Any Opinions on Octal Preamp Tubes?

[boots]

I just discovered I have a stash of 6SN7's and 6SL7's and I'm thinking about using octal preamp tubes in a tweed-ish build some day. Do you guys have any experience with these tubes? How do they compare to 12AX7? I have heard the octals have a fatter, richer sound.

How about the 6SC7? It was widely used in the old tweed amps. I would have to go on a hunt for some SC's, but if they are preferable to SN or SL, it might be worth finding some. Maybe there was a good reason all those old amps used 6SC7's?

[J M Fahey]

I have heard the octals have a fatter, richer sound.

Such a blanket statement sounds like myth (I could use a 4 letter word here, but we are among gentlemen)
I think they were good tubes, period, and later most were basically re-packaged in smaller, practical glass 9 pin envelopes.
But no Magic involved.

Maybe there was a good reason all those old amps used 6SC7's?

Sure.
6SC7, 6SN7, etc., that´s what you got from the suppliers way back then.

If you have a stash, use them.
I know I would.
They look lovely too.
Please post some pictures of what you build, the coolness factor will be high.

[Chuck H]

I do think it's interesting that you ask how they compare to a 12ax7. A 12ax7 has an amplification factor of 100. A 6sn7 has an amplification factor of 20. A 6sl7 has an amplification factor of 70. So as far as direct circuit replacement, nada. In other words, if you build a design intended for 12ax7's with your octal tubes it would be a very different amp. But, you can build an excellent amp with those tubes as long as the amp is designed to use them OR you can design around the difference in amplification factor yourself. I would say use the 6sl7's for amplifiers and limit use of the 6sn7's to cathode followers, cathodyne PI circuits and the like where amplification factor is not an issue.

JM2C

[Rhodesplyr]

6SN7s are beloved among audiophiles as output tube drivers in Hi-Fi amps and tend to last a long time. Many DIY Hi-Fi preamp builders still use 6SN7s. The 6CG7 is supposed to be a 6SN7 equivalent in a miniature envelope.

I've seen some vintage amps that used 6SL7s at first and, later, switched to 12AX7s with no component value changes. I'd assume that this changed the behavior somewhat, but I've never had the opportunity to do a side-by-side comparison.

6SC7s are problematic these days as they, AFAIK, haven't been in production for a long time, and we're getting close to the bottom of the barrel in terms of being able to find ones that aren't noisy and/or microphonic. Some people rewire 6SC7 sockets to use more widely available 6SL7s, which are very similar to 6SC7 specs, though not identical. I've had to put tube dampers on 6SC7s in combo amps, and I think that this was a problem with them from the get-go in combo guitar amps, the tendency to be microphonic, more so than later miniature tubes.

5751 miniature tubes are very close to 6SC7s in terms of mu, Rp, and transconductance, closer to 6SC7s even than 6SL7s. (Hmmm... does anyone make a 6SC7 -> 5751 adapter?)

In general, my sense is that while higher gain octal tubes (6SC7s, 6SL7s, and some pentodes) may have worked just fine in radios, Hi-Fi, organs, and heads, sometimes better than miniature tubes, there was always a problem of microphonics when using them in guitar combo amps. The reason I limit that comment to high-mu tubes is that I have three Leslie amps that use medium-mu 6SN7 drivers, and I've never had problems with them.

I think that one of the advantages of octal tubes is that the larger tube pins and tube socket contacts tend to provide better connections. I've rarely seen the contact problems you find with miniature tubes with octal tubes, though I have run across octal tubes where the pins weren't well-soldered to begin with.

[woodyc]

Sovtek was making 6SC7s which work ok. I think I last bought some a month or two ago.

About 6SN7s, I recently made the mistake of getting a bunch of those cheap Russian 6n8s tubes on ebay. They test great and have the right specs, but they all hum and tend to be microphonic too. They might be useful for something, but there is no way you can use them for example in a Reverberocket where half the tube is reverb recovery.

[Rhodesplyr]

Sovtek was making 6SC7s which work ok. I think I last bought some a month or two ago.

Interesting. I hadn't seen these before.

I notice that there are 6SL7s available under both Sovtek and Tung-Sol reissue brands. I would wager that Sovtek's 6SC7 is a repinned version of their 6SL7.

The only downside I can see is that in spots where the 6SC7's outer metal shield is important, you might have to improvise an octal tube shield. Sovtek's tube is technically a 6SC7-GT, which was available in the old days, though not that common.

[heavil]

I just discovered I have a stash of 6SN7's and 6SL7's and I'm thinking about using octal preamp tubes in a tweed-ish build some day. Do you guys have any experience with these tubes? How do they compare to 12AX7? I have heard the octals have a fatter, richer sound.

How about the 6SC7? It was widely used in the old tweed amps. I would have to go on a hunt for some SC's, but if they are preferable to SN or SL, it might be worth finding some. Maybe there was a good reason all those old amps used 6SC7's?

6SL7 looks more like a 12AX7 than 6SN7.
Octal tubes even so better to use in home audio.

[MWJB]

I would say that using 6SC7 in a new build is simply making a rod for your own back. The common cathode shared by both triodes & differing pin out mean that there are few, if any drop in options. No matter how good a tube sounded, if there was only one current production item that fitted in the socket, I wouldn't bother with it.,,too much risk of being stranded by short supplies & ending up with a 50lb door stop.

6SL7 will "work" in just about any circuit where a 12AX7 is specified, or even a 12AY7 for that matter. 6SN7 has the same pin out and can be considered a drop in, low mu, replacement as long as you consider the higher dissipation through the plate resistors.

