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Thread: Carvin Legacy 1 master volume mod problems

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    Carvin Legacy 1 master volume mod problems

    Hi,

    I have a Carvin Legacy 1 combo VL212 which have a master volume mod installed.
    I installed it a few years ago after reading a post at carvinbbs forum (which doesn’t exist anymore) and followed the instructions there.

    The C42 + C43 caps (473k each) on the picture were removed and replaced by the master volume which consist of these 4 wires hooked up to 4 caps Mallory 150's 0,1uf / 630VDC in series to a dual/stereo 1M log/audio pot.

    I need some help with this because it doesn’t work the way I want it to. If it’s possible to improve it please advice me

    The main problem I have is that the master volume pot is too sensitive in the first range. When turning the master volume a tiny bit from zero (silent amp) to 1 (8 o’clock) it gets too loud IMO, and with the knob at 11-12 o’clock the master volume seems to be inactive.
    Is it possible to improve it so it have a better wider range and easier to get a decent good tone at lower volume, because I mostly use this amp at home.
    The optimal would be if the master volume is inactive when turned completely clockwise so the amp behaves like it doesn’t have a master volume in that position.
    Should the caps or pot be another value or linear pot instead of log?

    Another issue with this mod is the clean channel breaking up at low volume. I’m not sure if this can be fixed.

    The third issue is a slight noticable hum from the amp, even with everything turned to zero and nothing plugged in. I first thought it was a problem with the power transformer or mismatched power tubes. All the tubes are replaced and biased correctly so that's not it. Do you think the master volume mod can cause this issue induced by the phase inverter?
    I'm not sure if a slight hum is normal on the Carvin Legacy amps. I can't remember how it was without the master volume mod.

    One more question, is high voltage passing to the master volume pot? or is it safe?

    Please see the attached pictures.

    https://ibb.co/6RjtNFV
    https://ibb.co/dmm6wq6
    https://ibb.co/8MbstnZ

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    Quote Originally Posted by MrTrapp View Post
    Hi,

    I have a Carvin Legacy 1 combo VL212 which have a master volume mod installed.
    I installed it a few years ago after reading a post at carvinbbs forum (which doesn’t exist anymore) and followed the instructions there.

    The C42 + C43 caps (473k each) on the picture were removed and replaced by the master volume which consist of these 4 wires hooked up to 4 caps Mallory 150's 0,1uf / 630VDC in series to a dual/stereo 1M log/audio pot.

    I need some help with this because it doesn’t work the way I want it to. If it’s possible to improve it please advice me

    The main problem I have is that the master volume pot is too sensitive in the first range. When turning the master volume a tiny bit from zero (silent amp) to 1 (8 o’clock) it gets too loud IMO, and with the knob at 11-12 o’clock the master volume seems to be inactive.
    Is it possible to improve it so it have a better wider range and easier to get a decent good tone at lower volume, because I mostly use this amp at home.
    The optimal would be if the master volume is inactive when turned completely clockwise so the amp behaves like it doesn’t have a master volume in that position.
    Should the caps or pot be another value or linear pot instead of log?

    Another issue with this mod is the clean channel breaking up at low volume. I’m not sure if this can be fixed.

    The third issue is a slight noticable hum from the amp, even with everything turned to zero and nothing plugged in. I first thought it was a problem with the power transformer or mismatched power tubes. All the tubes are replaced and biased correctly so that's not it. Do you think the master volume mod can cause this issue induced by the phase inverter?
    I'm not sure if a slight hum is normal on the Carvin Legacy amps. I can't remember how it was without the master volume mod.

    One more question, is high voltage passing to the master volume pot? or is it safe?

    Please see the attached pictures.

    https://ibb.co/6RjtNFV
    https://ibb.co/dmm6wq6
    https://ibb.co/8MbstnZ
    ppimv has all the problems you mentioned,despite all the rest of the world is convinced that it works perfect.
    Like attenuators and loads etc.
    Carvins have volumes for each channel,use those,my 2 cents.

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  3. #3
    Bent Member Chuck H's Avatar
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    Looking at your wiring I have some suggestions. I think what you wired up is one of the circuits below or something similar. Regardless of how you've connected your pot lugs the principles are the same. In diagram 1 you would have a variable resistor for attenuation of each PI output. In diagram 2 you would have a voltage divider for each PI output. Diagram 1 will not have a good taper for volume adjustment. So if you are wired up for a variable resistor type you could change it to the voltage divider type for better adjustment at lower pot settings.

