gary o

Vari Mu qestions Triode or Pentode ?
« on: April 27, 2008, 05:41:48 AM »
Ive seen some vari mu limiter circuits with the gain reduction?input/vari mu stage being a pair of triodes....I also see pairs of pentodes...

does one have any advantage over the other ?

Lots of circuits use the rare 6386 triode & I no a pair of 6AB6 can be wired as triode in the 6386 place....Iv also seen old circuits that used the 6AB6 as pentodes

same question really ?

Also do we need more parts for pentode circuit?

Im making a vari mu limiter...bit of a hybrid really,Im using 6AB for the gain reduction stage hence the reason for the questions stand by for input transformer questions hehe

Thanks for reading.


Vari Mu qestions Triode or Pentode ?
« Reply #1 on: April 27, 2008, 11:00:46 AM »
Hi gary o:
   What a great topic to discuss..   I know that pentodes are the tubes of choice for the application you have in mind..  Every time I made up pentode circuits I could never get them to behave as I thought they should..  I think it was the Screen circuit that gave me the issues..  The components of a pentode circuit must be so accurate..  Any component that is a little off tolerance will render your circuit useless..  for Audio, that is..  

          My opinion..  Of Course..  


   I forgot..   My interest for making up a pentode Pre-amp circuit was, to Use it for the Ribbon Mics I have..  I could never reach the potential that I know my ribbon's can achieve..

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #2 on: April 27, 2008, 12:09:09 PM »
Hi Bill, have search around here theres lots of discussions about ribbon mics and pres for them, I made up a RCA BA1A pentode pre ( I think its pentode) also a BA6A limiter & AM864 limiter they work great.


Vari Mu qestions Triode or Pentode ?
« Reply #3 on: April 27, 2008, 02:42:56 PM »
Read your Radiotron Designers handbook 4th edition.

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


Re. Radiotron Designer's Handbook..
« Reply #4 on: April 27, 2008, 03:08:01 PM »
Hi Gents:
  I have a Radiotron Designer's Handbook..  But it's the Third Edition..  

  It's the Third Impression   1941..  There was 3,000 copies printed..  

   I stand corrected..  RADAR was just coming on the horizon..  I'm too young to know for sure..  



Vari Mu qestions Triode or Pentode ?
« Reply #5 on: April 27, 2008, 04:13:34 PM »
Quote from: "emrr"
Read your Radiotron Designers handbook 4th edition.

Meaning the various types of gain reduction/expansion tubes and their relative advantages are addressed.

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #6 on: April 27, 2008, 08:51:24 PM »
Thanks Doug...

Had a very quick glance ....very interesting reading


Vari Mu qestions Triode or Pentode ?
« Reply #7 on: April 28, 2008, 03:16:57 PM »
hi there gary-o. ive made quite a few vari-mus using pairs of ef83 in the front. they need to be fairly closely matched of course or else they distort like mad when gain controlled.
a gain set voltage devider is set to the screen grid so as to allow fine adjustment to one of the pair. the components are just normal 1% tolerance and the whole thing works real good. maybe the same principle can be applied to your front end choice?

the ef83 is a remote cut off version of an ef86.
please check my (rarther scrappy) circuit and please to comment.
im always interested in making them better of course.
youll see my attack release circuit circuit is very simple and that seems to work well. i.e very 'effective'. ive altered the value of signal rectifier feed capacitors back to .01uf. so that any brutal bass dosent squash everything so much (in a mix). hope this all helps.

another thing that enables these pentodes to act smoothly is to preset their gain contol with a little bias to overcome their initial reluctunce to go into gain reduction. i hope my point is clear. the ef83 has a kind of hump ... and then its smooth. maybe your tube is similar?

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #8 on: April 29, 2008, 06:17:46 AM »
hey ekadek yr circuit looks great ...did you desgin it,I already had yr shematic on my PC ,dont no where I got it, sadly Im not clever enough to make it better maybe someone is lurking here who can help...I tend to copy circuits & try to imrove or should I say make them better for my purpose which is just to record vocals into PC, Im starting to understand how they work but not right down to the nuts & bolts...I copied the circuit for the BA6A a few years ago now, & have been using & tweaking it ever since I dont no how a real one sounds but mine sounds good to you I experimented with the diode feed capicitors I had bigger values for a while but yes the bigger values made it crush more due to bass, I do like to add switches tho so I can have this feature but I like to have a switch for the original sound too as Iv learnt I can wander off the path of how its meant to sound...I find my BA & other vari mus have way too much level for my modern sound card & have been experimenting with pads & a threashold level pot ( I see you include a threashold pot on yr vari mu) Im finding the pad alters the sound for the worse & having been thinking of designing my own vari mu....I know they need the power output stage to drive the rectifier to get quick enough attack so I was thinking of tapping my audio from earlyer in the get the levels low enough Im having to crush more than I i said I have my own threashold pot & if Iv done this correct Iv measured the bias voltage ??? which goes from 0V to 144V I think...In the BA manual it should be a fixed 66V as there is no threashold pot so I figure that if I set my pot so bias reads 66V then Im in the BA6 original threashold state ...I hope ? but in that condition I need to drive it harder cause more output.

