EMI RS124

Help Support GroupDIY:

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
The anode volts will change dramatically under compression.  Think about it, you are shutting the valve down quite a lot.
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,547
Location
UK.
If you use a 6ES8 valve instead of a 6BC8 you'll get more compression before is starts to get crunchy and crap out.  I believe that's what Rob used and is what I did too.
It's been a while since I measured one but I recall getting something like 12-14dB reduction max with a 6BC8 and some 18+dB with a 6ES8.
As a testing point,  I mentioned to Rob earlier in thread that the anode voltage at idle will be some 25 volts lower with the 6ES8, more like 50V rather than the 75V with the 6BC8.

It's normal that the anode voltage balance of the valve will get a bit off when it rises due to compression.  It never bothered me unless is was wildly off from side to side in which case I chucked that valve away and found  a good one.  I  suppose I could have selected valves by curve tracing them first but instead, I didn't. 

Otherwise, What Rob said. 
 

snod_donkey

Member
Joined
Dec 10, 2010
Messages
16
Winston O'Boogie said:
Shameless name drop:
Attached is a pic of Beatles engineer Ken Scott and I with one of my RS.124 comps.

When talking to Ken about the RS.124 in use at Abbey Road, he mentioned there were a couple of times when he was using them in the cutting/transfer room when the needles never strayed from max compression (he said 20dB) all the way through the track. 
Wish I could remember which particular Beatles tracks he was referring to but I'm not 100% sure now.


All very interesting stuff. I am learning quite a bit having this running on the bench too.  I actually have ordered some of those valves to try so will report back with anything i can find spec wise.

Might change the input transformer next too as 600 ohm primary might be a bit on the low side for some of the gear i want to run it with. I have a few Vortexion mixers which really want to see 10k or more to work best
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,547
Location
UK.
snod_donkey said:
Might change the input transformer next too as 600 ohm primary might be a bit on the low side for some of the gear i want to run it with. I have a few Vortexion mixers which really want to see 10k or more to work best

What you have'll work but, if it's a 600:10K in there,  you don't need that much of a step up on the front end. A 10K primary with dual 10K secondaries is more in line with what was there originally.
 

snod_donkey

Member
Joined
Dec 10, 2010
Messages
16
Winston O'Boogie said:
What you have'll work but, if it's a 600:10K in there,  you don't need that much of a step up on the front end. A 10K primary with dual 10K secondaries is more in line with what was there originally.

Thanks again for that.  Was thinking it may well have too much gain like that from the TX.  Will have a dig through some parts as i may have some old EMI transformers around that spec somewhere.

I do have a valve tester here so will see if i can get a few valves that are a bit more matched up too

Almost happy with it now anyway    :)
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
snod_donkey said:
I do have a valve tester here so will see if i can get a few valves that are a bit more matched up too

What you need really is a valve curve tracer.  The point is to get valves where their curves match so they are matched when the compression level changes.  If you have them matched when there is no compression then there is a good chance they won't be matched when you have 10dB of compression happening & the bias is completely different on them.  Something like a Utracer is a good test tool for this.
 

snod_donkey

Member
Joined
Dec 10, 2010
Messages
16
Rob Flinn said:
What you need really is a valve curve tracer.  The point is to get valves where their curves match so they are matched when the compression level changes.  If you have them matched when there is no compression then there is a good chance they won't be matched when you have 10dB of compression happening & the bias is completely different on them.  Something like a Utracer is a good test tool for this.


Nice idea.  I will look into this.  Have to say even when they are a bit off it is not obvious to the sound to my ears

thanks again ;)
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,547
Location
UK.
snod_donkey said:
Nice idea.  I will look into this.  Have to say even when they are a bit off it is not obvious to the sound to my ears

thanks again ;)

In the dozens of units I built in my time, I never noticed anything untoward either as long as both sides were decently balanced at idle.

At the time I started building these I coveted a Tektronix 570 curve tracer but, not having one, I did manually plot out a few valves at one point.  Putting the best matched valve I'd found in a unit,  I couldn't tell the difference in a blind test between that 'golden' valve and another that I'd done my, much easier,  routine on:

Basically, I'd put a valve in, let it heat up and settle, then put the balance pot approx in the middle and measure the two anode voltages at rest.
Providing that was good, I'd run a tone into the unit  at various levels and test the anode voltages at each point.  If they were within a few volts of each other as the anode V climbed, we were good go.



 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
Winston O'Boogie said:
In the dozens of units I built in my time, I never noticed anything untoward either as long as both sides were decently balanced at idle.

At the time I started building these I coveted a Tektronix 570 curve tracer but, not having one, I did manually plot out a few valves at one point.  Putting the best matched valve I'd found in a unit,  I couldn't tell the difference in a blind test between that 'golden' valve and another that I'd done my, much easier,  routine on:

Basically, I'd put a valve in, let it heat up and settle, then put the balance pot approx in the middle and measure the two anode voltages at rest.
Providing that was good, I'd run a tone into the unit  at various levels and test the anode voltages at each point.  If they were within a few volts of each other as the anode V climbed, we were good go.

