7,.

anyway, if you can get that part working, then measure next at the grids of the 6CB6, which should track the voltage from the 6AL5.

more negative means more gain reduction.

 

[thomasdf]

Hello everyone

I am building a stereo tube compressor which is very closely inspired by the EMI RS124.
I am supposed to use a 1:1 input transformer with a center tap.

I have a pair of Altec 15335A transformer ( 15K:15K) sitting here unused... Can I use them for such a duty, knowing that they don't have a center tap? Must I "simulate" a center tap of some sort with carefully chosen resistors (any help welcome!) or can I omit the center tap ?

Here is a bit of the schematics

 

[thomasdf]

The values you already have on your schematic look like a good place to start.

Ok, then can I omit the link between the two 39K resistors and the center tap of the input transformer, the two resistors acting as a false center tap?

Is the input attenuator before the transformer in your particular schematic?

Yes, absolutely... I was surprised at first but it is a praised compressor, so I guess it works just as well ?

 

[thomasdf]

Hello

Thanks for the detailled answer.
I am legally not autorised to post the schematics for everyone to see, but I can sure send you a DM with some files.

As for the virtual center tap, my transformer has a .5 ohms resistance over the secondary, so I guess 2x39K is way over the top?

 

[Rob Flinn]

Hello folks.

I'm just building a pair of these & have fired up the 1st one & am having some issues.

They are wired as in Winstons diagram, & I'm powering it from a bench PSU at the moment.  All the voltages are pretty much spot on.  The unit passes a signal ok. 

As I crank up the input I the sine wave starts to round and I can see compression happening, but I get to a point (roughly with the gain control at 12 o'clock)  when the waveform starts to become like in the attached pic.    On the Meter up until this point I can see some reduction, but past this point the gain reduction becomes less.  When the input control is fully up the waveform turns into the attached pic, & the meter goes back to zero.          The maximum control signal on the Earth side of the input pot is about -3.3v.    I have tried different valves in all positions especially the ecc18/6bc8 since I have quite a large stash of them.

Done quite a bit of head scratching on this & I'm just wondering if anyone has any ideas I could try ?

 

[Winston OBoogie]

Hey Rob,
can you determine whether the issue is in the audio amp or the control signal?  Pull the 6AL5 and see if you get a good waveform up until the audio amp and transformer starts to clip.
If that's good, look at your 6AL5 and also at the threshold voltage coming off the output valve's cathodes.
Otherwise, keep at it, you'll get there 

 

[Rob Flinn]

If I pull the 6AL5 I have the  waveform doing the same thing.    Does this mean that I have little or no compression happening ?  I have tried maybe 3 different 6AL5's.

 

[Winston OBoogie]

If I pull the 6AL5 I have the  waveform doing the same thing.    Does this mean that I have little or no compression happening ?  I have tried maybe 3 different 6AL5's.

If you pulled the 6AL5 and it's still the same problem then I would say there's an issue with the audio amp.  I assume you've looked through everything for wiring errors and checked the voltages yes?
Might be best to probe through the audio starting at the input to see exactly where it's getting squirly. 

P.S.  not really the same as having 'hands on' but, you could post a few high res pics to see if another set of eyes sees anything amiss?

 

[Rob Flinn]

Thanks John.  I will post some pics tomorrow if my time doesn't get utilised by the other half. 

Currently I'm using an Edcor output transformer.  Not sure if it's doing anything funky to the output, but I have pretty much spot on 145v on the anodes of the 6cg7.  Other than that all quiescent  Voltages seem to be good.

 

[Rob Flinn]

Seems to have been the output attenuator that I used, which someone posted earlier.      If I just strap a 620 on the output it seems much more comfortable.    I still get the funky waveform at max input, but the control signal will go to -18v.      Maybe the 6cg7 isn't quite ballsy enough for a low load. 

Not quite sure what is a good attenuator for this, there have been a few posted some using precision resistors which I don't really want to have to buy.  I may just try a couple of 300R with a 1k log between them, like you suggested to me years ago for a different project.     

Currently I don't have a meter attached, because the only one I have is 500uA F.S.D & that seems to screw with the biasing, somehow.  I have some el cheapo 200uA meters on order, but who knows when they are likely to arrive from the far east at the moment.

