Some more 9V valve/tube follies - a DC-coupled cathode follower

[samhay]

Johan's recent thread on a hybrid valve push-pull OD got me playing with a 12AU7 on the breadboard again.
http://www.diystompboxes.com/smfforum/index.php?topic=107868.0

In the aftermath, I thought I would share a couple of experiments that might have some potential. This one uses a single gain stage with a DC-coupled cathode follower. I am not terribly fond of low voltage valve designs, but this does something that is quite hard to achieve with solid state  - the compressed-top-of-the-peak thing (http://www.valvewizard.co.uk/dccf.html). It doesn't have much gain, but the clean tone is very nice and it overdrives fairly well. V1A uses a mixture of cathode and grid leak bias and the input impedance seems to be much higher than I was expecting, at roughly 1M.

I spent quite some time trying to get as much gain out of it as I could. Bootstrapping the anode resistor didn't help much in this department, but a cathode bypass cap on V1A works nicely. The schematic shows how I have been playing with positive feedback to give it a much more overdriven sound (doesn't add much volume as we soon run out of headroom). Not sure if it is a keeper, but might be a talking point.

No audio at the moment as I am away from the breadboard for a few weeks.

[samhay]

Back to the breadboard and have added a scaled Fender 5F2-A (Princeton) tone control and tweaked a few component values. The schematic has been updated.
Think I will box this up, but will do point-to-point, so no layout.
Will post audio if anybody is interested.

[peterg]

Looks interesting. Please post some audio.

[samhay]

OK - will do.
In the mean time, here is a capture of the output waveform (driven with a 440 Hz sine wave) when the feedback is dialed back.

[amptramp]

Judging by the thin part of the waveform at the bottom and the fatter part up top, I would say you are adding a fair amount of even-order harmonics to the signal.  The feedback control should give you some control of it.  It looks like you are on your way to a useful circuit.

[samhay]

That's the idea - the DC-coupled cathode follower is supposed to be good for this*, and it seems to work at 9V. I tried loading the follower down more with a smaller cathode resistor to get more distortion - it works, but you lose a lot of volume too.

Note that I am using postive feedback, so this is not going to reduce the distortion. With it dimed the waveform gets pretty ugly! I'm still not 100% about keepting this feature, but it is the only way I could get a decent amount of overdrive out of a single gain stage.

* I owe a lot of thanks to Merlin's writing on the subject: http://www.valvewizard.co.uk/dccf.html

[samhay]

Looks interesting. Please post some audio.

Sorry - it took a little longer than promised, but here is a quick clip.
First with feedback turned up pretty high, then with no feedback. Recorded straight from guitar into the valve OD, then into a simple cab sim before an audio interface. It's still on the breadboard, so may quite a bit less noisy once boxed up.

https://dl.dropboxusercontent.com/u/11996927/valve_OD_0.mp3

[samhay]

Updated the schematic to the version I have now built. Only change is the on-off-on switch that can switch in cathode bypass, feedback, or neither. Nice and versatile. 

[kingswayguitar]

great sustain - must sound good through a 4x12
you drop the overdrive at 0:50 i believe - still quite nice

[samhay]

Yeah the positive feedback gives you a lot of sustain, which can be fun to play with, and it does indeed sound a lot more interesting through a decent amp/cab that direct, as recorded.
As there is not much of a volume boost, it could be a nice feature to make selectable with a second foot switch. The cathode bypass cap must be switched out at the same time, but this can be done with a DPDT foot switch quite easily.