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JJ 6386 vs NOS GE/RCA
Old 1st April 2016
Gear Maniac

JJ 6386 vs NOS GE/RCA

Now that it's been a few years that those JJ 6386 have been released , i am curious to know what people are thinking about these tubes compared to some NOS original ones !
What is your experience with these tubes ?

How do you think they compare with some NOS RCA /GE ones ?

What do you think of them in some 670 ( repro or original ones ) limiters ?

I have been looking on the net to find some opinions about them but i was surprised to do not see more than what i found !
Old 2nd April 2016
Gear Maniac

Does anybody knows the value to setup to test those tubes with a tube tester ?
I have a gigital tester that should be able to test those tubes bt nothing is specified in the manual to test them !
I have checked the 6386 datasheet but i am not sure if i have to use 6,3 v for the heater !
My tester specifies to use 12,6v to test tubes like triodes 12ax7 and those types of tubes , so i am not sure if i have to use that value as well with the 6386 .
I have tried with 6,3v heater , 100v on the plate and -16v for the grid but i get very low Ia value, like 2mA instead of something around 10,5mA !
I tried with -2v on the grid and then i got 8mA for Ia , what seems much better but i need to be sure of the values to use to setup the tester to be sure the result is alright !
Old 4th April 2016
Lives for gear
gyraf's Avatar

My AVO Mk4 valve data manual, 17.edition, 1965:
Neg. grid: 2V
V(A): 100V
I(A): 9.6mA
S: 4mA/V

Jakob E.
Old 4th April 2016
Gear Maniac

Thanks !
I am surprised cause the 6386 datasheet specifies 100v plate and -16v for class A , with 9,6mA but for cascode it is 300v and -5 or 200v and -2 , with 10,5mA in the characteristics and typical operation ( and specifies 6,3v in the General specifications ).
That said ,it is with -2 and 100v that i got the most realistic results !
Thanks for your answer anyway .

But you are right , i had not thought about looking in an old Avo 160 Manual i had but that is what they specifies actualy !
Old 5th April 2016
Gear Maniac

Does anybody know how matched the NOS tubes need to be for a fairchild 670 repro ?
They are specified about 4000 micro ohm new but it seems there is quite some variation between tubes , so i wonder what is acceptable for it works correctly and what happens if they are not matched closely enough ?

Also, what the minimum acceptable for these tubes , is above 3000 still good enough to be used or 3500 micro ohm ?
Old 6th April 2016
Lives for gear

Vari Mu designs rely on matching between 'halves' to minimise distortion and 'control breakthrough'.
By having many valves im parallel (670) the average of the pairs is important which eases the requirements somewhat.
Matt S
Old 6th April 2016
Gear Maniac

Thanks for our answer.
I am not sure i understand perfectly , do you mean that each side of the triode needs to be matched with the other side of the triode but that the fact there are many used in parrallel , makes that they donnt need to be matched as much as if there was only one pair of tubes ?

How much do they have to be matched between each side of the triode ?
And how much dôthey need tôbe matched within a quad ?
Old 6th April 2016
Yes the 2 triodes sections of each tubes need to be matched, but it is less crucial that the tubes be a "matched pair".
Old 6th April 2016
Lives for gear

The more units (valves in this case) you have in parallel, the closer it gets to the statistical 'design value'. It also means that one single part 'drifting' in value will have less overall effect.
Matt S
Old 6th April 2016
Gear Maniac

Within what values does it have to be , to be matched , between triodes and between valves ?

I have some tubes with Ia going from 8 to 15mA , most beeing about 9 to 12mA .

