Frankenstein Vari mu limiter, lives....Collins 356E/BA9/Collins 26J/unilevel...

[gary o]

Iv been breadboarding......Just for fun..... I was lookin at Gates 26J/Collins 356E/unilevel/GEBA9 schematic & decided I wanted to braed board it, I dont understand too much about the theory so you may have to excuse me & please point out anything ya like as I am here to learn...

I have a Sowter input transformer 8540 & a Sowter 8K 600 R PP output tran the one they do for BA6A output {links below} wasnt sure if these would or should work in this circuit so I just tried it....I also dont have a 6386 tube so heres the frakenstein bit ....I breadboarded the input part of a western Electric RA 1593 A sadly I cant find the schematic here anymore il keep lookig & maybe scan later, it uses 2 6BA6 tubes that I have,for the level control section

I grafted them together not knowing what to expect....well its alive anyway I have audio flowing thru it Im not really sure what to do next Im gonna try and add a meter firstly but Im not sure whats goin on with the compression part of it.....I built it in three stages, I made the 26J output stage, had that running, built the input stage of the western had that running, connected those together got audio but way way tooo much....I put a 50K pot as a load i spose across the 2 6BA6 grids which brought levels down.....But I had a sort of compression going on at this stage before adding the daul diode stage at all, to test as I went along I had a mic plugged into a pre plugged into the frankenstein frank intosound card .....if I shout at the mic it distorts but compresses and then I hear the release ...about 3 or 4 secs ....creeps back up to normal level.

At this point the center tap of input tran had nowhere to go...

I then breadboarded the side chain of the 26J the 6al5 diode & connected the input trans centre tap & now I have much cleaner sound doesnt distort so much....when I turn threshold I get a better compression but its a bit distorted much better than with no side chain same slow release ....one thing I dont understand is I thought the 6AL5 got its power fromR 611 a 200K res but it works without that connected to the HT ? should it work ?{ Link to schematic below by the way}

http://img.photobucket.com/albums/v473/MikoKensington/Collins_356E-1.jpg

to test the effect of the side chain I shut of the heater voltage to the 6AL5 when off the audio goes back to crap.

Heres some questions firstly am I expecting too much to get this to sound useful ?....it sounds nice without compression if yr gentle.

Has anyone any ideas what I can try to tame the input a pad maybe... I forgot to say I have quite a bit of padding on the Op tran already something NYD & PRr helped me with a while back for the BA6A

Are my Input & output trans in the right ball park {Sowter Links below}

http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_PRO_VINTAGE_TRANSFORMERS_5.html#a275
http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_INPUT_TRANSFORMERS_11.html#a35

Ah I see the Sowter links dont work .....you may have to copy & paste  sorry about that {fixed links --PRR}

Any thought I would love to hear thanks.

 

[gary o]

Ok so Im talkin to myself ...no change there then...well if anyones reading & for my own records heres what Iv tried so far.....lookin at other vari mu circuits I see they have different means of taming the input , I tried a T pad followed by 500R pot before the input tran as in the 26J caseI then tried 100K daul pot after input tran then a 10K daul pot then a 25K daul pot as per my ba6a that seems to work best.

I dont understand but this thing seems to compress when the 6AL5 has no  HT voltage fed to it......&  if heaters are disconnected I loose all audio... I also have compression happening throughout the full range of the 100K threashold pot of the 26J circuit, tho little fully ant clock & way loads fully clockwise, so seems to work ok .... if I disconnect the 6AL5s 2 plates from the centre tap of input tran, I then have no compression which should be so

If control voltage from the 6AL5s plates is connected to either or both the input trans centre tap & or the centre of the daul pots then i works fine which I think is right, I notice that different old limiters do it either way to.

I messed with the caps that feed the signal to 6AL5s cathodes as expected the bigger cap values the more compression ....bass I spose.....I went smaller than the 26J circuits 0.1 uf to see if it would be bit gentler but doesnt seem to to my humble ears.

I also added a couple of 47K resistors to the Ra 1593s input circuit that I left out originally to see what they did.....they took level down a bit either way is good for me.

I also tried running the input part of this circuit on 280 volts right from my PSu or 150 volts from a Vr150 both work good 150V less output to be expected I suppose.

Some a you clever fellows are prob laughing at the stuff I dont know, ha I laugh with you Im learnin slowly & having fun and this little monter is sounding good now ....Im off to play with the time constants & gonna start thinking of bolting this thing together as it seems worth the effort, thanks for reading any comments welcome.

 

[emrr]

1)  it's a COLLINS 26J

2)  the DC fed to the 6AL5 is a bias voltage to control threshold and ratio.  Note when the pot is all the way down there is also no voltage.

3) side chain caps probably have to change by a full decade (0.01) to notice any real difference in comparative fashion.  But 0.1 should be small enough.  The 26U uses 1 mfd. 

Hard to follow a lot of what you are saying with no visuals or diagrams. Don't have the RA1593 in front of me. 

 

[gary o]

Hi Doug thanks for looking in .....Ah so the daul diode will be made to work without the bias voltage & in effect the the bias voltage at its highest will stop the limiting the threshold pot controls the voltage 0V to 285 ish in my case therefore the ratio .....its sinking in slowly takes me a while I think I understand now.

My scanner is broke only had camera & scissors & pen so please excuse this its what this circuit is not connected meter yet....

 

[gary o]

Yeah silly me its a Collins 26 J not gates .

