LA2A Questions

[Technetium]

ok,
i'm in the home stretch on my la2a. i dont have the T4b yet, but its passing audio, & outside a ground hum sounds great.

now that there are no more archives to search i have to ask some simple questions...

1) i bought a power supply that has no CT for for 6.3v secondary, i so i have 2 100ohm resistors going to ground. questions is, how are they oriented? i.e. one from each leg to ground, or both from the same leg? does it matter?

2) i also changed the topology to fit it in a 2RU case, so in order to be able to follow DJ's book, i DIY'ed a turret board from lexan & brass screws. it has 9 rows of 3 screws, replacing the (9) 3pin terminals (which i #'ed from 1-9), & 7 rows of 2 screws, replacing the (7) 2pin posts(that i #'ed from 1-7). i then followed DJ's wiring directions, keeping in mind CJ's list of errata about the book.
my question here is this: all pin 2's of the 3pin blocks should be run to ground, as well as all ground pins on posts 1-7, right?

i mean it seems like a simple question, (almost a stupid one) but i wanna eliminate the possibility of foolish mistakes before i rack my brain trying to find where this hum is coming from.

any help would be greatly appreciated,
thanks,
-dave

p.s. its great to see everybody reconviene @ the new TT home. u barely even missed a beat. it would have been a real shame if it all splintered after leaving RO.

 

[Rob Flinn]

.................."1) i bought a power supply that has no CT for for 6.3v secondary, i so i have 2 100ohm resistors going to ground. questions is, how are they oriented? i.e. one from each leg to ground, or both from the same leg? does it matter?"

You use a single resistor from each leg of the transformer to ground. This is so that you balance the supply round ground. I normally use a 100r wirewound pot, then you can tweak it around for the minimum hum.

 

[Jonathan Hayward]

Wahey, you found us Rob, welcome home! I didn't have your email so I couldn't tell you.

I'll shut up, back to LA2...

 

[Rob Flinn]

..........."Wahey, you found us Rob, welcome home! I didn't have your email so I couldn't tell you. "

Jonathon I`ve been here from the start just lurking in the background !

 

[Technetium]

You use a single resistor from each leg of the transformer to ground. This is so that you balance the supply round ground.

hey Rob,
yeah, thats what i figured/how it did it. guess i gotta start looking elsewhere for this ground hum.

I normally use a 100r wirewound pot, then you can tweak it around for the minimum hum.

never considered using a pot instead, thats a great idea.

thanks!
-dave

 

[Arrigotti]

I don't know if this would be Kosher in the Pro Audio world, but in the DIY Guitar Amp world, all of my amps have the heaters referenced to B+. This reduces filament hum. You do it by making a voltage divider (2 resistors) off of the B+ (get somewhere in the neighborhood of 10% of the B+ voltage). Attach the heaters to the middle of the voltage divider, and add a electrolytic cap on the ground leg of the voltage divider. I also kept the 100 ohms resistors from each leg of the heater wire in.

All of the Schematics from the AX84 site include this in the schematic. Some call for the PT to have a Center Tap, but there are a few that don't. I did this mod to a Fender reverb unit that I made, and it ended up being a very quiet unit.

I am starting work on my first LA2A, and would like to know if anyone has done this type of thing.

Hope this is useful, and thanks! :thumb:

 

[adrianh]

I sent a eamil to Rob
I knew he got it.
Just glad he popped up!

 

[gyraf]

The trick with referencing the heaters to a point offset to a positive voltage is mainly to overcome the problem with tubes having a limited heater-to-cathode differential voltage.

Lifting the heater offset voltage will allow you to use tubes with the cathode standing far above ground - like in SRPP circuits, without too much leakage.

I don't think it has anything to do with hum cancellation...

Jakob E.

 

[Winston OBoogie]

...

 

[The Kid]

" but that's another story..."

We're listening! :green:

 

[Rob Flinn]

Jakob


.........."The trick with referencing the heaters to a point offset to a positive voltage is mainly to overcome the problem with tubes having a limited heater-to-cathode differential voltage. "

"Lifting the heater offset voltage will allow you to use tubes with the cathode standing far above ground - like in SRPP circuits, without too much leakage. "

Yes this is true, a prime example being the PSU regulation in the Fairchild 670 where the EL34 heaters are referenced to about 200V

"I don't think it has anything to do with hum cancellation... "

I think it is to do with hum cancellation as well.

