PRR-176 REV 4 Build Thread ***Manual up!***1/16/14 - (chk 1st post)

[redmojosteve]

Hi
I am just getting to the stage where I am completing the build. For various reasons I have been distracted and it's taken me far longer than it could have done. However, having read the through the thread there seem to be a few different ideas regarding the types of pot to use and various values or log vs lin have been suggested compared to Abe's Rev 4 proposals. As it seems that a couple of builders have completed their build I was wondering what pots they used? I have got some pots for the task but before soldering away I would be happy to hear any suggestions.  I still have to buy the 10K rev log so I can order others at the same time if necessary.
Thanks for any advice
Steve

 

[anvl]

Hi Steve.  In my opinion, 10K Linear pots on both Gain In and Gain Out.

 

[sedit1]

I have a problem with my prr 176(rev 2)..the other channel is working perfectly (channel 2) but the first channel is not. The signal is very weak and distorted. I have swapped the tubes and tested with working channel cables, swapped op amps from channels and re-heated the soldering...

Only thing that seems weird to me is the heaters..The second channel is getting 6.3v to pin 4 of the tube just like it should be but the "broken cahnnel is getting 12.6v to pin4 and 6.3v to pin 5.. Do you have any Ideas what might be going on, I't out of ideas

Thanks!

 

[anvl]

Hi sedit1. 

I don't know much about PRR176 rev2, but this doesn't seem to be the problem.    Heaters need 6,3V between pin 4 and 5.  In your "broken" channel you have 12,6V - 6,3V = 6,3V.

You have problems with this channel even in bypass mode (no compression)?

 

[sedit1]

Hi sedit1. 

I don't know much about PRR176 rev2, but this doesn't seem to be the problem.    Heaters need 6,3V between pin 4 and 5.  In your "broken" channel you have 12,6V - 6,3V = 6,3V.

You have problems with this channel even in bypass mode (no compression)?

Thank you,
so it seems that is not the problem.
And yes the problem is also in bypass mode, no compression so bothways the sound is the same. Really low volume, singnal sounds like highpassed and distorted. I have lundahl trannys that I bought used coud there be some problems with those? Both cahnnels built at the same time from same components (just triple checked all the values). And no doa's in this build just ic's.

 

[anvl]

1.  In rev4 it was needed to add some jumpers in the PCB.  Something similar in rev2?

2.  Did you complete stereo linking connection?

 

[sedit1]

1.  In rev4 it was needed to add some jumpers in the PCB.  Something similar in rev2?

2.  Did you complete stereo linking connection?

1. No jumpers needed in rev2

2. Yes

 

[sedit1]

This might afterall be an broken transformer case..Just measured the interstage lundahl and in secondary pins 7/8 I get 48k resistance, should be around 800ohms like all the others..

 

[redmojosteve]

Thanks Anvl. I'll use the 10k lin and see how it runs. I am just about to power up the power supply for the first time to check the voltages. I can't run it on load yet as I haven't fitted the tubes owing to the reversed PCBs which I need to modify.
Cheers
Steve

 

[sedit1]

This might afterall be an broken transformer case..Just measured the interstage lundahl and in secondary pins 7/8 I get 48k resistance, should be around 800ohms like all the others..

I had a spare lundahl..Out from circuit the secondary pins 7/8 measures around 800r but in circuit it measures 48k..I justs cant get my head around it or what causes it..I once again cheked the resistors around it...Does this ring any bells for you guys?

Thank you!

 

[anvl]

Any impedance measured on the circuit is always different from a separate measurement.  I don't think this is your problem.

 

[sedit1]

Any impedance measured on the circuit is always different from a separate measurement.  I don't think this is your problem.

Thanks,

Yes the Lundahl is not the problem but the working circuit measures around 800r does this ring any bells?

 

[dribro]

Hi. Got a rev4 from a member and now hunting for some tubes. What is my best bet besides the usual 6bc8? Will a pair of 6n5p do the job ? Any recommendatios on european versions?

Regards
Adriano

 

[redmojosteve]

Powered up the 176 for the first time and no white smoke or melted components. I haven't sorted out my reversed valves, sorry  tubes, yet so there is no real load on the PSU but the voltages are fine except I'm getting 122V on the 100V line. As there is no load I would expect it to be higher but am I right in thinking this would be too high?
As always, thanks for the good advice.
Cheers
Steve

 

[anvl]

Hi Steve.  122V is not a bad value without load.  The most important thing is to reach 100V as exact as possible when everything is connected.  I'm working on a voltages map, it will be ready soon.

 

[redmojosteve]

Thanks Anvl.  A voltage map would prove very useful.
I've put power to the main board, without IC's or tubes at this stage. I've just found out I'm getting -18V for the meter LED's so I've connected them the wrong way round as I had assumed the LED voltage to be +ve. So, a bit of resoldering needed.
I may go for putting the heaters in parallel too as I am not too convinced about the series arrangement in terms of balancing the two channels. Mind you, I am a Mechanical Engineer so my view of electrical systems is sometimes a little different.
Cheers
Steve

 

[anvl]

Hi Steve.

I don't think this PCB provides a voltage for meter led.  The "meter" pad is only for signal meter, not for led.  They are different things.  I'm not sure, but probably you will have to make some troubleshooting work for your problem and also find out a solution for your meter leds.

Good luck.

 

[redmojosteve]

Thanks ANVL. I clearly misunderstood that. With the pad being marked "LED" I had thought it might drive a meter LED but I can sort it out another way.
Cheers
Steve

 

[redmojosteve]

I've got the tubes in and powered up. I'm getting around 116V for the HT so I think that will need taming a bit, and with all other PSU voltages being adjusted to the right values.  I haven't got any output yet but it's early days.
Cheers
Steve

 

[XAXAU]

abe how can I modify the compression ratio for this build? ie. make it go from whatever the stock compression ratio is to more of a limiter?

I don't really know. I know that it is a vari-mu and they have a very soft-knee, as in the harder you hit it, the more "ratio" you have. Me personally find no need for a ratio, if I want to be gentle stay in the 1-5db GR range, if I want to limit and smash things I hit it harder....very natural and smooth...maybe try it out and go from there?


also:
I would be interested as well, not necessarily to implement it but just to see. I know the 175? had a ratio feature and I believe it did this by essentially changing the threshold?ie just sending more gain to the sidechain. I'm not sure, I'll have to dig up the schematic. You might want to mount the threshold knob of the front of your case, cause messing with that it certainly feels like it controls some sort of rationess. (along with the level/distoriton ect ect..)

The original Urei 175 had a set ratio of 12:1. The 176 however had 2:1, 4:1, 8:1 and 12:1.

From UA webzine:

"After getting familiar with the unit, it becomes clear that the 175 and 176 truly have much in common with their offspring: the 1176. Threshold is set with the Input control, so setting your compression and output gain is a similar two-fisted operation. The 175 has a fixed ratio of 12:1—but similar to 1176, the 176 offers similar selectable ratios of 2:1, 4:1, 8:1 and 12:1; The multiple ratios are achieved by a multi-tapped transformer. Also similar to the 1176, Attack and Release controls are at their fastest when fully clockwise. The attack time is adjustable from 100 to 1000 microseconds, while release is adjustable from 27 to 527 milliseconds. Also, the compression circuit can be clicked off with a firm counter-clockwise twist, just like the 1176."

Is there a way to do this ratio switching in this rev 4 design? What is the actual ratio in this thing?