Collins 6Q-1

[Dr Gris]

Thanks Dave,

If it's possible to get the desired euphonic sound out of modern equivalents it would be great, NOS tubes of course.
I guess sourcing esoteric parts from Ebay would be fun, but for me the end result is most important.
And looks of course
Transformers I guess is the biggest question, finding good vintage Thoardson or Freed for this isn' t easy for someone like me who has no experience.
As for suppliers, I buy basic stuff from Mouser, Farnell and Elfa.

This is still a "bit of greek" for me, but I'm reading up and learning along the way. I' m still on kitbuilding level.
But tube stuff is what I ultimately want to build when I' m confident enough to handle P2P in HV circuits.

Kind regards
Magnus

 

[Dr Gris]

And thanks Doug,

I hear what you are saying, lots of options. But if i might be bold and ask you, any specific transformers you would recommend?

Kind regards
Magnus

 

[emrr]

You won't find originals, or even anything from those manufacturers easily.  In the US with new iron, I'd buy the Cinemag 150:50K input and try the 15K:600 Edcor.  The most common vintage iron would be UTC A-10 / A-24, but that's kinda silly cost wise.  2 channels of that would cost you more than a real 6Q-1, easily.  Sweden, plenty of Sowter options, also not cheap, not sure what Lundahl has. 

 

[lassoharp]

I'll throw in that Cinemag offers a couple of models with lower max level handling since it sounds like DrGris is going for easy saturation & overdrive.  OEP are less $ but not sure what level ratings are.  The old Beyer "peanut" transformers should saturate like crazy at the drop of a hat.  I see them on ebay fairly often.  I think they tend to get chased sometimes because of their use in some Ampex products.

 

[DaveP]

I have an Edcor 15k:600 on order at the moment to try in this circuit.

RS do a pretty good input TX 600:50k in mumetal case for £35.57 by OEP part number 210-6352
I've been using these since 1967!!!

best
DaveP

 

[Dr Gris]

A big thank you to you all!!

It' s true I want a character pre that can go from clean to sweet overdrive in a nice way.
That' s why Doug' s comments on the 6Q-1 over the years have caught my attention.

Dave, if you will build this it would be fantastic if you made a thread about it like your BA6A from scratch,
that was simply stunning. Now thats DIY on the highest level!!

Kind regards
Magnus

 

[DaveP]

In some spare moments I jury rigged a 6Q-1 to check the voltages, this is what I got:-

6AQ6 NOS Sylvania.
B+      140V
1uF    112V
Plate    84V
Cathode  1.066V
Current    0.595mA

6C4 NOS GE  Rk=150
B+          140V
1uF        115V
Plate      70V
Cathode  0.674V
Current  4.5mA

6C4 NOS GE  Rk=220
B+      140V
1uF    116V
Plate  74V
Cathode  0.941V
Current  4.2mA

Gain of 6C4 with various options:-
150 ohms      5.01x
150 plus cap  6.06x 
220 ohms      4.46x

The output resistances with these optons are:-
150 ohms      7k
150 plus cap  6k 
220 ohms      7.4k

Not a great deal of difference I guess.
best
DaveP

 

[Dr Gris]

So my lack of time, but mostly the lust of owning the real thing made me get two original 6Q-1's.
They' re on their way from US any day now. So much for building it myself  :-[
Yeah, I know I' m crazy, but what can I say, it' s all Dough' s (emrr) fault!! 
The second one was thought of as a spare if anything is wrong with either one of them.
But I' m thinking of getting them both racked up and working.
I hope I can pester you with questions about doing this the best way.
Any things to look out for with these originals?
What would be the best PSU solution for these?
Another thing I' ve been wondering about is how modern mics will work with
the input transformer wired for 50 or 250 Ω?

Best
Magnus

 

[DaveP]

Doc,
If I were you I would aim to make a stereo pair.
A dynamic mic will not like the low input Z although some people think it tames an SM57 for some apps.  A condenser mic should drive it ok I guess.

