EMI RS124

[gary o]

Im from the John Pertwee dimension....

Letter Im glad yr amused haha, I looked a tit in my crappy suit, it was a laugh tho .....I was singer song writer & Roy Orbersons guitarist.......Bobby Goldsboro...who?most people say at my gigs ...sung tear jerker Honey ...there was two of us I was the first one , I didnt sound that much like him on the night the later sounded nothing like him ha .....& Jean NO its not on you tube.....I hope......

My Alps 50K input pot came today...its now fitted along with neon balancer circuit.... my 6ES8 arrived too, sounds basically same to me slightly more level......I have my B+ down to 260V now BUT my plate voltages seem a little low maybe pins 1 & 6 of V1 with the 6BC8 they are 55V & with the 6ES8 they are 88V........Winstons schematics shows 6BC8 with voltages of 75V ? can it be that is showing voltage for 6ES8 in which case Im there abouts......its still working well tho.....

Anyone know if these any good
http://uraltone.com/kauppa/product_info.php?info=p4483_Hammond-109T-audio-transformer.html&XTCsid=qmeg0cc5g0cvbe2j9v6foqpke3

Thanks for reading.

 

[DaveP]

Gary,

The current draw on even the same tubes varies quite a lot, so I wouldn't worry as its in the right ballpark.  You may have to adjust the zero on your meter to compensate, that's all.  More important that both halves are the same.

The important voltage to get right is the 6CG7 as they picked 145/150V deliberately as the optimum operating point...see the mu/gm/rp curves on the data sheet.

Those little transformers are only good for speech or data, there is no bass response as it drops off at 300Hz on the spec.

Good work Gary, your posts are always entertaining!

best
DaveP

 

[gary o]

Thanks Dave the 6CG7 145/150V ....is that the plates or the CT od OP tran thanks.......

https://www.verical.com/part/413730-A262A6E#searchCriterion=mpnIDs&searchName=&_i_=1&searchTerm=413730&landingPage=catalogItemView

Wondered if this would be useable

Cheers.

 

[JayDubrek]

Hey Gary, your link's not working....

 

[gary o]

Hmmm workin this end Jay.....

Here more info & other transformers

 

[JayDubrek]

Hmmm .... maybe it's the browser on my iPhone...

 

[gary o]

http://uk.farnell.com/oep-oxford-electrical-products/a262a6e/transformer-audio-1-1-1-1/dp/1689033

meant to put this link in my last post

 

[Jean Clochet]

the 6CG7 145/150V ....is that the plates or the CT od OP tran thanks.......

Hey Gary,
The schematic shows 145V at the centre tap of the OP transformer primary and 142V on each anode of the 6CG7.

A bit more info if we need it is:
The current drawn by the complete 6CG7 push-pull stage is shown as 14.5mA so, with perfectly matched valve sections, it's 7.25mA per each 6CG7 triode section.

Voltage dropped through the output transformer is 145V - 142V = 3V

3V divided by 7.25mA = 414R which is the DCR of each half of the output transformer.

Don't know if this helps you at all though? 

Cheers mate.

 

[Jean Clochet]

... my 6ES8 arrived too, sounds basically same to me slightly more level...

I just caught what you wrote about that.
I found that the 6ES8 was less crunchy at the maximum end of compressing.  IOW, if there was a peak which caused the unit to hit the bottom stop on the meter, the 6ES8 stayed cleaner than a 6BC8.  I believe the trasconductance transfer function is a bit different and it could be that the 6ES8 and 6BC8 aren't compressing by the same amount?  Not sure. 
If you can, try both valves at maximum compression to see if there's a difference in distortion and let us know.  It could be that my memory is off!

Cheers. 

P.S.  I agree with Dave, don't worry about the anode voltage on the input valve too much - I'd say 55V to 85V is within spec for Rock 'n' Roll. 
As Dave said, as long as both sides are reasonably balanced throughout the range from idling @ 55V-85V up to 260V at max compression when the valve cuts off, you're good. 

 

[gary o]

Thanks Jean that helps im ma beginner.....still not sure where the optimum 145/150V is meant to be tho.... also dont know how to measure distortion dont think I have the right kit only rock n roll ears.....

I just found a 1:1 I think transformer in my junk box....got from ebay said was BBC ? I dunno but has NO CT but works well without nicer than my 1:2 Freed & as good as 1:4 Sowter......

thanks chaps....

 

[Jean Clochet]

Thanks Jean that helps im ma beginner.....still not sure where the optimum 145/150V is meant to be tho....

Valve's have quite a bit of leeway so, as long as you're close, it's fine.
Attached is a JPG showing the operating curves for the 6CG7 valve.  I've drawn a red crosshair where the RS124's 6CG7 valve is being biased according to a 145V anode voltage and loaded with a transformer.  Obviously, the original data sheet graph is for a single triode and there are two of them (dual triode in push-pull) in the RS124, so the 7.25mA plate current horizontal line is doubled to 14.5mA total current. 

