UA 610 Preamp - Transformer Substitute
« on: September 04, 2019, 02:23:33 PM »
I've got a set of UTC transformers that I have for an LA2A that I never got around to building.  They are the HA100X and A24.  I have some time to tinker the next couple days and was wondering if those would work in the UA610 circuit.  Attached is the circuit/schematic I found here for the 610 preamp.  Also not sure if any modifications would need to be made to make these input and output transformers work, if at all.

Thanks,
Nate


CJ

Re: UA 610 Preamp - Transformer Substitute
« Reply #1 on: September 04, 2019, 03:18:56 PM »
the input will work nicely, an improvement over the ouncer for sure, the output is not going to like the dc from the tube, the LA2a has a cap feeding the output so no dc, that is not the case with the 610.

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: UA 610 Preamp - Transformer Substitute
« Reply #2 on: September 04, 2019, 03:54:30 PM »
I just spoke with Cinemag and they recommend the CM-27101.  It's a 7:1 turns ratio 29.4K:600.  They can gap it but it will be all steel core, no nickel.  Does that sound like the way to go then for the output? 

Re: UA 610 Preamp - Transformer Substitute
« Reply #3 on: September 04, 2019, 08:10:34 PM »
I just spoke with Cinemag and they recommend the CM-27101.  It's a 7:1 turns ratio 29.4K:600.  They can gap it but it will be all steel core, no nickel.  Does that sound like the way to go then for the output?

The original 610 OP transformer was steel so no problem there. 
And as Doctor C.J. Coilmeister said,  the HA-100X is an improvement over the original 0-1.
Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #4 on: September 04, 2019, 08:44:58 PM »
Thanks for the help here.

CJ

Re: UA 610 Preamp - Transformer Substitute
« Reply #5 on: September 05, 2019, 04:11:52 AM »
you could use a UTC A-25 which takes 8 ma unbal dc, but is only good for 15 dbm input, and finding one on the cheap might be a problem, but that would be kind of retro to have a UTC front and back,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: UA 610 Preamp - Transformer Substitute
« Reply #6 on: September 05, 2019, 11:01:25 AM »
When I built mine I  used a Sowter 1160 for the OPT and it has the UTC O1 on input...some of the EQ switches I just left out of circuit.

Re: UA 610 Preamp - Transformer Substitute
« Reply #7 on: September 06, 2019, 09:28:34 PM »
That's good to know.  I placed an order with Cinemag for two of the gapped output transformers.   One to use and the other, as extra, just to have around in case someone else could use it or I decide to build two seperate units.   It will take some time for the transformers to arrive, maybe a month, so I may buy a Sowter as a quicker alternative.

I'd be interested to see your project if you have any photos or layout images.

Nate

Re: UA 610 Preamp - Transformer Substitute
« Reply #8 on: September 10, 2019, 12:34:32 PM »
I've attached the John Hinson 610-A schematic and attempted to make some modifications to it as per this gearslutz discussions:

https://www.gearslutz.com/board/geekslutz-forum/717343-ua-610-schematic.html

I'm not sure if I've implemented them correctly and if it will make much difference to the overall tone of the preamp.  I'd also like a primer on the eq circuit.

I also decided to pick up the UTC A-25 for the project and will substitute the Cinemag transformer when I get it to hear the difference.  UTC throughout sounded like a cool way to go.

Thanks again for the help here.

Nate
« Last Edit: September 11, 2019, 08:44:54 AM by dawsonaudio »

Re: UA 610 Preamp - Transformer Substitute
« Reply #9 on: September 10, 2019, 08:02:47 PM »
On second thought, I'll just stick to the John Hinson schematic instead of making those changes.
« Last Edit: September 11, 2019, 08:45:12 AM by dawsonaudio »


Re: UA 610 Preamp - Transformer Substitute
« Reply #10 on: September 10, 2019, 08:22:50 PM »
Well, that John Hinson on the schematic would be me.  I drew it from reversing an original module held at Universal Audio and also confirmed by examining the desk with 610a's owned by Neil Young.    I also was the engineer for the modern UA recreations. 

My advice is to build a circuit that is known to work first before changing anything.   I'm not saying it can't be improved any but, the schematic with the changes you posted doesn't look like it'll work at all. 

