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

[Dr Gris]

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

Were you able to fix the pulsating problem?
I still haven't had the time for mine yet and my workbench is crowded with two tube echos in pieces I'm servicing
when I have some spare time...

Cheers
//M

 

[kosi]

Update:
I had the same problems, oscillation past position 4.
I exchanged the BC140, which came with the kit for some from Reichelt. no change.

Today, about 1 year later, I changed them for the 3rd time, this time with NOS Siemens (ebay)
and now everythings tip top.
No oscillation at all, performed nice and quiet.

 

[skeeler87]

Are the boards for this still available?

 

[Winetree]

Update:
I had the same problems, oscillation past position 4.
I exchanged the BC140, which came with the kit for some from Reichelt. no change.

Today, about 1 year later, I changed them for the 3rd time, this time with NOS Siemens (ebay)
and now everythings tip top.
No oscillation at all, performed nice and quiet.

Where did you get the BC 140s on eBay?

 

[kosi]

here:
http://www.ebay.de/itm/172205048982

I wanted to make sure, that I get some older ones.

 

[[silent:arts]]

I exchanged the BC140, which came with the kit for some from Reichelt. no change.

Ok, the ones in the Kits have been from Reichelt, manufactured by CDIL.

Interesting. There must be a reason why Buerklin now stocks 3 different brands / manufactures (all for the same price)

 

[culteousness1]

I don't think that this problem is related to the brand of the transistors.

Got the oscillation problem one 51X series preamp built from Volker's kit as well as on one channel of my Silverline D-ASY26 built with transistors from Farnell. Maybe it's worth measuring with hFEs from the BC140-16s of a working unit. Unfortunately, I don't have a transistor tester at hand...

Cheers,
Carsten

 

[kosi]

hFE of my "old" and working ones is around 170, Farnells were higher
tested with a cheap chinese transistor tester

 

[adamboon]

So my pre-built JensAMP showed up.  It was oscillating, touchy and a drama.  I have fixed most things.  I replaced nearly all the Semi's with those parts listed as used in the BA31 / BA71 original. (2N3391 for Q1, a Russian 2N404 Equivalent for Q2 and Matched up NOS 2N2270's for the outputs).  It's all working (No Oscillation) now, but when I drive a sine into it hard (to see it distort it on the CRO) it's distorting rather asymmetrically in the second stage. RV6 controls my gain, bias and symmetry for the first stage - and I know it's not so much a JensAMP anymore (as I've changed all the semi's). but I thought I'd ask the advise for the more knowledgable here about which of the resistors around Q2 I should fiddle with to adjust my symmetry (and maximise headroom).  Or perhaps the answer is to wind my voltage up (to 30v like the RCA)... I'm not putting it in a 51x rack...

Adam

 

[adamboon]

For me stability came down to a careful balance of the value of RV6 (I used a 2K trimmer, probably ended up at less than 1K but I think the extra range was useful), the value of R7 (which I made bigger, I had 8K in there to make the Ge distort a sine symmetrically but found that when I kiss the top of the voltage rail at high gain the whole thing turns into a funky oscillator so settled back down to 5 or 6K) and the supply voltage, I found myself cranking up the supply to 30V - which fixed some things and was great, but Q4 and Q6 got rather roasty toasty and I found in the end (when I stopped chasing clipping symmetry) that winding the voltage back down to 24v make the thing more stable across all gain settings and input levels.

I've spent so much time fiddling with this thing.  I am pretty happy with it.  If anyone has given up on one or a pair and wanted to offload them I'de be interested.  I'd be even more interested if any "Super-Nerds" have ideas on any further tweaks to optimise the signal path headroom and/or general stability.