Any Opinions on Octal Preamp Tubes?

[jmaf]

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.

That's why you employ gravitron tubes in those cases. Gravitrons counter gravity by self-hysteresis, countering freddy currents by reducing random collision stress on the electron path up. You can think of electrons in a gravitron octal tube like salmon swimming up a stream, they jump from quantum level to the next reaching the top grid.

[Steve Conner]

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.

Sure, the spec has changed, but the physics of phono cartridges haven't. It was never so different that it was a treble boost instead of a low-pass filter.

Maybe the input had some weird response meant for use with a ceramic cartridge, but then it's not a "RIAA" input.

[soundguruman]

There have actually been several RIAA curves, at least one of which is the complete opposite of what is graphed above.
The per-emphasis of some early systems was the mirror image of what it is today. In other words the high frequencies were boosted instead.
I can recall that on early McIntosh phono preamps there were 6 different curves, selectable by knobs and switches.
This is because in the early days there were no single industry standards.
This makes for a really cool screaming treble boost, from early designs.

The Operadio used an LCR module to accomplish this curve, that used to be sold by Sprauge.

[J M Fahey]

Actually the RIAA EQ back then, when Operadios were built, was completely different .............. 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 ..............

No matter the minor adjustments, they ALWAYS increased Bass and decreased Treble on playback/reproduction.

"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".

Irrelevant, you referred to RIAA exclusively.

But you were in so much of a hurry to attack, you never bothered to consider this design innovation, which is indeed noteworthy.

I referred to your irresponsible distortion of what a RIAA curve means, anything else comes out of your mind.
Feeling attacked by what wasnīt even mentioned is usually described as one of the Paranoid Disorder traits.

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.

See last comment above.

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

Yes, there should be such a check.
Couldnīt agree more

As a side note, Iīm starting to feel sorry for music-electronics-forum , which I love.
People like you turn it either into an unreliable information source or an endless pissing contest when your unreliable data is corrected.
Both ways hurt it big way.

[jmaf]

I hope I'm not contributing to that lack of reliability on the data here. My information above is trustworthy and tested.

[J M Fahey]

Your "information" is fun, which is good.
Itīs clearly written smiling, tongue in cheek, not with crisped fists banging at the keyboard.

BANGING
banging : present participle of bang (Verb)
Verb:
1. Strike or put down (something) forcefully and noisily, typically in anger or in order to attract attention.
2. Come into contact with (something) suddenly and sharply, typically by accident: "I banged my head on the low beams".
More info ŧAnswers.com - Merriam-Webster - The Free Dictionary

[Chuck H]

I'm just here for the cake.

SGM... Honestly, if you have nothing to learn, are you only here to pontificate? this is an excellent resource. It's entirely possible to be wrong about something and still have a learning experience that ends in comradship and not disrespect. If your going to use a SN like soundguruman it's a good idea to actually KNOW what your talking about. You can learn that here. It's just best not to offer information that you can't be sure is factual. You can introduce such information as part of a question. As in, "Isn't there an RIAA curve that boosts treble where most didn't?" Then if it turns out to be the case you've improved the awareness of everyone reading. And if it's not the case then you've improved your own at least. Why you would refuse to be wrong when you are sometimes wrong is not our problem here. Please don't share it with us.

PS... You will be called on the floor if you do offer misinformation. A little humility can still render this an opportunity to learn. I'm not the most graceful person socially and even "I" get this.

[tedmich]

knife fight! JM's got the cape!

[J M Fahey]

Well, thatīs a very flattering portrait of me.
For accuracy you should add "a little" extra belly and pull "a little" hair and you would be quite close, thank you.

PD: if youīd replace knives with soldering irons, the whole image would be even better

[Enzo]

The only RIAA curve that would be "backwards" would be a record curve. There is absolutely no reason they would boost treble and cut bass is a playback curve. And no one here is building record lathes.

Why? The whole POINT of a RIAA curve is to get the groove to fit on the record. The lower the note, the wider the excursion of the stylus from the center line - just as in a speaker, the woofer cone moves further than the tweeter cone. The groove on a record cannot cross itself, so if they recorded flat response, the groove spacing would have to be extremely wide to make room for it. And that would mean there would not be very much room for time. 40 seconds and you reach the end of the disc.

SO they use the curve. They record things with a reduction in low freqs and an emphasis of high freqs. Plug a phonograph record into a flat amp with gain - it sounds very tinny and lacks bottom end. But play that through an RIAA preamp, and it boosts the weak bottoms and cuts the overly strong top. The net result is the record emphasis and the playback de-emphasis average out to flat. That is the resign intent.

And that is why you will NOT see a phonograph record playback curve with treble boost and bass cut. It would only make more exagerated the reduced bass and increased treble on the recording. And there may have been competing standards for phonograph playback response curves, but they all faced the same exact physical limkitations of spacing the groove on the cisc, and they all needed to do the same thing to accomplish this, reduce the bottom end. AN old Mac amp may have had selectable playback curves, but they wouldn't have had one that was the opposite of what was needed for playback from disc.

[jmaf]

Your "information" is fun, which is good.
Itīs clearly written smiling, tongue in cheek, not with crisped fists banging at the keyboard.

Of course, this forum never makes me bang at the keyboard, it's good vibes here. What does make me bang my keyboard is this computer program called MS Windows.

But seriously, some folks do take things to a certain unneeded extent. I realise that electronics and tech require a "serious" demeanor(math isn't exactly pub bar stuff) but that's precisely why I leave work aside, my tube amp bench, and come over here to be able to not be 100% serious.

[boots]

Wow! What an informative and entertaining thread this has been!

Thanks for the info Guys. If I didn't have a stash of octals, I would probably just stick with 12AX7's and not even think about the octals. After all, you can't beat a 12AX7 preamp. But since I am such a curious bastard, I will eventually build up a Champ or Deluxe type thing with 6SL7's. Just because I have some. And because I just have to know. I have been building too many amps lately, according to my wife and kids, so it may be a while. But I will definitely post the results here.

Thanks everybody - I appreciate the input. This is a great forum!

[Enzo]

Use them, have fun, they'll work.


The thread has wandered far afield, but since your opening premise was wanting to do something with octal tubes you already had, I guess advice to use 12AX7s is off base.

[Chuck H]

I guess advice to use 12AX7s is off base.

+1
Since you already have the tubes, that IS the point. There are ways to minimize hum in moderate gain amps without using parallel filaments (like a 12ax7). Lot's of great amps can be made with your tubes.

[Rhodesplyr]

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.

You're on the way to being right. Remember that 6J7s were designed as *radio* tubes. At radio frequencies, interelectrode capacitance matters much more than it does at audio frequencies. According to one source I have here in the study, 6J7s were designed with RF/IF receiver stages in mind and that's why it has the control grid at the top. If this source is incorrect, I hope someone will do me the favor of letting me know since I don't want to spread misinformation...

That the 6J7 datasheet calls it a "Triple-Grid Detector Amplifier" might be a clue to its original purpose.