Your output attenuator looks interesting ....what of attenuation levels does that give ? is that a T type atten ?

& whats the bottom 12AU7 is that like an active attack thingy ?

Is that a interstage transformer I see ?

& what transformers are you using ?

sorry for all the dumb questions hope you dont mind

Iv enot seen any limiters with EF83s in yet so thats interesting..I wired a pair of 6BA6s as triode and as if they were 1 X 6386 I grafted in onto the input of ny BA instead of its usual 12SK7s I tried different cathode resistors & so on I set the tubes up as in other circuits like the gates & collins but they didnt sound as good as the original I spose because it wasnt designed for those tubes...

Ive just been fiddling with a VK1 circuit.. copy of Altec 436C but with none vari mu ECC85 tube input ECC81 output ....the over all sound was great ( with BA6 output tran) & levels were more managable, but the limiting effect was crap for vocals maybe I could graft the 12SK7s BA input in place of the ECC85


Vari Mu qestions Triode or Pentode ?
« Reply #9 on: April 29, 2008, 06:03:16 PM »
many questions. i have a couple too. please put up your circuit so i mite understand more of what you say. i must admit to be being confused about your threshold describing.
yes. those are T attenuators on output. these amplifiers do need adjustable ouput levels for the reason you suggested. i.e too much grunt for average mixer input or soundcard. a properly designed T achieves this without undue loading of output traffo or following stage. it has no effect on frequency response  and is all good.  the 'within circuit ' signal level is high because of the design is reliant on that level to create a control signal large enough to enable gain control.

otherwise we'd  be making fairchilds with dedicated line gain controlled amplifiers with separate 20watt power amps to make that gain control signal. like u - that is my next limiter project too.
i have a few 6386s and a spare leak 20 . shell be big and clean.

the 12au7 u speak of is feeling for the difference between the no limit state and the limiting action state. a 1ma meter is wired across the e&a terminals on the two cathode resistors and shows limiting action.

the meter can also be switched across the cathode resistors of the ef83 tubes to indicate their current. by switching the meter back and forth between the tubes, you can align the gain of one tube to match the other. this fine adjustment is essential i reckon but still those ef83s need to be fairly closely matched to start with. the fine adjust AND the way the ef83s are normally biased  into gain reduction (approx -1.8v) help to smoothen the gain reduction curve. hence very low distortion both on reduction onset and deep exteme gain control.

my traffos are 600/68k c.t input. 16k c.t/600 push pull. and the gain cell uses a 470k c.t choke arrangement that is damped with those resistors across the halves. the original phillips 6n8 limiter amp used a similar thing and had r-c circuit (eq) across it. i deemed the eq useless because the new chokes were far superior to the 1950s material quality wound things.
(just old actually - old shellak on the little wires)


Vari Mu qestions Triode or Pentode ?
« Reply #10 on: April 29, 2008, 08:49:09 PM »
Welcome Wox!..It's Ray from over at the ntrack forum (years ago) ..good to have you aboard..there are a ton of nice people, designs and discussions around here...

 :guinness:  :guinness:

"orange whip? whip?...3 orange whips please"

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #11 on: April 30, 2008, 07:35:19 AM »
Sorry Ekadek I rambled away forgeting the schematic wasnt there... here it is

couple of the resistor values around the 12SK7s are wrong....not sure where brian sowter got this schematic but it doesnt match the one in the manual but I think its just those values that are wrong...I found out the hard way ha.

Il try to re explain my threashold arraingment with the help of the schematic sorry Im not to good at explaining what I mean .....
The 56K 33K 15K resistors in a row bottom right marked as diode bias......I removed these & put a 100K threashold pot in there place,I know from the manual the bias for the diode before my mod would be 66V so I figure my pot can set anywhere 0V to when I set the threashold pot to 66V it should be the same original threashold as the BA6 would have been & any setting lower than 66V lowers the seem below 17V it distorts ssssses quite bad...the reult for is I tend to compress more just to keep the output down a bit.

Check out the balance A & B pots on front end you can match any pair of 12SK7s should be able to do that with yr EF83s....theres a little test circuit built in sends a hummmm and you tune it out.

Ive had no luck yet with attenuation..I have a couple of different arraingments there some clever chaps assisted me wich uses a pot the other switchable, they do work a treat to a point but when i turn them right down to atten more ( when Im not compressing as much so output is higher) the sound looses bass goes horrible & thin.

the wise chap say balanced dont matter also 600 ohm dont matter to sound card, Il have to check back as to what i did but I think one of my attenuattors is 3 X 160 Ohm res in series across output of BA6 & attenuated output is taken across middle 160R res......Is that a U pad ??
Is it balanced ?? It would divide the output into one third I suppose...someone suggested i do it this way.
the other thing I have is a 10db H pad a 1K pot and couple other resistors I think its balanced its cross between newyok dave suggestion & a PRR suggestion ,,,those boys know there stuff.

I ramble on ha I like the sound of yr next project would be interested to hear more.

thanks for reading.


