Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RobBozic on January 15, 2013, 08:15:27 pm

Title: Frequency response calculation for 470k / 470k voltage divider
Post by: RobBozic on January 15, 2013, 08:15:27 pm

Hi gents,

Can anyone tell me how to calculate the frequency response when you have 470K / 470K voltage divider, or 470K / 470K mixer resistors, as in the plexi Marshall .

For a 1 meg volume, & a 2.2n cap it comes out at 69.7 Hz (1/2*pi*[1Meg + 38.5K]*0.0022)

Do I just calculate it with the 470k to ground?

Thanks for your help.
Rob

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: HotBluePlates on January 16, 2013, 12:15:08 am

Do you have a schematic to point to?

I ask because the resistance used (for an accurate calculation) is not just 1 resistance. But I think telling you what to use will be confusing outside of a concrete example.

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: RobBozic on January 16, 2013, 02:26:16 pm

HBP,

My amp is similar to the 6G16 Fender Vibroverb. Between the 2nd & 3rd gain stages there's a 470k/470k network which mixes the dry & reverb signals before the 3rd gain stage.
 The 3rd gain stage is biased like the Komet 60
Thanks
Rob

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: PRR on January 16, 2013, 08:57:43 pm

We could study fine detail of this network for weeks.

If you want to check the 0.022 cap:

The Grid is "infinite" (>100Meg), pretend it isn't there.

The reverb line is zero to 100K depending on pot setting.

The source is a 12AX7, a 82K and a 22K. This is essentially 19K.

So the 0.022 is in relation to 959K to 1,059K.

Which is all 1Meg, pretty near.

I can remember that 0.01uFd into 1Meg is 17Hz.

So twice that (0.022) is 8Hz.

Using math I get -3dB at 7.5Hz, -1dB at 15Hz, -0.5dB at 30Hz.

Which means this rig is FLAT to far below guitar, even for bass, and well below any loudspeaker you can carry.

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: HotBluePlates on January 16, 2013, 09:14:57 pm

Right.

The point I wanted you to see Rob is that the "R" which you use in the calculation is all resistance seen by a.c. from one leg of the cap to the other. So that includes resistances before and after the cap which go to ground or "a.c. ground" (a filter cap), as well as parallel paths presented by tube internal resistance.

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: RobBozic on January 17, 2013, 12:22:12 am

Thank gents,

I'll go through the math again.

So, at what frequencies should you trim the gain stages to? Low E is 82Hz, so if you trim to 82Hz for all gain stages the amp should still sound full and not flab out?

Thanks
Rob

Title: Re: Frequency response calculation for 470k / 470k voltage divider
Post by: tubenit on January 17, 2013, 05:29:31 am

I left the vibrato in, but you can easily edit it out.  Here is a SCH editable version of your schematic to use for your final version.

with respect, Tubenit