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Author Topic: Improvements to tonestack of HRM  (Read 17902 times)
Franc
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« on: December 13, 2009, 06:02:13 PM »
I'm looking for some improvements to the tonestack of the HRM.
It seems to me the Treble Middle and Bass controls are interacting too much.
The Mid knob hardly works when Bass is down and sounds like it is operating more in Treble areas than it should.
Is this the 'fault' of the internal tonestack that replaces the 0.005u cap that is there in the normal OTS?
Would it make sense to get rid of it and replace it with a 0.005u cap altogether?
Or is there a sweet spot for the internal 3 pots?

It's bugging me that the TMB controls are not responding the way I want them to...
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JD0x0
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« Reply #1 on: December 14, 2009, 03:33:09 AM »
im using a .008uf mid cap instead of the .022uf for more low mids. i also lowered the bass cap a little to keep it balanced. i raised the mid pot to a 100k from the 25K for more mids and a greater range. and lowered the treble cap a bit. IMO it sounds much better
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Franc
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« Reply #2 on: December 14, 2009, 07:49:37 AM »
Quote from: JD0x0 on December 14, 2009, 03:33:09 AM
im using a .008uf mid cap instead of the .022uf for more low mids. i also lowered the bass cap a little to keep it balanced. i raised the mid pot to a 100k from the 25K for more mids and a greater range. and lowered the treble cap a bit. IMO it sounds much better

Thanks for your insights! This is definately something I am going to try.
I tried to capture what you wrote in a pic:



What would be a recommended value for treble to try? Would 250p be a good idea? And to what value did you lower the bass cap?
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JD0x0
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« Reply #3 on: December 14, 2009, 08:12:57 PM »
i have a 200pf ceramic for the treble cap, you may want to go a little higher (i play teles)
the bass cap originally is .020 in my amp (not much different than the .022) the switch actually lowers the bass cap to around .009uf while also lowering the mid cap from .008 to .004uf. Basicly i was trying to achieve a low mid boost while also keeping the bass controlled and keeping the sound from being boomy or muddy.
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Franc
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« Reply #4 on: January 05, 2010, 08:15:57 AM »
I tried the changes, but we really didn't hear much difference Sad

The thing is, the amp barely reacts to the (front panel) Mid knob?
I have a second HRM 50W which is exactly the same, but this one DOES react to the Mid and sounds much better.

I have checked components in the mid section, but nothing seems to be way off.... What else can I check?
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bluesfendermanblues
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« Reply #5 on: January 05, 2010, 02:39:05 PM »
You check the difference between the two mid pots......

The one that is 'working' could be a 250K Linear
The one 'doesn't' could be a 250K Logaritmic

Maybe, thats the difference - check it! :-)
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bluesfendermanblues
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« Reply #6 on: January 05, 2010, 02:43:02 PM »
Regarding improving the tonestack

Try a 100k trimmer instead of the 33k slope resistor in the HRM circuit.

The HRM circuit (like every Fender or Marshall) tone stack has a mid dip. The 'dip' frequency can be adjusted with the slope trimmer - try going from 33k to 50k to 68k - which are common slope resistor values.

If you wish to simulate this: get the (free) Duncan TSC software. Works great.
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Franc
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« Reply #7 on: January 05, 2010, 03:01:22 PM »
Thanks for the suggestions!

Both pots are 250kA as per layout and even their actual values are close enough....

I will sure try the pot instead of the slope resistor to see what that brings me.
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Franc
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« Reply #8 on: January 05, 2010, 03:33:32 PM »
I have also downloaded the software to try and simulate what I am doing (and to understand Smiley )

But for some reason I now realise I can't get my head around how the HRM's extra piece of tonestacktrimmers incorporate into the 'standard' tone stack...

EDIT: Okay, I got my head around it Smiley I realise the HRM trim tonestack is really a standard Marshall tonestack, behind V2.
If I increase the slope resistor, the entire curve drops and the dip gets even deeper... How will this help make the front Mid control more effective?
« Last Edit: January 05, 2010, 04:21:25 PM by Franc » Logged
bluesfendermanblues
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« Reply #9 on: January 05, 2010, 05:32:25 PM »
I guess I prefer using a HRM amp in PAB mode, which in effect turn the amp into a marshall'esche circuit and tone. Don't expect great tones with both circuits (non-PAB mode)
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JD0x0
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« Reply #10 on: January 05, 2010, 09:15:39 PM »
the mid control may not make much of a difference until the lower end of it's range Ie. 9 oclock and below. Honestly these amps are pretty much set and forget type deals. Also dont forget about the treble and bass they work WITH the mid pot to get different sounds. If youre looking for less mids turn up the treble and bass which will be percieved as more scooped. Just do the opposite for more mids, lower the treble and bass.

Also if you have the treble and bass relativly low the mid control works less effectivly
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Franc
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« Reply #11 on: January 07, 2010, 06:42:06 PM »
Thanks all for your insights.

I agree that the range of the tone controls is limited in general, but the fact remains that I have 2 amps, built the same way and one of them is barely reacting to the MID control.

Which is odd, because the MID is the most straight forward of them all. It's a plate signal going through 150k and a 10n cap...  what could go wrong???
So it is still puzzling me why I can barely hear a difference when I am turning the MID control on one amp and a clear difference when doing it on the other (with all other settings the same, treble and bass at about 12 o'clock).

There must be something I am overlooking....
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JD0x0
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« Reply #12 on: January 07, 2010, 08:25:13 PM »
Quote from: Franc on January 07, 2010, 06:42:06 PM
Thanks all for your insights.

I agree that the range of the tone controls is limited in general, but the fact remains that I have 2 amps, built the same way and one of them is barely reacting to the MID control.

Which is odd, because the MID is the most straight forward of them all. It's a plate signal going through 150k and a 10n cap...  what could go wrong???
So it is still puzzling me why I can barely hear a difference when I am turning the MID control on one amp and a clear difference when doing it on the other (with all other settings the same, treble and bass at about 12 o'clock).

There must be something I am overlooking....

250K is used on most dumbles and gives you a much bigger range. that could be the problem
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Franc
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« Reply #13 on: January 07, 2010, 08:28:21 PM »
Really? You're not kidding? The Ceriatone layout says 150k....
I'm gonna try that as a last resort, but wouldn't it influence the Bass to much?
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erwin_ve
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« Reply #14 on: January 07, 2010, 10:58:07 PM »
I think Franc is reffering to the slope resistor not the mid pot!
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