Universal Audio UA 176 - Limiting Amplifier

[ilfungo]

Sorry JBB
Retro 176 have this functions...

 

[JBB]

Retro 176 also has a "side-chain". He realized by changing the nominal C6 and C7.

Look for similar schemes on the internet.

 

[kobydid]

Very nice JBB!
I have been dreaming of building one of these on a turret board. Never got around to actually drawing up a layout. I guess I have no excuse now!
Thanks for sharing!

 

[ilfungo]

Very nice JBB!
I have been dreaming of building one of these on a turret board. Never got around to actually drawing up a layout. I guess I have no excuse now!
Thanks for sharing!

Hi JBB I've discovered this for Side chain HPF...
I need just a little help.
I'm using hammond 124E as interstage ( Drip alternative).
It's 15k :135K, It must be reversed as UTC A19?
THANKS!!!

 

[ilfungo]

This is the hammond Interstage...
Thanks

 

[David Kulka]

It looks nice, but I don't see a ratio switch. If so, doesn't that make it a 175?

 

[ilfungo]

Hi David you are talking about the output trnansformer?
This is the interstage transformer....

 

[pH]

I think David is referring to the output transformer. The 175 had a fixed compression ratio, whereas the 176 had a different transformer with multiple windings to change the ratio.

 

[ilfungo]

I'm using :
UTC A10 as input
sowter 1285 with turn ratio for the output
Hammond 124 ...

Can someone help me with hammond wirings?
Thanks!!!

 

[alexc]

The interstage should be a step down ratio

ie. input side to the interstage is the higher impedance side, output side is the low impedance side.

Run the input side (HiZ) centre tap as B+ supply to the vari-gm tube complementary pair.

The centre tap of the ouotput side (LoZ) is not used. The output side (LoZ) feeds the make up amp.

 

[ilfungo]

Thanks alexc
can you explain it more simple for me?
Can you tell me how to connect on the schematics?
Thanks

 

[David Kulka]

I think David is referring to the output transformer. The 175 had a fixed compression ratio, whereas the 176 had a different transformer with multiple windings to change the ratio.

Yes, that's exactly right. The 175-B (I don't think there ever was a 175) was the most common of the 3 models. It had a fixed ratio of 12:1. The 176 which came later used a complex output transformer with 4 sets of sidechain winding taps, and had a ratio switch that selected among those 4. The ratios were 1:2, 4, 8 , and 12. The 1176 Rev A, which came a few years later was modeled after the 176, but it used a very different method of developing the sidechain signal..

The Retro 176 uses a similar transformer but I think that an ordinary human would have a lot of trouble acquiring a transformer like that. So I'm guessing that the the OP has actually built himself a 175-B, which is still a pretty nice unit of course.

There was also the 177, which is pretty rare. That one had 4 ratios like the 176, but no VU meter.

 

[0dbfs]

The front panel on the first post describes it as a 175b but the thread title says "176"..

Looks like some fun in any case!

Cheers,
jb

 

[dmp]

The Retro 176 uses a similar transformer but I think that an ordinary human would have a lot of trouble acquiring a transformer like that.

The Sowter 1285e has the primary taps to switch to the sidechain for different ratios. Not hard to get (if you can spend the money).
If you look at the 176 layout & schematic that JBB posted you will see that he used this Sowter transformer and he did in fact design a 176 with four selectable ratios.  Maybe he built a 176 and a 175b?

 

[JBB]

))))

 

[alexc]

Thanks alexc
can you explain it more simple for me?
Can you tell me how to connect on the schematics?
Thanks

On the schematic of the 176, the interstage transformer is connected with the hiZ windings connected to the gain reduction tubes plates and the loZ windings connected to the gain make up amp tubes grids.

In this case, the B+ (high voltage) is connected to the gain reduction tube plates via some resistors (R12, R14 and R13) which are across the interstage primary (HiZ winding).

This is an alternative to feeding the B+ to those plates by using the centre tap of the interstage primary (hiZ winding).
The R13 variable resistor is used to achieve a 'balance' of the B+ high voltage to each plate.

There are reasons for doing it that way - it changes the impedance  seen by the plates and can be an 'easier' (higher) or more 'predictable' load to the gain reduction tubes, whose own impedance varies alot over the gain-reduction 'duty cycle'.

So, you need to know the HiZ winding labels for the interstage traffo and connect those to the 'plate feeding resistors' R12, R14.

The Hammond 124E is described as a 15K : 33K8 (secondary windings parallel'd) or a 15K:135K (secondary windings series).

The original 175/176 was a UTC a-19, which is 15K:80K, used backwards for the step down ie. 80K:15K

So, I would use the Hammond 124-E as a 135K:15K step down configuration.

ie.  connect the hiZ windings in series  -> join BLACK and GRN/YEL together  to give the GREEN and BLK/YEL wires as the hiZ interstage side of 135K

The loZ side 15K is BROWN and BLUE. The RED ct is not used.

In the Drip boards you need then to map these wires to the hookup pds on the pcb. I'd have to check all that again.
The best way is to use a continuity test to work out which hookup pads go where and then map those to the transformer leads.

Hope that helps!

Regards
AlexC

 

[alexc]

Oh yes and boy does it sound good! My drip175 with Sowters  is fabulous.

But, I still think my rca ba25 style (pentode vari gm tubes, utc+sowter traffos)  is better tho'   

That one just plain ridiculously good sounding. So smmoooooootttthhh.
It's like waiting to exhale then instant catharsis.    Audio opiates.

 

[ilfungo]

thanks alexc!!!

 

[ilfungo]

I'm ready!!! ;D

 

[ilfungo]

very DIY!!!