For the power supply you could do worse than copy other power supplies from well known mic-pre's, do some research in the technical docs section.  I might be tempted to try a voltage doubler like the 436 supply cos were not talking heavy current here, advantage of that is that the hum is 50Hz not 100Hz, but the main thing is to have lots of smoothing stages before you get to 140V.  A pair of Toroid transformers wired 230/9 to 9/230 like the Gyraf design works well for small current supplies.  Pay careful attention to the earthing arrangements when you get to put it all together.

Hope that gives you food for thought!
best
DaveP

 

[emrr]

I run them at 200V when racking. Much higher and the current may be borderline without replacing some resistors.  I always use International Power linear for B+. 

2 modules is 4 channels; quadraphonic!

Open grid, no loading issues.  250 fine for every mic I've tried.  50 is a huge eq on most mics, a fun band pass filter at times.  I don't know that I'd bother wiring every channel for 50 option. 

You must replace V2 grid resistor with a 250k log pot for volume control, with layout such that lead extensions are short as possible. 20 dB input pad will be mandatory for many uses as well. 

 

[Dr Gris]

Got some iPhone pics if anyone is interested. Let' s see if this works...

#1: https://www.dropbox.com/sh/iwa848686xhb1ob/3O1Jo7Rc1G?m
#2: https://www.dropbox.com/sh/z7d07f9dkms69e4/fGaLzSkJww?m

As you can see there are some differences.

Cheers
M

 

[Dr Gris]

Hi again,

Things move extremely slow with this racking project, no time, no money...
To make me feel a little better I started in the opposite end of this, and bought a nice Collins winged emblem
to put on the front panel when it' s done 8)
At the moment I' m thinking of getting a 3U 11" deep basic case from GroupDIY member Dan Deurloo,
his cases look great (Collective Cases)! What do you think about putting the power supply in his external
PSU floorbox version? http://www.groupdiy.com/index.php?topic=41963.msg618199#msg618199
And run a 5´cable between them?
My plan would be to put a IHB200-0.12 (175-210V @ 0.12A) from International Power(Thanks Dough) together with a Hammond 266M6 (6.3V @ 3A CT) for the heaters in that box.
Or is that a bad idea??

Maybe DC is better for the heaters?

Going for 250Ω input I' m supposed to change the (C 101/C 107) .0047uF to .0022uF, any suggested cap
I can use here? The original being, if I' m correct, a Molded Mica.

What do you think?
More questions to come

Best regards
Magnus

 

[emrr]

Look at my rack again, power supply in the same case, no hum at all.  Collins later started ignoring the front end cap change, it's really for fine tuning treble response.  You can use anything. 

 

[Dr Gris]

Oh boy have I looked at your photos of restoration and racking.
What a fantastic collection of beautiful work!

My thought were that if I put two 6 Q' s in one rack it would be to crammed in there,
but if you say it will work to put the PSU inside I will definitely go with that.

When I look at your pics it looks like you have a AC to DC supply for the heaters?
Is that something you would recommend in this circuit?
Since this became my latest project I have read pages here and elsewhere about PSU design,
but I haven' t found a conclusive answer.

Another thing I see, if I' m correct, is that you take the phantom supply from the B+ supply?
I haven' t thought about that since my condenser mics only consists of some vintage U87' s
and my beloved KSM 141' s (best bang for the buck mics I own!) Looks simple and elegant.
Have to read up on that.

One of my 6Q' s don' t have the big Micas???

Like I' ve said before, I started building kits one year ago and know very little about this.
I' m old and stupid (what can I say, I chose the Hammond as my main instrument  )
But I' m slowly learning and I' m very thankful and inspired by all the helpful people here!

And Doug, if you ever need some B3 tracks, let me know

 

[emrr]

My 6Q rack pics show a DC filament supply.  I did that because at the time I had a lot of them as surplus, cost me $3 each, less than a filament transformer!  New they are about $50 today.  I've heard them with AC filament and that is fine too. 

