The Big Three: a nice sounding space charge tube circuit.

[brad]

I've been playing around with 12AJ6 tubes on my breadboard lately and came up with this nice sounding circuit.  I call it 'The Big Three' because a) the tubes were used in car radios in the 50's and b) there's three of them!

I initially started out with two stages that used regular cathode biasing with resistors ect, but just as I was about to add a third stage, I remembered reading about vacuum tubes being used as bias diodes in some old tube book.  I hooked the diodes in a third tube up to the cathodes without expecting it to work at all, and low-and-behold...the circuit worked perfectly!    In fact, I actually think it sounds better this way.

Does anyone have any suggestions for further improvements or can think of a reason not to leave the bias diodes in place? Here's the datasheet: link.  Credit goes to Stephane for turning me onto these tubes!

(Btw, I did finally manage to get these tubes to clip, but it didn't sound very good.)

[dano12]

Wow, that looks very intriguing! (I love simplicity). So does the circuit provide a clean boost?

[StephenGiles]

There was a great English band called the Big Three back in 62-64, check them out on the Merseybeat site.

[bancika]

How 'bout a clip?

[StephenGiles]

http://www.merseybeatnostalgia.co.uk/html/the_big_three.html

[brad]

Wow, that looks very intriguing! (I love simplicity). So does the circuit provide a clean boost?

Yes, it can be used as a clean boost although I like it just as tone colouration at unity gain.  It really improves the sound of my solid state Fender bass amp, so I think it would probably be something I'd leave on all the time.

Considering the diodes share the same cathode, I wonder if I shouldn't just link the two annodes or just use one of them to bias both tubes.  I'm not sure if I should leave the plate and grid open like that either.  From what little I can find about this type of biasing on the internet, the consensus seems to be that it has less distortion than using a bypassed cathode resistor.  I guess it has more puretube™ tone! 

This is what the tube looks like for all you tubegeeks.  They have a small 7 pin base and are about 4cm x 1.4cm, so they're smaller than 12AX7s.  Another good thing about them is that they're designed to run at their optimum at 12v.  You can get them for around US$3 each on eBay but a couple of the ones I've bought have had slipped bases, making them pretty useless for a final build unless you like the look of crooked tubes.  I've seen a range of brands available such as Tung-Sol, GM/Delco, GE, Sylvania...and the ones I have are CBS-Hytron and RCA.

[dano12]

Wow, that looks very intriguing! (I love simplicity). So does the circuit provide a clean boost?

This is what the tube looks like for all you tubegeeks.  They have a small 7 pin base and are about 4cm x 1.4cm, so they're smaller than 12AX7s.  Another good thing about them is that they're designed to run at their optimum at 12v.  You can get them for around US$3 each on eBay but a couple of the ones I've bought have had slipped bases, making them pretty useless for a final build unless you like the look of crooked tubes.  I've seen a range of brands available such as Tung-Sol, GM/Delco, GE, Sylvania...and the ones I have are CBS-Hytron and RCA.

I'd love to try a project with low voltage tubes. Apparently Tube Depot sells the 12AJ6 for around 4 bucks per. Do you have any suggestions on where to look for sockets for those? Thx!

[brad]

You can often find a variety of 7 pin sockets on eBay.  I won a lot of 10 ceramic chassis mount sockets for US$10 and they're very good quality.  Surplus Sales of Nebraska also have some great sockets as well as a range of IERC and push/twist shields for several types of 7 pin tubes ( www.surplussales.com ).  They also ship internationally.

[dano12]

Thanks much for the pointers--definatley something I want to start mucking around with.

I also found the Tube Depot sells a variety of sockets:
http://store.tubedepot.com/sk-7pin.html

12AJ7's at Tube Depot:
http://store.tubedepot.com/nos-12aj7.html

[StephenGiles]

Do try the Big Three link - as I have said before, the solo in Some Other Guy is very good for 1963. I don't think US guitarists played like that then.

[brad]

12AJ7's at Tube Depot:
http://store.tubedepot.com/nos-12aj7.html

You don't want 12AJ7s.  They're regular high voltage tubes (and pentodes too I think).  I've never tried Tube Depot, although the only reason why I use eBay for tubes is because sellers usually show photos 

Do try the Big Three link - as I have said before, the solo in Some Other Guy is very good for 1963. I don't think US guitarists played like that then.

Catchy tune! 

[dano12]

12AJ7's at Tube Depot:
http://store.tubedepot.com/nos-12aj7.html

You don't want 12AJ7s.  They're regular high voltage tubes (and pentodes too I think).  I've never tried Tube Depot, although the only reason why I use eBay for tubes is because sellers usually show photos 

Being a bit dsylexic today, sorry. Yes, it is 12AJ6. They have them at Tube Depot also. Thanks for the catch.

[rmo]

Antique electronic supply has 12aj6's for $2.20 a piece. http://www.tubesandmore.com

[brad]

Just a note: although this circuit works as it is, I'm no expert on the theory behind it and was kind of hoping a tube guru would know whether it needs further tweaking or not 

[brad]

These are some readings I took with my new oscilloscope...

Left:  input - 440hz sine wave
Middle: output of second stage before the level pot
Right: output of level pot (set at about 1/4 rotation) after overdriving the input

I guess you could interpret these results as showing that it gives quite a lot of clean boost.  The last trace is weird though...because the top of the waveform began to clip quite a while before the bottom half.  I suppose this is why the overdriven tone isn't very nice.

[gez]

To my way of thinking (flawed at best) the fact that one gets a 'space charge effect' at low plate voltages suggests that one or two electrons are surplus to requirement.  With this in mind I started starving the heaters and, sure enough, things cleaned up in this respect and the valves (sorry, it's a British thing) worked as normal (up to a point, you can't go haywire with this approach). 

With such low plate voltages I seriously doubt cathode stripping would be a concern, so why does everyone run the heaters at full blast in these circuits?  I didn't bother continuing my experiments so who knows if this will shorten life...just thought I'd mention it.

PS  Not advisable to do in this circuit as it relies on space charge for bias.

[brett]

Hi

The last trace is weird though...because the top of the waveform began to clip quite a while before the bottom half.  I suppose this is why the overdriven tone isn't very nice.

But this is an excellent waveform!  Just what a tube amp or Fuzzface gives in the early stages of overdrive.  Nice harmonics from this, for sure.

Very cool result, and an intriguing concept.

cheers 

[brad]

Thanks Brett!

Perhaps all that's needed to get it sounding better when overdriven is a few tweaks of the coupling caps.  Less low-end may open up the sound more.  Promising signs for a dedicated 12AJ6 overdrive design in the future though! 

Oh, and I've been meaning to say:  the 66K input resistor is actually two 33K resistors.  The closest standard value should be ok...33K was just the first thing I happened to pick up.

[comfortably_numb]

Please keep us updated on your experimentation.  I'm very interested in this circuit, as I have been looking at building something exactly as you described - a sweetener for my solidstate amp.  I've been looking at the real mctube, and will probably build it, but this is very intriguing as well.  Thanks very much for sharing.

[StephenGiles]

The Big Three of course used 100 watt amps built by their lead guitarist - pretty loud by 1963 standards.