Cheers  :)
Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #11 on: September 10, 2019, 08:27:56 PM »


P.S. Somewhere posted on here is a small schematic I drew with a slight rebiasing change of the input stage on a few of Neil's modules.  I believe it was an official UA change, not something done by Wally Heider or Neil's techs. 
Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #12 on: September 11, 2019, 12:36:36 AM »
My apologies for making a mess of your schematic.   I know I butchered it!

Thank you for sharing your schematic of the 610.  I really appreciate it.   I do have the small schematic as well which I'll implement.

Thanks again.

Re: UA 610 Preamp - Transformer Substitute
« Reply #13 on: September 11, 2019, 02:55:02 AM »
My apologies for making a mess of your schematic.   I know I butchered it!

No problem.  I think the big mistake I saw (no input to the 1st 6072 grid) was really just an oversight on your part but ya never know who might download it in future and try to make it work. 

FWIW you can, as you show on your schematic, vary the feedback/gain of the 1st stage within reason, and that's what happens in the M610, 2-610 etc. 

Good luck, post pictures dude  :)
Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #14 on: September 12, 2019, 06:10:08 PM »
I'm preparing for the build this weekend and just wanted to ask if S2 is a double pole/triple throw rotary switch. 

Also, the echo out portion...what would that be used for in 'modern' times.  Maybe it was feeding part of the desk...Should that be omitted all together?

Other things I'm want to add are a phase switch, and maybe a pad or two at the front.   I've got an external phantom supply, but may build something inside the box so it's self contained.   Any suggestions?

One other thing...the lo and high gain...is that a switch as well?  Pictures I've seen of the desk look like the channel strip has lcd panning, lf rotary switch, hf rotary switch, mic/line switch, gain potentiometer and echo out. 

Just trying to understand the circuit.  Thank you.

Nate

Re: UA 610 Preamp - Transformer Substitute
« Reply #15 on: September 12, 2019, 07:00:44 PM »
if S2 is a double pole/triple throw rotary switch. 

S2, the high EQ, is 1 pole, 4 position rotary
S3, the low EQ, is 2 pole, 3 position rotary.
Make before break for both.


Also, the echo out portion...what would that be used for in 'modern' times.  Maybe it was feeding part of the desk...Should that be omitted all together?


I would omit it unless you were building a desk.


Other things I'm want to add are a phase switch, and maybe a pad or two at the front.   I've got an external phantom supply, but may build something inside the box so it's self contained.   Any suggestions?


Polarity reversal on the output, and pad on the input as you said.  How many dB do you want?

One other thing...the lo and high gain...is that a switch as well?

No that was a jumper on the PCB. FWIW, all the ones I looked at were in high gain position.

Note to self:  Don't let c**ts wind you up

CJ

Re: UA 610 Preamp - Transformer Substitute
« Reply #16 on: September 12, 2019, 10:46:54 PM »
where was that desk,  at the ranch or at Wally Heider?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: UA 610 Preamp - Transformer Substitute
« Reply #17 on: September 13, 2019, 08:26:39 AM »
where was that desk,  at the ranch or at Wally Heider?

A couple of us went to Neil Young's place to see it.  The desk had originally been @ Wally Heider's.

Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #18 on: September 18, 2019, 12:02:20 AM »
I'm getting ready to put the finishing touches on my 610 build.  I'm adopting the below power supply from the LA2A I never built.  The power transformer is 500vdc at the secondary.  I just want to make sure that the B+ won't be too high.   I know the voltage will drop a bit under load but not sure if it will come down that much.  I haven't connected the B+ yet as I'm going to triple check all of my connections.

Maybe I should use a different power supply design...?  Just not sure where to look as the 610 schematic doesn't have much to reference from.

Thanks again for the help here.  I'll be sure to post some photos tomorrow or next once I get it working.

Nate

Re: UA 610 Preamp - Transformer Substitute
« Reply #19 on: September 18, 2019, 11:25:34 AM »
One other question with regards to the input alterations to the original schematic:  The 82pf cap that's eliminated  from the second 12ax7 triode cathode to the 12ay7 cathode and the .002 cap that's also elminated from the second 12ax7 triode cathode...does that instead connect with a jumper or is that connection completely eliminated?



 

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