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Post subject: Copying DRRI tonestack in modern Fender amp
Posted: Mon Jan 13, 2014 12:10 am
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I've got a Bandmaster VM (head version of the Deluxe VM), and I've been looking and researching stuff for hours on trying to improve it. I know it will never sound like a DRRI, blah, blah blah, but I just bought a Fender Excelsior and you can't have everything!

This amp is very similar to HRD, although it has a SS preamp for the clean channel.
I've heard HRDs with some mods that I like, (and I LOVE Billm's blues jr mods, I'm not sure if I can apply some of that to the VM, does anybody know?), so I figure I can maybe get it headed in that direction. I've looked at the HRD schematic, but because there is no mid-control, I'm also keeping an eye on the amp schematics of the DRRI and the 1965 Bandmaster (what I'm aiming for).

Here I'm comparing this modern Fender amp based off a Deluxe Reverb 65 RI tone stack, as they both use a fixed resistor in place of a mid control and I like its usable tone stack. I will also note that I'm relatively new at this and have a alot of questions.

Bandmaster/Deluxe VM Schematic: http://support.fender.com/schematics/gu ... ematic.pdf (save it to view offline, and it will be easier to zoom in)

DRRI schematic:
http://ampwares.com/schematics/65_Deluxe_Reverb_RI.pdf

So we don't get confused I'll be referencing the tone stack on the Vibrato channel of the DRRI, although for these purposes it could've easily been the "normal" channel.


MID-CONTROL-RESISTOR MOD
DRRI - 6.8k ohm resistor (R9)
BMVM - 3.3k ohm resistor (R132)

The Bandmaster VM seems to feel very much big bottom/top heavy (“PA speakers type sound)”. A schematic shows that the DR has a 6.8k fixed resistor (R9) controlling the mids, while the Bandmaster has 3.3k resistor (R132). I've head that this value is akin to having a mid-control on the DR permanently set at 6-7. Would it follow that the reason that the Bandmaster is so big bottom/top heavy, because with its smaller resistor value it's akin to a mid-control permanently at around 3? Would it make sense to try a 6.8kohm resistor at R132 for this Bandmaster?

TREBLE TONE POT $@!&# RESISTOR
DRRI - 100Kohm (R18)
BMVM - 22K (R129)

The DRRI has a 100Kohm resistor labeled R18.
The BMVM has a 22k resistor labeled R129. Is this the same as a 220K ohm resistor? Or is it really 22Kohm? Because that's way less, and the only reason I can imagine is that without a tube driving the preamp they thought they needed to keep the highs down to prevent it from being sterile. If I wanted to help remove some of the brightness from the amp I would want a lower value resistor, correct?

TREBLE TONE CONTROL CAPACITOR

DRRI has 250pF capacitor labeled C7
The BMVM has .001 cap labeled C77. Is .001 a 100pF capacitor?

I'm getting confused at to pF and uF.

With a higher capacitor value in this position would be possible to roll off more of the high frequencies, correct? (according to “The Tube Amp Back”)

COUPLING CAPACITORS
According to “The Tube Amp Book” suggests changing the three coupling caps on a DRRI (C7, C8, C9) all to a .02uF value to help get rid of the mid-light “PA system” sound, for a more a natural sound. Could this work on the BMVM in the equivilent (C77, C74, C75)?

I know that the .02uF value on the bass coupling cap was successful for getting rid of the boominess on a HRD (http://steelguitarforum.com/Forum11/HTML/009817.html) And the BMVM with the cab is even boomier than my stock HRD was.

Comparing the BMVM and the DRRI in this part of the tone stack has me very confused:

DRRI:
C7=250pF
C8=.1uF
C9=.047uF

Equivilent spots on BMVM
C77=.001
C74=.47
C75=.22

To me it looks like they used almost the same values as the DRRI, but they mixed up the positions. Is there an explanation I'm missing?

TONE POTS
DRRI
Treble=250K ohms 30 A, (R19)
Bass=250K ohms 10 A, (R20)

BMVM
Treble=50K B, (R135)
Bass=50K 15 A, (R136)

“B” would refer to a linear pot. So the larger pot on the DRRI will allow more detailed control, correct? So I could use larger pots on the BMVM tonestack to make those controls more usable? Any ideas as to why they have the treble knob on a linear pot?


Also, what parts of the schematic should I be looking at to see what other things could be affecting this signal in regards to the tone stack? I know the VM goes through alot of op-amps from the DSP). I know that the "Duncan Tone Stack" calculator can be used in figuring out some of these differences, but I don't know how you actually use it.

Thanks for helping me out!


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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Mon Jan 13, 2014 12:30 am
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No law against changing resistor and cap values to experiment with tone. Have at it.

Most of the time, a cap value will be in microfarads (µF), unless otherwise noted. So, a cap listed as .001, would be .001 µF, which is 1,000 nanofarads (nF), 1,000,000 picofarads (pF).

