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Lets talk about replacements for 6lf6 tubes

jtrouter

Active Member
Jul 7, 2015
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Hello,
I have a couple of 1970's tube amps that i am slowly working thru and rebuilding. I have a Palomar Skipper 300 and a PAL 351BDX and
they both use 6lf6 tubes. At first i was going to rewire them to use higher voltage tubes but then i was looking at the 6KN6 tubes and they
do show as a replacement for the 6LF6 but not the "preferred" option? Other than a little lower power output is there any other reasons to
not use the 6KN6 or are there any changes i have to make to the amplifiers to use the 6KN6? Thank you and as always look forward to the information
i get from this forum and with your help i continue to grow in knowledge each day.
 

jtrouter

Active Member
Jul 7, 2015
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Yes you are correct, the link that you listed is the one that i read and was talking about.
Prefered Substitutes ........................6LF6, 6KD6, 6LX6, 6MH6

Substitutes ................................ 6JS6 6KN6, 6LB6, 6LR6
As you can see it shows Prefered and just substitutes and the 6KN6 does show as a
PLAIN Substitute. That is what i was asking are there any changes that i would need to make to optimize its use?
 
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nomadradio

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Apr 3, 2005
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The 6KN6 isn't just one tube, it's two.

Literally. One version looks just like any 6LF6 or 6KD6 on the inside. A single gray sheet-metal anode.

The other version of the 6KN6 is literally two 15-Watt tubes in parallel, inside one glass envelope.

I have never seen a proper explanation of why this was done. Smacks of a patent dodge, meant to evade a patent-licensing fee, maybe?

Inside a color TV this difference would not be significant.

But to a linear at 27 MHz, the two-section tube will have more capacitance to ground that the conventional single-section tube adds to the tuned circuits.

If your 6KN6 looks like any typical 6KD6/6LF6 on the inside, you're probably good to go.

The capacitance difference matters most when more tubes are in parallel. One or two tubes of the 'twin' type might tune up okay with only a small adjustment to the plate coil. But four or six of them in parallel could make adjusting that coil a bit touchy.

73
 

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jtrouter

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Jul 7, 2015
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So let me see if i got this, IF i want to use the 6KN6 tubes in place of 6LF6's i want to make sure that it is the single version not the double version. And it is going to be a test as I found a source that i can by unused NOS tubes for under $15ea or good pulls for half of that. And for that price it is well worth checking out.
The 6KN6 isn't just one tube, it's two.

Literally. One version looks just like any 6LF6 or 6KD6 on the inside. A single gray sheet-metal anode.

The other version of the 6KN6 is literally two 15-Watt tubes in parallel, inside one glass envelope.

I have never seen a proper explanation of why this was done. Smacks of a patent dodge, meant to evade a patent-licensing fee, maybe?

Inside a color TV this difference would not be significant.

But to a linear at 27 MHz, the two-section tube will have more capacitance to ground that the conventional single-section tube adds to the tuned circuits.

If your 6KN6 looks like any typical 6KD6/6LF6 on the inside, you're probably good to go.

The capacitance difference matters most when more tubes are in parallel. One or two tubes of the 'twin' type might tune up okay with only a small adjustment to the plate coil. But four or six of them in parallel could make adjusting that coil a bit touchy.

73
So to make sure i understand i want to buy the ones that are the single version and not the double, and it should look like a 6LF6. I found a seller that has them that are unused NOS really cheap so that if they do work i will stock up to last me forever and if not im not out much and even then would still be happy just for testing use. Thank you guys as always you are a great help!
 

Shockwave

Sr. Member
Sep 19, 2009
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Hmm. Just noticed one difference in the spec sheets. The 6KN6 draws 3 Amps from the 6.3 Volt supply. 6KD6 only draws 2 Amps. Never noticed that before. Could make a difference, seems like.

73
Those 3 amps of filament current, also give the 6KN6, a whooping 1.5 amps of peak cathode current! It has the widest oxide coated cathode of any sweep tube. I bought sleeves of these tubes years ago for $4.99 per tube. I've been using them ever since in every application from a single tube grid driven amplifier, to six in parallel. As Nomad mentioned, the version with the single tube inside the glass envelope, is preferable. I've noticed the double tube version runs hotter, sometimes with a red plate, even when the circuit is retuned to compensate for the capacitance.
 

jtrouter

Active Member
Jul 7, 2015
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Those 3 amps of filament current, also give the 6KN6, a whooping 1.5 amps of peak cathode current! It has the widest oxide coated cathode of any sweep tube. I bought sleeves of these tubes years ago for $4.99 per tube. I've been using them ever since in every application from a single tube grid driven amplifier, to six in parallel. As Nomad mentioned, the version with the single tube inside the glass envelope, is preferable. I've noticed the double tube version runs hotter, sometimes with a red plate, even when the circuit is retuned to compensate for the capacitance.
Well then i have to ask how can i tell single vs double? I am game to try a few as they can be had much cheaper then the 6lf6 or other replacements?
 

nomadradio

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I should snap a pic next time I get to work. Pretty sure we had one of each.

The distinction seems simple enough. If you see two side-by-side copies of the same gray structure you have the "twin" version. And if there's just one large gray structure, you have the single version.

73
 

jtrouter

Active Member
Jul 7, 2015
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I should snap a pic next time I get to work. Pretty sure we had one of each.

The distinction seems simple enough. If you see two side-by-side copies of the same gray structure you have the "twin" version. And if there's just one large gray structure, you have the single version.

73
iS ONE BRAND MORE LIKLY TO HAVE A DOUBLE VS A SINGLE?
 

nomadradio

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Apr 3, 2005
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Brand name printed on a retail carton, or painted onto the tube may or may not tell you who actually made the tube. It was common for factories to sell large wholesale lots to the sales company with their competitor's name on it. Tubes left the factory with nothing but the type number printed on them, packed in 12-by-12 egg-crate cartons.

Consider: The GE plant in Owensboro has a sales division. But they're a totally separate business unit. They just happen to have first dibs on the tubes from that factory. The GE sales department sees that they will run out of 6L6GC tubes in a few weeks. But the next production run for that tube in Owensboro is over two months away. Any sales company knows that telling a customer "tough luck we ran out" is no way to make money. To fill the gap, the sales company will buy what it needs wherever it can find what it needs. Might mean getting them from RCA in New Jersey, Sylvania in Pennsylvania, Amperex in New York, or from some foreign broker outside the country. They don't care, they just want to fill orders.

Remember that the brand name gets printed onto the tube by the distributor, the people who print the cardboard carton.

And that name may or may not have anything to do with who actually built the tube.

With a little practice, you can identify a factory by the type number. If it's sandblasted through a stencil, that tube came from GE in Owensboro, KY. If there is an eight-sided 'stop sign' frame around the type number, that would be RCA. Gray numbers printed in some kind of gray ceramic slip would be Amperex. And gray numbers with no outline, but flat and smooth would likely be Sylvania, later ECG.

Somebody (else) should post a video with examples.

73
 
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