Universal Audio UA 176 - Limiting Amplifier

[erozas]

Hey JBB, really superb work. I just wanted to congratulate you on the quality of it. I saw some of the amps you do too and I think they're absolutely gorgeous.
I would love to have the neatness you have, I know I will at least try.

Thanks by the way for posting this on the net.

 

[ilfungo]

Hi
I've upgraded my 176 with UTC A19 as interstage ( before I've hammond 124E).
The sound is much better !!!
More low end and more character...

 

[JBB]

Low frequencies him and so abundant.

 

[ilfungo]

Hi JBB
I've just one problem
I'm using Hammond  370bx and in some point of schematic my tension are bit low...
I don't know if is a problem but I don't understand why in some point are very near and in other point not...
the big problem is 256V instead 295v (12bh7 pin 1-6) and
38v instead 45v (6aL5 pin 1-5 / ratio 12:1)
I've checked all but no luck...
Any suggestions?
Thanks!!!

 

[JBB]

hi,

256V on 12bh7 pin 1-6 in the normal range.

20.5v on 6aL5 pin 1-5 is not very good.

I also used Hammond 370bx. I have all the normal voltage.

What is the voltage on the GZ34 pin 8?

All other parameters of voltage normal?

What is the output transformer used?

 

[ilfungo]

All the tension are very near
GZ34 pin 8 - 361v



Ok I've seen an old post that say 45v is wrong and the real tension could be 38v!!!
now I've rewired the primary of the hammond for 220v operation and the tension are all better:
275v on 12bh7 instead 295v ( I'm in 10% of tollerance).
Last question, after one hour of work my hammond 370bX is very hot...

 

[JBB]

*** Last question, after one hour of work my hammond 370bX is very hot ...

- Nothing dangerous.

- The hotter the compressor, the warmer sound))

 

[ilfungo]

OK many thanks!!!

 

[ilfungo]

Hi  JBB
Have you tried IN/GR/OUT meter connections?
Have you used the original schematics connections or your layout connection? (18K* =???)
For me work only in GR mode....

 

[JBB]

hi,

*** Have you used the original schematics connections or your layout connection?
- Not used. There's some nonsense.

- This part of the scheme I have developed on their own, as I was comfortable.

18K need to choose depending on your operating voltage.
In good need to put another selector for this resistor values for -10db, 0db, + 4db, + 10db and so on ...

 

[Rompstomp89]

Universal Audio UA 176 - Limiting Amplifier


Implemented another project:

Photos of the project: http://jbb.ru/176/

Layout UA 176: http://jbb.ru/schematics/176-layout.htm

Scheme UA 176: http://jbb.ru/schematics/176-shem.htm

Layout UA 175b: http://jbb.ru/schematics/175b-layout.htm

Scheme UA 175b: http://jbb.ru/schematics/175b-shem.htm

BOM UA 175b/176 http://jbb.ru/schematics/UA_175_176_BOM.xlsx


***********************************************

Hey JBB!

Im having a little difficulty visiting these links,  Funny enough they worked last week for me but now can seem to visit your site.  Plan to use your layout for my new build!  Would you mind uploading them?

Thanks!

Romp

 

[Seabstudio]

Hi !
I will start building the Vari Mu UA175b version soon and begin to find parts .
Thanks JBB !
http://jbb.ru/schematics/175b-shem.htm

I already have 2 Cinemag transformers ( Input and interstage ) and Sowter 8650E for the push pull output and all tubes
I hesitate to remove the Tube rectifier and use only a simple bridge rectifier with 4 diodes + CLC ( 10H too ) then classic RC chain with a toroid transfomer and keep the OB2 for regulation
It can change the sound or character of the Vari MU ?

If I decide to use the GZ34 , why the shematic indicate 325V at secondary but the Hammond 370BX recommended in BOM have 550V C.T. @ 58ma 5V C.T. @ 2A 6.3V C.T. @ 2A ( Sorry it s the first time I will use a Tube Rectifier )

For the Input / output attenuator , today JBB confirm my research this morning . The Capi Gear/ TPad attenuator 600ohms can be used , but do they sound as good as a classic Daven design circuit ?

That the substitute recommended :
https://www.hairballaudio.com/.../bourns-600-t-pad...
https://capi-gear.com/catalog/product_info.php...
Any opinion / test ?

