[BUILD] 1176LN Rev D DIY

[strangeandbouncy]

Thanks Safe as Milk/CJ!

can i safely deduce that it is wired backwards from the diagram? ie 500:250? I do hope so, 'cos Ive got a spare pair of Gardners 600:300 he he he.

Thanks guys,

ANdyP

 

[strangeandbouncy]

flaheu,#


i think he is refering to the trimpots mounted on the pcb, not the input attenuator . . .

please someone correct me if I'm wrong. . . .


AndyP

 

[Lowfreq]

I've got one of those cinemag CMLI-15/15B input transformers sitting round here, which from what I can gather is what UA are using in their reissues.

But it's 15k:15k, And from what I've read (a few of W.O.B & Ed's posts) UA are not doing the T attenuator thing. Just using an input pot.

Even tho there have been a few discussions about using the tranno,I can't seem to find what pot I should use. The G1176 uses a 10K audio for the input. I'm thinking just doing the same and wiring the tranno 2:1.

Does anyone have specifics on how UA work around the CMLI-15/15B?
I can't justify spending money on more iron, and I'd prefer to use what I've got. Save the $$ for the correct output transformer.

I know it's not the authentic way of doing it, but I've still never used a real 1176, and this will be my third 1176 clone, :shock: so I'm not too fussed getting it exactly like a revD. Just close................ :roll:

 

[enthalpystudios]

the attenuator is most important...... thats really what we should iron out first. but i'm not sure where to begin on something like that guy.

maybe we could use 24pos switches ;]


thats a lot of resistors though. i dunno.. there's got to be a way to get them.

but for reference, this rev d. board is not really anything like the UA stuff. this is basically the same circuit that used the utc o-12 input transformer.

it was 500z:200z. 600:150 or similar should be just fine. the important thing is having a passive attenuator up front, before the input transformer.

 

[Lowfreq]

mmmmm, ok........... I think once again, I need to carry on doing some more reading.........and learning :wink:

So the UA claims of being based of rev d/e is quite a loose claim then?


This is some of what I found in some other threads.
[quote author="Winston O'Boogie"]
Family Hoof wrote:
[...and shunted the secondary with a pot (instead of placing a T-attenuator BEFORE the transformer. According to most users this doesn't sound as good, or even similar.


Not quite correct. I recall doing two versions of "new" front end for UA using that 1:1 Cinemag.

One version (1176LN) terminated the secondary with a fixed R and, on the primary, had a pair of series R's feeding a shunt pot, then a pair of smaller R's to feed the winding. The combination of the series R's and the pot (set for no attenuation) gave about a 4dB loss, same as the 0-12 was. The load on the source was not constant but was always fairly low. FWIW, the Cinemag seemed to be fine with the shifting input Z.
This might not sound the same as a 'T' atten and 0-12 but it is an alternative. Maybe not a good one if you're building a "clone" but, IMHO, fine for DIYing etc.

The other version (6176) had a pot on the secondary with a series R feeding the top of the pot to attain the same 4dB drop and also terminate the transformer effectively. This arrangement presented a constant Hi-Z load to the source...

gyraf wrote:
- it will present a gentler load to the previous stage. Not all equipment sounds good when loaded that hard..
Jakob E.


And some stuff sounds better with a low load which is why, on the 6176, you could switch in the 620R primary loading resistor that I shoved on a switch on the back panel... [/quote]

But again, I'm still getting my head round trasnformers, attenuators etc, so I could be pissing into the wind..... :roll:

 

[JdJ]

Here's some interesting stuff on T Attenuators:

Floating?

Why it's important

More with a schem by NYD

Hopefully some of this is useful...

I'm still not sure how I'm going to pull it off. Some threat around here had someone suggesting going to the source for UA with the requirements. Group buy?

Cheers,

J

 

[tommypiper]

I'm wondering the same things about the T. Making a DIY attenuator looks to be too much so far.

Doubt if going to UA would yeild anything fruitful; shudder.

Maybe there is a way we could make a simple 3 position attenuator? Hi, med, lo? With a few switches and resistors? Just to have ballpark input sensitivities which would suffice in most cases?

How much attenuation (in dB) are we talking about? I've seen old surplus Ts but have no idea what range we need.

 

[JdJ]

Ooops-

Meant to post this one-

NYD's Bridged T Schem

More T Talk

Sorry- I just wanted to have these all in one place...

-J

 

[strangeandbouncy]

I have just downloaded the Purple MC77 documentation, and they seem to have used a simple 2-ganged pot(value not given, but I guess 500R) and 2 resistors befor the o-12 or equivalent transformer. LAter versions use a 3-deck pot, with 500r x 2 and 5k.

Sorry, i am crap at posting links - they never work for me.

Could someone help with a "loading" question? i actually have a pair of t-attenuators, and a pair of Gardners 600:300 transformers. Would I need to increase the value of R4(270R) if the transformer is 300R?

Also, if I might be so presumptious, if i don't use the Cinemag or whatever equivalent, and ignore the feedback tap, what kind of transformer should it be? 150/150:600R?

Many thanks,

ANdyP

Many thanks,

ANdyP

 

[tommypiper]

Can anyone summarize the situation and provide specs, schems, info? Is everyone as confused on the Ts as me?

:guinness:



Edited for brevity.

 

[edanderson]

over the life of the products, the purple MC76 and MC77 used both the "original style" bridged T attenuators (two decks + two fixed resistors) and a non-bridged T attenuator (three decks). these two devices are essentially electrically equivalent, in that when they are at full CW rotation, there is almost no attenuation, and at full CCW, there is more than 50db of attenuation. at any position in the rotation the impedance is approximately 600ohms.

today there are two available choices, neither of which is cheap. the three deck non-bridged T attenuator is made by state electronics for purple, using clarostat 70 series mod pots. they feel smooth and should last forever, but they are not cheap. i know at one point andrew was asking $70ea for them, and i'm not sure he'll be selling them at all any more.

