Rob Flinn

Re: EMI RS124
« Reply #500 on: May 30, 2020, 08:13:09 AM »
The anode volts will change dramatically under compression.  Think about it, you are shutting the valve down quite a lot.
regards Rob


Re: EMI RS124
« Reply #501 on: May 30, 2020, 08:28:31 AM »
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. 

Re: EMI RS124
« Reply #502 on: May 30, 2020, 01:55:42 PM »
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

Re: EMI RS124
« Reply #503 on: May 30, 2020, 10:13:10 PM »

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.

Re: EMI RS124
« Reply #504 on: May 31, 2020, 03:14:40 AM »
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

Re: EMI RS124
« Reply #505 on: May 31, 2020, 05:57:04 AM »
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.
regards Rob

Re: EMI RS124
« Reply #506 on: May 31, 2020, 11:44:20 AM »
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 ;)

Re: EMI RS124
« Reply #507 on: May 31, 2020, 12:58:58 PM »

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

Re: EMI RS124
« Reply #508 on: May 31, 2020, 01:40:08 PM »
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.
regards Rob

Re: EMI RS124
« Reply #509 on: May 31, 2020, 03:26:02 PM »
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.   
 


 

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