Help with Gates tube mic preamp???

[rascalseven]

That's a bottomless can of worms I won't go into. Except: if I was going there, I would not start here.

I'm game.  What do you suggest?  I only picked this design because of the busted mixer my buddy gave me hoping I could rack some pre's for him.  But I'm more concerned about ultimately building a quality circuit than I am in building this particular circuit.  I've been putting off the tube thing for awhile and figure this is a great opportunity to start learning about it. 

I'm not married to this circuit, however, though I do like the apparent (though still misunderstood by me) simplicity. 

I've no doubt that I can build any of a number of circuits, including NYD's MILA or One-Bottle, UA 610, Hamptone tuber, Altec 1566A, etc., but I'd like to understand the design function of what I'm building too, so for the sake of my understanding I'm thinking simpler is better.

JC

 

[emrr]

Are you sure about the 0.001u at the input? That's absurdly high.

I believe that's correct.  A large-ish ceramic disk with ".001 10%" marked on it.  Didn't cut it loose and measure it since the markings are so clear.  Same on every channel.

I've never seen a ceramic disc cap in anything that old.  Someone added that later, I would bet.

I've never met 0.1u mica in audio; gotta be paper or plastic.

Actually these are new caps on all but two channels... look exactly like big micas to me, but I suppose could be some brand of film.  The two oddball channels appear to have original Sprague caps that are plasic shells (black plastic with yellow printing) that still .1uF 400v caps. 

Plastic black Spragues are old replacements, not original.  Those didn't exist when these were made.  I will bet $$ on a 1 mfd output originally, and Gates would have claimed reasonably flat down to 30 Hz. 

I could buy that the 470K's were sorted incorrectly.  I'd more expect to see the 16K as input to the filter cap from the outside world, and the 40K and 100K branching from that point. 

Tube rect will never give you sag here, but I like them anyway.

Altec 1566?  Please never build that piece of junk.  Just my opinion.  I will be very cross with you if you hack this nice thing into that. 

If you must hack, something like the MILA will be the most like what you want, but I'd change it up to work with octals.  That may be beyond your ability.  Compare the circuit as is to the RCA BA-2, which is a 50 dB classic.   That's the easiest low damage hack. 

------BIG EDIT-------

I have the manual.  This is the 31A or 31B console from 1946.  There are no specs quoted.
The SA series console manual shows a low end boost in the program amp.  Could be the case here, and it may make up the bottom end in the preamp.

The layout is correct. 
The 0.02 is correct.  All the other cap values are correct, except the input sec cap which isn't supposed to be there. 
Input sec resistor is supposed to be 200K.   The 16K is supposed to be 15K.  All other resistors are correct.
The output tube is supposed to be a 6C5. 

Is this thing really individual channel chassis, or is it a single chassis?  The only picture I've found is a single chassis for the whole console.  The later SA types were made both ways. 

Gates also called HS cased UTC iron by the name LS-whatever.  The transformers listed in the manual are ADC's.   The pic I have has a mix of things, some replacements. 

Do you have the program amps too? 

 

[gary o]

Cool stuff   JC I too been searchin for my perfect tube pre amp for a while now I learn very slowly you seem to be doing ok ...I learn best when trying things out as you seen  Ive read lots of stuff what other people use or have made I started off with the Altec 1566A ediie colletti article thing Im glad I did I still use it sometimes Ive heard people comment that that circuit is noisey, works well in my mixes...sadly I only have modern transformers all be it good ones but what I mean is i dont really no what the originals sound like so I make the circuit & audition the transformers I have....heres the circuits I have tried using Sowter 9045 mic transformer 70 pounds worth quite expensive Ive tried Oep but sounds shit compared to sowter its cheap tho....

Altec 1566a 8540 OP tran
RCA Bc2B never really liked this much Big OP tran was 100 pounds
RCa BA21A wrong op tran no feed back  sowter 9050a
BA1A big op tran as BC2B
BA2a same op tran as above
Red 47 sowter 8540 OP tran
Mccurdy AU300 3 tubes  in this one sowter 8650 { I think} I really like the same of this loads of level controled by different resistor values
Hamptone ... broke this one up again didnt like thin sound
M610
Urie UA 1016 push pull  sowter 9050a OP tran
Pultec Mb1
Westerex 1474a sowter 9050a Op tran
My fav V72 Sowter input tran (not the 9045 ) sowter OP tran designed for V72 and Sowter choke for v72

I think theres couple more I made too but deconstructed them....like I say none of these have original transformers...but my favorite as I say is v72 closely followed by Mccurdy maybe Ba2 or red 47 next I have couple of PSUs 12.6 VDC & various HT 100 to 300 VDC I experiment with all the parts capacitors carbon resistors metal film I usually have switches on the feed back so I can have none & I usually prefer no feed back...I no that the guys who designed these were aiming for the least distortion but I prefer some

My search goes on as I get hold of transformers and other parts I audition them to find a home for them & as new old schematics pop up as did your Gates thanks once again   then out comes the soldering iron theres not many Ive made & not found a use for they sound different for different songs /mixes so I end up keeping them all for there different flavours...I just need to find a home for the Gates soon...maybe a biscuit tin or something

Cheers.