PEC in canada makes a reasonable facsimile of the two deck bridged-T attenuator, and supplies them to UA. i think i heard about someone getting one from UA for ~$50? but then i believe the same person said it was scratchy after six months. i got a quote from precisionsales.com and the 50 pcs price was $35ea. the part number is 05-0007 if anyone wants to go that route. you are unlikely to find anyone else willing to make the unusual tapers that are required for this part.

ed

 

[JdJ]

Perhaps we can start here:

[quote author="edanderson"][quote author="EP"]You will need 2- 500 ohms, and 1- 1k ohm.
Configuration 500-1000-500 ohm[/quote]

for a 500 ohm (close enuff to 600) config, the 1k will have to be a log/audio or what clarostat calls "Z" taper. otherwise the impedance and level change will not be how you want. also, a 1/2w rated pot could theoretically be burnt out by driving ridiculously large signals into it......

ed[/quote]

[quote author="edanderson"]it is a custom 70 series clarostat 3 deck pot with a two 500 ohm decks and one 1kZ deck. they aren't cheap, and we do have to wait for them to be made to order. we've had more problems with getting the meter and ratio switches than any other part.

ed[/quote]

Sorry for all the confusing links- I just figured it was best to have a few sets of eyes on this stuff as I am not as good at figuring out details as many...

Sounds like getting Ed in here would be helpful.

BTW the MC77 Schematic is here on page 14. It shows both the old 2 deck and newer 3 deck jobbies.
:guinness:

-Josh

EDIT: dang! his ears must have been burning!! snuck in under the wire...

 

[edanderson]

from john's ("winston") post, i would guess that he was talking about basically an H-pad with a pot acting as a variable shunt, as sketched above. this, together with a 1:1 transformer is a valid way of doing a pot as attenuator into the transformer. it is not the same as what was originally done, and wouldn't necessarily sound the same (though it could in some cases). the big advantage of this approach is that the parts cost would be lower and the parts much easier to get.

ed

 

[edanderson]

in the case of using a non original input transformer (anything other than an O-12 or the equivalent), the termination resistor across the secondary would have to change. the "correct" value is dependent on the characteristics of the transformer itself, and the way the attenuator is done.

pretty much the best advice i can give is to wire up whatever you have and see what you get. then tweak to taste.

ed

 

[JdJ]

Thanks Ed!

It's always a pleasure to get this stuff from the source... And a viable alternative to the T!

:guinness: :guinness: :guinness:

-Josh

 

[tomcat]

[quote author="tommypiper"]I think we need to establish a starting point, a knowledge base and facts, before we can all safely start figuring out (so many different) solutions.[/quote]

Would love to see that

Im completely lost when its going to things like T-Attens., In- and Output Impedances and so on....

 

[edanderson]

the output circuit of the rev D/E 1176 is specifically designed to work with the original output transformer. if you don't intend to get the original transformer, and you want something like an 1176, i would suggest not building the rev D/E version at all and building a rev F style G1176 instead.

if, however, you are interested in getting something going that may behave rather differently than the original circuit, i would say to start with a gapped, M6 steel core, 1:1 600ohm output transformer.

the JLM111DC might work, but will cost the same as the original part, and not have the additional windings. a LO1166 type *might* work in a 200:150 configuration, but i wouldn't expect stellar performance. i can't think of anything in the standard range from sowter, but i'm sure they could make a simple gapped 1:1 on request. i would guess that lundahl could adapt one of their tube interstage transformers to roughly fit, but it would be quite expensive.

when the cinemag outputs are ready, i will put up an ad in the black market. it should be towards the end of this month. if i come up with a less costly solution that would work for budget DIYers, i will post that also.

ed

 

[tomcat]

Thanks for the info. Im waiting already for the cinemags since i read some post about it some time ago. So far the output is solved.

The big :?: for me is still ... what to do with the input? Some sort of 1176D input for dummies info... :wink:

 

[strangeandbouncy]

Ed,



i had no idea I was so near to the "source" . . . Thank you so much for all your invaluable advice. It is surely greatly apreciated.

just one thing, i am not able to see how a 600:600 output tranny would work, since there appear to be 2 seperate windings on the primary? Do you meen 150/150:600, or am I missing something here? I would be very interested in any group buy for da real deal . . .

I am not a "clone purist". I merely like great sounding gear. Slight diffferences are to be encoursaged in my book. i have lots of "unique" lash-ups in the studio! Right now, I am considering a 2520 driving a 10klog pot for the input . . . .


All th ebest.,

ANdyP

 

[mnats]

just one thing, i am not able to see how a 600:600 output tranny would work, since there appear to be 2 seperate windings on the primary?

I think Ed was referring to alternatives to the correct part, so these alternatives would not have the same features as the original or the copies he's going to offer.

The second "primary" I think that you are referring to is a winding that takes the V+ current "backwards", offsetting the voltage produced by the output of the class A amplifier stage (if my analysis is correct). If you fed that 30V into two equivalent primaries like the 150/150 you gave as an example, you would be overcompensating for the small voltage at the output.

A gapped transformer doesn't care if there is a small amount of DC at the output. So you would just forget about the other winding and feed the 30V directly into the line output stage (shorting together brown and black as shown on my and the Purple document).

For the input it seems to me the answer is clear: if you want something like the original find a 600 ohm T attenuator and a UTC O-12. It might take a little time and effort, but like I've said earlier this ain't a G1176.

If you want have some fun and try something new, try some of the things that have already been suggested. You could even feed an unbalanced signal through a pot and into the board directly.