I am surprised that they could have matched the tubes really closely in the 60's !
Were they really matching tubes that much in the 670 ?
Old 6th April 2016
Lives for gear

If you consider how accurate the mechanical tolerances of valves are, and they were assembled by hand it is pretty easy to match valves 'electrically'.
If you work out the 'average' anode voltage and current for the GROUP, you can measure all sections of all valves and record the operating current. Then split them so you get equal results for the best match overall.
If they are not 'equal' you get more thump and distortion and it is up to you to decide when it is too far out.
Matt S
Old 7th April 2016
Gear Maniac

Thanks for your answer.
I have a pretty good digital tester , that gives Ia at 0,1 mA and a way to calculate Gm by setting the grid voltage one v lower than the grid voltage used for Ia , so the difference between Ia test and Gm test will give the Gm voltage in mA , so i can get the transconductance value but i thought there were some sort of datasheet , saying what would be acceptable and what's not !
As i said, most of my tubes have 9 to 12mA for Ia but i was wondering if there was any infos saying how much the tubes need to be matched between triodes and between tubes , to have an idea about what is acceptable , better than testing every tubes in the machine one by one !
Old 12th October 2016
Here for the gear

A note on 6386 tube alternatives

WARNING!! Here’s a note for all you out there looking to replace the 6386 tubes in your variable mu limiter. JJ seems to be the only manufacturer that can supply these tubes new. I’m here to tell you, they are NOT the same as the original 6386. NOT.

I ordered four tubes (yes, they cost $120 each) for a Fairchild 670 for one of my high profile clients. I installed the tubes I received from Tube Depot in one channel. They behaved very differently from the original tubes in the other channel. You’ll say “Well, they’re new. Duh”. And they did perform well with respect to distortion and noise.

The problem is, the transconductance profile is vastly different from the original. The first indication is that it draws a lot more current at idle than the original – so much so that with all four new JJs in one channel, it was impossible for me to “zero” the unit in question. I finally chose to use two original 6386s and two new JJs in each channel. With this arrangement I was able to zero it. Even so, as I called for gain reduction I found that the meter reading had no relationship to the actual amount of gain reduction. With five dB of GR, the meter indicated 10dB. My client decided to go with the discrepancy and get on with life. But what happens when the other original 6386s wink out and need to be replaced?

JJ includes a “helpful” little sheet that provides the test results for the particular tube you have bought. It indicated that across the four tubes I had in my possession, there was a fairly large difference, from a quiescent current draw of 14mA (not too far off the original) to 20mA. I contacted Tube Depot with my concerns. I wanted Tube Depot, or JJ, to hand select a set of tubes that conformed more to the 14mA end of the spectrum. I discovered quickly that Tube Depot is just there to sell you the tube – after that, best of luck. They did provide me with an email address for the folks at JJ in the Slovak Republic, and I began a dialog with them, once again asking for a set of hand selected tubes. Maybe there was a snag in translation, but no matter how I worded the request in multiple email, the answer I received was that “JJ would not change the way it manufactured its tubes” The final exchange wrapped up with the technician berating me for my lack of knowledge of tubes and vowing not to respond any further to my emails. He has held true to his word. I promised him I would bring this all to the attention of the pro audio community – here we go.

Conclusion: Buying JJ 6386s is just as much a roll of the dice as any other solution.
Old 12th October 2016
Lives for gear

When fitting the new valves did you test the 'balance' between each half of each valve?
You should measure the anode current for each section and then split them into pairs. NOTE that of the 4 sockets per channel 2 have one half on the 'left' as you could say, and 2 on the right. This can present an interesting puzzle as to which valve should be placed where but I would expect there is a tech document somewhere that states how the various valves should be paired up.
Valves can have quite a wide tolerance compared to some parts used nowadays but Fairchild's engineers were pretty cunning in their design and using a set of valves, perhaps preselected from a big batch of 6386 these will work properly. Matching and characteristics of the valves needs to be made at the expected voltage and current conditions of course.
To expect some newly manufactured valves to be anything like a 'well worn' set of originals seems unlikely.
If transconductance is 'out' I would anticipate some way to compensate for it but not having a unit to 'play' with I can't say.
Matt S
Old 13th October 2016
Lives for gear
emrr's Avatar
I guess I'll follow your repost around some with my repost:

Seems like results are a mixed bag, with lots of people having similar experiences to yours, and others having no problems. Given the cost, yes, both vendors should be able to supply you with reasonably matching tubes. Current draw differences of that magnitude appear unacceptable, that's a much larger variation than one would expect from a dozen lot of 6V6's or 6L6's, which draw 3x - 4x more current overall.