    Your leads are excessively long and routed over the power supply. This is likely where any extra noise and hum is coming from. The leads for this circuit need to be as short as possible and routed AWAY from the power supply. In fact this circuit often uses shielded leads to combat noise and potential instability.

    You also have the circuit grounded with the power supply. This is another potential source of hum both because this is a high current ground point and you are using a long lead AND because it creates a ground loop between the board ground for the PI outputs and the chassis ground you are using for the mater volume circuit. Your master volume circuit should be grounded on the board with any accessible ground point used for the PI.

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Bent Member Chuck H's Avatar
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    Oh... And regarding your other inquiry about this amp from 2008/2017...

    Quote Originally Posted by MrTrapp View Post
    Hi,

    Very old thread, but did you get this sorted? If so, please let me know how. I have a Legacy 1 combo which I need to convert to 240V.
    It looks like Carvin had a specific transformer for domestic and another for export. So there is no way to convert your amp without replacing the power transformer. It's almost certain that the export transformer for your amp is an "obsolete" part. Meaning you're not likely to find one new or used. I think the converter you've been using will have to do. Sorry no one responded to that other post. That's not like us typically.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Senior Member Pedro Vecino's Avatar
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    I don't see adecuate install a post PI master volume on that amplifier. Lead channel has infinite gain and this type of master volume does not work well with them.
    If your interest is in the control over the volume (lead channel is really uncontrollable to play at low volume) there is a simple solution: replace R58 resistor (47K) with a 560K resistor and master volume potentiometer (1M Lin) with a 500K Logarithmic. That will allow you perfect control over the volume.
    The presence potentiometer is not accommodated for the new circuit but works equally well with it.

    You can do the same on the clean channel simply by replacing the linear 1M with a logarithmic 1M.

    Click image for larger version. 

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    Quote Originally Posted by Pedro Vecino View Post
    I don't see adecuate install a post PI master volume on that amplifier. Lead channel has infinite gain and this type of master volume does not work well with them.
    If your interest is in the control over the volume (lead channel is really uncontrollable to play at low volume) there is a simple solution: replace R58 resistor (47K) with a 560K resistor and master volume potentiometer (1M Lin) with a 500K Logarithmic. That will allow you perfect control over the volume.
    The presence potentiometer is not accommodated for the new circuit but works equally well with it.

    You can do the same on the clean channel simply by replacing the linear 1M with a logarithmic 1M.

    Click image for larger version. 

Name:	carvinL.JPG 
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    Well I agree the amp isn't suitable for a master volume, but that's up to MrTrapp. He knows that the amp has separate channel volumes and he knows how much gain the preamp has but decided to install this thing anyway.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    Looking at your wiring I have some suggestions. I think what you wired up is one of the circuits below or something similar. Regardless of how you've connected your pot lugs the principles are the same. In diagram 1 you would have a variable resistor for attenuation of each PI output. In diagram 2 you would have a voltage divider for each PI output. Diagram 1 will not have a good taper for volume adjustment. So if you are wired up for a variable resistor type you could change it to the voltage divider type for better adjustment at lower pot settings.

    Your leads are excessively long and routed over the power supply. This is likely where any extra noise and hum is coming from. The leads for this circuit need to be as short as possible and routed AWAY from the power supply. In fact this circuit often uses shielded leads to combat noise and potential instability.

    You also have the circuit grounded with the power supply. This is another potential source of hum both because this is a high current ground point and you are using a long lead AND because it creates a ground loop between the board ground for the PI outputs and the chassis ground you are using for the mater volume circuit. Your master volume circuit should be grounded on the board with any accessible ground point used for the PI.

    Thank you for the suggestions! It is wired as diagram 2 already. I will re-route the cables after they are shortened and also find another ground point.

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    I will re-route the cables after they are shortened and also find another ground point.
    I strongly recommend to use shielded wires as they are passing the PT, which emits strong hum fields.

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    Quote Originally Posted by Chuck H View Post
    Oh... And regarding your other inquiry about this amp from 2008/2017...



