Best way to mod an amp??

Drew_TNBDDrew_TNBD Frets: 22445
Lets say I wanted to put some resistors into the signal path of an amp, but I wasn't sure what values I wanted. Could I use a pot wired in place to determine the values and switch it out later once I knew them?

I tried modding my 50-watt 5150III to get rid of the volume drop. I managed to lower the gain, but I didn't manage to eliminate the volume drop. Thinking it's a case of dialing in the correct resistor values.
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Comments

  • ICBMICBM Frets: 71950
    That sounds like you changed the wrong resistor…

    Yes, using a pot is a good way of dialing in a value, although it's usually a good idea to put a 'lower limit' value in series with it if you don't know exactly what you're going to affect, just in case a too-low value causes trouble. Half the original value is probably a good place to start, with a pot of roughly the same or greater than the original value, that will let you dial down to about 50% or up to 150% (or higher).

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Drew_TNBDDrew_TNBD Frets: 22445
    ICBM said:
    That sounds like you changed the wrong resistor…

    Yes, using a pot is a good way of dialing in a value, although it's usually a good idea to put a 'lower limit' value in series with it if you don't know exactly what you're going to affect, just in case a too-low value causes trouble. Half the original value is probably a good place to start, with a pot of roughly the same or greater than the original value, that will let you dial down to about 50% or up to 150% (or higher).

    Hmmm... I followed the instructions I've dug up fairly accurately, but didn't use the exact values. f I connect resistors in series to change the resulting value, is that any different to using a single resistor rated for that value?

    Here is what I sourced from Howard Kaplan and Youtube:

    Okay... the gear page can be useful sometimes. I pm'd Howard Kaplan the designer of the 5150III and asked him what he'd recommend modding to even out the levels on the 50-watter... he said this:

    There is no way to increase the level of CH1 without raising its distortion (i.e. - more gain) with the topology as configured. The other option is to lower the gain of CH2, but this will of course change the distortion characteristic of this channel. If you can live with a lower amount of gain on CH2 you can try installing R27 (should be empty as shipped) at 180K or as low as 100K and see how it sounds to you.

    Channel 2 was the most important one to Ed when the amp was developed and he also wanted a cleaner (than the 100W Head), so the this is how the jump got into the amp. If switching between CH1 and CH2 is important to you in your performances you could use an effects processor device in the loop to adjust the level as you switch from CH1 to CH2 using the MIDI interface to control the 50W Head. The only real way to solve the issue is to have a dual (concentric) gain control and a dual volume control for the CH1 and CH2.

    Have your tech install 100K at R27 and maybe put R79 back to 10k (more clean). Doing the dual concentric control change is very involved. Not feasible without a large tear down of the amp. Are you in the UK? Curious about the band name - Tacoma Narrows Bridge Disaster. I lived in the Seattle-Tacoma area for 28 years. Was not there when the bridge went down, but did see 3 other major bridge disasters while I lived there. Hood Canal bridge sinking; I-90 floating bridge span sinking and the West Seattle Bridge hit by a ship and forcing a 3 year replacement project. I love the music by the way!!! Listening now.


    -----

    From Youtube ():

    R27 - 150K resistor (reduces gain and volume of the crunch channel)
    22K resistor in parallel with R44 (increases volume of clean channel)



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  • ICBMICBM Frets: 71950
    Is the 50W one different from the 100 (apart from the power output)? I have a schematic for the 100, but not the 50 - and that doesn't really make sense, R44 is the cathode-follower resistor for Channel 3 and you definitely wouldn't want to put a 22K in parallel with that…

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Drew_TNBDDrew_TNBD Frets: 22445
    ICBM said:
    Is the 50W one different from the 100 (apart from the power output)? I have a schematic for the 100, but not the 50 - and that doesn't really make sense, R44 is the cathode-follower resistor for Channel 3 and you definitely wouldn't want to put a 22K in parallel with that…
    Yes I think they're fairly different.

    http://support.evhgear.com/schematics/EVH® 5150III 50W_Mexico.pdf




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  • Drew_TNBDDrew_TNBD Frets: 22445
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  • ICBMICBM Frets: 71950
    edited March 2017
    Those two are basically the same.

    Why is he talking about R44 though? That's near the start of the channel and will alter gain not volume.

    To reduce the volume of Ch 2 you're going to have to do something at the end of the channel, around the tone stack. Increasing the value or R97 would do, although it will also change the voicing of the channel since it's part of the R97/R104/C58 divider - you may have to increase R104 as well.

    For what it's worth this is an incredibly illogically-numbered schematic! The component numbers appear to be scattered at random rather than following any sort of sequence, and it's quite hard to find anything .

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Drew_TNBDDrew_TNBD Frets: 22445
    Yep. I agree. Can't make head nor tails of it to be honest!

    I've reverted my changes. Gonna leave it to the experts :)

    Thanks for the help J! You're a legend.
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