A couple of quotes:

1. Newnes Wireless Constructors Encyclopaedia 1935

Smoothing the H.T Supply

A potentiometer may also be included with advantage across the filament of the usual full wave rectifier valve, the H.T positive lead being taken form the slider instead of the transformer. This will balance out any hum that would otherwise enter the filter circuit.

2. Radio Designers Handbook 4th edition P785

12. A low resistance potentiometer may be connected across the heater supply, with the moving arm earthed ? this may be adjusted for minimum hum. In some cases less hum may be obtained with the moving arm returned to a point of positive (or sometimes negative) voltage to the order of 5 to 50 volts. Minimum hum may be obtained by optimum adjustment of both potentiometer & voltage.

I have experienced hum reduction by simply connecting the centre tap of a filament supply to earth & having a badly audible hum vanish.

 

[punk]

I'm curious, why didn't you use the power xformer from Allied Electronics as it is very economical?

I would suspect that you will solve your hum issue when you've added the resistors. The la2's I've built are all very quiet. Surprising when you consider how the circuit design 'looks'.

FWIW, I modded my single-ended Kalamazoo guitar amp with the heaters referenced to B+ thinking it would eliminate the hum but I couldn't perceive any improvement.

Sometimes it pays to follow a project as close as possible the first time. You may have to determine if your hum has been caused by changes in layout or component/wiring errors.

good luck,

byron

 

[Winston OBoogie]

...

 

[Technetium]

i had already put the 100ohm resistors in (one from each leg) but still had hum. i was just checking to make sure i did it right. there are defintely a few places it could be coming from so i'll just have to hunker down & do some investigating...i dont mind, its a good way to learn.

i didn't use the allied transformer because it wasnt listed in DJ's book & at the time i was an absolute newbie just didnt know any better. i wasnt even aware of this forum back then, but over ther last 9 months of reading TT (now the lab) i have learned SO MUCH its incredible. i would never approach this project the same way ever again...live & learn i guess.

anyway, like i had said, i changed the topology & DIY'ed a turret board to match DJ's plans. i also used mogami mic cable for all connections, w/ the sheilds draining to the star ground. (they're only connected on one end, @ the star ground)

another thing i also notice... the 6AQ5a gets noticably hotter than the other tubes, is that normal?
also, does anyone have a list of what my voltages should be in key points? DJ's book makes no mention.

thanks again, u guys rock. :guinness:
-dave

 

[Rob Flinn]

..........."another thing i also notice... the 6AQ5a gets noticably hotter than the other tubes, is that normal?
. "

From memory the 6AQ5 is a power valve & from memory draws more heater current.
Power = current x voltage. So therefore you would expect it to get hotter than the 12AX7/ECC83 that are in circuit

 

[Technetium]

oh good.
thats one less thing i think i did wrong :grin:

thanx rob.
-dave

 

[punk]

here's a link to an la2 schematic with voltages

http://www.waltzingbear.com/Schematics/Urei/LA_2.htm

a great site btw for equipment schematics.

DJ's book does have voltages for the tubes in the appendix.


It's good to see you've got a positive attitude. It can be very frustrating troubleshooting but it's an education you never forget.

byron

 

[kent]

...Hey Byron! Long time no hear.

My LA2 had a ground loop hum which I elliminated by seperate local grounds of the amp section and sidechain section. Granted, I did not use the Jansen book (I didn't know about it at the time) so I had my own layout. If you're interested my stuff is http://www.conditionedresponse.com/DIY/DIY.html under the LA2 section. My layout is in there.

cheers,
kent

 

[cayocosta]

Here's the la2a "1968" layout:

http://www.vacuumbrain.com/TA/LA2/la2a_layout2.jpg

Assembled pics here:

http://www.vacuumbrain.com/TA/Rons_LA2a/

 

[Technetium]

hey byron, kent & cayocosta.

once again thanks for the info!
first thing i wanna do is check my voltages, just to make sure all that is ok, then i will move on to the ground hum. since i did a bunch of home runs to a single star ground, i guess i can just start attacking them one @ a time. if that doesnt work, i'll move on to more drastic measures :grin:

It's good to see you've got a positive attitude. It can be very frustrating troubleshooting but it's an education you never forget.

it definitely IS frustrating, but i dont really have much choice i guess. i need to learn more & really want it to work. i knew when i started this, it wasnt gonna be a walk in the park. actually since the SSL comp is highly regarded as a good newbie project, i put the LA2a on the back burner for a while & moved over to that. now that that's done (& sounds great) i'm re-energized, a little more confident, & ready to finish up the LA2a.

kudos to all you guys, its great to see some many people willing to help others who they have never met.