    It looks like Carvin had a specific transformer for domestic and another for export. So there is no way to convert your amp without replacing the power transformer. It's almost certain that the export transformer for your amp is an "obsolete" part. Meaning you're not likely to find one new or used. I think the converter you've been using will have to do. Sorry no one responded to that other post. That's not like us typically.
    Actually this was solved a few days after that post. I nice tech at Carvin sent me a description how to fix it. It's the same transformer on both the US and export models. I think it's called multi-tap transformer. The procedure was to cut two cables from the transformer and wire them together. Also changed the fuse from 6 amp to 3 amp.

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    I know it's not the best amp with a master volume but I love the tone from this amp and I think the tone is actually ok with the master volume, but I just need to tweak/fix it some so it works better at low volumes and also the hum.. will be fixed soon

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    Quote Originally Posted by Helmholtz View Post
    I strongly recommend to use shielded wires as they are passing the PT, which emits strong hum fields.
    Yes I will definitely change to shielded wires.

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    Last edited by MrTrapp; 01-16-2020 at 12:03 AM.

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    Maybe a few stuped questions here but I'm not an expert in amp electronics as you guys are.

    What will happen if I change the 1M pot to 500k? Will this give me a better adjustment of the pot or it will it cause internal damage or change the tone to worse? Because it would be nice to have the master volume off when turned all the way clockwise if you getting me.

    At the moment my 1M pot is audio taper. Would a linear pot be better?

    Is high voltage passing through this master volume curcuit? If so, should I add some safety resistors in it?

    Another wild idea I just got, adding a switch for the master volume. If it would be possible with a push-pull stereo pot? The original caps and the master volume curcuit connected to one push-pull pot? If I get in a case I want to be sure that the master volume is completely bypassed.

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    Changing the pot from 1M to 500k won't make much difference without other changes because the division RATIO won't change much.

    Stay with the audio taper pot.

    There is high voltage on the leads that go from each PI output to the first .1 capacitor. After those capacitors there is no high voltage on the leads or the dual pot.

    You shouldn't need a switch. There is no significant loading of the signal with that master volume for that amp design when the master volume is turned up to 10. Assuming you're using a dual 1M pot.

    Is your dual pot for the master an audio taper? If yes, probably the standard Alpha? If yes it's likely to be a 15% taper. That's not bad and should be performing acceptably for this circuit.

    Helmholtz is correct about the use of shielded leads and the power supply. You will be A LOT better when you shorten the leads and route them straight over the board and flat to the face plate to reach the dual pot. Even if you don't use shielded leads this may be sufficient.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    If no one has changed the lead channel volume potentiometer it must be linear (1MB). It´s the logarithmic (audio) that facilitates fine adjustments. The idea of changing it for one of 500KA raising the value of the resistor in series is to achieve a slightly better curve than simply changing it to 1MA. The combination of resistor and potentiometer recovers the original value of 1M. 560K+500K ≈ 47K + 1M.
    This is the volume with which I played this morning. Gain 6 / volume 2,5. The volume is manageable in this way. With the original 1MB potentiometer it would be impossible to play with that setting at home.

    Click image for larger version. 

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    If you wanted to have absolute control over the final volume of the amp, you could install that master volume. Although I do not particularly consider it necessary with the above.

    Click image for larger version. 

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    EDIT: We have posted at once.

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    Quote Originally Posted by Pedro Vecino View Post
    If no one has changed the lead channel volume potentiometer it must be linear (1MB). It´s the logarithmic (audio) that facilitates fine adjustments. The idea of changing it for one of 500KA raising the value of the resistor in series is to achieve a slightly better curve than simply changing it to 1MA. The combination of resistor and potentiometer recovers the original value of 1M. 560K+500K ≈ 47K + 1M.
    This is the volume with which I played this morning. Gain 6 / volume 2,5. The volume is manageable in this way. With the original 1MB potentiometer it would be impossible to play with that setting at home.

    Click image for larger version. 

Name:	carvin_.jpg 
Views:	18 
Size:	53.6 KB 
ID:	56659

    If you wanted to have absolute control over the final volume of the amp, you could install that master volume. Although I do not particularly consider it necessary with the above.

    Click image for larger version. 

Name:	carvin__.JPG 
Views:	23 
Size:	153.5 KB 
ID:	56660

    EDIT: We have posted at once.
    Thanks! That sounds interesting! I need to try this as well

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