It would appear you need to buy 3-4x the number needed for a 670 to really get good selection, and I would imagine that was also true to some degree with GE's or RCA's back in the 1960's.

My only JJ experience was with a pair in a stereo Collins 26U-2, and they performed better than any of a 10-12 RCA or GE 6386's I had on hand to compare with. The limiter had never sounded better. At the time original GE's or RCA's were selling over $200 apiece, they seem to have fallen back to something more reasonable now, but for how long? I'd certainly consider going to some of the dual tube variants that remain dirt cheap, and putting the investment into a rig for testing/matching, making some dual socket adapters, etc.

I like gridcurrent's response here:
Old 13th October 2016
Lives for gear

matching should not be an issue as a proper balance stage will have bias trim adjustments. Its more predominate in balanced pentode circuits because dual triodes don't very as much.

The best way I found is to set the tube up in a test circuit (class A grounded cathode amplifier per section) and measure the idle current on both cathodes and then inject a signal and measure the peak to peak voltage on the plate. Then you compare the two readings: I compare the voltage gains to the idle currents of the two sections.

when I build a stage like the 670 Two double triodes in parallel, I will swap the second tube's pin set so that the second triode section on the second tube is on the non inverting signal path.
Old 13th October 2016
Lives for gear

As far as the 6386 tube being overpriced. Yes it is as there is quite a few of VHF tubes that can be used. No I won't list them because I don't want those $5 tubes to go up in price
Old 4 weeks ago
Here for the gear

Some of the Manley units use 2 6BA6 tubes in a "T-bar" adapter which makes matching easier. You just test a bunch and get matched pairs that way.

BTW, a well known Mixer dude told me he tried the JJ 6386 in his 670 and they sounded just as good as the originals. On the other hand, a techie friend who worked for a major brand of tubes told me they were not the same. Buy maybe JJ engineers got them repro-ed well enough, and the Fairchild engineers accounted for all these matching issues when they designed the 670 and 660....
BTW one tip I have for 670 original and clone owners: the tubes run hot and anything you can do to get the heat off is a good idea...a small quiet fan blowing the heat away shouldn't be a big problem to mount. I'll do some research and report any findings i come up with. And I have a batch of 6386 NOS right now(May 2019). contact me at [email protected] if interested.
cheers RR
Old 3 weeks ago
Gear Head
wavenurse's Avatar

In symmetrical compressor conception, it's better to have balanced tubes AND matched tubes if you can! It's really the remote cutoff tube(s) that dictate the overall behavior of the compressor. Even if it's less important in parallel design you can gain more stability.
For example i try in an AM-864 matched 6SK7 with balanced 6SN7 at the output. Just for the fun i replace one of the 6SK7 with another that draws more current at same voltage. Immediately, i notice more distortion with same input level and no compression (worse with compression).

So if we want to preserve the signal it's the better (but costly) approach.
Balancing and matching is a paid option with tube depot and any other tube provider.
I supposed there is also a trend to overprice some tube. But when you see the retail price of an original fairchild 670 or just the replica, we can't blame tube manufacturer for this.
Differences observed in trans-conductance profile may lead to bad gain reduction response and the datasheet of the jj6386 and original 6386 show that.
Real alternative is the one that richmix propose. 6ba6 or ef93 (triode wired) with an adapter, but again this is not the same as original 6386.

So mainly it's the owner choice who must decide if overall specifications must be maintained or if he can tolerate disparities (to gain some money).

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