Amp voicing mods

T

tjnx

Active member
I started poking around a bit at modifying my tone stack and was thinking of doing a few options on a switch. Would I get more bang for the buck replacing components elsewhere first? ie cathode / plate or coupling caps or a slope resistor on a pot vs some marshall / soldano tone stack values on a switch.

Going to order a few parts to tinker, any thoughts are appreciated!
 
FourT6and2 said:
Slope resistor, treble cap, NFB resistor/tap. Those three should yield large changes and are easy to experiment with.
Click to expand...
Thanks! I'm looking to brighten it up just a little. I know there is more to it than just the tone stack, but it looks like I could shift the mids pretty simply as that is where I think the problem lies.

I compared it to a JCM and SLO 100. I can line up the mids to the Marshall with a 28k slope and then it looks to be just 2 components off the soldano. I attached the schematic at the bottom.

Here is the link I used for tsc online, it defaults to the marshall:
https://www.guitarscience.net/tsc/m...3=22n&RB_pot=LogB&RM_pot=Linear&RT_pot=Linear

The first image compares the mid points of the amps at stock values, with current values being the Randall RD45H. The next two images are the rd45h with adjustments to match the other 2 amps.

jcm_slo_rd45_midpoints.jpg
rd45_28k_slope.png
rd45_to_slo.png
 

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Leave the slope at 47K and try raising the value of the treble cap from 470pF instead. Sky's the limit. Can even go as high as 4700pF if you wanted to. But try 510pF, 680pF, 820pF, 1000pF, 2200pF, 3300pF, 4700pF. Alternatively, you can lower slope resistor to 39K or 33K for more mids. But might become too honky. Only one way to find out.

Neither of these will "brighten up the amp" so much as they alter the center frequency of the tone controls, raising or lower the mid scoop that's present in the tone stack. If you want to actually brighten the amp, you can play with any low-pass filters that are in the circuit (if any), but like I said above, NFB is also a powerful tool. But it will change more than just treble response.

I don't have time to really dig deep into the amp's schematic. So you'll have to find areas that are cutting treble and mess with those.
 
FourT6and2 said:
Leave the slope at 47K and try raising the value of the treble cap from 470pF instead. Sky's the limit. Can even go as high as 4700pF if you wanted to. But try 510pF, 680pF, 820pF, 1000pF, 2200pF, 3300pF, 4700pF.

Alternatively, you can lower slope resistor to 39K or 33K for more mids. But might become too honky. Only one way to find out.

Like I said above, NFB is also a powerful tool.
Click to expand...
Cool! so like this right? I need to look in to how to properly do a NFB and figure it out on this amp. This is my first go on any voicing, I've done caps, resistors and jacks before but this is all new.

treble_caps_rd45h.png
 
tjnx said:
Cool! so like this right? I need to look in to how to properly do a NFB and figure it out on this amp. This is my first go on any voicing, I've done caps, resistors and jacks before but this is all new.

View attachment 421210
Click to expand...

Yes, you got it.

RE: NFB... looks like your amp has 100K/4ohm tap. That's about as bright/loose as I'd want it, so no need to mess with that.
 
tjnx said:
This is what I was thinking of doing to mess with the slope resistor values
Click to expand...


Sure, replacing the slope resistor with a pot is a simple and easy way of doing it. Just find the lowest value you'd ever want and hardwire that in series with a pot of some value to get the maximum resistance you want. Pretty easy.

Hell... then you can just replace the treble cap with a 6-way rotary with various cap values and you've a mostly variable tone stack.
 
FourT6and2 said:
Yes, you got it.

RE: NFB... looks like your amp has 100K/4ohm tap. That's about as bright/loose as I'd want it, so no need to mess with that.
Click to expand...
Ok thanks!

FourT6and2 said:
Sure, replacing the slope resistor with a pot is a simple and easy way of doing it. Just find the lowest value you'd ever want and hardwire that in series with a pot of some value to get the maximum resistance you want. Pretty easy.

Hell... then you can just replace the treble cap with a 6-way rotary with various cap values and you've a mostly variable tone stack.
Click to expand...
Yes! Exactly what I was thinking, again was going to copy what this guy did when doing a bright cap but I hadn't considered the treble until you mentioned it. Thanks man, I've got a very good jumping off point now of things to try!

 
FourT6and2 said:
Neither of these will "brighten up the amp" so much as they alter the center frequency of the tone controls, raising or lower the mid scoop that's present in the tone stack. If you want to actually brighten the amp, you can play with any low-pass filters that are in the circuit (if any)
Click to expand...
Ok just for my reference later, I think I misunderstood the tone stack graphs as a final eq curve vs the effect of the controls on the available (post-filtered) frequencies. I don't remember which video (dsl 100?), but Headfirst Amplification talked about this in a mod video about the Soldano vs Friedman approach and the influence of filter caps on the plate and the cathode.

If I understood correctly Friedman was rounding off highs via plate caps on the anode and Soldano was letting as many frequencies through the gain stage then cutting them out later.

With a 4700pf cap vs the stock 470pf the mid scoop is re-positioned and I should have access to more of the upper frequencies available post filtering via the controls.

4700pf sweep
4700pf sweep.png
470pf sweep (stock)
470pf sweep.png
 
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psychodave said:
Are there any caps across the resistors in V1 creating a low-pass filter?
Click to expand...
Yes, take a look
rd45h_v1.png


The amp sounds similar to the 'Friedman Style' 45k 4 Ohm nfb resistor in this video, so maybe less nfb would help? I almost never use the presence below half way and usually turn it up in a band setting.

 
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The nfb circuit looks like this
rd45h_nfb.png



The results of adjusting it seem to be in the direction I'd like to go, but given the advice above (from someone that knows better) idk if thats a good idea. Found this video for adding a response and depth control that I would likely adapt

 
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