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Octal Preamp tubes

bmcmusic

Member
Messages
87
Some early Fenders and Gibsons like the GA20 used octal preamp tubes like the 6SJ7. they have a wonderful full round sound. Has anyone used them in more modern higher gain amps? I am curious as if there is a technical reason or that just the 12AX7 became more readily available, kinda like beta vs VHS.
 

Trout

Member
Messages
7,551
I might be wrong, but doesn't Red Iron Amplification use octal pres in their amps?

Seems to me I saw that also, Their clips sound amazing.
3-6SL7GT in the T-Rex


I think there is a good bit of misconception on what exactly a microphonic tube is.

I do not feel tapping on a tube and hearing a tingy noise means its microphonic. Tubes were never designed to be beat on. Mechanically induced noise does not always mean the tube is microphonic.

If the noise continues after a light tap, now thats a potential microphonic problem.

I have on several occasions seen pre-amp tubes that inheriently squeal, the 12AV7 comes to mind. You do not have to tap on it for the noise/squeal to occur.

I have seen some of the older metal octal pre-amps tubes do this as well.
 

bmcmusic

Member
Messages
87
How did I know the gear page member would come through. As always, this is the place for those who know their tone. Any others?
 

oldschoolguy

Member
Messages
2,253
The Siegmund Midnight Special uses octal tubes. Wonderful amp all around, just perfect for what I need it to do--very 3D sounding. Depending on which tubes one uses, it can be 14-30 watts. An extrelely well thought out amplifier and expertly executed. Touch sensitivity defined.
 

schmidlin

Member
Messages
5,258
+1 on all comments. Octal pre's are delicious. Bit of an issue with Trout's harmonic definition tho. Speakers shake tubes just like a pencil tap can. You can get that 'telephone' two-tone sound, especially noticable on single notes. Trust but verify.
 

wyatt

Member
Messages
4,169
Some early Fenders and Gibsons like the GA20 used octal preamp tubes like the 6SJ7. they have a wonderful full round sound. Has anyone used them in more modern higher gain amps? I am curious as if there is a technical reason or that just the 12AX7 became more readily available, kinda like beta vs VHS.
The 12A*7's tubes after significantly more robust construction and better durability, as well as more gain all in a compact size. It was really a no-brainer for guitar amps to make the switch. It is also understandable that the hi-fi switched as well, it was just too convenient a package not to use, though audiophiles will often lament the change (12AX7's have a upper midrange notch that they feel screws up RIAA equalization). Let's use computer monitors or TV's as an example -- big, bulky, heavy CRT's affer deeper and truer color, but the overall benefits of flatpanels outweight that enough to drive CRT's almost out of the market.

The really noisy octals are the 6SJ7's that Gibson and Valco prefered, but that had more to do with the fact that they were true pentodes. Pentodes are inherently noisey tubes because of their construction, it's not just microphonics, but hum, static, extra clicks and clacks, it can be real hard to sort out the good ones...whether it be 6SJ7's, EF86's or 5879's.

Microphonics can be another issue, 6SJ7's, 6SC7's and 6SN7's, as well as their 12.6V counterparts (12SC7, etc.) are mostly really old tubes and may were deisgned to sit in a parlour, in radio playing at moderate volumes, guitar amps (especially combos) often shake these tubes a lot more than intended (Gibson used to rivet their sockets in, instead of bolting, so that they were springy, almost shock mounted from the chassis).

Anyway, most of the above factors makes these old octals typically unsuitable for high-gain amps where one little click can be compressed to the volume of a power chord. In a high gain amp like a SLO, one messy wire placement can create so much noise the amp is unsuable, so you have to be careful.

Now you can hunt down the JAN versions of many of these tubes and they are built to run in conditions that could feature artillery barrages, but they'll cost more (though the RCA 569* series is designed to run 10,000 hours, well worth the expense). Of course, pretty muc all US NOS 12A*7's tubes came out the assembly line meeting the same JAN requirements.

