EMI RS124

[lassoharp]

my BA2A sounds great with the BA6A but really thin bass

So, are you saying the BA2 only loses bass when connected to the BA6A(and RS124)?  Does the BA2 have full bass when just ran into DAW? 

 

[gary o]

Hi Lasso BA2A only lacks bass when connected to the RS124 NOT BA6A, I have point to point DIY versions of REDD47, V72, Altec 1566A,Pultec MB1 ALL sound full a nice into Both limiters

Good news about meet up.....

 

[gary o]

Just did one last experiment with RS I added a voltage reg tube tried a OA2 & OB2 as in BA6 Gates STA & other limiters.........as I think Jean pointed out didnt make any difference I could hear with rock n roll ears, pretty colors tho.....

going to address the BA2A problem now ......thanks for reading....

 

[letterbeacon]

I'm sure greater minds will be along in a minute, but it sounds like maybe an impedance issue?

 

[gary o]

Forgot to say lasso yeah BA2A sounds good without any limiter.....

Suspecting my BBC input tran that I dont know much about, I swapped for Sowter 8540 1:4 from my BA6A ......... now the BA2A into the RS124 sounds full and nice, more level due to it being larger step up ......so why does the BBC sound good on all other pres BUT not the BA2A

I re tried my Freed 15K 60K... again DOESNT sound full bass with BA2A.......

I notice the polarity makes a big difference in sound & how the vari mu compresses, Im not quite understanding whats going on .....some basics Ive never really got to grips with.....something to do with phase ?....audio is AC tho right ?.........I notice if I record on my DAW with what I would call polarity round wrong way the compressor only compress half of the waveform that I can see in my DAW...  I have tried the limiters and the pres in different polarities but still BA2A sounds weird thru the RS124

one last thing the centre tap on the RS124 does NOTHING..... I thought it was essential to a vari mu push pull circuit ?

Thanks for baring with me on this.....

 

[DaveP]

Gary,

have you got one of the transformer pairs reversed?  That would try to cancel out a phase, more noticeable on bass, also check that the in/outs are all balanced not one side earthed.

With the swapping of transformers, you might have mis-wired.....easily done 

best
DaveP

 

[gary o]

Thanks Dave I no what you  mean easily done have tried all wireing combinations on two transformers....

 

[lassoharp]

Suspecting my BBC input tran that I dont know much about, I swapped for Sowter 8540 1:4 from my BA6A ......... now the BA2A into the RS124 sounds full and nice, more level due to it being larger step up ......so why does the BBC sound good on all other pres BUT not the BA2A

Only thing I can think of at the moment is that possibly the BA2 effective output impedance is higher than what the BBCs primary wants to see for full response. 

You could try putting a U-pad in front of the RS 124 using a very small shunt resistor (75r or 50r) to set a lower source impedance and see if that changes anything response wise.

Also, you didn't mention whether the BA2 was using a termination resistor on the sec during any of the interfacing.  I'd say always terminate until you pinpoint the problem. 

 

[gary o]

Thanks Lasso I wasnt using a resistor across the output of the BA2A also not on my other pre amps.......this is some basics Ive never really got to grips with........I just tried a 620R across BA2 op tran & into RS124, I loose a lot of level and some top but bass is better......I put a 47K pot wired as reostat to adjust resistance across op tran but cant get a good sound its better than lack of bass tho,. I just dont understand how this works....... when I do this but into the BA6A its doesnt seem to effect the frequency response so much

thanks

 

[emrr]

you probably lose about 6 dB because the ba2 is loaded to a matching impedance, which is likely also the impedance the BBC input wants to see for full bass response.  You have plenty of gain to play with, so the loss doesn't matter given that the bass corrects.  The ba6 input would have a different source Z requirement for full bass, thus the difference you hear.  It's useful to do these experiments, and put in a switchable output load if differences noted. 

 

[lassoharp]

I put a 47K pot wired as rheostat

Probably want to use a 5 or 10K pot there.


As emrr has said and shown in several sets of sweeps, the effects of varying the sec loading can be different for different amps.  The general thing to be aware of is that loading the sec affects the load the pri presents to the tube.  So a 15K:600 will give ~ 15K AC load impedance when the sec is terminated with 600r.  Just pulling out a 15K:600 OT and testing,  I get 14.8K when sec is loaded with 602r.  When I put a 10K r on the sec I get 3.8K (at 120Hz) on the pri.  There are numerous other interactions to consider that will affect the outcome but the simplified basic situation is that with a 10K load on a 15K:600 OT, your bass frequencies now have a much lower load to work into which would suggest you will have greater loss at lower frequencies.  AND, in addition - the AC load impedance at 1K (with 10K sec loading) has now jumped up to 87K!!  It may not effectively be that high in a real circuit but you can see from the numbers what is happening - that great shift in loading will likely make the resulting output sound shift towards being very toppy/thin - like turning the knob on a tilt EQ towards the treble.  But, as emrr also pointed out, some OTs seem impervious to loading.  I'm not sure why that is.

I loose a lot of level and some top but bass is better

The level loss is normal.  When you readjust everything to make up for the level loss you may find the overall sound more balanced.

 

[gary o]

Thanks fellas I will need to digest this buts its making some sense I do struggle with this and other techy stuff...... Yes  losing the level is not a problem  when I re balanced the levels it was much better fuller tho some top had gone it was like my condenser mic was now a ribbon..... I will experiment with pots later today.....this is really intersting....I will also dust off my LA2A and see how it deals with the BA2A ....

I dont use L A2A much I may borrow its tranformer for the RS124 I also have a 012 type transformer in a 1176 clone I dont use much , I wonder if that would work....

Thanks once again.....

 

[DoingItMyself]

I was wondering, would I have to do anything to the circuit if I wanted to use a ECC189 (6ES8) tube instead of the 6BC8 or is it OK to simply just swap it out?  Thanks

 

[lassoharp]

They are compatible electrically and in functional context.  Just yank and swap.

 

[DerEber]

I tryed the ecc189 in my poorman instead of the 6bc8 but had no luck. I had a lots of current flowing, but no audio.
But I can´t confirm that my tubes were good. I only had two.

 

[DoingItMyself]

They are compatible electrically and in functional context.  Just yank and swap.

Thanks 

 

[lassoharp]

I've never tried the ecc189 but have used both 6ES8 and 6BC8 on DIY Altec compressor.

 

[DerEber]

Maybe a stupid question, but I never got into neon lamps befor.
What exactly does the b2a neon do in this circuit other than glow.
It´s not the one on the frontpanel, isn´t it?
I need to substitude it with some other type, thats why I am asking.

 

[DaveP]

The neon is part of a simple oscillator circuit, as it goes to both sides of the push pull circuit it is possible to cancel the signal when the tube halves are in perfect balance.  It was designed as a simple set-up procedure that could be done in the control room with just a screwdriver,  dismantling in the workshop was thus avoided along with the consequent loss of time.  Other designs like the BA-6A use a reduced mains frequency from the heater circuit for the same purpose.

best
DaveP

 

[DerEber]

Thanks Dave,

I was thinking of the lamp maybe being part of the oszilator, but yet not 100% sure how it works.
Doesn´t it simply go on and off in 100Hz?
The capacitator is charging until the Lamp goes on. Then Lamp goes off again.
Maybe it produces more harmonics this way?

best greetings,
Stephan