Sta-level JJ 6386 Thump

[schrobbelbop]

Just finished Drips dual sta level.

When compression kicks in i'm getting low end thumbs, my fairchild is doing
the exact same thing when not balanced properly, one of my fairchild channel is
always doing it a bit

Since it's the same thumps like the fairchild im suspecting the JJ's causing this.

Any one any suggestions? I see a 100R balance near the 6386 and 12au7 how do
i set this correctly? I'm about to source for nos tubes and see where that goes..

 

[Rob Flinn]

I normally set it so I get the same voltage on the cathodes of the 6386 and that seems to work for me, but I'm not using JJ valves.  another way to do it could be to set the unit into compression and adjust for minimum thumping.

 

[schrobbelbop]

Ok i did that and its a little better but still thumping and distorting on faster release times.

voltages seem a little high on the 6368 end

C 1.2 = 1.4
A 60 = 70

Testpoint 105 = 115

psu b+ = 300

Teatpoint 2.4v (annode 12at7) = 2.45

maybe i need to drop that first so change the R12 20K 2watt to 35K something.
Or add a choke?

 

[dmp]

Generally for Varimus, you would send the same signal to both grids of the varimu tube (6386) and null out the response. That doesn't mean it is possible though with a really bad tube.
A 'thump' means the two halves of the varimu tube are reacting differently to the control voltage (the two halves see the same control voltage as the DC of the grid coming from the sidechain).  A change in the control voltage changes the tube gain (e.g. how the thing achieves compression)
If the two halves of the varimu tube  are really different, you may not be able to 'balance' them with trimmers. Meaning if the control voltage changes by 1 volt and the gain of the two tube halves is wildly different, you're not going to be able to get rid of the thump with the trimmer.
Another option to the trimmers would be to buy several JJ 6386 and select the best one (make sure you can return the ones you don't want though, since they're $120 each).

 

[Rob Flinn]

Did you take into account the 2 notes on the circuit diagram ?

What DMP says is very true.  The JJ valves have quite a poor reputation for consistancy, which is rather odd considering their price.

 

[Rob Flinn]

Take a look at POM's page http://www.fairchild-recording-equipment.com/Tube%20Supply/
He seems to have had very mixed results with JJ tubes & lists some batch numbers that weren't good.

 

[schrobbelbop]

Did you take into account the 2 notes on the circuit diagram ?

What DMP says is very true.  The JJ valves have quite a poor reputation for consistancy, which is rather odd considering their price.

Wich 2 notes?

 

[Rob Flinn]

this diagram

 

[schrobbelbop]

yes i did.

im gonna let my 6386 checked...

 

[dmp]

An option is to go to two separate tubes like Bernbrue did in his Sta Level clone with an adaptor. Then you can buy a bunch of $2 tubes and find two that match.

http://twin-x.com/groupdiy/albums/userpics/6386-Subsitution-Adapter%5B1%5D.pdf

 

[schrobbelbop]

An option is to go to two separate tubes like Bernbrue did in his Sta Level clone with an adaptor. Then you can buy a bunch of $2 tubes and find two that match.

http://twin-x.com/groupdiy/albums/userpics/6386-Subsitution-Adapter%5B1%5D.pdf

that seems a solution instead of the stupidly expensive JJ's do you know if there is a pcb available for the T-bar mod?

 

[dmp]

I'm not sure - maybe Bernbrue would know

 

[emrr]

I must've gotten lucky with the JJ's, I never had any funky ones.  Last one's I tried were around 2011 I think.  I believe they had current measurements on the box?  Maybe they used to match them?  I dunno.  The ones I had beat every 'good - tested' vintage 6386 I had around. 

 

[dmp]

Well, Manley will sell you the tbar kit for only $300...

https://www.manley.com/tech-pages/tbar/

You could make it on a board with turrets and mounted sockets.  To plug into the 6386 socket, use some thing like this guy:

https://www.tubesandmore.com/products/socket-saver-9-pin-miniature

 

[schrobbelbop]

Well, Manley will sell you the tbar kit for only $300...

https://www.manley.com/tech-pages/tbar/

You could make it on a board with turrets and mounted sockets.  To plug into the 6386 socket, use some thing like this guy:

https://www.tubesandmore.com/products/socket-saver-9-pin-miniature

I'm gonna make my own T-bars found some cheap matched pairs ef93/6ba6's.. i'll make a the whole thing for 30 euros
must worth a try, keep you posted!

POM had very good tips about balancing de JJ's and that they can be terrible... he has to order large amounts to have decent quad pairs...

 

[Sorr]

With no input signal. I feed 60-100Hz to the common point of the dual 100k input pot. Measure the AC signal between the two 6V6 anodes (you can listen to the O/P as well.) I adjust R41, the pot on the 6V6 cathodes. For minimum signal. 
This control is to reduce the common mode, ie the thump from the dynamic DC shift,  of the whole amplifier.
Which includes the 12AT7 and 6V6's.

Those JJ 6386 are  normally very well matched. Probably what you pay for as they may have to reject lots.  If you use 6BA6's or even the premium 5749, you still have to match them for best results. They can be all over the place.
If you had a large imbalance with the 12AT7 and/or the 6V6's you may not be able to balance it adequately.  Which means you still will get thumps when compressing, say a drum with fast attack and release.
I measure and select the 12AT7 and 6V6's for DC current and gm. I have found this helps when using NOS 6386's  as my choices are limited.
If you dont have access to a suitable valve tester. Then I suggest you get the New Sensor platinum matched  6V6's ie 6V6EH.  Whilst this is no guaranty. It gives you much better odds than other "matched" valves. Then you can try swapping different 12AT7's.

I let the unit warm for up for at least an hour before making any adjustments.

 

[schrobbelbop]

I just saw that my 12at7eh is not a balanced version of the tube. ill order some balanced ones...

 

[schrobbelbop]

Installed the T-bar mod, with 2x 6BA6 tubes, still the same thump.
I've noticed that it's only an attack thump, with fast release times its worse.
I think it's just part of the unit's sound and design. compression is really clean but the attack thumps stay
whatever i try.

Seems that the Retro sta level has the same issue, as quoted from the sound on sound magazine:

"The vocal example shows how effortlessly the Sta-Level can handle rather hefty amounts of gain reduction. The Triple mode controls the transients in a very effective yet quite natural-sounding manner. The Double and Single mode open up the signal, and a slow release also adds something to the end result. However, at times the “thump” is clearly audible, and visible on a meter. A low cut in the chain after the Sta-Level would probably be a good idea."

 

[emrr]

Try other 6AL5’s?  I’ve seen that be an audible problem. 

 

[schrobbelbop]

Try other 6AL5’s?  I’ve seen that be an audible problem.

I did, no diffence.