johnnytoobad

Tube mic preamp UA610a LA2A inspired
« on: March 10, 2020, 12:06:22 PM »
Hi!
A friend asked me to build him a microphone tube preamp that he can also use as a DI for his Helix and his acoustic guitar.
Talking together about what he likes and wants we went for kind of modified LA2A compressor but without compression, just the amplifier part, but telling me he wants to use ribbon microphones, I was scare it missed some gain so i added a first stage.
Heaters are DC and elevated.
So now it almost  sounds fine but I got strange behaviors.
When turning first potentiometer I got some noise but noise disappears when it is all the way up.
And on second potentiometer I got oscillation after turning it up to 1/3.
Do you have any suggestions?
Thank you!
« Last Edit: March 10, 2020, 05:05:22 PM by johnnytoobad »
I'm not free but i'm cheap...


andyfromdenver

Re: Tube mic preamp UA610a LA2A inspired
« Reply #1 on: March 10, 2020, 04:37:04 PM »
Hi, what a cool project.  I was unable to view the link said 404 fnf
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
1997-2020 fav artist: Erik Satie

johnnytoobad

Re: Tube mic preamp UA610a LA2A inspired
« Reply #2 on: March 10, 2020, 05:07:20 PM »
Ok fixed now youcansee it! And you can see th wiring...
I'm not free but i'm cheap...

andyfromdenver

Re: Tube mic preamp UA610a LA2A inspired
« Reply #3 on: March 10, 2020, 11:02:27 PM »
Tube circuits are hard to experiment on, unlike circuits that use breadboards. 
It seems like you could just run the first gainstage without the volume then.  Just a series resistor and 1M to ground from the first grid.  Then, what if you tried rewiring that second 12ax7 as a single parallel gain stage (instead of the two) and see if that works?  These are prelim thoughts and I always have to experiment. 
Andrew
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
1997-2020 fav artist: Erik Satie

Re: Tube mic preamp UA610a LA2A inspired
« Reply #4 on: March 11, 2020, 12:39:09 AM »
Your worst noise from the input pot will be at about the -6dB point as it presents the highest source resistance to the 1st valve. 
With your shown 1M pot, this extra resistance will be in the 250K range. 
Unfortunately, using a 1:10 input transformer, there really isn't a practical value of pot to use as a secondary termination that won't result in extra noise when it's turned down to the -6dB point.   
You'll also find that the interaction of this changing source R with the miller capacitance of the 1st valve stage will be messing with the bandwidth and, to an extent, the distortion at higher frequencies.
Sorry  :(

BluegrassDan

Re: Tube mic preamp UA610a LA2A inspired
« Reply #5 on: March 11, 2020, 01:02:46 AM »
I’d say get rid of the first pot and convert those 12ax7s to 12ay7s. Miller capacitance is gonna be an issue with this design, and 12ax7s will exasperate it.


BluegrassDan

Re: Tube mic preamp UA610a LA2A inspired
« Reply #6 on: March 11, 2020, 01:08:01 AM »
And I can’t really tell from the drawing how much NF you’re inserting, but two 12ax7s after a 1:10 transformer is probably gonna be a ton of gain.

johnnytoobad

Re: Tube mic preamp UA610a LA2A inspired
« Reply #7 on: March 13, 2020, 01:52:08 AM »
So I made some math and changed  some coupling caps (for a low cut at 60Hz), I changed first potentiometer from 1M to 500k.
I changed where I toook the ground for the shielded wire to the first potentiometer.
That solved the problem for the first potentiometer.
But the oscillation was still there and it was everywhere looking with the oscilloscope.
Touching with my hand accidentally the output transformer when placing the oscilloscope probe made it isdappeared so I realized it as not grounded because all the parts of the chassis are anodized.
So i wired it all to ground and no more oscillation!
Now changing couplin capacitors back to 20Hz.
Thank you!
I'm not free but i'm cheap...

andyfromdenver

Re: Tube mic preamp UA610a LA2A inspired
« Reply #8 on: March 17, 2020, 10:30:19 AM »
Nice detective work!  Are you happy with the amt of gain?  And the sound and freq response? 
I have been wanting to do my first preamp but it will have to wait til school is done.  I really like having low cut/ adjustable.
What are the controls?  two gain pots, 48 phantom, anything else? 

Thanks and be well :)
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
1997-2020 fav artist: Erik Satie

Gert

Re: Tube mic preamp UA610a LA2A inspired
« Reply #9 on: March 20, 2020, 06:01:25 AM »
Hello,
Are you shure  about those diodes in the supply??


Re: Tube mic preamp UA610a LA2A inspired
« Reply #10 on: March 21, 2020, 11:28:50 PM »
Still get rid of the first pot. No reputable manufacturer would use one in that position! Also reverse the diode in the negative side of the power supply; it is reversed.
« Last Edit: March 21, 2020, 11:33:38 PM by Bill Wilson »

andyfromdenver

Re: Tube mic preamp UA610a LA2A inspired
« Reply #11 on: March 29, 2020, 12:34:11 AM »
Still get rid of the first pot. No reputable manufacturer would use one in that position! Also reverse the diode in the negative side of the power supply; it is reversed.
I'm not familiar with this half wave with needs-to-be-reversed diode.  Is there a solution given the PT specs to wire a fullwave instead? Add more PI filters to drob B+, I guess? 
Also, johnnytoobad, hope you are well and can you share the DC voltage you are getting after rectification and  on the first preamp triode plate before and after plate load resistor, I'm curious.  Thanks!
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
1997-2020 fav artist: Erik Satie

PRR

Re: Tube mic preamp UA610a LA2A inspired
« Reply #12 on: March 30, 2020, 02:35:50 PM »
> Are you shure  about those diodes in the supply??
> reverse the diode in the negative side of the power supply; it is reversed.


Read the small print carefully. "150-0-150". He has omitted drawing the center tap.

andyfromdenver

Re: Tube mic preamp UA610a LA2A inspired
« Reply #13 on: March 31, 2020, 12:02:28 PM »
> Are you shure  about those diodes in the supply??
> reverse the diode in the negative side of the power supply; it is reversed.


Read the small print carefully. "150-0-150". He has omitted drawing the center tap.
Thats what I thought, but if so he needs to move the bottom diode.
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
1997-2020 fav artist: Erik Satie

PRR

Re: Tube mic preamp UA610a LA2A inspired
« Reply #14 on: April 01, 2020, 02:44:36 AM »
Yes, the drawing of the power section is clearly wrong several ways. However he made it squeal, and then not-squeal. So probably the build is right and the drawing is wrong. For a one-off, that's not unusual, and not terribly fatal.

analag

Re: Tube mic preamp UA610a LA2A inspired New
« Reply #15 on: April 03, 2020, 12:22:44 AM »
A 1:10 input transformer followed by four stages of 12AX7's. Too much gain, too many caps in the audio path for my taste.
« Last Edit: April 03, 2020, 12:57:53 AM by analag »
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.


 

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