For resistors / Capacitors / Potentiometers etc ...
BOM UA 175b/176 http://jbb.ru/schematics/UA_175_176_BOM.xlsx

Why a 2W potentiometer are recommended in BOM for major potentiometer ?

R6 100 ohm cathode balance don t have a big watt /voltage in it ?
R10 use for Vu meter - it can just a trimmer ?
R13 Plate balance I understand that 1W or more is a way to go
R33 ( Attack ) / R34 ( release ) can be a classic potentiometer 16 or 24mm ?

Thanks for helping in advance !
Seb

 

[beatnik]

If using tube rectifier I suggest Hammond 372X instead, it will give a higher b+ voltage. You might still need to tweak r44 value, perhaps 1k2 or even 1k to get closer to the original voltages.

The schematic you are referencing to uses a 50uF capacitor for c13 after the tube rectifier, this exceeds the recommended ratings for the GZ34 and on the long term it will make the tube die prematurely and that can even take out the power transformer as a consequence. I recommend sticking to the recommended value of 20uF that is also what the original 175b uses and using a 500v rated cap for a longer life.

If you choose solid state bridge rectifier instead, a lower ht secondary will be ok and you should use more capacitance in the filters.
These are the formulas you can use to calculate the transformer specs based on the rectifier configuration and the required dc voltage :

https://www.sowter.co.uk/rectifier-transformer-calculation.php
The hairball/capi attenuators will work fine however bear in mind the power rating is not very high and the output one can overheat or even get damaged with very high levels and sustained tones. for normal operation they will work fine.

If you wanna save the trouble of sourcing the 600ohm attenuators you could probably change the input to a post transformer attenuation with a more common dual 25k pot (check the rca ba-6a schematic) and change the output to a h-pad with a single 1k pot (check the gates sta-level schematic)

For the remaining you don't need 2w pots , 1/2w will be fine. Mounting r10 on the front panel is recommended because the vu meter tends to drift from zero quite a bit

Double check with the original 175b schematic because the attack pot is actually 25k with a 10k between wiper and ccw, and not 1m like shown in the schematic you are using. Also on the original there is a switch on the pot to disable the compression.

There are more differences compared to the original 175b schematic, r27 and r29 are listed as 120k whereas in the original they are 1.2m and also r18 and r21 should be rated 1w instead of 1/2w

Good luck with the build

 

[Seabstudio]

If using tube rectifier I suggest Hammond 372X instead, it will give a higher b+ voltage. You might still need to tweak r44 value, perhaps 1k2 or even 1k to get closer to the original voltages.

I still don t understand why we need a secondary with more than 500V . Need to study that Tube rectifier and how to calculate the AC secondary needed and DC result after the choke

The schematic you are referencing to uses a 50uF capacitor for c13 after the tube rectifier, this exceeds the recommended ratings for the GZ34 and on the long term it will make the tube die prematurely and that can even take out the power transformer as a consequence. I recommend sticking to the recommended value of 20uF that is also what the original 175b uses and using a 500v rated cap for a longer life.

OK I understand why I see in previous message that people "complain" about the warm of transformer , maybe it s related .

If you choose solid state bridge rectifier instead, a lower ht secondary will be ok and you should use more capacitance in the filters.
These are the formulas you can use to calculate the transformer specs based on the rectifier configuration and the required dc voltage :

https://www.sowter.co.uk/rectifier-transformer-calculation.php

Thanks for link . Actually in my valve spring reverb for exemple I use a Bridge rectifier with 4X UF4007 + a gyrator choke ( equivalent to 100H ) . For reservoir I use 2 capacitors in serie with bleed resistors ( so the voltage limit is large ) then after the active choke , again the 2 capacitors in serie and bleed resistors .
I understand the "sag" change in an amplifier when push hard , but a Vari Mu react a lot if it s Tube recifier VS SS rectifier ? Sure it more easy / filtering with the SS

The hairball/capi attenuators will work fine however bear in mind the power rating is not very high and the output one can overheat or even get damaged with very high levels and sustained tones. for normal operation they will work fine.

If you wanna save the trouble of sourcing the 600ohm attenuators you could probably change the input to a post transformer attenuation with a more common dual 25k pot (check the rca ba-6a schematic) and change the output to a h-pad with a single 1k pot (check the gates sta-level schematic)

A friend will give me 600ohms attenuators Solved

For the remaining you don't need 2w pots , 1/2w will be fine. Mounting r10 on the front panel is recommended because the vu meter tends to drift from zero quite a bit

Good tips , ok I will put a screw trimmers with tiny hole in frontplate !
Ok for the rest of potentiometers thanks!