Vari Mu qestions Triode or Pentode ?
« Reply #12 on: April 30, 2008, 09:39:15 AM »
apoligies back. i cant  find that circuit at sowters.
i need a better link or something.  thanks mate.

i think u need to attenuate the output right back
and use the limiting less. theyre only really designed to turn down the peaks.
to make the difference between the softest sounds and the loudest sounds - less. to limit the programme dynamic range. yeah i know they are good distortos but it doesnt seem like you want THAT sound on your voices.

sounds like u need one of those radio station stay level volumiser thingeys with super fast transparent attack and long release AND it expands when signal drops as well. designed to keep all the signal dynamic range within 3 db or something no matter what the signal input is. like an automatic volume control.  :idea: a little volume slavegirl.

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #13 on: April 30, 2008, 12:24:28 PM »
No worries the should take you to the Sowter page ....on the left of page click pro vintage at bottom of that page on the left is a link to schematic.

Sorry about that I thought it would take you direct to schematic but it doesnt.

Yeah your right thats what Im tryin to do take level right down but cant seem to do that without losing the bass.

The thing sound great when compessing a lot I love it but sometime I want a more gentle effect.


gary o

Vari Mu qestions Triode or Pentode ?
« Reply #15 on: April 30, 2008, 03:22:30 PM »
Thanks Rafa see if I can post a clean version of the manual schematic has all correct component values on it.

Refering to my last post...a little correction reguarding my diode bias ....... I left the 15K res & replaced the 56K & 33K with a 100K pot to give me my adjustable threashold & rather than connect one end to the 15K i went to ground to give a wider range tho its this extra range than distorts.

I read from the radiotron manual PDFs (link in prev post) very interesting stuff some mention of the double choke thingy...I think the Altec 322 has one a those too.



Vari Mu qestions Triode or Pentode ?
« Reply #16 on: April 30, 2008, 08:34:19 PM »
yes. thanx rafa. i looked and looked at that diagram.
something wasnt right.
so i went into my stash to find the circuit of a limiter i had once.
the AWA 2G58250. its a direct copied RCA BA-6a design
with a few differences and australian ialterations.
those guys were very good.

i made a page  of the bits of the diagam.
the front end one is the most revelvant to you maybe.

I reckon the drawing from sowters that u been using is incorrect.
the cathodes resistors on the gain cell pair of 6SK7 should be 15ohm.
the tubes would be set up all wrong with 56Kilohms.
someones handwriten those numbers in just a bit wrong.
maybe they were meaning 56 ohm?. dont know.
heres a link to the page i made this morning after a little photo session.
(sorry. i dont have a really reliable scanner)

theres a few differences between the ba6a sowter version
theres quite a few details that you could use there i reckon.
the gain setups of all the stages is a little different.
feedback eq in the make up stage.
output attenuator details etc.

the awa limiter was amazing to use. so richified.
really nice dynamic control. very smooth.
i remember tho that i did lack low low bass.
the awa had .1uf feeding the power output stage.
the sowter drawing says .01uf.
that might sort it to open it up even more.
.22 or .47. ???
i never experimented with that before i let it go.
i do miss it actually. aah well. its only gear eh.

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #17 on: April 30, 2008, 09:54:18 PM »
Fantastic ! :grin:  how interesting....Think you misunderstood me a little I knew the sowter diagram wasnt 100% I did point out in my post with the link about the cathode reistors being wrong values, yr right they meant to be 16 ohms I havent been using Sowter schematic ..A few years back when I 1st made my BA6 I had a grainy copy of the manual schematic & some component values werent visible & this schematic helped to fill in the blanks...& I did try out the 56K cathode resistors as I didnt no which schematic was the right one when it didnt work I ccarried on DIY using grainy schematic..I only sent you the link or tried to coz i knew where it was on the net (sorry to confuse) I will scan my now clean 100% correct manual schematic & post it here , my BA6 is madeas i say per the manual schematic with Sowter transformers all my voltages are well within the 10% of the manual voltages.. and it does sound lovely, unlike yourself I dont have or know how the real vintage things sound...I only have homemade stuff to compare but the BA6a blows away my LA2A with Sowter I said Im just trying to find best way of matching the vintage to modern sound levels really

Your Awa is so interesting to me ...tweaks i can try...awa has 6SJ7s in 2nd gain stage BA6 has 6J7s thats interesting...I wonder if it sounds different to the BA with those tubes....the 0.01 caps are the same....I have good bass , I was told the caps on last 2 stages cathode add some bass ?...theres a clearer pic of the balanced T attenuattor on your schematic & 10 db pad .....Im gonna study yr pictures closely

Il get the100% BA6A schematic up here in the morn so you can have a look...If yr interested do a search (BA6A) my & others old posts are here mainly me asking silly questions.. Im still so green... nice guys helping me out..CJ reversed engineered a BA interstage transformer as brian sowter didnt have specs to make me one

thanks so much for posting & bringing this info to light

cheers.......... Gary O.

gary o

Vari Mu qestions Triode or Pentode ?
« Reply #18 on: May 01, 2008, 07:28:28 AM »
Hopefully this should be the BA6A schematic 100% correct


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