Finally getting around to ordering parts.... but I am green as can be...

[barrymclark]

I have always wanted to build an amp. The one I want to build really isn't a big deal. Just an oversized Champ really.

But, my question at the moment regards the resistor at the input.

The schematics for the Champ 5F1 and the Vox AC30 (one of my favorites) call for a 68k right after the input. Another one of my favorites, a Gibson BR-1, calls for a 100k. As elementary a question as I am sure it is, what does that value difference mean with regards to the integrity of the system and, further, the tone? Is there a min/max value that can be used?

Thanks!

Barry

[nickb]

It's there primarily to suppress radio interference. 68k - 100k range is typical. 69k-10 Meg is the range but you'll loose high frequencies as he value gets bigger

[echuta13]

I personally just use a 10K resistor there. Some do not use anything.

[barrymclark]

So then is it fair to say that the greater the value, the greater the radio interference resistance and the opposite true for lower values? Also, might the exclusion or lower values mentioned - 10k to no resistor- be due to new instrument cables and guitar wiring being more resistant to radio interference?

Thanks!

[woodyc]

...69k-10 Meg is the range but you'll loose high frequencies as he value gets bigger

No kidding.

The grid stopper forms a lopass filter with the input capacitance of the stage. The -3dB point of the filter you're creating is f = 1/(2*pi*R*C).

The relevant capacitance is the grid-to-plate value you can look up on the tube data sheet times the gain of the stage. (The gain dependance is called the Miller effect.)

For a typical 12AX7 the effective capacitance is about 100pF so you want something bigger than 10K to cut AM radio. If you go bigger than 82K, you start cutting audio frequencies.

[pdf64]

With the 'classic' two input sockets per channel arrangement, when the main / high sensitivity input only is used, the two 68k resistors are switched into parallel, resulting in an effective resistance of 34k.
This resistor is a source of noise, and the noise reduces with its value, hence ~10k with high gain amps; there's been an excellent recent thread on this topic http://music-electronics-forum.com/t30275/
Pete