I dropped phantom from B+ in those with power resistors and zeners, they eat about 25 mA a channel if I recall, but the supply had it to burn.  Maybe not the best way to do it, but they've run 10 years fine with that load.  I calculated R watts for surrounding element short condition at the time, not knowing any more fail-safe method then. 

I would do a layout leaving room for power supply, and hook power up and check hum, without making power supply position permanent yet.  If hum is a problem, finish it outboard. 

 

[Dr Gris]

I' ve finally supplied power to them on the bench.
No meltdowns, fire or explosions and I' m still alive!!
I hope I have the time tomorrow to take some measurements and to actually put a signal through them.

I' m not sure I understand Doug' s 20dB pad implementation.
Is the pad a 47K resistor in form of a grid stopper at V1, and then you add feedback from V2 grid
through a 56K resistor giving you back the 20dB the grid stopper took away?

I' m sorry for my stupid question, just trying to learn....

Best
Magnus

 

[DaveP]

Magnus,
Grid stoppers don't reduce gain, they only reduce radio interference and instability.  The 20dB cut  just switches in feedback thats all.
best
DaveP

 

[emrr]

Everyone should ignore my foolish feedback based pad.  Really.  Years ago I wanted to know if it would work, I made a drawing, it's all over the internet, don't do it.  It completely erases what qualities these are good for.  Use a standard passive pad, and hack in an interstage volume control. 

You have to add a resistance between FB and input transformer because otherwise you are driving gain from V1 back into the mic, totally changes the mic load.  It's NFB isolation for the input transformer. 

 

[Dr Gris]

Thanks again for making me understand and sorting things out for me.
I' ve read so much about tube/valve design/topology my head spins and I mix stuff up...
Maybe I should write things down as I find them important and helpful.

Anyway, while still on the bench I soldered XLR ins and outs and connected a mic I had nearby (Beyer M88).
One of them just gave me mains hum and no signal, but after inspection I found out the input transformers
weren' t connected properly. After the fix I had sound through both of them. And what a sound,
they are absolutely incredible!! They sound so full. After some tube swapping between the channels to
see if there were any big differences in sound I made some measurements.

I could clearly hear that one of the 6Q' s, lets call this 6Q#1 had more gain than 6Q#2.
Channel 2 in 6Q#1 had most gain and also most distortion.

6Q#1 has 220Ω cathode resistor on 6C4 and the 680k feedback resistors.
6Q#2 has 1k cathode resistor on 6C4, no feedback and no cathode bypass cap on 6AQ6
and some parallel resistor arrangements to the plates that I calculate are not far from stock value.

Measured some voltages at 200V B+, these changed when I swapped tubes but at the moment they are:

                              6Q#1                                                6Q#2
              Channel 1            Channel 2                Channel 3                Channel 4
          Plate      Cathode    Plate    Cathode      Plate    Cathode        Plate        Cathode 
6AQ6: 113.5      1.284      98.9      0.651        96.5      0.753          97.1        0.791
  6C4:  92.5      1.070        88.4      1.079        105        3.487        107.3      3.304

Maybe I should replace all those carbon comp resistors to even out the readings?
I don' t plan to use these in a matched stereo signal chain scenario, recording classical so a little
difference is ok with me.

Any comments?

I' ll have to learn about switches know, DPDT and what not.... I really need a -20dB pad like you said.
I have a case from Dan Deurloo arriving any day and the pots are here.
My plan is to put some nice Daka-Ware knobs on the pots.

Cheers
Magnus

 

[Matt C]

I'm getting ready to build one of these preamps and it seems straight forward enough, but I have a couple questions. 

first, more just for my own curiosity, is there any real point to using the 6AQ6 with the two diodes in it?  am I wrong in thinking they're not doing anything?

for an input pad, is it easiest to just slap a generic switchable U-pad on the primary side of the input transformer?

the output will be sent to new-ish gear with high impednace inputs,  should I use an output transformer with different specs? or use a 15k:600 as Doug recommends and load the secondary with 600R?

thanks.