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SOHA II Builders Thread - Page 90

post #1336 of 1688
Quote:
Originally Posted by stringgz301 View Post

Another SOHA lives!  Just got it up and running and (no surprise) it sounds great.

 

Quick question.  I built it incorporating Regal's mod from the start.  B+ is set to 85v and the tail current set to 2.75 mA per tube section.  However, the difference between TB+L and TPL is 22.6V (not 19V per the design website).  Is this correct?



Mine is about the same,  with this the danger is going below the 19V, a few volt's higher hasn't hurt.   Only disclaimer is that if the voltage drop gets too high the upper tansistors could potentilly overheat.  With the 50% open top rule you should be fine.  I'm guessing that you could lower B+ a few volts and the 22.6V will come down.

 

You did change R5?  Probably a good idea to measure the plate voltage on each section of the tube to make sure they are close,   if not you would want to adjust R5,  but the tube sections might not be matched perfectly,   I don't think its a thing to worry about too much.    Just make sure to check these voltages whenever you tube role.  


Edited by regal - 5/8/10 at 10:42am
post #1337 of 1688

progress!

 

So Alcaudon recommended I try take measurements with the potentiometer connected, and I gave that a shot.

 

Instead of reading only a few millivolt drop from TB+R/L to TP R/L, I am now reading 11v drop on the left side and 15.6v on the right side.

That's still not 19v, but its the first change I've gotten in a while.

 

Something weird is going on though.

When I measure the TB+R and TPR with respect to ground, I read 60.5V and 43.0V respectively.  However, the reading I get when I put the dmm leads across the two test points is 15.6V.  (i'm using the same scale/range).

Last time I checked, 60.5 - 43.0 = 17.5, not 15.6.

 

Same thing happened on the left side.  TB+L and TPL reads 60.6v and 47.0v respectively.  Reading the voltage across the two, I get 11.6v.

60.6 - 47.0 = 13.6 != 11.6.

 

I know the multimeter has a finite internal resistance, but should it make that much of a difference?

 

I measured TL+, TL-, TH, TB+, R10, and R6 as well and the measurements haven't changed from before.

post #1338 of 1688

Can use a 100volt Wima for C13P?

post #1339 of 1688

Very interested in this project, but I am also having trouble sourcing the linear regulators for the power supply. From the looks of it, I'll be waiting until July 1st if I want to pick them up from either Digikey or Mouser unless I can find a suitable alternative. 

 

Does anyone have any ideas for alternatives to the linear regulators listed on BOM or is it better just to wait for stocks to be replenished?

 

Thanks

-SWP

post #1340 of 1688

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7815CTFS-ND

(this is 7815)

 

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1477-5-ND

(this is 7915)

 

 

 

no need to wait just need to brush up on your digikey searching techniques

post #1341 of 1688

Hum - Argh!!!  My SOHA II build was dead quiet for the first two weeks.  Today, I did some minor surgery and replaced the big power caps (C3P and C4P).  It was quiet initially and then I walked away for an hour or so.  Came back and now have hum in the right channel only.  Definitely some kind of ground noise (when I touch the pot or other parts hum decreases a bit).  Some diagnostics:

 

- removed the tube -> hum remains

- disconnected C1R -> hum disappears

 

Any other debugging ideas?

 

Thanks in advance.

post #1342 of 1688

Did you swap the tube left to right to see if the problem follows the tube?

 

Did you discharge all the PS caps before you did this surgery?  If not you may have blown some silicone,  hard to tell which.   But I would replace everything silicone from MPSA42 foward to the tube on the bad channel.

post #1343 of 1688

Changing/removing the tubes doesn't have any impact.  I can start changing silicone, but before I do that, just ran through all the setup measurements and they're 100% normal.  What are the "normal" readings at the individual tube pins?

post #1344 of 1688

OK, more debugging.  I now have hum in both channels.

 

And riddle me this, Batman: on 6 of the seven heatsinked transistors I have continuity from the heatsink to ground, but on LV+ I don't.  However, I do have continuity from the metal tab of LV+ to ground (but not on the metal tabs of LV- or H-).  What is going on?

 

Help!

post #1345 of 1688

The way to check your heatsinks for proper insulation is to check for continuity between the heatsink and the metal tab of the TO-220 device.   If there is continuity you need to check your washer, pad, and or screw because you want isolation not continuity.   Do this before you solder the heatsink to the board.   Also make sure you are contacting a bare spot on the heatsink (not coated.)

 

In your case checking for continuity to ground from the heatsink tells us nothing really.

The regulators are different,  some the metal tab is ground others not.  Check the datasheets and which pin is has continuity to the tab.  Either way you want isolation.  And all this needs done prior to soldering down the heatsinks,  else troublshooting becomes very difficult.

 

I tell everyone who wants to build a SOHAII that they need to understand the principles of heatsink isolation as it is the most difficult part of the build.

 

Quote:
Originally Posted by stringgz301 View Post

OK, more debugging.  I now have hum in both channels.

 

And riddle me this, Batman: on 6 of the seven heatsinked transistors I have continuity from the heatsink to ground, but on LV+ I don't.  However, I do have continuity from the metal tab of LV+ to ground (but not on the metal tabs of LV- or H-).  What is going on?

 

Help!

post #1346 of 1688

Thanks for the reminder.  I paid a lot of attention to the heatsink isolation during the build and it appears they are working properly.  I guess my hum is coming from elsewhere.  Any thoughts about how to isolate/pinpoint the source?

post #1347 of 1688

I have my SOHA II together trying to run through the voltage checks.  1st step checks of the power supply were ok, but I get 0mv when trying to set the buffer on either channel.  I have jumpers 1 and 2 connected.  Any ideas as to the most likely reason?

 

Never mind... I had OPA2134's instead of OPA134's.   All is well.


Edited by huskydawg9 - 5/22/10 at 5:10am
post #1348 of 1688

Resolved my issues.  Noticed that there was a lot of noise and random e12 trips when touching the volume knob or tapping the pcb.  Reflowed the volume pot and tube socket joints.  Fully up and running again.  Back to enjoying the music. 

 

Thanks to those who offered help.  If I mess with it again please slap me upside the head.


Edited by stringgz301 - 5/23/10 at 7:41am
post #1349 of 1688



You're not alone there.  I have an amp that I continually upgrade and always miss silly things like that.  My wife says if I open the lid one more time she is going to sell it on ebay!

Quote:
Originally Posted by stringgz301 View Post

   If I mess with it again please slap me upside the head.

post #1350 of 1688

Just finished my build but I'm having problems.

My R3P resistor gets really hot and starts to smoke, I've tried looking for shorts but I do not see any.

I orginally had LV- and H switched around and it was working okay there, but after I corrected it this happened.

 

EDIT: Fixed it forgot to put the plastic washer back onto the H reg I guess I should get some rest haha..


Edited by Jaivan - 6/9/10 at 5:57am
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