Me, I'd stick with 12A#7 type tubes, for their flexibility & interchangeability...even if you can't find a working tube of the exact type you want, you'll often find something that will at least function in that socket and "get you home" in an emergency. Plus, no matter how carefully you design, you should never underestimate the tonal contribution that a particular tube might contribute to a circuit...you don't know until you try and once you have tried you might be inclined to optiomise for that particular tube type...or not.

I'm with JM Fahay on the octal vs noval "myth", I did a lot of experimenting with an Ampeg Reverberocket and some "substitubes"...my conclusion was that different tubes sound different (the original 6SL7 sounded vastly different to the Sylvania NOS versions of the same tube, for instance). 12A#7 are also typically wired humbucking in guitar amps and can be less prone to hum & noise.

The idea that octals have "a particular sound" carries no water with me, especially if one could be comparing, say a 6SJ7 pentode with a 12AX7 dual triode, or an EF86 with a 6S/L/C/N7 as guys often do.

[soundguruman]

The Operadio schematics show the founding father's circuits for these tubes,
it starts with a 6J7, which is a high gain pentode (grinds like a chainsaw), then a 6SL7 for V2, then 6SN7 for the phase inverter, then 2X 6L6 outputs.
The 6J7 has a cool grid cap. This may contribute to it's insanely out of control gain. This type amp sounds good for an Elmo James model.

[Chuck H]

it starts with a 6J7, which is a high gain pentode (grinds like a chainsaw)

Can the 6j7 be driven into clipping by a guitar when used as the first gain stage?

The 6J7 has a cool grid cap. This may contribute to it's insanely out of control gain.

Does this connection contribute to gain?

[Steve Conner]

Of course it does. The grid connection is at the top, so the extra weight of electrons piling down into the tube makes a real heavy distortion.

Unless you use it in a "tubes down" combo in which case the electrons fall out and it sounds plain anemic.

[soundguruman]

Can the 6j7 be driven into clipping by a guitar when used as the first gain stage?

depends on the plate resistor, and the plate supply volts I think you could, but its not practical without oscillations to make it clip out
better to keep the gain at 50 or below, although I think it can go to 200
It was not uncommon back then to use a 500K or 1 M plate resistor, that was mainly for a carbon microphone input
and it was not uncommon to use a 5M grid bleed resistor, but the grid is recommended capacitor isolated from the input.

Does this connection contribute to gain?

Well I suppose the designers put the grid on top to get more gain with less noise.
Yes the design of the tube with the grid on top helps it sound wilder for the guitar, it's just about out of control if you set it up that way
but it's out of control in a very cool way. In those days the highest gain preamp tubes had grid caps.
The medium gain preamp tubes had a base connection for the grid back then
better use DC heaters for V1 cause the pentode tends to hum a lot with AC heaters, but has a really crunchy type of attack that is unique
this is the same preamp tube as a Hammond B3 organ, so you can kind of get the idea of how it has a "grinding" chainsaw sound, than no other tube really has.

The best ones are JAN 6J7, with a metal envelope. I don't think the glass ones sound the same.
If you build the Operaidio input stage, use the phono input design, other than the mic design. The phono input has some cool old style RIAA EQ, that allows ear splitting treble boosting.
I suppose it's not for everyone compared to 12AX7

[soundguruman]

The octals do have a richer sound, for hi fi they sound killer

[J M Fahey]

The phono input has some cool old style RIAA EQ, that allows ear splitting treble boosting.

As usual, you got it wrong.
Absolutely wrong; the exact opposite in fact.
No "RIAA equalized Phono Input" can provide treble boosting , ear splitting or otherwise, because it does the exact opposite: boosts lows up to 20dB and cuts highs up to 20dB.
The curve itself is as follows:

Since reading curves is maybe too complex for you, here is the text that accompanies this graph:

RIAA equalization is a form of preemphasis on recording, and deemphasis on playback. A record is cut with the low frequencies reduced and the high frequencies boosted, and on playback the opposite occurs.
..........................
RIAA equalization is not a simple low-pass filter. It carefully defines transition points in three places: 75 µs, 318 µs and 3180 µs, which correspond to 2122 Hz, 500 Hz and 50 Hz.

Of course, Phono Preamps are used for playback, so they boost lows and cut highs.

How dare you post such nonsense everyday without limit?
People with genuine needs and doubts comes here to get accurate information on Electronics and Sound, you are "burning" this forum as a reliable data source.
I do forgive ignorance, nobody is born knowing, but not such stubborn imbecility.

[soundguruman]

Actually the RIAA EQ back then, when Operadios were built, was completely different,
but you would not bother to check that. The RIAA has been changed several times since then, and you are referring to the most recent standard, not the one in very old designs.
You are just interested in attacking posters, as quickly as possible, to make up for your inferiority complex. And this complex is rather obsessive, I would gather.
And undoubtedly, you have never even glanced at the design of the Operadio phono stage.

"Especially from 1940, each record company applied its own equalization; there were over 100 combinations of turnover and rolloff frequencies in use, the main ones being Columbia-78, Decca-U.S., European (various), Victor-78 (various), Associated, BBC, NAB, Orthacoustic, World, Columbia LP, FFRR-78 and microgroove, and AES".

And yes the tube was designed this way to allow more gain in a pentode preamp tube, with less noise or oscillation. The Grid cap input was developed to put some physical space between the input and the output.
It was a compromise, as all designs are. At the time, it was state of the art. The grid could be connected with a shielded wire that comes directly from the input jack area, away from the inside of the chassis.
The output was on the inside of the chassis, and so there was a steel wall between the input and output connections, clever method.
But you were in so much of a hurry to attack, you never bothered to consider this design innovation, which is indeed noteworthy.

How dare I? That's hilarious. Sounds like you are a big fan of Republican debate strategies. First you attack everybody who is different from you, before declaring your personal greatness.

There should be an ego check in the lobby instead of a hat check, especially made for you.