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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Tue Jan 14, 2014 7:12 pm
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fenderludwig: I am neither electronically or musically talented, but that will not deter me from offering my sage advice. If you are embarking on amp mods, please have fun with it. It will cost you untold amounts of time, effort, and MONEY searching for "tone"; but that "tone" goal you are reaching for changes .... probably daily. So just enjoy the ride (and the never ending amount of funds you will devote to it).

If you are like me, you will try and try to make Brand X amp sound like a Deluxe Reverb RI. Then, after about this many dollars = $$$$$$$$$$$$$$$$$$, you will relent, and find a used DRRI at Guitar Center for $650. And guess what, it will sound like exactly what you were trying to achieve in the first place .......... and then it won't, because .... you know .... it's Thursday. So out will come the Credit Cards ... and Paypal on Fleabay ... and you will be back to trying to get that DRRI to sound even more perfect.

And THEN .......................... well, seriously, it never ends. Sooooo, that's why I'm saying: just have fun (while you are burning through your kids college education). Because, really, it is very fun .... and certainly better than practicing.

Want to try my latest obsession?: Xotic EP Booster. No soldering required.

Or, when you get tired of the electronic mods, give me a holler. I have some seven or so speakers in my garage you can try. And you WILL be hitting that speaker aspect next. Hard.............

Now, if I can just get Arjay to sell me one of his minty, original JBL D120's for $100, I just KNOW my DRRI will finally be PERFECT!

Well ....................... until it isn't. It's Thursday, after all.


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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Tue Jan 14, 2014 8:57 pm
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Thanks for the tip shimmilou!

Hey WhatsThatSmell,
Pretty much everything you said resonates with what I've been thinking. That reasoning is actually what has led to me to keeping this amp and making the best of it as opposed to going out and buying a DRRI or a SuperSonic, only to do the same things. My plan is to improve this amp as much as I can and then just get on with it and enjoy playing.

Was actually considering a RC booster, will have to look at the EP. It seems like everyday I see another pro with one of those Xotic booster pedals on their board! For speakers I've already decided and I'm trying not to budge. ha!

As far as the mods go, increasing the tone pots from 50k to 250k should allow them to do more right?


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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Tue Jan 14, 2014 9:50 pm
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fenderludwig wrote:
...increasing the tone pots from 50k to 250k should allow them to do more right?


I don't know what you mean by more, but it will change the frequency response of that particular pot and the overall tone stack. Each pot works with the cap(s) as an "RC" circuit (resistor, capacitor), picked for a certain range of frequencies, so if you change the pot or cap(s) values, the frequencies will change. You probably shouldn't change the pot values very much, as the interaction/overlap with the other pots might be too much.

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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Tue Jan 14, 2014 11:42 pm
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fenderludwig wrote:
I know that the "Duncan Tone Stack" calculator can be used in figuring out some of these differences, but I don't know how you actually use it.


If you are planning on making these changes and experimenting with different component values, you really need to learn how to use the calculator. Otherwise you risk tearing up the PCB swapping components too many times. At least with the calculator you can get a general idea of the impact of each change you are thinking about making. Each component can be changed simply by double clicking it. I have no idea how you calculate the source and load impedance of the surrounding ICs (well, I used to, many years ago). You could find it in a text book and do it manually. Even if you get it wrong it doesn't affect the shape of the tone curve greatly, just the overall gain.

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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Fri Jan 17, 2014 2:13 am
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Thanks for all the help, was playing around with the duncan tone stack calc last night, having more fun than I thought I would with a calculator, and I discovered some values that look that they would help alleviate some issues too.

Just making sure I got this right...
so really changing these values doesn't necessarily affect tone quality, but rather what frequencies are allowed to pass through. This could be accomplished by using an EQ pedal correct?

So if you love how the tone stack already functions, but wanted to improve tone, you could keep the same values but replace with a higher quality cap... that's where the whole (Orange cap vs sprague vs mallory vs ___) debate comes in?

Coupling caps aside, from what I've read it seems that as far as electrolytic capacitors go, most people think they don't have much impact on tone, and for a newer amp without problems wouldn't need to be replaced. Any thoughts guys?

:D


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Post subject: Re: Copying DRRI tonestack in modern Fender amp
Posted: Fri Jan 17, 2014 2:23 am
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fenderludwig wrote:
Just making sure I got this right...
so really changing these values doesn't necessarily affect tone quality, but rather what frequencies are allowed to pass through. This could be accomplished by using an EQ pedal correct?


Yes.

And with a helluva lot less attendant aggravation than trying to mod the flimsy PCB's used in these modern amps.

For more more precise control over the frequency spectrum, a parametric equalizer will provide more sonic flexibility than a simple graphic EQ pedal.

Arjay

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