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Old 28th March 2007, 07:48 PM   #1
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Default 5879 Experience?

Anybody have any experience with these tubes? Datasheet suggests audio use, but maybe that doesn't mean anything.
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Old 28th March 2007, 08:00 PM   #2
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Yes...

http://www.pmillett.com/pentode_phono_preamp.htm

It is designed for audio use as far as I know. It's OK, not spectacular. Gm is a little low for my tastes.

Pete
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Old 28th March 2007, 08:08 PM   #3
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Thanks Pete, did you happen to try it triode strapped?

And, what does it mean by "sharp cutoff" pentode?
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Old 28th March 2007, 09:31 PM   #4
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Nope, didn't try it as a triode.

"Sharp cutoff" refers to gain change with grid bias point (you see this as more constant spacing between the lines on the plate curves). For low-distortion use (audio) you always want sharp cutoff.

Remote or semi-remote cutoff tubes are used for gain cointrolled IF stages, mostly.

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Old 28th March 2007, 09:40 PM   #5
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I have a bunch of these tubes, but never got around to testing any. The old tube manuals that I have say that these were designed for microphone preamp use.

A sharp cutoff pentode is the normal variety. The control grid is wound in a constant spiral. They are intended for linear operation at a fixed operating bias. These are what you want for audio.

A remote cutoff pentode has the control grid wound in a way where the spacing varies along its length. As the bias voltage goes more negative the closely wound part of the grid will cut off that part of the tubes current making the tube appear smaller. These were originally intended for AGC applications in radio receivers. The RF or IF stage gain could be varied by adjusting the grid bias. This was used to reduce the gain on strong signals to avoid overload and keep the radios volume level constant as you changed stations.

These tubes have an intentional nonlinearity which makes them unsuitable for large signal audio applications. In many cases there is an equivalent remote cutoff pentode substitute for a common sharp cutoff pentode (6SK7 - 6SJ7 and 6AU6 - 6BA6) puting the remote cutoff tube in place of the sharp cutoff tube will generate distortion. Sometimes this can be used in a guitar amp. The tube will sound clean at low drive levels, but get absolutely nasty when driven hard.

There are triodes with similar characteristics. They are often just called "triodes, for gain controlled applications" or AGC triode.
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Old 28th March 2007, 09:56 PM   #6
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Interesting, thanks. I also have a stack of 5749's that might get tried in a guitar amp.
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Old 29th March 2007, 12:21 AM   #7
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Quote:
I also have a stack of 5749's that might get tried in a guitar amp.
5749 = 6BA6, I have a lot of these too. I also have about 100 pounds of 6AU6's, mostly used ones. I tested some of the 6AU6's for audio use in pentode and triode connection. Some are noisy, some distort, but some work pretty good. While I had the setup, I plugged in some 6BA6's. In addition to the noisy ones, none would deliver distortion under 2%, some were over 5%.

I used to build Fender Champ clones several years ago. I built a few of the 5C1 version that uses a pentode wired 6SJ7 for the input tube. Grid leak bias with a 5 meg resistor is used. It also uses a 2 meg screen resistor. With values like these it is real easy to overload the input tube. I know I used different values, but I can't remember what they were, and I wasn't smart enough to write things down back then. I know that I had a combination that could be driven hard enough with an overdrive pedal to generate distortion in the output tube first. Then with a simple tube swap to a 6SK7 the tonal charachter of the amp changed. It could be played cleanly with the guitars volume set on 3 or 4, but got into some serious fuzzy distortion by turning up the guitars volume knob. The amp got some serious distortion, but didn't get much louder because the distortion was happening in the input stage.
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Old 11th April 2007, 11:38 AM   #8
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NOS EF86 are getting too expensive to acquire and I'm lucky to have a stash of NOS RCA 5879's at my disposal. One fine day, I found its a fine replacement for the EF86 in my clone Leak TL25+, and so, with re-routing the wiring, its performs to my expectation.
I've decided to stick to this tube..in my opinion, its just as good if not better and without breaking the bank.
I've had a little bad experience with a handful of NOS EF86's being microphonic and some background hiss..and now hesitate buying anymore 86's in future.

I did read somewhere, 5879 is the American version of EF86 and should perform similar, albeit different pin designation the only difference. For now, its still very cheap and plentiful to obtain..from Ebay.
I can wholeheartedly highly recommend this tube as a great alternative to the overpriced NOS EF86.
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Old 11th April 2007, 01:02 PM   #9
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For EF86 replacement, I've switched over to the EF40. Worst part is sourcing the B8A tube socket - but you can find them from UK sellers.

The EF40 is an earlier version (rimlock socket) version of the EF86. They seem to (completely subjective here) sound a touch better than the EF86 - part of that could be from the socket which actually 'locks' the tube in using a centering nub on the glass base. Best part about the EF40 is it is dirt cheap - $5-$10 for 50s Dutch or French made ones.

http://www.r-type.org/exhib/aad0126.htm
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Old 11th April 2007, 01:18 PM   #10
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Default 5879

The 5879 seems to be a miniature version of the 6J7 pentode. I think RCA did this because they used 6J7/1620 tubes in a lot of their mixers and mic preamps, and thus wouldn't have to change the circuit design much when "miniaturizing". Thus the triode-connected mu is low (20) and the transconductance is also low. This makes it lower gain than the EF86. However, within these constraints, it is a pretty good sounding tube. I used a triode-connected 5879 as the first tube in my RF-powered 2A3 SE amp, and this amp is one of the best-sounding ones I've made. However, why not use the original 6J7? They are fairly available, and sound good, too.

- John Atwood
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