Double check with the original 175b schematic because the attack pot is actually 25k with a 10k between wiper and ccw, and not 1m like shown in the schematic you are using. Also on the original there is a switch on the pot to disable the compression.

Oh yes ! It can be great to disable compression and just have the analog print
Don t know why JBB change the value / remove certains part , maybe a sort of mods

There are more differences compared to the original 175b schematic, r27 and r29 are listed as 120k whereas in the original they are 1.2m and also r18 and r21 should be rated 1w instead of 1/2w

Good luck with the build

Many thanks for great and detailed advice !

 

[beatnik]

I still don t understand why we need a secondary with more than 500V . Need to study that Tube rectifier and how to calculate the AC secondary needed and DC result after the choke

The secondary is centre tapped. You also have to take into account the loss in the transformer and the voltage drop across the tube rectifier, which I believe is around 30V for the GZ34 and add that to the drop across the choke and the resistor to get to roughly 300V at the 12BH7 plates.
There is a handy software Duncan psu designer that allows to simulate various psu configurations.
I can guarantee you that the Hammond 372X works well with this circuit because I have built one with that and the voltages are spot on - you might need lowering R44 slightly perhaps 1K2

OK I understand why I see in previous message that people "complain" about the warm of transformer , maybe it s related .

Nope, a warm transformer likely indicates the current rating is too low. The problem I mention results in arcing inside the rectifier tube, without any warning signs unfortunately. This will shorten the life of the tube which in turn can break the power transformer. It will happen after a while

 

[HerbertR]

Hi guys. I will start this ptp build as 175b the next weeks. But as beatnik said there is a difference if you compare the two schematics.

-why did UA set the plate Voltage originally only to 80V with R27+29 at 1.2M? Thats for my understandment not a good range. 100V on the plates is on the low side for a 12AX7 as I know and it should be difficult to bias them very well. 12AX7s generally should run anywhere between 100V and 250V what I know... The russian schematics with R27+29 at 120k with 180V Plate Voltage looks much better to me. I think the tone should be richer. But I am really not a tube expert....

-it is also not clear why the original UA has for C13..."ripple" caps with "only" 20uF in combination with the GZ34 tube. The russian schematic shows 50uF. GZ34 datasheets say generally max 60uF or recommend about 50uF... within the used Voltage range of about 380V at the Cap.

It will get an interesting build...

 

[emrr]

Many vintage pro things run in the 50-80V range.

 

[Heikki]

Nothing wrong with 80V on 12AX7 plate but I think it might be a typo. With 275V B+, about 2V bias voltage and 220k plate resistor I would expect the plate voltage to be nearer to 180V than 80V.

For reliability would be more comfortable with 20u after GZ34 than 50u unless the power transformer has high winding resistance. In guitar amps rectifier tubes fail often and from what I've been told in vintage pro equipment never. The ancient people often knew more than we do.

 

[HerbertR]

Nothing wrong with 80V on 12AX7 plate but I think it might be a typo. With 275V B+, about 2V bias voltage and 220k plate resistor I would expect the plate voltage to be nearer to 180V than 80V.

For reliability would be more comfortable with 20u after GZ34 than 50u unless the power transformer has high winding resistance. In guitar amps rectifier tubes fail often and from what I've been told in vintage pro equipment never. The ancient people often knew more than we do.

Awesome! Thank you!

 

[HerbertR]

Okay, then I have done it. I took JBB's Layout as submisson but I used the values close to the original 175B schematic.
I am using an Input TX from UTM 2510 LA2A/175B TX with 600/50K. The Interstage is a Carnhill 2424 80K/20K "high hue" (UTCs A18/19 offers were way to expensive...) and for the Output TX a Sowter 9650e from DONs - and I also tested a UTM 3580 Mu Output in 10:1 successfully. The PSU transformer is the Hammond 370 BX. I connected the 220V primary to get all Voltages correctly within 10 percent. The sound is fantastic, it is really a fast compressor, gentle, super on fast vocals. Makes more silky density compared to a Sta.
Totally quiet, no hum, no hiss. Noisefloor is with fully open In/Out pot about -79dB. Cool ..... Thanks to all great engineers with their ultimative knowledge here - -