I actually agree with you.  I have built dozens of vari mu compressors of all different types & repaired loads.  I have an AVO Mk4 & a Utracer & have never needed to use either of them the get the units working well.  Some compressors like a gates sa39 or 38 are a bit more fussy, but I have always got a match fairly easily by trial & error.
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,547
Location
UK.
Rob Flinn said:
I actually agree with you.  I have built dozens of vari mu compressors of all different types & repaired loads.  I have an AVO Mk4 & a Utracer & have never needed to use either of them the get the units working well.  Some compressors like a gates sa39 or 38 are a bit more fussy, but I have always got a match fairly easily by trial & error.

Yep, I think you're spot on.   
I don't know what stock is like these days on 6ES8's but,  I would typically buy 25 or so at once from the supplier and, for the most part I got a pretty good percentage of pass rate.  Maybe 4 of the 25 would be thrown out?  They used to cost (equiv of about) £5 each so not that big a deal.   
 

Juanaca

Well-known member
Joined
Dec 8, 2019
Messages
69
Hello good afternoon, I am a fan of audio electronics, I need a couple of compressors vari mu my study but I do not have the comp economy to buy them or send them to build, so I only have the option to assemble them myself.
I have read the whole thread and I have not been able to find how many amps the transformer needs in its two secondary windings.
the voltage of one winding is 6.3v and the other is 117v, but I can't find any amperage.
that is one of my thousands of doubts.
thank you
 

snod_donkey

Member
Joined
Dec 10, 2010
Messages
16
Juanaca said:
Hello good afternoon, I am a fan of audio electronics, I need a couple of compressors vari mu my study but I do not have the comp economy to buy them or send them to build, so I only have the option to assemble them myself.
I have read the whole thread and I have not been able to find how many amps the transformer needs in its two secondary windings.
the voltage of one winding is 6.3v and the other is 117v, but I can't find any amperage.
that is one of my thousands of doubts.
thank you

Take a look at the valve data sheets that you want to use. This will give you the HT and heater current ratings you require
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
snod_donkey said:
Take a look at the valve data sheets that you want to use. This will give you the HT and heater current ratings you require

I don't think the data sheets will give you the h.t current, but if you look at the diagram and use the voltages marked on it, then one can calculate the h.t using ohms law.
 

Juanaca

Well-known member
Joined
Dec 8, 2019
Messages
69
Hi Rob, thanks for the pointers, this is not a thread for learners so excuse me.
However, to calculate the ohms law I think you have to know the resistance of each circuit, I have 120v but I don't know what resistance each winding has to calculate the intensity.
thank you very much
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
Juanaca said:
Hi Rob, thanks for the pointers, this is not a thread for learners so excuse me.
However, to calculate the ohms law I think you have to know the resistance of each circuit, I have 120v but I don't know what resistance each winding has to calculate the intensity.
thank you very much


Why is this thread not for learners ?
It's nothing to do with the resistances of the windings.  If you actually look at the diagram it tells you that 14.5mA is going through V2.    V1 is simple.  Look at resistor R25.  One side is says 0.2v the other side of it it says 1.1v.  Therefore the resistance across it is 1.1-0.2v.  Using ohms law V=IR you can determine the current that is going through the resistor which is common to both halves of the valve.  All you do is work out what current each valve is drawing in this way & add them up.
 

Juanaca

Well-known member
Joined
Dec 8, 2019
Messages
69
So I think that in the 6.3v section with 2A it is enough, and with about 50ma in the 120v section it can work well.
I think I have the first step, now I need to decipher the rest of the scheme.
There won't be a thread to learn to read schemes of this type?
thank you very much
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
Juanaca said:
So I think that in the 6.3v section with 2A it is enough, and with about 50ma in the 120v section it can work well.
I think I have the first step, now I need to decipher the rest of the scheme.
There won't be a thread to learn to read schemes of this type?
thank you very much

That sounds about right.  I found it was easier to find a transformer with about 240v secondary for the h.t, & just  subbed in a circuit with a full wave rectifier.
 

Juanaca

Well-known member
Joined
Dec 8, 2019
Messages
69
Can you explain that better, Rob?
I have a toroidal transformer with secondary1 of 6.3v and secondary2 with 220v, how can I lower the voltage of this second to 120v without having to remove turns of wire and in the way that puts the least risk to my life?
Thank you very much
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,504
Location
Sussex, UK
Juanaca said:
Can you explain that better, Rob?
I have a toroidal transformer with secondary1 of 6.3v and secondary2 with 220v, how can I lower the voltage of this second to 120v without having to remove turns of wire and in the way that puts the least risk to my life?
Thank you very much

The original circuit uses a voltage doubler to increase the transformer voltage from 115v (from memory) to 230v.  Get rid of the voltage doubler & use a bridge rectifier, then the 220v straight from the transformer will be close enough to 230v to not make any difference.
 

Juanaca

Well-known member
Joined
Dec 8, 2019
Messages
69
Rob I have been thinking about what you comment but I am not able to see it in the circuit, where the voltage is doubled?
the original circuit already brings a kind of rectifier bridge.
I'm sending you what I've deduced to see if it's okay.
Thanks again
 

Attachments

  • Captura de pantalla 2020-10-10 a las 12.40.46.png
    Captura de pantalla 2020-10-10 a las 12.40.46.png
    142.4 KB · Views: 25

Latest posts

Top