 

[Winston OBoogie]

OK that sounds much better, great stuff. 
I  don't think you can get much more out of a 6BC8 than the 18 odd volts CV you're measuring now.  A 6ES8 will get you to a bit over 20 volts and starts off with a higher standing current (lower anode voltage) so you can squeeze a bit more out of the unit if you want.

The simplest output attenuator would be a 1K audio taper pot across the transformer so you might give that a go.  It's been  years since but I think that's what I did on the Lisson Grove comp that was made for a while.

Edit:  forgot to say that, with  a meter to see what's going on, you'd probably be indicating near enough to -20dB gain reduction with that -18V on the grids.  The 6BC8 just has nowhere left to go as the plates are right up near the rails at that point so that's why your waveform is looking stranger there.  Still, that's a lot of Gain reduction so... 

Secondly, I'm not quite sure why your output attenuator was causing the control voltage to only rise to 3.3 volts tbh? 

 

[Rob Flinn]

Secondly, I'm not quite sure why your output attenuator was causing the control voltage to only rise to 3.3 volts tbh?

Me neither.    Although the transformer is supposed to be 600R out & the attenuator measured 300R in all positions.    I realised I made an error with one of the resistors  but swapping it for the correct value which gave me 600R  still messed it all up.

Anyway thanks for the encouragement.

 

[letterbeacon]

The simplest output attenuator would be a 1K audio taper pot across the transformer so you might give that a go.  It's been  years since but I think that's what I did on the Lisson Grove comp that was made for a while.

This is a stupid question, but how would you connect a 1k pot across the transformer? Do you mean a stereo pot? I'm using the Sowter transformer designed for the Altec 436.

 

[Rob Flinn]

This is a stupid question, but how would you connect a 1k pot across the transformer? Do you mean a stereo pot? I'm using the Sowter transformer designed for the Altec 436.

See attached. This is how I've done it in the past.

You take your output across the pot.    You can turn the signal all the way down , but the output transformer always sees at least 600R.  You could probably use just one 620R resistor, but I've always done it this way.

 

[letterbeacon]

Oh great, thanks for this - will give it a go.

My RS124 has been sitting unfinished for a while, with similar problems to yours, so hoping to crack on with it now/

 

[Rob Flinn]

Mine are both now working, but, I only have one output transformer & I've been waiting on the right spec meters from China for sometime now.  I ordered some Sowter outs last week but they had a 4 week lead time when I ordered them which has probably now turned into several months, so I have switched to other unfinished projects while I'm waiting.  Once they are both working I will get some decent panels made for them too.

 

[letterbeacon]

Just out of interest, did you build the power supply as it is on Winston's schematic? What power transformer did you use?

 

[Rob Flinn]

A good question.    I have a pair of Parmeko vintage mains transformers in there at the moment.  The problem is they are 300v C.T.  I was hoping to build a psu like the original altec one, in the hope that the load is so low the voltage that the 150v would sit up to be correct.  This doesn't work, so I can get it to run with a bridge rectifieracross the 300v a la RS124 but I need a hell of a big dropper resistor.  So, I'm not quite sure what I'm going to do in the long run.  I could either change the transformers or possibly mess around with the 10k & 6k8 resistiors in the psu.  Transformers are so expensive compared to a few years back, which is why I wanted to use these ones.

Anyway they're on the shelf until the o/p transofmers get delivered.

 

[letterbeacon]

So now I have some free time on my hands, I've decided to try and get my RS124 fully working.

I've swapped out the old 600ohm attenuatator on the output with Rob's 1k pot + 2 300ohm resistors so that the output pot now matches the action of the input pot (a small thing, but it was bugging me).

There seems to be two problems so far - it's passing audio, but low cutting it at the same time.

I'm not getting any compression at all.

I've decided to try and fix the low cutting first, and then worry about the lack of compression after that.

All my voltages match the voltages on the schematic with no compression.

Here's me running white noise into the unit, and you can see on this eq that I'm getting a big low cut across the spectrum.


I'm not sure where to begin with this one. I don't seem to remember there being a low cut when I had the old 600 attenuator in, but then I didn't really play with it too much. Do you think it could be to do with that?

I'm running the RS124 into a RME Fireface 400.