Valve caster as a preamp. (caps and vu-meter questions)

[Beros]

Hi..

I'm thinking about building a valve caster to put in front of my amp. The main reason for this is that i record with an isolated speaker cab, and that causes the tone to get somewhat boomy and bassy.

I'm thinking of putting a 6 position rotary switch in the circuit to make it bassier or treblier.
I've looked at Anon's mod in the third post in the thread:
http://www.diystompboxes.com/smfforum/index.php?topic=63479.2140
Is this the best way to make the bass treble response switchable? How about input and output caps? Why not change them? What's the difference?

I'd like much headroom. Will powering the circuit with 18v give it more headroom? Or maybe lowering the input resistor?



Thanks!
Beros

[petemoore]

I'm thinking of putting a 6 position rotary switch in the circuit to make it bassier or treblier.
  Perhaps a 2pole 3 position suggestion, this way 2 values can be switched simultaneously ?
   Like a 3 speed bike, if the gears are the right ratio...
  Otherwise 6 selections at 1 position...
I've looked at Anon's mod in the third post in the thread:
http://www.diystompboxes.com/smfforum/index.php?topic=63479.2140
Is this the best way to make the bass treble response switchable? How about input and output caps?
  These two or any other staging capacitors in the signal path [Signal-C-Path]do what capacitors do in any position: rising impedance with lower frequency, signal going through them equates to HP filter.
  Why not change them? What's the difference?
   Can't argue, I did some cap-swappin' to tighten bass [smaller staging values and cathode capacitor tweeking.
  I'd like much headroom. Will powering the circuit with 18v give it more headroom?
  Starved plate = loss of headroom = low plate voltage.
  Or maybe lowering the input resistor?
  There's an input cap pulldown resistor, 1meg presents only a small load to what feeds the circuit.
   

[Beros]

  These two or any other staging capacitors in the signal path [Signal-C-Path]do what capacitors do in any position: rising impedance with lower frequency, signal going through them equates to HP filter.
  Why not change them? What's the difference?
   Can't argue, I did some cap-swappin' to tighten bass [smaller staging values and cathode capacitor tweeking.

Thanks.

But, what would the technical and audible difference be if i swapped the input cap, versus the output cap, or as in Anons mod, I add a cap to to pin 3 on the tube socket?

Are there any drawbacks with any of the ways?

[FiveseveN]

I'd like much headroom.

More headroom for your current gear or more headroom than a stock Valvecaster?
For the former, adding a gain stage would be counterproductive. You can "fake" extra headroom by using an expander.
For the latter, yes, a higher supply voltage is one way to go. But it seems that what you need is a simple high-pass filter and there are many ways to go about using one. You could even use a mic with lower bass response in your iso cab. Or ad a cap before your amp's input (would probably fit inside a jack plug).
You really need to give us more information about your rig and how you use it.

[ultran8]

I JUST PUT A SWITCH ON MY VALVECASTER FOR FAT AND SKINNY . YOU CAN CHECK OUT THE WAY IT SOUNDS ON YOUTUBE AT EFFORTEQUALSRESULTS. RUNNIN MINE WITH A 12AX7 AT 12 VOLTS . DO TRY MANY TUBES . EACH ONE WILL DO SOMETHING WAY DIFFERENT TO THE PEDAL. MAY TRY THAT TWO BEFORE U GET IT ALL WIRED UP WITH A ROTARY.

[Beros]

I'd like much headroom.

More headroom for your current gear or more headroom than a stock Valvecaster?
For the former, adding a gain stage would be counterproductive. You can "fake" extra headroom by using an expander.
For the latter, yes, a higher supply voltage is one way to go. But it seems that what you need is a simple high-pass filter and there are many ways to go about using one. You could even use a mic with lower bass response in your iso cab. Or ad a cap before your amp's input (would probably fit inside a jack plug).
You really need to give us more information about your rig and how you use it.

Yeah I meant more headroom than a stock valvecaster.

My rig is LPB-1 to 18w marshall to celestion loaded isocab with two SM57, to mic pres and cubase.
I use the lpb1 to push the amp into more overdrive when soloing. I've tried an old MXR EQ after it to get rid of some of the bass, but it's too noisy. And a EQ in cubase still leaves the sound muffled.

I would like to be able to use the valvecaster both as a variable lo pass filter, and as a booster, whenever i'd like one. Maybe I'll add the tone control to make it high pass as well...

[Steben]

You could use lower gain tube (fe. 12AT7).

12AX7 > 12AU7 > 12AT7
(100) > (60) > (20)

It will give as much raise in level (max 400x), but less distortion

As I mentioned earlier on this forum, high gain tubes as 12AX7 / ECC83 are used in amps with +200V headroom.
When using lower voltage (less then a tenth) youl will easily get loads of distortion if you use the same gain settings.

[CynicalMan]

Steben, you have the 12AU7 and 12AT7 mixed up. The 12AU7 is has the lower amplification factor (mu), of around 20. The 12AT7's is around 60.
http://www.nj7p.org/Tube4.php?tube=12AU7
http://www.nj7p.org/Tube4.php?tube=12AT7

The Valvecaster is meant to be an overdrive, with low headroom. Maybe you want something more along these lines:
http://www.diystompboxes.com/smfforum/index.php?topic=71381.msg578340#msg578340
http://www.diystompboxes.com/smfforum/index.php?topic=76356.0