A note on 6386 replacement tubes

[Trouble Report]

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.

 

[pucho812]

I have not had such issues with the jj 6386  replacement tubes.

There is more going on in that fairchild then just the 6386's tubes.  If you are having problems  even after a tube replacement, then you really need to  get into it further then just swapping tubes because the unit is misbehaving...

 

[emrr]

Oh well.  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.

 

[gridcurrent]

have to take your complaint with a large grain of salt.
too many good reports of  JJ 6386 tubes in 670's.
what value is meter calibration resistor?
nearly all 670's in LA have had that cal resistor messed with.
A&M went so far as to replace it with a pot on their myriad of units,
just so the meter would "zero".

A crude but effective way to sorting 6386's for a 670 is to test them, just one at a time in the unit, documenting emission/meter reading and balance.
40+ years ago, one would have to go through over 20  6386's to find 8 that would "zero" and balance with the factory 4700 ohm meter resistor.
don't expect new JJ's to be much different.

 

[Trouble Report]

Just to clear, the unit in question is stock - no mods of any kind. Further, a channel with four reasonable NOS 6386s aligns to "0" properly and the meter tracking is acceptable. It's when the JJs are introduced into the equation that things start to get iffy. Don't get me wrong, they sound fine. My contention is that they are not a spec-for-spec replacement, and that this can cause - has caused -  some issues.

 

[gyraf]

..or maybe you, as a private customer, got to buy the tubes that were de-selected by the firstpickers (companies that buy large amounts very often gets allowed to select/reject).

I often suspected something like this may be reason for the extreme inconsistencies in current tube production.

Jakob E.

 

[lassoharp]

My contention is that they are not a spec-for-spec replacement, and that this can cause - has caused -  some issues.

If you went back in time and bought four brand new GE  6386 from the local tube shop and tested their idle and told me they were all sitting at 14 ma, I'd call you a liar . . . . . . But you're not a liar and neither are tube sepc sheets - they're just overidealized - and most of the time not accurate   This is something true to most all tubes though.  14 ma spec quote is virtually meaningless now.

The excess current draw is disparaging and nothing can really be done about it, but I dont think it means you cant get performance from them.  With vari-mus its more about how they behave under GR, and that's not something you can easily glean from transconductance specs or simple gain and emissions testing.  You'll swear you have them properly sorted and matched, then they will start thumping under the stress of GR.  There are no curve sheets to follow and you simply have to sub and observe.  Meter calibration you can always adjust with shunts - thumping and related distortion artifacts from unmatched tubes you can't.    With parallel design you have more latitude than with singles since unmatched combos can balance each other out.

And if you buy NOS 6386 on the open market, there's every chance that the many techs and clone designers have had them, tested them and found them difficult to match and sent them on their way for others to take a chance on.  They'll measure strong on the checkers and the ebay sellers will be bragging on the specs. . . . .and you'll be asked to pay top dollar . . . . .  just saying.

 

[PRR]

It isn't that hard to get a 250V supply, a -50V supply with pot, and a mA meter, plot Vg versus Ik from ~~15mA down to 0.1mA, and graph it.

Does anybody do that??

 

[radardoug]

It isn't that hard to get a 250V supply, a -50V supply with pot, and a mA meter, plot Vg versus Ik from ~~15mA down to 0.1mA, and graph it.

Does anybody do that??

Thats far too hard for these guys hehe!

 

[lassoharp]

It isn't that hard to get a 250V supply, a -50V supply with pot, and a mA meter, plot Vg versus Ik from ~~15mA down to 0.1mA, and graph it.

Does anybody do that??

I have thought of attempting a test rig similar to what you describe but wasn't sure if the results would be telling me what I needed to know.  Apparently it will so thanks PRR! 

Should be a nice way to compare different tube types too.

 

[emrr]

It isn't that hard to get a 250V supply, a -50V supply with pot, and a mA meter, plot Vg versus Ik from ~~15mA down to 0.1mA, and graph it.

Does anybody do that??

oh, that's what you do with those old Eico/Heathkit bench supplies they say to themselves.....

 

[bruno2000]

http://www.dos4ever.com/uTracer3/uTracer3_pag0.html

I own one and like it.
Best,
Bruno2000

 

[pucho812]

Thats far too hard for these guys hehe!

speak for yourself... 

 

[CJ]

build a jig with a variable cathode resistor,

plug and play, vary the input with an ac signal,

using 6BA6 mil grade over here,

 

[Trouble Report]

If you went back in time and bought four brand new GE  6386 from the local tube shop and tested their idle and told me they were all sitting at 14 ma, I'd call you a liar . . . . . . But you're not a liar and neither are tube sepc sheets - they're just overidealized - and most of the time not accurate   This is something true to most all tubes though.  14 ma spec quote is virtually meaningless now.

The excess current draw is disparaging and nothing can really be done about it, but I dont think it means you cant get performance from them.  With vari-mus its more about how they behave under GR, and that's not something you can easily glean from transconductance specs or simple gain and emissions testing.  You'll swear you have them properly sorted and matched, then they will start thumping under the stress of GR.  There are no curve sheets to follow and you simply have to sub and observe.  Meter calibration you can always adjust with shunts - thumping and related distortion artifacts from unmatched tubes you can't.    With parallel design you have more latitude than with singles since unmatched combos can balance each other out.

And if you buy NOS 6386 on the open market, there's every chance that the many techs and clone designers have had them, tested them and found them difficult to match and sent them on their way for others to take a chance on.  They'll measure strong on the checkers and the ebay sellers will be bragging on the specs. . . . .and you'll be asked to pay top dollar . . . . .  just saying.

It's true that just because you buy NOS does not mean that you are guaranteed matching tubes, or even tubes that work well. Ebay, et al, is the other alligator pond you can find yourself in. You would probably have to buy 12 tubes to get 4 that were in the same ballpark. My objection is that JJ presents its product as a direct and unassailable replacement, "the answer", when the truth is that it's just another roll of the dice.  Although they probably wouldn't sell very many tubes if their claim was "They're close. Give 'em a try".

Also: yes, one could "zero" the meter by adding a shunt,  but as Lassoharp pointed out, once you introduce GR into the equation, the current profile, and its reflection on the meter reading, becomes a very different ball game. In the case of the JJs, it's a ball game using JJ's rules.

 

[radardoug]

Ummm, every component you buy has tolerances. They can range from small (.1% resistors) to large (20% electros). Thats a fact of life. You need to work within the rules. Fairchild worked within the rules by parallelling tubes to even out the spread. There is no point in you criticising JJ, they are making tubes that work within their tolerances, which I am sure they will supply.
You want matched pairs? Buy lots and match them.  As Barak Obama says, stop whining!

 

[CJ]

Rein Narma said in the interview that the 6386 varied the least out of all the remote cutoffs he tested,

back then you you could get hold of  big lots of mil surplus stuff,  that way you can screen tubes and not lose too much money,

 

[Ihearttubes]

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.

I realize this thread is very old, but I am experiencing the exact meter discrepancy you are having with JJ’s. ~10db of gain reduction on the meter with only 5db of actual GR. My build is brand new (newly completed Drip V2) and otherwise functioning perfectly. The JJ tubes test and sound fine. Curious if there is any workaround for this issue or if this is actually normal behavior?

 

[VertigoRecording]

June 2022 update. The JJ 6386 tubes are expensive and they still suck and they still will not work in fully functioning vintage Fairchild 670 or 660. Trust me. They suck.

 

[gyraf]

Agree