Whats an octal tube?

Bud Angelotti · Post by **Bud Angelotti** » 3 Jun 2013 5:19 pm

I learn so much here about electronics, as well as loads of other stuff, mostly stuff i'll never use or do, but it's very interesting either way.
Can anybody tell me what is an octal tube and whats the difference between an octal and a "standard" tube. Whats the difference in sound and why is the octal tube "better".
Thanks in advance.

Craig Baker · Post by **Craig Baker** » 3 Jun 2013 5:40 pm

Bud, the term "octal" refers to the tube's 8-pin base connections. Some common octal tubes would be a 5U4, 6L6, 6V6, 6SN7 etc. They're were hundreds of high quality octal tube types and they remained popular up until miniature tubes came along, such as the 12AX7, 9-pin miniature, or 6AQ5 7-pin miniature, etc. The octal tubes are still hard to beat.

Craig

Greg Cutshaw · Post by **Greg Cutshaw** » 3 Jun 2013 6:00 pm

Wiki has excellent descriptions of all types of tubes!

6SN7 Octal:

http://en.wikipedia.org/wiki/6SN7

Tubes in general:

http://en.wikipedia.org/wiki/Vacuum_tube

Greg

Tim Marcus · Post by **Tim Marcus** » 3 Jun 2013 7:47 pm

octal tubes are not better - but they are different.

Fender stopped using them in their amplifiers in the mid 50's because they were much less reliable than the new 9 pin tubes that were being made by RCA. For years 9 mini dual triode tubes were the standard. These are tubes like 12AX7, and the like.

Many years later - long after RCA stopped making tubes - 9 Pin tubes became very valuable to audiophiles and electric guitar players. They were buying up the old stock, hoarding, using them in amps (ie, wearing them out) and driving up the price.

The 8 pin octal tubes then became an inexpensive alternative. Many could be used in the same circuit with just a socket adapter. A 6SL7 has the same gain as a 12AX7 but can be had for 1/3 the price.

Some amp makers started using the 8 pin tubes again. Some companies (Sovtek in particular) are even producing the octal tubes again.

However, the engineering of the octal tube has not changed. They are still just as problematic as they were in the 1950s in guitar amplifier circuits. There are a few reasons why most amplifier manufacturers avoid them:

1. microphonics - because of the large assembly inside of the bottle, the tubes are highly sensitive to vibration. Any amp over about 5W will degrade them quickly compared to the 9 pin tubes.

2. Noise - octal tubes do not have a center tap for the cathode heaters like the 9 pin mini tubes do. (that is actually Pin 9 on a 12AX7 - a humbucking filament works like a humbucking pickup in a guitar)

3. Shared cathode - some octal tubes, the 6SC7 in particular has a shared cathode which means that you can not split the tube into 2 halves and put a tone stack in between them. This leads to some rudimentary designs.

Obviously, I am a bit biased against them, but its only because of my experience with them.

In certain applications they do ok - in a low gain setting where there is DC on the heaters like the Sarno preamp for example. However, in an amplifier with high wattage (especially a combo amp where the tubes are near a speaker) they are best avoided.

Some think that they sound better and are more sensitive to touch, but I do not agree with that. I think that the output section of a tube amplifier is much more important than the preamp section for touch sensitivity. Many of the octal based amplifiers are cathode biased on the output side and this is what gives the amp the "feel"

Bud Angelotti · Post by **Bud Angelotti** » 4 Jun 2013 5:06 am

So, they are a physically larger, older technology that has benefits as well as drawbacks in their use with "modern" electronics, yes?

Len Amaral · Post by **Len Amaral** » 4 Jun 2013 5:16 am

Tim Marcus wrote "The 8 pin octal tubes then became an inexpensive alternative. Many could be used in the same circuit with just a socket adapter. A 6SL7 has the same gain as a 12AX7 but can be had for 1/3 the price."

I recently bought the converter socket for 8 pin to 9 pin phase inverter and preamp socket. Had two bad experiences with a new amp with the 8 pin octal tubes and it's a crap shoot buying new tubes. Haven'y tried the new sockets yet but plan to do so in the near future.

