Octal Preamp Amp Builds or Mods?

and THAT'S the trick. A ton of the octals I come across, at least the older ones, are noisier than I'd like. There's reasons they switched over to 12ax7 when they did.

 

You might want to take a look at Carl's Custom Amps. He builds both a Princeton and a Deluxe with the octal pre-amp tube and there's a little bit of info on his website. I've not heard one in person but have heard positive feedback from those who have. I believe Brent Jesse (www.audiotubes.com) may carry the tubes and could be a source of information. (FYI I have no affiliation with either of these business's, just another gear guy...)

http://carlscustomamps.com/octal-princeton
http://www.audiotubes.com/6sl7.htm

 

I've done two octal preamp builds that were inspired by fender designs, a Princeton and a deluxe. In my experience the difference isn't about feel so much as the tubes sounding different, a little warmer. Also keep in mind that the commonly used octal tubes have a slightly lower gain than a 12ax7, so that contributes to the sound as well. Additionally in these archaic amp designs there's more than just the tubes to consider, many have different plate and cathode resistor values than the more modern familiar circuits and they may also have things going on like grid leak bias, very small filter cap values, and paraphase inverters which will also make for a different sound than we are used to in more "modern" amps.

Finding good tubes can be a bit of a hassle but it can be part of the fun if you're into bargain-hunting tubes on eBay. If you're just going to shell out for in NOS tubes from a "reputable dealer" then it's going to cost you some money but on the other hand you can hold them accountable for getting you tubes that work well. My approach has been to buy ANOS 6sl, sc, and sj7 tubes whenever I can find them at a decent price so that I have plenty to swap out when I need to find one that will work in a particular build, I try to pay $5 or less each and I've gotten quite a few out of old radios and other tube audio gear I found cheap on Craigslist so I've got a pretty good stock at this point. Also keep in mind that certain tube positions are more critical than others and a tube that is problematically microphonic as an input may work fine as a phase inverter etcetera.

 

The 6SL7 tube is a close match, now to fit it...

I've built a champ clone with an extra socket fitted for the power tube so I can use a noval EL84 or an octal 6V6GT (only 1 tube at a time tho!)

Quite easy to do, just run short lengths wires from one socket to the other.

 

There is also the loctal socket type 7C6

I built a champ type amp from a 1950s Philco radio with loctal tubes, sound clips here
https://www.tdpri.com/threads/radio-to-guitar-amp-1st-schematic-and-sound-demo.1011298/#post-9681114

 

I'm working (slowly) on a Hammond AO-15 conversion, it'll be all octal. Here's a shot of work in progress. 6SL7 or 6SJ7 input, the two will feed tone/volume & mix back together- anyway that's the plan.

 

Will be anxious to hear of your progress on the project. Good luck!

 

The 12a_7 9 pin targeted the problems with the octals. They had the technology to build smaller tubes and they wanted/needed smaller lighter radios. (Think WWII airplanes.) With the smaller tubes the microphonics were less due to several factors, size being one of them.

They are different tubes but the new ones were designed to replace the old so it is not much more variation than we have between 12a_7 types.

I think you may be more interested in the pentode type tubes Gibson was using in the preamp. Pull up some youtube and give those a listen. The pentode can supply twice the gain of a 12ax7 so reducing gain will be of interest. Guitarists have sorta chosen with their dollars the fender/marshall sound but the blues guys might just want to give the pentodes another listen. OMMV.

 

^
This

 

If you're interested, these threads document three of my builds with octal preamps.

Octal Princeton

Octal Deluxe

And a single ended build that became something more original.

 

Octal is a tube base, introduced in 1935. It presence alone holds no significant indication as to what the tube might actually be.

So it could be any tube after that date, old or modern, glass envelope or metal.

Diode, triode, tetrode, pentode, have beam plates, pentagrid set-up.............High power low power....any sort of Gm or mu....some have an extra connection for the plate element on top

I have built several amps with pentode octals in the preamp, but never one with triode octals; which are also said to offer great results as Nickfl shows us, but would be quite different in circuitry, gain structure and resulting tone.

So what is it you are interested in?