NOS Tung-Sol 6L6GC

[martin manning]

...they sound most similar to the GE but the hump is in a different place and on the soft to hard scale they are soft to medium.
...So I ordered a pair of medium matched for 450Vp...

David can you translate "hump is in a different place"? And what does matched for 450V mean? Quiescent current at 450V and some Vg1, or something more than that?

[David Root]

Martin, as you know the 6L6GC is not particularly linear in frequency response. The hump, or kink as they called it in the UK (Kinkless Tetrode KT66) is at a different frequency in the Tung-Sol than in the GE. I should have asked him where, above or below the GE but I didn't.

Matched for 450V means he matches the pair for quiescent current draw in an amp he designed for matching tubes, at 450V plate, (and presumably ~450V screen too, since 6L6GC is usually used Fender-style thru a low resistance choke).

[martin manning]

Martin, as you know the 6L6GC is not particularly linear in frequency response. The hump, or kink as they called it in the UK (Kinkless Tetrode KT66) is at a different frequency in the Tung-Sol than in the GE. I should have asked him where, above or below the GE but I didn't.

Hmm... The tetrode kink shows up in the static anode characteristics, so that would not be part of frequency response, and anyway these are beam tetrodes, with almost no kink at all. In fact an EL34 pentode shows as much or more of a kink than a 6L6 does. If I compare the GE and Tung-Sol distortion curves, they are slightly different, whereas the anode curves appear to be the same. It is only very a slight difference, but probably more than I'd expect from even a careless copyist. But if all the data sheets are really the same, how does this guy know that the GE and Tung-Sol tubes have different characteristics? I remain very leery of antique tube dealers... I think most of what they are selling is BS.

[David Root]

Well I guess I got the kink part wrong. If it doesn't affect frequency response why was the KT66 developed?

My source listens to his tubes very carefully in amps and has been doing it for a long time. He doesn't gush about his stuff, and always gives what seem to me to be measured comparisons based on his (and no doubt his customers') listening.

In fact I have on numerous occasions related to him opinions about one make vs another and very detailed related opinions from folks here on TAG, and he invariably will say straight out if he agrees or does not and why, usually in great detail.

So I guess I trust the guy. He tried to dissuade me from buying a Sylvania short black plate 12AX7 recently because he didn't agree with some opinions I had heard about this rather rare tube. I bought it anyway, it is a gain monster.

[martin manning]

It was developed to remove the negative resistance characteristic (the "kink," due to secondary emission) that tetrodes exhibit when the plate voltage is pulled down below a certain point. See: http://en.wikipedia.org/wiki/Beam_tetrode

[David Root]

Postscript--Just received the pair I bought from Hi-Test, see pic. Look just like the one dreric posted. Three getters including a top getter. The number "3" next to "U.S.A." beneath the octagon is clearly visible, this indicates Tung-Sol manufacture according to Hi-Test.

The boxes look the same as dreric's pic too.

Will compare them with RCA blackplates in my second gen ODS 50W and report back.

[jelle]

Neither the 6L6GC nor the KT66 and EL34 amplify all frequencies equally or linear in our push pull systems. The 6L6GC has a noticable mid dip and excels more at the bass and treble frequencies compared to a KT66 or EL34. Regardless of the secondary emission reduction. My guess is that the vendor used incorrect terminology to describe what he heard. Happens.

[talbany]

Sonic differences
EL-34's and other TRUE Pentode tubes will tend to generate more 3rd order harmonics than a Tetrode..
Tony