Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: octal on February 22, 2016, 07:36:35 pm

Title: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: octal on February 22, 2016, 07:36:35 pm

I'm trying to use the AMZ tone control circuit (see attachment) - it's pretty much the same style as a "bigmuff" tone control or a Supro Thunderbolt tone stack, but with the values tweaked to allow a mid boost hump. The circuit is designed to work with a solid state source with a 1K ohm output impedance. I'm trying to drive it with a circuit with an output impedance of around 7k. (Using a triode that's roughly like a 12AU7)


Can anyone explain how to tweak the circuit values to make the response the same with the higher source impedance? I get (or at least I THINK that I get) that this circuit is sort of a first order high pass filter and a first order low pass filter with their outputs summed through the "tone" pot, but I'm struggling to put it all together in my mind. I played around with values in the Duncan tone stack simulator, but everything is so interactive that I'm floundering.  I'm guessing that the mid hump comes from from spreading the corner frequencies of the high pass and low pass filters so that just the mids come through.

Title: Re: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: jjasilli on February 22, 2016, 08:46:15 pm

Looks like you'll need a cathode follower to drive the tonestacjk.  See, e.g.,:  https://www.ampbooks.com/mobile/amplifier-calculators/cathode-follower/ (https://www.ampbooks.com/mobile/amplifier-calculators/cathode-follower/)  You'll have to delve into how to calculate component values.  Looks like a 12ax7 can get to about 617Ω which is getting in the 1K ballpark.

Title: Re: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: octal on February 23, 2016, 07:17:13 am

That's a good idea, I had considered using a CF, but would prefer not have to use an extra triode section if possible....  it seems like there has got to be some way to scale up the values to make the stack easier to drive with the same freq response. 

Title: Re: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: jjasilli on February 23, 2016, 08:17:56 am

I assumed you wanted a tube approach.  There are some posts on the Forum re "mosfet followers", where a transistor driven by a tube is used in place of a tube cathode follower.  Probably, a pure SS circuit, or hybrid tube-SS circuit will work.

Title: Re: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: PRR on February 23, 2016, 04:14:01 pm

I'm not sure it needs to be tweaked.

In Duncan TSC, when I mock that up and change source from 1K to 7K, the response changes very little, and a small turn of the knob brings it back near-parallel.

The "obvious" change is to see that the 470K leg is already super-high compared to 1K-7K source, leave it be. The other leg is much lower. Change 0.015u to 10X smaller (higher impedance), 1,500pFd. Change 3K+25K to 33K+250K. I wonder if the pot should also go bigger, but then the load (1Meg) should be bigger, so I would not go too far.

Title: Re: Scaling the AMZ single knob tone control circuit for a higher source impedance.
Post by: octal on February 23, 2016, 10:05:09 pm

Thanks, PRR.  You're probably right about it being "close enough for guitar" as it stands, I'll also run your suggested values through the sim and see how it looks.