Looking at the graph, you have some margin without worrying about exceeding the max. dissipation of the valve or worrying about it getting non linear (more distorted). 
The only thing which may change a little with a slightly different B+ voltage on the 6CG7 is the threshold at which compression happens.  This is because the threshold of compression take-off is set by a voltage applied to the 6AL5 diodes and, in the RS124, this voltage is tapped from the cathodes of the 6CG7 which is about 3.5V DC.  It's only taken from the 6CG7 cathodes because it was convenient and didn't require more parts (resistors) to derive it. 
In any case, it won't change much, if at all, and I wouldn't worry about it.

also dont know how to measure distortion dont think I have the right kit only rock n roll ears.....

I meant for you to use your Rock 'n' Roll ears    I do most of the time too unless I need a critical measurement.  I usually prefer listening to music rather than sine waves so I think ears make pretty good test instruments in general 

Just crank the crap out of the input so it compresses right to the bottom of the meter and see if you hear more distortion with a 6BC8 valve or a 6ES8 valve.  Or not if you don't care  8)

Cheers.

 

[gary o]

Thanks Jean crank the crap out of it using rock nroll ears that is my kinda test , I will be trying it as soon as I find my missing in action 6SE8......its here somewhere did it with 6BZ7 sounded like shit haha...

I have just tried the different value caps for the side chain feed..... .022, .047 ( didnt have 2 .05s) and those two values in parallel & for fun a 1uf......may have a little rock n roll video of it

 

[gary o]

Messing about with RS124 changing caps very in scientific rock n roll test of side chain caps just for fun

http://youtu.be/p42A6jnIiCU

 

[DaveP]

Hi Gary,

Blown away by your voice, you should be on stage instead of the bench!

BTW. I think you proved why EMI/Altec chose low values of caps.

To be honest, a voice is too high in the frequency range to test the low end where the caps are affecting the performance, bass guitar would be ideal.

Anyway, very impressed by your gear, far tidier than you led us to believe! ;D

Great stuff
DaveP

 

[MagnetoSound]

Hi Gary,

Blown away by your voice, you should be on stage instead of the bench!

Seconded!

 

[letterbeacon]

Hi Gary,

Blown away by your voice, you should be on stage instead of the bench!

Thirded!

Great video Gary and thank you for the BA-6A bonus test.  What pre amp were you using, out of interest?

I reckon all us UK DIYers should get together for a meet up this summer - perhaps find a studio somewhere where we can audition our kit.  Somewhere with a pub nearby...

 

[gary o]

Wow thanks for the compliments guys I just about make a living from singing each week in pubs n clubs its getting harder tho.....

Got loads of songs on you tube & some silly things check it out if you have a minute or two...

http://www.youtube.com/user/gary0liver?feature=mhee

Thanks Dave I might fit a switch to my RS to switch between some cap values so I can give it a better go maybe with a Jim Reeves song I take your point about the bass you see...its fun to actually do these experiments good or bad .....I see some other vari mu s have different value caps one of the Gates 26U I think has 1 uf but i guess it has more power to drive a faster attack.....

Matt the pre I used was based on  Collins single ended pre with good quality Sowters

http://www.groupdiy.com/index.php?topic=3397.0

That brilliant idea about the UK meet up...... bring your lovely RCA transformers & I will bring my crock clips hehe.....seriously it would be great....I tend to find I read lots a stuff about a classic bit a kit & then have to fork out for the parts to make it just to hear what it sounds like, thats why I make my stuff really rough & ready out of junk & Im lucky I can do it really...

 

[lassoharp]

maybe with a Jim Reeves song

Well if you're in that neck of the woods - throw in some Marty Robbins, Don Gibson and Ray Price.

Better Yet, how would the pub crowds react to some Scott Walker renditions?

 

[gary o]

Oo good choices Lassoharp Ive  song devil woman, oh lonsome, sea of heartbreak, for the goodtimes was that Ray Price.......not sung a Scott Walker but his voice is more more natural tone....how about bit Eddie Arnold....I love these classic singers.....

Anyway I forgot to add while having mini vari mu off between the RS124 & BA6A kind off because Letter beacon was asking about a BA2A ish pre I build ( Matt has made a gorgeous looking BA2A with real RCA iron, jealous)..... In my video & other tests I used a collins DIY single ended pre thru both vari mu s & they sound pretty similar BUT my BA2A sounds great with the BA6A but really thin bass less thru the RS124 ??? at the moment I have I think a 1:1 transformer BBC input tran & it sounded similar with my 1:2 Freed input tran ? the Ba6A has 1:4 the Brian Sowter sold to me years back when I made the BA6A & say its perfect for the limiter & to be fair every pre I have sounds great thru the BA6A.....Just wondering why the RS124 doesnt like the BA2A........

any ideas what am I missing......

 

[DaveP]

I reckon all us UK DIYers should get together for a meet up this summer - perhaps find a studio somewhere where we can audition our kit.  Somewhere with a pub nearby...

I'm up for that!
DaveP