JENSAMP-51X (Germanium Mic-Pre by Darius Kubarth)

[Upacesky]

I contacted Volker and he told me all the rev04 he's built are working. That's already a good news, we're not doomed.
I do get the same problem when I drive the pre too hard on low volume settings.

Let's try to debug it, I'm a newbie, so stop me if I'm wrong.
I thinks we can agree that those are oscillations. Wikipedia tells me oscillations (well at least those put put oscillations) are sometimes caused by low frequency (under 10 Hz) feedback. My pc audio analyser confirms this, there's a peak at those really low frequencies.
If I understand feedback, it's a bit of audio that goes again to the input of the pre and gets amplified again, just like feedback with a mic and loudspeakers right?
I don't know how it helps.

Some say in a tube amp troubleshooting manual to change the decoupling caps (http://www.geofex.com/ampdbug/mtrboat.htm)
I can't read a circuit, well I don't understand it. Could it apply here?
If I remember right from the german forums, Volker changed the value of C13 in order to prevent those problems. Is it a lead?

Anyone ideas?

 

[Dr Gris]

So I reflowed all pads on one of them today. The good thing is that I can go two steps more than before (seven clicks up) and then it pulsates as before. When I turn the gain down again step by step it continues to pulsate and ads some sqealing until it stops at first position. If I don't go all the way down to first position it keeps on pulsating. This goes for both DI and XLR.
If I go further than when the pulsating starts, it goes nuts....that's a technical term, right?

I feel a bit pissed atm but I'm gonna do the same thing to the other and see what happens,
who knows, I might get lucky... 8)

Cheers
//Magnus

 

[Dr Gris]

Ok, reflowed the second one. Large pop between 3 and 4, both ways and the pulsation starts at 4'th position.

I guess I've been lucky with all the other stuff I've built so far and it's my time to hit a snag.

Now what? Change transistors? I socketed the ASY's but not the others, bummer!!
Any hints are welcome.

Best
//Magnus

 

[culteousness1]

I am not sure if this will help, but I can stop the oscillation by touching the cans
of C2 and C7. See picture:

EDIT: Picture removed.

Maybe this is a hint...

Cheers,
Carsten

 

[Dr Gris]

Great, I"ll try that on mine. The only signal I've sent through them so far is witht the DI,
and before they freak out they sound great.

Cheers
//M

 

[tzman]

So I reflowed all pads on one of them today. The good thing is that I can go two steps more than before (seven clicks up) and then it pulsates as before. When I turn the gain down again step by step it continues to pulsate and ads some sqealing until it stops at first position. If I don't go all the way down to first position it keeps on pulsating. This goes for both DI and XLR.
If I go further than when the pulsating starts, it goes nuts....that's a technical term, right?

I feel a bit pissed atm but I'm gonna do the same thing to the other and see what happens,
who knows, I might get lucky... 8)

Cheers
//Magnus

The fact that the symptoms on your units changed after reflowing, made me think that this maybe is a built error after all... So I reflowed mine and I can get one unit up to 11 and the other up to 10 before I get the exact same noises you are getting. They both went to 7 before reflowing.
I also changed C13 to 1000pF which made the oscillation start at the 12th position. Didn't bother doing it on the second unit as it didn't solve the problems and I don't know how high I can take this capacitor...(what are the symptoms I could get? At which value would it eat to much into my bandwidth?)

 

[Dr Gris]

Well, I touched those caps and on one unit it didn't help, on the other it did but problems came back
when I raised the gain, still touching those caps. I started touching the other caps in that row of caps
in different combinations and it made the pulsating character change ever so slightly....

//M

 

[culteousness1]

Ok, no worries  ;D

Seems more that it only helped accidentally on my unit.
But I remember that reflowing the solder joints helped too.

Maybe this is worth trying...

Carsten

 

[tzman]

I forgot to mention that touching the caps like culteousness1 did, didn't help on mine too.

BUT

I've got them working!

After desoldering so many parts already I decided to go a step further. I ordered some BC140's and BC109's thinking that Volker may have gotten a bad batch of those. So I replaced the BC140's and guess what: I works.

After thinking this over, I got to three possibilities:
1. The BC140 are indeed a bad batch
2. I "fixed" the unit by soldering in the new components, but in reality the old 140's had one or more cold solder joints (although I reflowed every joint twice...)
3. I've broken one or more BC140's by putting the heatsinks on after soldering them in. I now put the heatsinks on them before soldering them in, avoiding stressing the component.

So now I only have two questions left:
- Do the BC140's have to be matched in any way?
- And why are there 11 dots on the frontpanel, but 12 gain steps?! This has to be asked before, but I couldn't find an answer.

 

[Dr Gris]

That's good news!!!
I think I can get BC140's locally, I'll try changing them as the next step when time allows.

//M

 

[Dr Gris]

So I changed the BC140's. This time I put sockets where they go. No luck....
About the same as before exept on one unit the last three steps don't pulsate.
I did measure the transistors, but only resistance and the diod test with my DDM, didn't build a test-jig.
The old ones measured about the same as the new ones.

Since I'm confident in my soldering skills by now, measure components and know how to put them in the
right position I'm quite irritated by now.....and since I'm no engineer I'm lost.

Somebody mentioned a discussion on a Geman forum, any news from there or hints from
Kubi or Volker,  'cause here they're rather [silent] 

Regards
//Magnus

 

[Dr Gris]

Hopefully I will have some spare time during the upcoming holidays for these.
It's been very quiet here, have you people with the same oscillation problem got your's working?

Cheers
//M

 

[Upacesky]

I changed the BC140 and now mine is working. Youhou  . I'll put it to the grill next week end.

 

[buckethead]

I changed the BC140s too, works fine now

 

[Upacesky]

I did a recall sheet (.pdf) for the Jensamp 51X. Feel free to download, modify and use it freely. PM me if you want the source file.
Jensamp 51X recall sheet

 

[Dr Gris]

I'm glad you got it working. I'll try and replace those BC140's once more.
They really sound great before they crap out and the craftmanship of the kit is superb.
And the front panel design is the coolest of them all!!!

Cheers
//M

 

[delmonton]

Hello Dr Gris and Co. .. seems that everyone is using their jensamp with success …

cause none post a problem in the last 4 months … or the discussion is somewhere else ? ..

well .. my pair has the same problem .. pulse noises hearable on the module once you turn on the psu ..

I'm curious about Dr Gris … do you solved the problem changing the BC140 ASY26 and BC109 ??

thanks to everyone

 

[Dr Gris]

I haven't changed the BC140's one more time. I measured the ones that came with the kit,
and the new ones, they're all about the same. Never ordered new BC109's or ASY26's to try.

I had them in my rack for some time doing nothing thinking the "electronics might settle"...
When I tried them again after a while, same thing.

I reflowed my boards and changed the BC140's once and somehow I managed to change where
the oscillation started on the gainswitch ( with no signal), but last time I tried them they crapped
out at the same gain as when I tried them the first time (4:th click if I remember correctly)
when I gave them a healthy hot signal...

Since then I haven't had time/inspiration to deal with them cause they'll probably make me mad again...
Don't expect any help from the guys behind the Jensamp here.

Too many people has had problems with this amp and I would love to hear how they fixed theirs,
is changing the BC140's until you find some from a "good batch" the answer??????

Best of luck!
//Magnus

 

[[silent:arts]]

Gentlemen, I'm not getting tired to repeat: the Jensamp doesn't like hot signals ...

 

[Winetree]

Gave up on mine on Dec 2011 ( Read Post # 112) Still sitting there.
Maybe I'll try again with replacing some parts.