Now there are some Russian equivalents for some of the old octal tubes, the Sovtek 6SL7GT is actually a relabeled Russian 6H9C or 6N9S or whatever, so they are widely available in bulk as a new tubes, so it is an ideal tube for a manufacturer to try and use if they want to go octal. It's a dual triode, but its gain factor (46-54) is only in the range of of say a 12AY7 or 12AT7, so obviously it's not as efficient for building hi-gain amps as a 12AX7 (~100 gain factor).

Octals have definitely made a comeback in the home hi-fi market, but those are low gain amps that are built to sit rigid, isolated for the speakers, preferably on a bed of glass beads. Myself, I use 5693 (RCA's JA 6SJ7) instead of a normal 6SJ7 in my GA-20 and my hi-fi amp and quality NOS 6SL7GT's in both as well.
 
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epluribus

Member
Messages
9,170
Cool stuff...I'm in the midst of doing a bench-test amp with a switchable socket just for a 6SJ7. Handy stuff.

--Ray
 

epluribus

Member
Messages
9,170
Au contraire, we're not the Slept-at-a-Holiday-Inn types at all. We'd be the ones up all night building things. :)

HTH
 

teleache

Member
Messages
153
Ok, no one has yet mentioned the octal high gain pentode 6J7.... Much like the EF86. But the 6J7 yields much more low mids and wider bandwith. The old Supro/valco 1X10 combo's from the 40's and early 50's are total tone monsters ! Like DR. Z and matchess ef86 preamp's are doing, it has already been done in the 40's with the 6J7 tube. If you lucky enough to fine one ! And if it sounds muffled and choked. It's the Cheaply made rola field coil speaker. Remove and save the original speaker and Get a Fender deluxe output transformer and 10" Jensen, eminence or weber alnico and have an amp tech put the output Tran's screwed onto the speaker. Then this tiny little Class A 1X10 combo will breath FIRE ! Not just a cool harp amp anymore...It's now a studio gem for guitar players now!!! Just make sure you find an old stock quite 6J7 and you'll have a Nashville country picker dream amp !


Here's a pic of my modded supro. fieldcoil was removed and reissue jensen p10R with a output transformer attached:
 
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plexi67

Member
Messages
359
Komet amps used an octal.. Many early amps used octal preamp tubes, the early tweed fender twin..very sweet sounding amp. The Ampeg B15..just too many to list.
 

deeval

Gold Supporting Member
Messages
2,158
My Alessandro BnT uses 6sL7,s and I have not had a Peep of Microphonic,s sqeal,s from those tubes and this amp has some high gain if needed.
 

thesedaze

Member
Messages
1,682
The Siegmund Midnight Special uses octal tubes. Wonderful amp all around, just perfect for what I need it to do--very 3D sounding. Depending on which tubes one uses, it can be 14-30 watts. An extrelely well thought out amplifier and expertly executed. Touch sensitivity defined.
15-30W? 15W Max. Down to 4W.

The Midnight Blues is more of a 15-30W for the small power tranny depending on the output tubes. I run octal preamp tubes on my Midnight Blues.
 

oldschoolguy

Member
Messages
2,253
Yes, he's absolutely correct. The Blues is the bigger brother to the Midnight Special. I had just finished reading about it on the Siegmund cite when I responded to that remark by the op and obviously 15-30 had stuck in my head. Didn't realize that that was what I'd written. The Special only uses one Power Tube and so could not be 30. My bad.
 

oldschoolguy

Member
Messages
2,253
Thesedaze, tell me about your Midnight Blues. Could one use it effectively as a quasi-bedroom amp. I mean do you have to turn it up really loud in order to get the goods out of it? And if so, does the master thin the sound out if you use it? I don't have to have a really low volume truly "bedroom" amp but I would like a little more power than the Midnight Special ocassionally. I was thinking of Alessandro but now after hearing them, I'm thinking of the Midnight Blues more and more.
 




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