 

[NewYorkDave]

Altec 1566?  Please never build that piece of junk.

Shhhh! I still have one laying around that I'd like to sell one of these days

something like the MILA will be the most like what you want, but I'd change it up to work with octals. 

Off-topic, but I was planning to try the MILA topology with a 6SL7 and a 6SN7. Haven't gotten around to it, though. Time for tinkering is in short supply lately.

 

[gary o]

Hehe I like my DIY 1566A.....Not arguing but would like to no whats bad about them ? parts, construction , circuit, transformers....

 

[rascalseven]

Doug, that's fantastic!

Yes, it's a whole console, not individual channels.  That's why my buddy didn't want it -- no place to put it.  Awfully big and HEAVVVYYYY for just a few preamps.  He was just hoping for pre's from it.

I don't like the idea of hacking up the whole board myself, but it's missing the VU meter, the wiring needs to be completely redone (clearly been messed with over the years, and much of it is just hanging lose now).  So much of it all needs to be replaced, and a couple of the channels have already been dismantled (when he gave it to me several transformers had been yanked, though he had them).  Also a few of the Davens have been replaced and a couple are falling apart.  The chassis has been drilled for mounting flashing, colored lights (must have been a prop/conversation piece for someone at some point).  If it were to ever be a fully-functioning console again it would require a complete dismantling, cleaning, stripping, new paint, and total reassembly with many new parts (like the Davens).  I just thought it'd be a good opportunity to learn about tubes and get a few decent (hopefully stellar) pre's in the process.

Glad to know I got the layout right -- a big encouragement to me, thank you!

Until your last post I was beginning to consider making something like NYD's One Bottle, but with these two octals instead, no doubt having to adjust component values for these tubes, particularly the 6F5 which I think is higher gain than the 12AV7.  If that is a good idea I'd appreciate some pointers as to how to proceed.

BTW, one of the things that had been removed from the console looked like a UTC HA transformer case, but it had screws in the side of it that held the pinboard on.  It's much lighter -- felt hollow somehow.  I removed the endpiece (just undid the screws, didn't damage anything) and it is what I'm guessing is a filter network of some kind that appears to be potted in beeswax or similar material.  Haven't investigated that further.

Any chance I could get a copy of that manual?  Scans would be great or even hard copy.  Would be more than happy to pay for shipping if you went with a hard copy.

Also, could you point me to the schem for the RCA BA-2?  Or can you email it?  My email is r7fm (at) tx [dot] rr (dot) com.  50dB would be sufficient, methinks.

Yes, I have the program amps -- the whole console, really, except the VU meter.  I have the iron that was removed too, but the board is useless as is, not to mention a potential life-ender if I tried to power it up in its current condition (exposed wiring and all).  I don't want to tear apart something that would be a treasure to someone else, but the iron is great, and I'd much rather put it to use than have it sitting in my garage or waiting around for a year to find someone who'll come get it and pay me enough to be able to buy similar iron to make some tube pre's. 

I guess I could use the parts to make some pre's and just keep everything else in case I feel compelled someday to restore the whole console... hmmm.

So, Doug, any chance I can get that info from you?

Thanks tons!

JC

PS - re: Altec 1566A, I used to have three of them, and I liked them.  Distorted very easily, and a bit too much color for most things (had 4722 and 15095 iron, IIRC), but they had their own charm.  I wouldn't mind something that sort of resembled them in my rack again now for some options.  Not about to pay what they go for now, however.  I'm hoping the Gates bits can be made into nicer pre's than those.

 

[emrr]

If that thing is headed for pieces, the manual is not of much use.  And it would take me most of a day to scan.  The (rickety and torn) schematic is a 2 foot by 3 foot foldout of THE WHOLE CONSOLE (unscannable), and has no parts values listed, only part #s.  So what you've drawn combined with what I have added is accurate and better. 

