Controlling tube pre amp levels & transformer help

[gary o]

Hi Iv being trying to DIY the best vocal channel I can for the money, I have DIYed many tube pres and some tube limiters & have come to the conclusion that a BA6A limiter is the best sound Iv ever heard so I will use just that, all the pres have there merits, so I want to use them all however some have level control which is good, & others dont & I need to tame them before the limiter does its bit..trying not to use pads.

I have Sowter mic transformers 9045 on my tube pres that sound great but for the hotter pres without level controls ect V72, BA21A, Mcurrdy A300

I think I need maybe lower ratio trans...
is that wise ?

I have a little tran from a audix transistor pre ,seems to have lower ratio than sowter I also have a OEP A262A3E but am not sure I understand best ways to wire it ?? if anyone could give me some pointer that would be good

I want to make a means of plugin in different trans to different pres..

That brings me to my next questions...upon reading here about the A262A3E tran, I read about Zobel networks & stuff, sadly i find it quite hard to understand lots of the techy stuff thats here but im trying & I like to experiment & use my ears...some 1 was experimenting putting different resistors across the output of the tranformer ...
I put a 200K pot across the output of one of my transformers Is this not good idea to leave it there to control the level to the pres ?? I dont no how to use my scope yet but to my ears it sounds pretty good still.

I have a Gardners GR118066 AF matching tran Pri 6.25/25K Sec 100K .......
Can I use this for anything ???
Also a RCA MI12399 pri 250 Ohms centre tapped Sec 50K 1:14 ratio...what could I use this for ??

Thanks for reading sorry the what might seem to you silly questions

Many thanks .....................Gary O.

 

[rodabod]

[quote author="gary o"]
I think I need maybe lower ratio trans...
is that wise ?[/quote]

That'll get you lower gain, but you might want to consider the change in input impedance caused by the swap. A lower ratio will present a higher impedance at the input - eg. if you moved from a 1:4 transforner to a 1:2 transformer, then your input impedance would multiply by four.

Why do you dislike pads?

The Zobel stuff isn't too complicated - they are basically just filters to tame resonance (peaks in response).

 

[skipwave]

Rod is right, the transformer's job is impedance. The voltage gain is just a side effect.

Pads seem like the best possible way to maintain the tone you love and lower the levels to where you need them. If designed to emulate the impedances that are normally seen by your outputs. It is true that a pad which presents a very different load to an output transformer can certainly alter the tone. Just model your pads after the setup that you have obtained best results from in the past.

 

[emrr]

Fixed gain pres are exactly that; fixed. pads are your only option, unless you wish to re-design, and then what was the point of bulding a clone of a classic fixed gain pre in the first place?

Think about the input and output padding on the BA-6A. You are probably throwing away all the gain with the input and output attenuators; measure it. . They don't adjust gain, they pad input and output levels.

 

[gary o]

Thanks for responsese fellas, yeah I do struggle to get my head around impedance im afraid but I take on board what yr saying, Im not worried about modding the amps and limiters a little from there original design I already have all sorts of mods to all of them as i dont suppose they sound as original due to the different transformers, mostly sowters that brian suggested would work in the various circuits and they do...
It just didnt seem right that all that powers is kinda wasted...& to pad say a pre just before the limiters input tran brings level up again the BA6 is 10watt amp, I understand now it was designed for long longs from studio to radio transmitters.

I think Im gonna make a better input dual pot control for the BA as the one I first made isnt good enough.. so its kinda like padding then output of a fixed gain pre...as I now have a threshold control on the limiter plus attack release & various daul constanants networks I knicked from other classic limiters.
I may need to tweak the output of limiter as I couldnt get proper daven D type attenuators & had to make 3 position balanced attenuator

Juat saw this on Sowter web site , wondered if it would be good for BA6A input

click on attenuators on left of page ,hadnt seen these before or am I barking up the wrong tree again

http://www.sowter.co.uk/


Thanks again for your valued thoughts.....Gary O.

 

[emrr]

Or go with a no-gain / low-gain input transformer on the BA-6A; same discussion goes on and on endlessly in the LA-2A threads. That BA-6A input transformer is adding approximately 20-25 db of gain. So, if your input attenuator is always set at -20 or lower, you're just adding it up and then throwing it away every time. Limiters like that in many cases replaced the program amps in consoles, and were fed mix bus signals in the -25dbm and lower range, thus the available gain.

 

[gary o]

Yeah thats exactlty what I mean its kind of wasteful, yes as you say same for LA2A I may buy a hi quality center tapped 1:1, I can then try it in both my BA6A and LA2A...Iv got some little beyerdynamic ones but no centre tap could try it in my LA2A or maybe I could split the phase for the BA6A with resistors as in the Altec 438A, even just as a level experiment for BA using Beyer 1:1

Is it better to split phase with input tran rather than resistor network ?

Thanks im learning lots

 

[emrr]

Splitting phase works fine either way I think. What I wonder about is the affect on bias current and timing constants with resistive center tap versus transformer winding path. Altec advertises same spec in both versions, so probably very little difference in practice for gain reduction purposes. Seems it may make much more difference with something like the Fairchild 660, which delivers very large current to the GR tubes(?).

I don't think those little Beyers can handle the levels. Simple cheapo is that Altec 15K:15K octal can thing which is meant for line levels.