modding a honeytone 9v guitar amp to be cleaner

[kerryandray]

Hi, all.

This isn't technically a stomp box question, so if there's a better place to post these questions, please let me know.

OK...I have a travel guitar I made with a boss SD1 and phase 90 effects from GGG and with a danelectro honeytone amp in it.  You can read a bit about it in this topic, where you all helped me get the P90 working: https://www.diystompboxes.com/smfforum/index.php?topic=124461.0

It's actually pretty awesome as is, but the amp sounds like a box of buzzing bees.  It's a filthy little thing, and I'd like to clean it up and get as much headroom as I can. I would like to modify it to better suit my needs.

1. Clean up the amp to have as much headroom as possible.
There are two bits I will do:

   1A. Provide the preamp chip with 18V using a 1044MAX chip (as described in Mod 10 of this post: https://www.angelfire.com/music/harmonica/honeytone.html).  This part I can figure out on my own.

   2A. It's got an overdrive function I'd like to eliminate (because I already have a Boss overdrive effect built into the guitar along with a Phase 90 effect).

2. Change the tone control from a single knob tone to a 2-knob bass/treble type of control.
Why?  Because when I eliminate the overdrive knob, there'll be an unused, potentiometer-sized hole in my guitar...so I might as well!  ;-) 

So...how do I modify the honeytone circuit between the two halves of the TL072 op amp to remove the Overdrive and to change the single tone knob into a 2-knob tone circuit such as found on this page: https://monster.partyhat.co/article/amplifier-tone-stacks/ ?

Below is the Honeytone schematic with a red circle showing the parts that need to be changed to remove the overdrive function.  The blue circle indicates the parts that need changing for the tone stack. 



So...in terms of removing the overdrive and getting as much clean headroom as possible...what should I do with the parts in the red circle?  Do I just use the same as are used on the other half of the TL072 op amp (i.e. the 2k2 and 1k resistors and the 470n capacitor)?

In terms of the tone stack, do I just retain the 10uf coupling capacitor coming off the first half of the TL072 op amp and then tack on the fender brownface tone stack, or the bandaxall? 

Any suggestions as how to proceed would be greatly appreciated. 

Here's a sample modded schematic that assumes I can use the same components on the first half of the TL072 as appear on the second half and that I can just retain the 10uf coupling capacitor, replacing the honeytone tone stack with the fender brownface. 



Would this work?  If not, what do I need to tweak?  Thanks all!

Ray

[antonis]

I should suggest to maintain 100k Gain pot and raise 1k resistor value up to 10k while reducing 1μF cap value down to 100nF..

For Bass/Treble I should suggest a Baxandall EQ..
https://www.diystompboxes.com/smfforum/index.php?topic=96844.0
(the above is just a random thread - there are various types of Bax EQ, including 1/10 or equal resistor/capacitor values, single cap for Bass and/or Treble, etc..)

[kerryandray]

I should suggest to maintain 100k Gain pot and raise 1k resistor value up to 10k while reducing 1μF cap value down to 100nF..

Thanks!

Why would you leave the gain pot?  I'm always going to leave it at it's lowest gain setting because I use the boss sd1 effect for overdrive.  So can't I hardwire it to lowest gain?  To do that, can I just make the changes you suggest but either A) drop the gain pot or B) replace the pot with a resistor of some value to get the same effect as having the gain turned all the way down?  What resistor value would I need?  100k?

Also, the 2-tone idea is only to fill the hole left by the missing gain pot, LOL.

[antonis]

Why would you leave the gain pot?

Both for precise gain setting ability ( x1 - x11 for the whole shaft rotation) and for filling the respective empty hole..

Of course, if you're sure about first stage desirable gain, replace it with a single resistor..
Gain is set by (1 + R_Feedback / R_Gain), where R_Feedback = 100k pot & R_Gain = 1k resistor..
(like 1 + 2k2/1k of second stage..)

[kerryandray]

Of course, if you're sure about first stage desirable gain, replace it with a single resistor..
Gain is set by (1 + R_Feedback / R_Gain), where R_Feedback = 100k pot & R_Gain = 1k resistor..
(like 1 + 2k2/1k of second stage..)

Awesome,  thanks! That helps a lot.  I'm going to need to have a think about that equation while looking at the schematic...

But for now...from a theory perspective...if I basically wanted the same amp to be hardwired to lowest gain, I could just replace the 100k gain pot with a resistor.  In this case, at what resistance is the gain pot when set to the cleanest setting?  Is it's at zero ohms or 100k when gain is turned all the way down ?  I'm going to guess it's at zero ohms,  thereby allowing for a lot of feedback?

[kerryandray]

In this case, at what resistance is the gain pot when set to the cleanest setting?  Is it's at zero ohms or 100k when gain is turned all the way down ?  I'm going to guess it's at zero ohms,  thereby allowing for a lot of feedback?

Yeah, it's gotta be at zero ohms.   That would produce a gain of 1 using the equation you supplied.  Right?

[antonis]

Quite right..!! 
(either for pot resistance set to zero ohms or in the absence of 1k resistor, stage gain is unity due to non-inverting configuration..)

P.S.
The above is called "voltage follower" (or "buffer" by its intimates..)

[kerryandray]

Sweet!  Thanks Antonis!

So...looking at your suggestions again...you suggested changing the 1k resistor to 10k.  Given the equation, that would have the effect of making the 0 - 100k sweep of the gain pot have 1/10th the overall gain compared to the stock amp...all spread across the same rotation.  And THAT'S why you made the comment about precise gain control?  LOL.  I just got that joke now 

So you made some other suggested changes to the capacitor on the first stage?  What effect does the capacitor change you suggested have?

[antonis]

1μF capacitor & 1k resistor form a High Pass Filter of -3db signal attenuation (Vout/Vin  = 0.707) at 159Hz corner frequency..
If you wish to keep the same corner frequency you have to maintain R x C product the same as before..

Same stands for 100k & 100pF but in the reverse action (Low Pass filter) and for 15.9kHz..

P.S.
Formula for -3db corner frequency is f = 0.159/(R X C), where f = frequency, R in Ohms, C in Farads and 0.159 is 1/2π
Formula for voltage attenuation (in db) is 20 X log₁₀(Vout/Vin)

[kerryandray]

Whatever tone stack I choose...do I just slap it in there as is?  Like in my sample schematic...I just plopped the fender brownface tone stack in there with the same values including the 250k pots.  Is that right?  The honeytone had a 100k pot for the tone control...

I did take a look at the baxandall but think a guitar amp eq might be a better place to start for this. 

Any comments or thoughts on how to implement the stock Brownback fender tone stack shown in my sample schematic would be greatly appreciated.   Thanks!

Ray

[kerryandray]

I should suggest to maintain 100k Gain pot and raise 1k resistor value up to 10k while reducing 1μF cap value down to 100nF..

Another question.  From a pure theory perspective, would I get the same reduction in gain...with comically precise gain control...by simply changing the pot from 100k to 10k?  And would that mean I'd have to change the 100p capacitor to a 1000p to keep the same corner frequency?

And further, if both approaches have the same effect on gain without changing the corner frequencies...is there a reason to choose approach over the other (i.e. raising the gain resistor versus lowering the feedback pot) ?