Dave, don't worry.  Anything I might say about a 1566A will never influence the lemming buyers. 

 

[gary o]

Ba2a bottom of page...

http://www.coutant.org/ba2c/index.html

 

[rascalseven]

Gary, that schematic is for the BA-2C.  It uses a pentode, apparently.  Not sure how to use that as a basis for my two single triodes.

Is the BA-2 different than the BA-2C?  if so, Doug, can I get the BA-2 schematic?

Thanks!

JC

 

[emrr]

same thing.  note it's run as a triode.

 

[rascalseven]

Ahhh... Okay.  I think I see that now.  :  (Wow am I a tube noob or what?!)

Thanks for pointing that out!  ;D

So is there anything one should know or be aware of when subtituting honest-to-goodness triodes for a design the uses pentode wired as triodes? 

Thanks tons!

JC

 

[emrr]

You could use 6C5's directly I think.  You could go 6SJ7 if you want to avoid the grid cap connection.  Grab yerself a tube manual and look through the relative type charts in the back. 

 

[emrr]

While we're on Gates, here's the SA series design from 1947, made on through at least '53.  Started with the big UTC's, and cheapened down to the lesser Triad's later.  The Triad types lack the bottom of  the UTC, and sound more shrill and sharp. I didn't keep any Triad!  Some consoles made with a unibody chassis, some made with individual modules like this.

What follows is all 4 basic circuit variations I've seen in the field.  #3 is my photoshop hack, #4 is the Triad transformer variation.  Note the feedback tailoring differences between #2 and #4. 

 

[gary o]

Oh Bugger ! more soldering..... Im running out of biscuit tins

 

[rascalseven]

Regarding the differences between the feedback filter networks on drawings 2 and 4 -- was this difference a result of the switch from UTC's to Triads?

And your photoshop hack, that only removed the filter, but not the feedback, right?  Removing R4 and R8 (or just cutting the joint between them) removes the feedback?

JC

 

[emrr]

Right, and right again.  Never seen one in the field lacking feedback, though I tried a variant using 77 and 37 tubes without it.  Which I liked. 

As the Radio News article states, the #3 version with no DC blocking (low freq tailoring) cap also raises B+ to V1 slightly.    Rare to see this sort of plate to plate feedback coupling; usually plate to cathode. 

Notice the 1947 version starts with a 0.5 mfd output cap (never seen one) and they raised it to 1 mfd pretty quickly.  I think I have 1948 or 1949 modules using 1 mfd there. 

Go Gary go!

 

[gary o]

Ok was bit slow doin this one had a salsa lesson.....but have cobbled together the third schematic down a quick A B with the 6SN7 Gates using same transformers Is the latest sounds fuller more bass just nicer bit less output, I like the sound a lot.....thanks Doug for another pre flavour.....il mess with the slightly different feedback version & have a listen I prob end up with swithes there to choose flavor of feedback I fancy on the day .....Im off to eat a lot of biscuits so I have a tin to bolt this thing into....

 

[emrr]

So that's your secret; salsa!  No wonder you build faster than anyone else.

 

[gary o]

Hehe yeah can only solder on beats 123 567 tho.....Well this thing sounds nice I tried the different feedback arrangments not that much in it to my humble ears I have glass & metal 6C4s I like the look of the glass ones had slight trouble with the old top cap I have being so dirty gave me distortion might think about a 6SJ7.....now wheres those biscuits....

 

[rascalseven]

Okay, I built the SA-70, the third schematic (no feedback filtering), using 6J7 and 6J5 (in place of the 6C5 that I am lacking). 

Not the slickest build, I know, but it sounds great. 

Power transformer is an old Chicago PV-50A that a friend gave me.  Chassis is salvage, also given to me -- just something to put it in to try it out.  I added a switch to lift the feedback to give me a 50dB setting too, so I have two gain settings.  Pot is an audio taper.  No pad or phantom for this (I've got both as separate devices for testing, and will add them when I rack some of these for real).

I've only listened to it with an SM58 (no phantom supply here at the house), and it sounds wonderful.  The feedback setting feels more constricted somehow (or just less full/round?).  I'm only using headphones (on my Mbox2... all I have at the house).  I'm in sessions at the studio all day tomorrow, so I'll put it through the ringer there, but I can already tell I'm gonna like this. 

It's dead freakin' quiet too.  Somewhat surprising to me, and very welcome!

Before I commit to making more of these I'm gonna build the RCA BA-2 and NYD's One Bottle to compare.

Thanks so much for your info and help, Doug!  I'm very happy!

JC