Craig Baker · Post by **Craig Baker** » 4 Jun 2013 5:17 am

Yes. Old guys like us still like them. I guess it's fun to open the lid of a short wave set and see them glow, and they do keep the room warm on a winter evening, but truth be told, Tim's post is 101% accurate.

Ken Fox · Post by **Ken Fox** » 4 Jun 2013 5:28 am

I have a Gibson GA-75 (1952) with all the classic octal tubes mentioned. Tim is right on money on most everything. I feel that the amp has a total different touch sensitivity than anything else I have used. On it it's own it is very warm, low power (about 20-25 watts with 2 x 6L6GC power tubes)amp. Not an amp I want to use without one of my Fender clones teamed up with it. I use it and A 1965 Princeton reverb clone for guitar. I love the combination of the two.

You can see Andy using one and a 1953 (I think) Fender Bassman on some of the early Time Jumper videos.

http://www.youtube.com/watch?v=MRU8CjjE054

I like Brad's idea and have also heard his new octal preamp at the Dallas Show, it rules! You can get the warmth and use a separate power amp for the power needed for your steel or guitar rig.

Is this everyone's idea of the perfect steel amp? No. I have spent many hours talking with the pro players and I can say each have their signature tone and amplifier needs. You gotta find find what works for you, not what works someone else. Lots of great choices out there, but make your own choice!

Olli Haavisto · Post by **Olli Haavisto** » 4 Jun 2013 6:07 am

I bought an adapter on ebay and a new TungSol 6SL7 so I can use an octal in a 9 pin slot.
It does sound more like a hifi tube to me in my custom amp`s Fenderish channel.
A lot different than a 10 dollar 12ax7 but not not that much different than a pristine early 70`s RCA 12ax7.No microphonic problems so far, a little hum, though. An extender is also needed for Fender amps and similar with a "wall" around the 9 pin socket. A cheap way to get a glimpse of the octal experience....

Brett Lanier · Post by **Brett Lanier** » 4 Jun 2013 6:31 am

Olli Haavisto wrote: It does sound more like a hifi tube to me(...)

I'll agree with that. When A/Bing the two types of tubes in the exact same amp, I hear a more broad EQ spectrum with the 6SL7's.

I also strongly agree with Tim's comments about the power section of an amplifier. A good power amp can make all the difference.

John Limbach · Post by **John Limbach** » 4 Jun 2013 8:16 am

This is not meant to disparage the originator of this post, but it instantly reminds me how old I am when that question even has to be asked.

First time I knew I was "old" was a couple years ago when my grandson was driving me berserk in the car saying the same thing over and over. So I said to him, "Jesse, you're just like a broken record". A long silence from the back seat and then he replied, "Grampa, what's a record?"

Ah yes, Tempus Fugit.

Post by **Dave Mudgett** » 4 Jun 2013 8:41 am

Purely my experience with them - octal tubes are different, for sure. Better or worse is strictly application and personal taste. I've owned a bunch of guitar amps over the years with octal tubes, typically early Gibsons, tweed Fenders, and some Valco-made amps. I personally like them a lot for certain things. I don't think most modern amps can touch those early amps for that old smoky jazz guitar sound. They're great for blues guitar or harp. They can also tame the brightest Telecaster - I currently have a '53 5B5 Fender Pro that just tears it up for country music with a bright Tele that just spews shards of glass with, let's say, a Twin Reverb or even a Deluxe/Vibrolux Reverb. My 5D3 tweed Deluxe sounds very different. Circuit and speaker differences, I know, but I've messed with octal-to-miniature-9-pin converters and run each amp into the other's speaker, and the tubes really do sound different back-to-back in the same amp.

Getting quiet, non-microphonic octal tubes can be a bit of an issue. Most of the originals were metal-can tubes, and they can be real noisy and microphonic. If they're cheap, I'll grab a bunch and sort through them, but that's only useful for buying at a hamfest or something like that where the prices are realistic. You tend to pay a premium for the glass-bottle tubes, which can still have issues, as Tim mentions. All of this of course limits the gain possible in such an amp. But that doesn't bother me in the slightest - when you use octal tubes that warm up the sound along with one of the crude early phase inverter designs, you don't really need to crank the gain to get the sweetness. To me, the combination of the octal preamps with simply turning the volume up on one of these simple circuits gets all the juice I'll ever need. No, it's not gonna give you Marshall juice, but neither is a later Fender. It is its own thing.

