Another ValveCaster question

[TarheelTechie]

Hi folks! I'm newish to this forum but I've been tinkering with electronics for along time.
The valvecaster is my first build from scratch. Though the schematic didn't include several things such as a by-pass switch or a on-off switch for the power, I added them.
Also I included a regulated 9v power supply, really cleaned up the hiss, with a 7809 and a 12v transformer.
Basically my question is my pedal has alot of compression. Makes for great sustain but seems to muffle the hell out of it.
What values can be changed such as a cap or resistor to fine tune the pedal?
Thanks for all help,Greg

[GibsonGM]

http://www.diystompboxes.com/smfforum/index.php?topic=63479.0    Pretty much sums up the Valvecaster!

You didn't mention what tube you're using - 12AU7?  Sometimes ppl try other tubes.  12AX7 = poo, to my ears.

You're running a tube intended to be used at a couple hundred volts on 9V.   That starves it. This places its operating point in a very narrow little slice of its curve; I would expect that massive compression would be one result of this.  The more drive, the more compression...  A way to see if anything can be done would be to raise your power supply to 18V, if you're on 9V, and note if it opens up more.   Be sure you use caps rated higher than 18V, of course 

Following the discussion linked above might reveal a few more clues for you - they've about exhausted it, I think!!! 

I'm doing one similar to this, but with FOUR gain stages (well, 3 and a follower) and running at 100volts via a multiplier.   I'll write it up when done if it's any good.

[TarheelTechie]

Thanks for the reply, Mike. Yeah I found that thread but found it a bit LONG,lol.
I am using the 12au7 like in the schematic. Guess I didn't think about the tube being starved, the whole idea behind the build.
I think a FET driven tube pedal will do better. Once again thanks for the reply,Greg

[rutabaga bob]

I don't know if it will help the compression factor, but you can use a 9AU7 instead of a 12AU7, so the heater won't be running on starved voltage.  From there, you might think about a voltage multiplier for the plates.
Welcome to the forum!      Larry

[GibsonGM]

The "starve" part is more to do with the load line for plate voltage/current...if you check out the data sheet, look at how if you pick a point at say 10ma on the Y/130V on the X, so the bias is at maybe -2V, you will have a pretty flat (but angled, yes) line to work from as your input signal causes current variations around that point....very linear.

If you use very low voltages with MOST tubes (low voltage tubes being the obvious exception), the grid lines all bunch up at the bottom and are much more 'curvy' in the low voltage range...you get odd effects as a result (too much compression being one).  Very  non-linear, unpredictable.  But often produces some pretty good OD/distortion, so we build a few things like this.  Plus, it's fun, ha ha.   Most tube circuits are run at high voltages just because of this fact of tube characteristics.

But, for advancement, I'm with rutabaga - work your way in, learn about load lines, and either multiply your voltage or get a PT...you'll surely want to learn a lot more first, though...tube work is tricky and uses a lot of potentially dangerous high voltage.  And is very challenging (read that as 'rewarding', ha ha!). 

This dude knows TONS about audio tube work - spend some time on this site!! 
http://www.valvewizard.co.uk/