Tube tech LCA2B low frequency "thumbs"

i got some trouble with my 2 lca2b's. when compression is taking place there are quite high low frequency "thumbs" at the output of the unit (both channels) when compression drops in. it was/is not possible to run the compressor without a lo cut filter patched afterwards.
yesterday i put in 2 new special selected and symetry matched v1/101 vca ecc81 tubes (50 €) from tube amp doctor to one lca2b and did a vca balance adjustment described in the manual.
after this it is the same stupid thing...
so i'm wondering ; is this a usual behavior with the lca2b? does anybody here have noticed something similar??

Sounds like it  needs calibrating - see the bottom of the link below:

http://www.tube-tech.com/lca-2b-dual-channel-tube-vca-compressor-15.html

Cheers

Ian

ruffrecords said:

Sounds like it  needs calibrating - see the bottom of the link below:

http://www.tube-tech.com/lca-2b-dual-channel-tube-vca-compressor-15.html

Cheers

Ian

Click to expand...

i did the calibrating like stated before...

robomix said:

ruffrecords said:

Sounds like it  needs calibrating - see the bottom of the link below:

http://www.tube-tech.com/lca-2b-dual-channel-tube-vca-compressor-15.html

Cheers

Ian

Click to expand...

i did the calibrating like stated before...

Click to expand...

Ah, I missed that bit!! So, you changed the tube and calibrated but it still does the same thing. Looks like there is something else wrong with it then. Did you have any problems doing the vca balance adjustment?

Cheers

Ian

you mean "thumps" , instead of "thumbs"

do not know how to fix your problem but here is a schematic>


http://schematic.danrudin.com/index.php?dir=Tube-Tech/

more stuff here


http://schematic.danrudin.com/index.php?dir=

Did you adjust P2 (100R) - mu-stage-balance?

I have a LC2B that I have retubed  and calibrated many times.  The tubes used in the Mu circuite ECC81  have a range of ability to cut level.  Depends on the mu and transconductance of tube used, in the variable mu part of the circuit.  Tube tech selects tubes for a working range that they discuss in the calibration description.  ( look at the last cal page step 7 and 8 for the working ranges of the tubes when unit is calibrated). 

I've had tubes before that would attenuate to much (too good) and create thumping problems.    In other words the tubes parameters were too good for the working range of the circuit.  You might want to order select tubes from Tube Tech direct for just the 12AT7's that they have selected before going on a circuit chase. 

Of course there could be parts out of spec.  There are electrolytic cathode bypass caps that are fed from the tertiary winding on the output transformer.  Caps could be bad.  I recently had to replace an output transformer because the tertiary winding was intermittent causing jumps in level after it warmed up.  Chased that one for a long time 

Just saying the Tubes used could be balanced but out of the working range of the circuit.      Also I have found that these adjustment might need to be gone through a few times to get the calibration to center while the tube is heating up and breaking in. 

fazer said:

Of course there could be parts out of spec.  There are electrolytic cathode bypass caps that are fed from the tertiary winding on the output transformer.  Caps could be bad.  I recently had to replace an output transformer because the tertiary winding was intermittent causing jumps in level after it warmed up.  Chased that one for a long time 

Click to expand...

I have also replaced an output transformer in one of these units.  Of course it is the last thing that you would suspect, so yes, it takes a while to find.  Thankfully comparing the voltages of the two channels makes it easier.

gyraf said:

Did you adjust P2 (100R) - mu-stage-balance?

Click to expand...

yes i did ;-)

i think fazer is right with "tubes parameters were too good".

i wrote with tubetech; they answered "The selection procedure for the vac tube is pretty tough!". Hopefully they could sell me some suitable tubes.
the lca2b is definately no "easy-swap-tubes-unit"!

thanks a lot for all your replies!!

i will let you know when i got it working without thumps ;-)

just to keep you informed; i lastly managed to get some "tube tech selected for LCA2B" china tubes (dragon) from the german service audiowerk and found some time for the adjustment procedure.
the tubes were less than 90 € (incl. shipping)
after some hours of turning pots (finally doing some fine tuning by ear ) my first LCA2B is working quite well now.

BUT; the "thumps" doesn't went away completely!
i think this design has always these side effects of a little low frequency "rumble" (caused by the control voltage?) which is hardly audible at normal operation with proper adjustment/good tubes. if you send a highpassed music signal through the compressor and watch the analyzer it clearly visible...

i would like to know how other "vari-mu" compressor perform in this regard???

conclusion: i wouldn't buy this high price unit again
if i was the tube tech designer i would have build in an auto-adjustment-circuit. 

With every vintage vari-mu compressor I've serviced, subsonic thump is still visible to some degree with careful tube selection and calibration.  It's a case of "good enough" in my opinion.  It's rare to find a case where it can't be made inaudible.