To me, it's just another sound in the palette, and a damned useful one to have. As Ken mentions, these old octal-preamp amps mix in fantastically with a modern amp. I love mixing, let's say, an old octal-preamp Fender or Gibson with a Princeton or Deluxe/Vibrolux Reverb. My main guitar amp rig for years was an early Pro (or Magnatone 260, not octal, but a similar vibe) run parallel with a late-50s tweed Tremolux or blackface Deluxe/Vibrolux Reverb. The old Magnatone was one miniature-preamp-tube amp that had that more-midrangey hi-fi sound that mixes fantastically with the later more scooped-mids sound, and of course then you have the Magnatone vibrato. For lower-volume, I'd use a small old Gibson and a Princeton Reverb.

I probably wouldn't use one of these early octal amps for pedal steel, they just don't have the glass I want. But I've heard Paul with his Little Walter live several times, and it really does sound fantastic, obviously there are some important design changes. I'd go so far as to say the best I've ever heard him use - it adds a level of sweetness that just works, to me. Of course, I don't think you can beat an early Fender Pro or one of the early big Gibson 15" amps with octal tubes for console steel. Those amps just flat out nail the sound I'm looking for.

Different strokes, sometimes the same folks. I like a lot of different approaches to sound.

[Edited - I misread Ken's point about Andy Reiss, yeah that's an old Gibson, looks like a big ole GA-75 indeed. That and the octal-preamp Pro and Bassman are just fantastic jazz guitar amps.]

Steve Lipsey · Post by **Steve Lipsey** » 4 Jun 2013 9:53 am

Don't discount Tim;s comment about a lot of the touch sensitivity coming from bias choice....many of those old amps (and some modern amps, esp. octal ones) were cathode biased, and most modern amps opt for the increased power of fixed bias....

Tim built my Milkman Sideman with a bias switch, and the cathode bias side definitely has a very different feel from the fixed bias side....a little more compressed, more responsive to touch....the fixed bias side is more crisp (esp. on those low bass notes), more headroom....both have their moments...

Tim Marcus · Post by **Tim Marcus** » 4 Jun 2013 10:23 am

and the Dan Lurie FYD amp has octal preamp tubes with a fixed bias output... so there you go! Plenty of ways to do it.

also - no one has mentioned this but most power tubes are octal. 6L6, 6V6, 6550, KT88 etc... they all have an octal base but are pentodes.

In a power application, noise and microphonics are not nearly as present. Also, in a push pull amp the tubes are run out of phase so any hum from heaters would get cancelled out.

Honestly, I tried octal preamp tubes in my 85W pedal steel amp model, and after about 5 minutes of playing they were nearly destroyed. An expensive experiment. However I had a lot of steel players ask me about them because of other amplifiers marketed to steel players that use them. The difference in sound was slight, but the difference in performance (in a combo amp) was not slight. Since I primarily build grab and go amplifiers, I will not use them.

That said - if you have time and resources to roll enough octal tubes through a combo amp you will eventually find a set that will hold up. At least for a little while

Len Amaral · Post by **Len Amaral** » 4 Jun 2013 11:04 am

I also like Brad's Idea of an Octal preamp and use your power amp of choice to push the sound out at whatever volume you are looking for. I have a Tonic preamp, several Revlation preamps, SMS Classic a two Black Boxes. I play musical chairs (pardon the pun) with these preamps and often find a different color that with different power amps/speakers,etc.

BTW, I have one of Brad's Octal preamps on order

John Billings · Post by **John Billings** » 4 Jun 2013 12:23 pm

My tech made me some adapters for my '52 Pro Amp. We put them in, with really good 12AX7s. I played for about a minute. We looked at each other and said, "No good!" Back to the octal pres. Amp sounded much better with them.

Bud Angelotti · Post by **Bud Angelotti** » 4 Jun 2013 6:32 pm

Well, as I said in the original post

I learn so much here about electronics, as well as loads of other stuff, mostly stuff i'll never use or do, but it's very interesting either way.

Just learned some more stuff from ya'll.
Thanks!

PS - You're all invited to the beach in the fall.
We'll catch some bluefish! Yummy!

