jj 6386 problem in sta-level clone - SOLVED or ....

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takisti

Member
Joined
Dec 4, 2008
Messages
24
Location
Estonia
Hi,
I have messed with this all last week.. :mad:Here is the problem:

I bought  two jj6386 tubes for Sta-level compressor clones.Both tubes are working and compressing fine, but there is strange anomaly...When compressor compress hard and signal stops and compressor meter needle comes back to zero (control voltage decreases to 0v) there is strange and annoying noise "bump".Both tubes make it ,but bit different way.

Compressor signal amp and sidechain are 1:1 to original ,all voltages are ok,all tubes are in balance.Compressor working without any issues with so called "T-bar" (  pair of 6BA6 connected into triode mode instead 6386)

Can it be tube quality issue ?Does anyone had same problems?Is it possible to do something with this?

I know that todays tubes are not very good compared to nos but jj claims that they test and choose every 6386 ...and yes,they were not cheap.
 
The tubes might be "matched" and balanced in an operating point, but the "curves" along the negative grid variation are too different to be used in a varimu. It is rare with good tubes, specially new, but does happens. In fact, In our STA-Level, I tried five vintage tubes, used and new and all worked with no thumps after balancing. It means that maybe JJ tubes is a no go for varimu compressor circuits. If they sold it for this application, and said these were tubes with matched triodes, call them and have it replaced, or ask for your money back. Matching should go all the way down along the curve. My tubes does -40dB or more with no audible thumps. But these are GE 6386s. Thumps are really small, and are down at 5Hz or something. I mean, there is always some thump, you know. I generally cut below 10Hz for better thump rejection and to save speakers hard work in a transformerless digital signal chain that lets all low end down to 1Hz go through after the compressor. If you can hear it, there is a problem, and you could try a 20Hz or 30Hz low cut or even higher depending on the track and listen for if it gets rid of it. Or use smaller value coupling caps in the compressor circuit to make it persistent, with less low-end response. But if it doesn't work, after all this, tubes are really no good.

Other circuits thumps less than a STA-Level. Interstage transformer after compressing stage helps, and caps in the cathodes. Also check the balance of output tubes, and use matched pentodes, because that's where the thumps are really rejected in this circuit.
 
rafafredd said:
It means that maybe JJ tubes is a no go for varimu compressor circuits.

Definitely not the case with these tubes. I've had success building a few vari-mu circuits with the new JJ6386 without any issues. Haven't done the Sta-Level yet, but cant imagine it would be any different.
 
I dont believe its output tubes balance that makes this noise..(Its works fine with pair of 6k4p-ev even if I dont select them )
If I take off R35 and give negative control voltage from separate power supply( without input signal) ,waiting few seconds and then switch off control voltage ,then I hear same "bump".The higher the control voltage the louder the "bump"

I have made lot of varimu compressors with ecc82,6bc8 and 6k4p ,but first time with 6386 (and I have first time this problem)
 
i remember something about the jj6386 being different from the GE right out of the box, but after a little burn in, the cathode gets going and the tube then acts like the GE. might have been the difference in chemicals used, many chemicals used in NOS tubes are no longer used because of toxicity issues,

have you burned in the tubes for long?

you could plot grid volts vs plate ma at the rated voltage,

what kind of transformers?

some of these compressors depend on high dcr to swamp out the thumps a bit, some models include resistors in series with the pri  to further the effect,

 
Tubes have been in use  max. 10 hrs  - time when I testing compressor after experimenting with different resistor values,caps,transformers
Input transformer is edcor 600:10k ( I tried old Tesla 1:4 one too- with same result)
Output is edcor 8k:600


 
!0 hrs isn't enough burn in is it?
Not sure how long for new tubes to burn in but i always burn NOS for about 2 to 3 days - usually by then all the low level noise, farts and pops have gone - if the tube is good!
 
All problems cured with NOS RCA 6386.I bought four of them and they all are excellent right from the box.Maybe I had especially bad luck with two new JJ6386 or ...
 
I put the JJ's in a stereo Collins 26U, and they were flawless, better than any RCA or GE I could find.  26U is a much tougher test on the tubes also.  Who knows. 
 
you could graph Ip vs Vgk using the same plate voltage as the Sta-Level.

see if their is a kink in the graph somewhere,

JJ makes good stuff, i love their 6V6, and i hear the KT88 is great also.

you could use the Sta-Level chassis for the tube socket, heater and plate voltage, disconnect the grid circuit, add a series resistor to the grid,  then connect a DC  pwr supply backwards to the grid to vary Vpk.



 
CJ said:
you could graph Ip vs Vgk using the same plate voltage as the Sta-Level.

Indeed. I find it strange with all the blind tube swapping and spending money when it's so easy to measure and match the curves to perfection with a very simple grid voltage vs. current eating measurement.

Now we have an unfortunate situation that lots of people will find this thread and read it "JJ tubes crap, random NOS tubes from ebay good"  :(

I wouldn't exactly write in the thread title anything like "solved!" when it's all just random luck at the moment.
 
Well unfortunately, not everyone has the expertise or measurement equipment to objectively figure out what the deal is.

But you're right, swapping out the tubes = problem solved, doesn't saying anything about anything.
 
desol said:
Well unfortunately, not everyone has the expertise or measurement equipment to objectively figure out what the deal is.

Actually, everyone on this forum does. We all must have a multimeter and each and every one of those is capable of measuring current. Then just put any negative voltage into the tube grid and measure plate current. I'm almost certain everyone also has access to a PSU that outputs negative DC voltages. Measure at multiple voltage points, you choose the steps, more is better. Simple as that.
 
you can use a battery and a pot for the minus voltage,

then just measure voltage drop across a resistor to get the current,

info >

http://www.groupdiy.com/index.php?topic=8506.20
 
Kingston said:
desol said:
Well unfortunately, not everyone has the expertise or measurement equipment to objectively figure out what the deal is.

Actually, everyone on this forum does. We all must have a multimeter and each and every one of those is capable of measuring current. Then just put any negative voltage into the tube grid and measure plate current. I'm almost certain everyone also has access to a PSU that outputs negative DC voltages. Measure at multiple voltage points, you choose the steps, more is better. Simple as that.

That's the beauty about relativity, is assuming what you can or can't do based on the amount of theory you possess...in this case. My point was more centered around agreeing with your previous 'sad faced comment, that the outcome of the issue didn't prove anything...and could possible have a negative influence on the reputation of JJ tubes. Don't get me wrong..i love learning!
 
it solved problem for me..I bought new tubes after days with experimenting ,measuring and testing.I did simple testing unit by putting jj tube between two transformers and simulated negative control voltage with adjustable dc power supply. It still did this "bump" when I increase or decrease cv.I heard clearly when one side of triode makes this noise then other..changing anode or cathode resistors  didnt help,as I remember.
And,yes maybe we talk about different things..Im not sure it was so called "thumping"(coming from imbalance?)...as I said compressor works fine and sounds good with jj tubes.Only problem was this bump-sound what I heard when input signal stops and cv decreases (it do this when cv increases too but its not a problem in reality because its relatively quiet compared to signal)

In any case I dont want to say that buying NOS 6386 from ebay is way to go - they can be fake , out of balance or simply died.





 
I've heard quite a few complaints about the JJ 6386s and performance issues, though it's not every tube, just a high percentage of problem pieces. 

Pardon the hijacking, but rather than make another thread I figured this would be a good place to ask: I have a friend whose original BA-6A (edit: it's his BA-25a) thumps and no changing of 6386 tubes will resolve it.  Any suggestions?
 

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