VVR (cathode bias) suitable for preamp fixed bias amp

[guile]

Thanks! Will look into it and report back.

[guile]

Martin, would it be possible for you to draw the above in the layout I posted earlier?

[martin manning]

Electrically it would go like this, but you could locate the 100n cap/1M resistor pairs near the tube sockets.

[guile]

Wow thanks Martin, what service!

[guile]

It works flawlessly! Completely silent and no popping sounds from TB pedals.

Do any of the named parts need to comply to a certain standard besides the values?

[martin manning]

I would just use good quality film caps and metal film resistors. The voltage rating does not need to be very high since they are on grid leads, but I would choose 200-600V caps.

This is an interesting experiment. Do you like the sound you are getting? Does it change dramatically from low to high voltage settings?

[guile]

I would just use good quality film caps and metal film resistors. The voltage rating does not need to be very high since they are on grid leads, but I would choose 200-600V caps.

This is an interesting experiment. Do you like the sound you are getting? Does it change dramatically from low to high voltage settings?

I will use orange drops then. Can you explain why you suggest 100n for de cap?

I like the sound that I'm getting a lot. I can only test at room level, but I like it so far. It's so much more lively. My pedals also all sound amazing.
Concerning the high- and the low voltage, there is about the same difference; no dramatic difference.

[martin manning]

The value of the caps is large to avoid rolling off any low frequency content. You might not notice any difference using 47n or even 22n caps; try it and see.

[guile]

The value of the caps is large to avoid rolling off any low frequency content. You might not notice any difference using 47n or even 22n caps; try it and see.

Thanks Martin. Please explain 'low frequency content'.

[martin manning]

Low frequency content is bass notes. A cap-resistor combination as used here to keep DC off of the guitar pots and amp volume pot makes a low-pass filter, which will have the effect of reducing the bass as the value of the cap is reduced.

[guile]

Low frequency content is bass notes. A cap-resistor combination as used here to keep DC off of the guitar pots and amp volume pot makes a low-pass filter, which will have the effect of reducing the bass as the value of the cap is reduced.

Thanks, less complicated than I thought.
I used 22n orange drops because I had them lying around. It sounds great, but I wouldn't want to change the tonal character of the amp. Do you think 22n is too much or shouldn't it matter? I tried 100n, 68n, 33n and 22n and I couldn't hear any difference. But I tested at room-level, so could be different in live situations.

[martin manning]

You can make a quick calc of the frequency where such a filter will reduce the power level by half: 1/(2*Pi*R*C), or 159155/(k ohms * nF). For a 1M resistor and 22n cap, the so-called corner frequency is 7.2 Hz, but the roll-off actually begins a couple of octaves higher, so around 30 Hz. I think you are fine using 22n.

[guile]

Thanks Martin, impressive stuff!

I read somewhere 'not to use a cap of lower value than the largest coupling cap in the amp circuit'.
Is this true?

[martin manning]

You can't generalize like that. in each specific instance the surrounding circuitry affects the frequency response.