Ken Fox · Post by **Ken Fox** » 5 Jun 2013 5:56 am

Her is a nice demo of the Gibson GA-75. It will go from clean to a very nice distortion. I also use the microphone channel for guitar. 6SJ7 on the mic channel and 6SC7 for the remaining preamp tubes. 6L6GA power tubes and 5U4G rectifier

http://www.youtube.com/watch?v=Ln1SJYFjp3E

Len Amaral · Post by **Len Amaral** » 5 Jun 2013 8:55 am

Update on octal sockets that convert to standard preamp tubes. The ony thing I can say and it's my opinion along with a friend who is an experienced technician is
very impressive.

Changed the Phase invertor 8 pin socket octal to a 12AT7 and the Preamp 8 pin octal socket to a 12AX7.

Result: Much more headroom and IMO more usable tones and still touch sensitive.
I am not going back to the standard octal tubes in this amp and although I
bought it for steel it will remain a guitar amp. I also changed the 5u4
rectifier tube to a SS module.

Bud Angelotti · Post by **Bud Angelotti** » 5 Jun 2013 12:29 pm

So, with my very limited knowledge of this stuff, it seems that these octal tubes generally have a wider dynamic range, if thats even the proper term.
At the same time, since they are an older technology,
they can be "pursnickity." check my spelling on that one
So, and correct me if I'm wrong, this is part of the evolution of the tube. Some folks really like the old stuff. Thats cool!

Donny Hinson · Post by **Donny Hinson** » 5 Jun 2013 1:54 pm

I think the octal technology was more expensive, plain and simple, and that's why it went away. The 7 & 9 pin miniatures required no bakelite base, no hollow pins that need forming, staking and soldering, and all their internal elements were far smaller (read: less metal, and less glass, too). One last benefit was cheaper shipping and storing, due to the reduced size and weight.

The low noise and high reliability aspects first took the form of the metal envelopes in the octal design, and later as the loctal types, with their designed-in retention ring center post. These found wide use in car radios before transistors, and it took many years for the solid state radios to approach the fidelity of the tube sets.

Tim Marcus · Post by **Tim Marcus** » 5 Jun 2013 2:08 pm

also, you can find a small mini dual triode with a flat frequency response like an octal tube.

12AY7 would be a good choice for that

Another history lesson - the midrange dip on a Fender amp is there not because it sounds good, but because it matches the midrange spike of the 12AX7A or 7025 tubes that RCA was making at the time.

12AX7s have color - they make an electric instrument sound alive - but they are not traditionally used in hi-fi amps. 12AU7, 12DW7, 12AT7... these are all commonly used in applications where the amp should remain flat

Dan Tyack · Post by **Dan Tyack** » 7 Jun 2013 7:31 pm

I'm not an amp builder or an expert on antique engineering, but I have used many, many tube amps over my career.

In my recent experience with octal based amps (several Little Walter amps and Brad Sarno's new octal pre) I have noticed a real difference compared to the 9 pin based designs that I've used. They sound great. I'm not going to try to describe it, but I know my ears, and there is a difference, even compared to lower gain 9 pin tubes.

In terms of potential issues:

Reliability: I've used the same preamp tubes in my LW 50 for 2 years. I have spares, but so far, not needed.

Noise: The LW amps and Brads octal pres are some of the lowest noise amps I've ever played through. I did have a very slightly microphonic tube initially in my LW 50, but I've also seen this in other 50s NOS preamp tubes.

It could be that there are issues with using octals with preamp designs that are 9 pin based, but both the LW and Sarno designs are octal based, and I haven't seen any issues.

Tim Marcus · Post by **Tim Marcus** » 7 Jun 2013 9:13 pm

I believe that

I think that the issues begin when the amp is in a combo setup, as opposed to a separate cabinet for the head and speaker. Same goes with Brad's preamp - there is usually some distance between that box and a speaker.

Fender style amps, where the tubes hang below the chassis and next to the speaker will destroy even the best octal tubes and make quick work of it. Especially an amp over 50W.

The Sarno preamp has DC heaters - not sure if the LW does or not - but if you use DC on tube heaters the result is dead silence other than some minor hiss when the gain is wide open.