The Huffer: space charge tube buffer.

[brad]

...I was hoping to get some more testing done before posting this, but what the heck.  I call it a "buffer" because it only produces a small amount of boost by itself (well, not enough to be called a boost pedal anyway).  If you're thinking it looks almost identical to the first stage of The Big Three circuit I recently posted...that's because it basically is!  After getting my hands on a bit more info about space charge tubes, I found the suggestion that cathode biasing isn't needed at all on these tubes and that the cathode should be grounded along with "a large resistor from the grid circuit".  It sounds really good to my ears, but I'm open to any suggestions for improvements!  I'd like to turn it into a real pedal, and will probably use an internal trimmer for the output pot.

This is what happens if you drive the input with a boost.  The pic on the far left is the output of the circuit with just a guitar level input signal...and each successive pic shows what happens when the input signal is boosted by 3dB...all the way to 15dB:



(The first waveform only looks weak because I had to use a less sensitive setting on my oscilloscope to make sure the final waveform didn't go off screen.  In reality, the unboosted output waveform is about 3 times larger than the guitar signal)

[brad]

Oh...I forgot to mention:  the tube is still a 12AJ6.

[Peter Snowberg]

Very nice!

One suggestion to try... change that grid stopper to 33K or even lower. It's there to prevent oscillation. With such low Mu, there isn't the same need for a stopper.

Amps with two input jacks (hi and low) which use 68K resistors on the input (like Fender amps) generally have two of them. One jack places the two 68K resistors in parallel, connecting to the grid, and a 1M heads to ground for low level signals while plugging into the other jack connects one 68K to the grid and the other 68K to ground so you get a low impedance input and only 50% of the signal hitting the tube.

[brett]

Hi.
I agree with Peter.  The tone of my Marshall 18W pre-amp is excellent without a grid resistor; just 470k holding it to ground.

RG's comments about grid conduction in his RangeMaster article seem to indicate that boosters with low output impedance might get better tone if there's no resistor to reduce the grid current.  At least, I think that's what he says.  (I'm a tube newb)
cheers

[brad]

Thanks guys!  I'll definitely have to experiment with the grid resistor over the next few days then.  I would have thought removing it altogether may allow radio frequencies through, but if not...then I suppose that's one less component to worry about!    Can't argue with Jim Marshall!

Another tweak that's suggested in nearly every 12AJ6 datasheet is to use a 1M plate resistor with a 1M grid resistor to get a gain of 16.  I tried it, but I think I was so used to the boosted tone that I didn't like the change.  It seems like there are so many possible variables that it's hard to have confidence in any one of them being "right"...even when they sound good.

May I also add that I'm in gratitude to Stephane from sopht.ca for introducing me to space charge tubes.

[bradd lustfünd]

Well...I tried grid resistors of 66K, 33K, and none...and this is what I heard (using my trusty p-bass):

None - bass heavy muffled tone similar to rolling off a tone control
33K - a good balance of lows, mids, and highs.  A "full tone" if I may say.
66K - less lows and more of a "stringy" sound.

I wasn't expecting the tone to change just by messing with the grid resistor, but a switch would be useful to swap between 33K and 66K because they're both useable tones.  The volume didn't seem to change, although I'll check with my scope later.

[brett]

Hi

I wasn't expecting the tone to change just by messing with the grid resistor

Good results.  I'm surprised too, that the difference is so much.  What type of pickups are you using?

[bradd lustfünd]

The bass is a made in Mexico Fender P-Bass...and the pickups are whatever the stock ones are.  The amp I'm using is a solid state 100w Fender BXR combo with a 15" speaker.  I'll start thinking about a layout and how to box this circuit up soon.  I don't know how it would sound with other setups or on guitar, but it makes my rig sound way better.

[bradd lustfünd]

I've added the final version to my journal and added a pair of new traces showing the output during normal use.

Now I can finally rest