Re: UA 610 Preamp - Transformer Substitute
« Reply #60 on: September 30, 2019, 12:04:30 AM »
I’m kind of figuring this out as I go so I’ll figure out a better grounding scheme,  Could you recommend a reference? Thanks for your help here.


radardoug

Re: UA 610 Preamp - Transformer Substitute
« Reply #61 on: September 30, 2019, 04:17:41 PM »
Star grounding is always a good way. The way you have it, there could be currents flowing that do not reference to the ground the rest of the tube is seeing. At the very least, star ground around that input stage. And any grid lead is at high impedance so should be as short as possible or shielded.

Re: UA 610 Preamp - Transformer Substitute
« Reply #62 on: September 30, 2019, 10:53:01 PM »
Thanks for the info.  I will try to address those issues.

Re: UA 610 Preamp - Transformer Substitute
« Reply #63 on: October 05, 2019, 11:33:08 AM »
If you're going to wire point-to-point then there's a book by Morgan Jones that goes into wiring a valve circuit.  Actually, there are two books by him.  One is the theory stuff, and the other concerns actually 'Building Valve Amplifiers'".  This is written from the perspective of Hi-Fi stuff but a phono preamp is dealing with similar low voltages to a mic amp so it's useful.
There was a book by Allen Wright I had many years ago dealing with pre amps...  "Pre Amp Cookbook" maybe?  This explains how you should keep the current loop around a stage as small and tight as possible.  Always good to do this, regardless of if it's a point to point or a PCB layout. 
There's a book by Merlin Blencowe (sp?) which I haven't seen but it may also be useful. 

Anyway, you'll get there mate, keep at it 😊

« Last Edit: October 05, 2019, 11:41:26 AM by Winston O'Boogie »
Note to self:  Don't let c**ts wind you up

Re: UA 610 Preamp - Transformer Substitute
« Reply #64 on: October 07, 2019, 11:54:47 PM »
Just coming back to the preamp build.

I appreciate the information on the reading materials.  I've browsed some of the 'Building Valve Amplifiers.'   There's a lot there.

I'm a little confused as to why I'm not able to get this preamp working without noise and hum.  I've eliminated the eq section entirely.  Bypassed the 250k potentiometer (added a 250k resistor from pin 2 to ground/ .2 cap going to pin 2)but still have an enormous amount of hum.  There is some kind of ground loop happening....?   The preamp is working, just with hum in the background...It also sounds just a touch thin.  Not a lot of low end, but that could be just that I'm testing using a 57.  Still, it seems a bit on the thin side.

I re-star grounded the preamp at two points, input valve and output valve.  Then they both separately connect to power supply cap ground. 

Maybe it's my layout?  I've built a couple of la2a's and never had this issue.  Maybe the output transformer A25's ground shouldn't be connected...?  Any ideas?   Maybe I should I use a turret board instead?

Nate
« Last Edit: October 08, 2019, 12:35:03 AM by dawsonaudio »

CJ

Re: UA 610 Preamp - Transformer Substitute
« Reply #65 on: October 08, 2019, 02:56:58 AM »
you could build an external power supply to run the preamp,

then you would probably have a piece of equipment with lower hum than the original.

and the same power supply could be used for future projects.

pick a chassis with a lot of room so you can add another supply for solid state equipment.

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 #66 on: October 08, 2019, 09:55:29 AM »
Damn, I'm sorry you're still having noise problems, must be frustrating.

I do think CJ's idea for an external supply is a good one. 

Although the original  LA-2a does work as wired, the fact is that it isn't a particularly good model to reference for grounding schemes.  There are places where the chassis is used as a ground return which is not a good idea.
When at Universal Audio, as experiment I built up a unit from scratch-n-dent parts lying around and used a proper grounding scheme.   When done and tested on the AP system 2, the noise was some 4dB lower.  Not necessarily a huge improvement but it does illustrate that there is a difference.
The 610 has higher gain than an LA-2a and we're also starting out with much smaller signal voltages.  Any noise picked up or induced into the front end will be amplified.
First off, assuming you are, I think it's probably a bit masochistic to use AC on the heaters of a valve mic pre these days 😉.  You can certainly minimize hum but your job is harder.
If you do have AC, have you installed a humbucker pot across the line?  Typically this would be a 250 ohm wire wound.   If you don't have a pot, you could try a couple of resistors of 125 ohm nominal with the centre point grounded.
Wiring all heater wiring before anything else and keeping it down against chassis and run off, away from signal wiring is important.
Keeping wiring from the input transformer to the first grid shielded or as a twisted pair, and keeping that loop of transformer, valve and passive parts as small as humanly possible.  Visualize or draw out the complete current loop for it so you know what's included in that loop.

Maybe your ammo box isn't giving you enough room to wire this comfortably?  It will certainly couple any and all hum from your power transformer into your signal so, again, a remote supply ameliorates that.


A thin sounding signal isn't good, and isn't how it should sound.   I don't know what you mean by grounding the output transformer though, do you mean the transformer shielding?
Note to self:  Don't let c**ts wind you up

mrclunk

Re: UA 610 Preamp - Transformer Substitute
« Reply #67 on: October 08, 2019, 10:44:24 AM »
Thanks for the info.  I will try to address those issues.
Ian's grounding doc is a great read.
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf
I built a 610 clone and didn't have any probs with hum. If you're still stuck after reading that i can open it up and see what i did but sure it's just standard star ground.

edit: and this is my goto for heater troubleshooting.
Nice explanation of the humbucker pot Winston mentioned.
http://www.valvewizard.co.uk/heater.html
« Last Edit: October 08, 2019, 11:01:28 AM by mrclunk »

Re: UA 610 Preamp - Transformer Substitute
« Reply #68 on: October 08, 2019, 10:53:57 AM »
Ian's grounding doc is a great read.
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf
I built a 610 clone and didn't have any probs with hum. If you're still stuck after reading that i can open it up and see what i did but sure it's just standard star ground.

Cool  8) 
Note to self:  Don't let c**ts wind you up

vitopower

Re: UA 610 Preamp - Transformer Substitute
« Reply #69 on: June 27, 2020, 10:32:24 AM »
Just coming back to the preamp build.

I appreciate the information on the reading materials.  I've browsed some of the 'Building Valve Amplifiers.'   There's a lot there.

I'm a little confused as to why I'm not able to get this preamp working without noise and hum.  I've eliminated the eq section entirely.  Bypassed the 250k potentiometer (added a 250k resistor from pin 2 to ground/ .2 cap going to pin 2)but still have an enormous amount of hum.  There is some kind of ground loop happening....?   The preamp is working, just with hum in the background...It also sounds just a touch thin.  Not a lot of low end, but that could be just that I'm testing using a 57.  Still, it seems a bit on the thin side.

I re-star grounded the preamp at two points, input valve and output valve.  Then they both separately connect to power supply cap ground. 

Maybe it's my layout?  I've built a couple of la2a's and never had this issue.  Maybe the output transformer A25's ground shouldn't be connected...?  Any ideas?   Maybe I should I use a turret board instead?

Nate
Did this ever get sorted out?  Would be interested to know your thoughts on Cinemag vs UTC output in this circuit.  I really enjoyed the UA plug, now I'm itching to build one.


 

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