Now I understand ish the threshold voltage... Im feeding threshold my full 290V and the pot works right now ant clock no comp at all ...clockwise full biffter !!

Also discorvered silly mistake I had a 22K plate res intead of 2K2 ....I have more level now

I did try 0.01 uf to feed the side chain before & didnt seem to make much difference & is working good as is.

I have higher output level & voltages come right as per original schematics if i leave R7 & R10 out ,tho does work & sound good with them in....the output will be attuated anyway so prob leave em out & save 60 pence woo hoo.

I added the daul & average switch and time constanants ...It does start to fart on the average setting at lowest threshold at higher input levels now a bit.....not too bad... tuning the balance pot helps a little
I read the attack time for the collins 26J inthe average time constanant mode would be the faster at 11 millsecs 62 ms daul mode
The Western RA 1593 would have had an attack of 1.5 milliseconds but that had little amp driving a transformer to drive its daul diode..Im not clever enough to work out how to calculate Franks ( thats its name now} attack time.....PRR are you there .....something to do with the tubes impedance & sum of total resistance then theres the power....I dont no it sounds good for vocals...

I added a meter with pot on so I can zero it , it works well

All in all a good days work turned a another pile a junk into a limiter of sorts Im happy ...Im off to find some sort of case maybe a biscuit tin or something.

Thanks for your interest.

 

[PRR]

> only had camera & scissors

 

[gary o]

Maybe some sticky tape....

 

[gary o]

Heres a scary pic....I know I said breadboarded it I meant crock clipped it....yeah I know its dangerous.



Looking bit tidyer in old case I found in the shed ...





I reported a weired thing in earlyer post that I loose audio when I turn off the 6AL5 s heater power......silly idiot the pre I was using to test was usining same supply  :-[ ......silly boy.

I hooked up a OEP A262A3E as input tran that seems to work good Im gonner try a little beyer 1:1 I have might not be man enough tho.

Was wondering if I should make a H ladder ettenuator for the input instead of pot....any thoughts ?

I also may try other attack release times from the Collins 26J manual, theres a link round here somewhere for that, its seem changing 2 resistors will different release times in daul & averge mode ,could use a double pole switch & have a selection.

Theres room in franks new home for another stage of amplification, could be interesting experiment to add a stage from another vari mu limiter maybe 12AY7 stage, If I find one that I can get working I could make in switchable I might even be able to figure out how to add some feedback....I think Im getting carried away

 

[analag]

Excellent deduction and problem solving...you didn't need our help my friend. Keep going, experiment.

 

[gary o]

Analag what a nice thing to say thankyou very much experiment on I will....

Further experiments to come ......Im gonna add a middle stage gonna crock clip it & patch it in once its bit tidyer.

I found a a copy of a manual for GE BA9 smae as Collins but on its schematic someone  has added some feedback it drawn on in pen, dont no who added it but before I add my middle stage I will try it.... 2 x resistors 1 x capacitor per 6V6

Cheers       

 

[emrr]

If you actually had a proper Unilevel or Collins output transformer, there'd be an unused tertiary winding you could use for feedback control.  Analag posted something somewhere recently about a Cinemag with that sort of possibility. 

 

[gary o]

Interesting Doug I wonder why they didnt use it in the first place ....you say the BA6a Op tran has tertiary winding too... I no the BA6A has capacitor resistor feedback so why not use the tertiary winding for that too....Is it not better ?

Any ideas

 

[gary o]

Just tried the feedback as drawn by someone on my schematic... as follows plates of 6V6s to 33K to 47K res to .47 u cap all in series back to 6BA6 plates in my case.....I wonder is there a reason to use two resistors in series rather than one 80K res per side....

Over all level went right down as expected not a major difference in sound but does soumd cleaner I suppose could be handy to tame the levels I might put a 2 pole switch in there..

Gonna add a third stage now to see how that sounds.....

 

[gary o]

Ah I see the Collins 26U has tertiary winding on tran & is used .....Ithink Doug has mentioned this before..

 

[emrr]

They're just using the same transformer in multiple pieces, some with and some without the tertiary.  Gates used the tertiary on the 1st versions of the SA-39, and did away with it by revision #2.  Switched to resistor and cap path.  I'd assume phase considerations played a part in these decisions, and possibly only with regard to radio transmission requirements. 

Gates SA-94 program amps were done with two different sets of transformers, one with tertiary and one without.  Different feedback schemes to achieve same basic result. 

 

[gary o]

Wow interesting stuff Doug I see lots of old mic pre circuits with tertiary windings like the BA21A for one I know you have some iron for those I ve often wondered how much differebce those winding made to the sound....when Iv DIYed stuff with feedback I find I usually prefer the sound without the feedback I guess its more distorted.

I just crock clipped a 12 AT7 as per Gates Sta level....it works lovely....I didnt connect the Sta levels feed back system a 150K res and .1 u cap at first as I say sounded nice....I then added the feedback part ,levels dropped as expected, trouble is I have to turn my input pot and pre right up to get same level but now with threashold right down it compresses way too much.....gotta get my head round this...I think I understand maybe keep input pot & pre down and make a smaller output attenuator.

Gotta experiment with the caps that couple the 1st to 2nd stage I have .1 uf Sta level has .02uf.

I also swapped the 12AT7 for 12AY7 I need to do some recording to find the combinations of parts I like the sound of best, I spose ive ended up with a STA level after all all be it a ghetto junk version