ckaudio

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Hi all -

I’ve been a Studio Vintage SV20 owner for a couple of years now. While I quite love the amp I’ve never truly been able to love the bright channel as much as I’d like to. I felt like the normal channel was too dark and the bright channel was too bright, and when cranking the bright channel I just got more gain than I wanted, and when mixing the channels I couldn’t get a balance I was happy with without it being too bright.

I’m not a metal guy, not really into the 80’s hot rodded plexi sound, I was going for more of a 70’s classic Marshall crunch, which is why I picked the SV20 over the SC20. I wanted to be able to play Jimi Hendrix, Zeppelin, AC/DC etc., and at the time the SV20 was a lot more affordable than a 1987X. I picked it up when it was $1,300 new.

I think that I was hesitant to make a mod to it because of how many people claim that the Plexi is supposed to sound like that and if it doesn’t sound good you’re not running it right. I spent a lot of time with this amp. I ran the amp volume high and rolled off the guitar volume etc, etc, and frankly these 20W models don’t sound like a 50W/100W model. The small transformers are likely part of the reason, as well as the low plate voltage running the tubes cold to keep the output at 20W.

Anyway… Today, I finally ended up changing the bright cap from the stock 4.7nF down to 100pF, and I must say I’m honestly impressed with how much better the amp feels. I first removed the cap and ran 2 wires from the pcb, and used alligator clips to try about 7 diffferent values ranging from no cap to 4.7nF (stock). I ended up going with 100pF because it gave back some of the clarity and air without feeling like the top end was falling apart.

Clipping the cap definitely felt too dark to me. It’s still brighter than the normal channel because of the 470pF cap on the channel mixing resistor, but with it completely removed it definitely felt like it lost a little magic.

I am running the amp head through an Origin 2x12 vertical cab which I replaced the stock Seventy-Eighty speakers (junk) with modern day G12-M Greenbacks. It now has the thick creamy bluesy overdrive that I was wanting. It does the AC/DC thing, Zeppelin riffs sound incredible as well, I run the High Treble volume on about 6-8, and it’s awesome. The 20W seems to move enough air to make me feel the magic without pissing off the neighbors.

If you’re an SV20 owner who feels the same way I did about your amp, I highly suggest you try the mod. You simply replace C101 on the control panel PCB. The whole PCB comes off by removing nuts from the pots, and pulling a couple of wire connectors.

I documented the whole process with photos if anyone is interested in a walkthrough. If you’re savvy, you’ve probably already done this, if you’re at all handy with a solder iron, it’ll be a breeze!
 

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marshallmellowed

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Hi all -

I’ve been a Studio Vintage SV20 owner for a couple of years now. While I quite love the amp I’ve never truly been able to love the bright channel as much as I’d like to. I felt like the normal channel was too dark and the bright channel was too bright, and when cranking the bright channel I just got more gain than I wanted, and when mixing the channels I couldn’t get a balance I was happy with without it being too bright.

I’m not a metal guy, not really into the 80’s hot rodded plexi sound, I was going for more of a 70’s classic Marshall crunch, which is why I picked the SV20 over the SC20. I wanted to be able to play Jimi Hendrix, Zeppelin, AC/DC etc., and at the time the SV20 was a lot more affordable than a 1987X. I picked it up when it was $1,300 new.

I think that I was hesitant to make a mod to it because of how many people claim that the Plexi is supposed to sound like that and if it doesn’t sound good you’re not running it right. I spent a lot of time with this amp. I ran the amp volume high and rolled off the guitar volume etc, etc, and frankly these 20W models don’t sound like a 50W/100W model. The small transformers are likely part of the reason, as well as the low plate voltage running the tubes cold to keep the output at 20W.

Anyway… Today, I finally ended up changing the bright cap from the stock 4.7nF down to 100pF, and I must say I’m honestly impressed with how much better the amp feels. I first removed the cap and ran 2 wires from the pcb, and used alligator clips to try about 7 diffferent values ranging from no cap to 4.7nF (stock). I ended up going with 100pF because it gave back some of the clarity and air without feeling like the top end was falling apart.

Clipping the cap definitely felt too dark to me. It’s still brighter than the normal channel because of the 470pF cap on the channel mixing resistor, but with it completely removed it definitely felt like it lost a little magic.

I am running the amp head through an Origin 2x12 vertical cab which I replaced the stock Seventy-Eighty speakers (junk) with modern day G12-M Greenbacks. It now has the thick creamy bluesy overdrive that I was wanting. It does the AC/DC thing, Zeppelin riffs sound incredible as well, I run the High Treble volume on about 6-8, and it’s awesome. The 20W seems to move enough air to make me feel the magic without pissing off the neighbors.

If you’re an SV20 owner who feels the same way I did about your amp, I highly suggest you try the mod. You simply replace C101 on the control panel PCB. The whole PCB comes off by removing nuts from the pots, and pulling a couple of wire connectors.

I documented the whole process with photos if anyone is interested in a walkthrough. If you’re savvy, you’ve probably already done this, if you’re at all handy with a solder iron, it’ll be a breeze!
Thanks for the info, and sounds like you did it right by trying several values. A lot of guys just go into an amp and clip, or break off the bright cap, and claim it's "perfect now". I've not owned a Marshall that sounded it's best with the bright cap completely removed (my opinion), but I've changed the bright cap in every non-master volume Marshall I've owned to get it "just right" (at least for what I like). Glad you found what you were looking for with that minor change.
 
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Matthews Guitars

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It's well worth it to experiment with bright cap values. They can totally change the character of the amp.

In the case of a Superlead, at least, the difference can be amazing. I changed the bright cap from 150 pF all the way up to 5600 pF, and it's a Jekyll and Hyde level of transformation. It's SO aggressive! Love it like that! For a more classic tonality I can just use my other Superlead which has the stock value bright cap.
 

ckaudio

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Thanks for the info, and sounds like you did it right by trying several values. A lot of guys just go into an amp and clip, or break off the bright cap, and claim it's "perfect now". I've not owned a Marshall that sounded it's best with the bright cap completely removed (my opinion), but I've changed the bright cap in every non-master volume Marshall I've owned to get it "just right" (at least for what I like). Glad you found what you were looking for with that minor change.

I haven’t owned any other actual tube Marshall’s except for a JCM2000, so I can’t compare this one to a “real” Marshall. But I’d reckon part of bright cap sound comes down to personal taste and the sound one is after, so it’s nice to be able to tweak the value as one wishes! I’ve debated selling the SV20 now that the resale value is higher, and use the money to build a 50W clone of some sort, but I think I’m happy with it now, it’s a really well built amp for the most part.
 
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Ramhead

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I documented the whole process with photos if anyone is interested in a walkthrough. If you’re savvy, you’ve probably already done this, if you’re at all handy with a solder iron, it’ll be a breeze!

I would love to do that on mine, so please share the whole process. I know how to do it on vintage amps, but on this one I don't know where its located.
It will come in handy to many other users. Thanks!
 

FleshOnGear

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Hi all -

I’ve been a Studio Vintage SV20 owner for a couple of years now. While I quite love the amp I’ve never truly been able to love the bright channel as much as I’d like to. I felt like the normal channel was too dark and the bright channel was too bright, and when cranking the bright channel I just got more gain than I wanted, and when mixing the channels I couldn’t get a balance I was happy with without it being too bright.

I’m not a metal guy, not really into the 80’s hot rodded plexi sound, I was going for more of a 70’s classic Marshall crunch, which is why I picked the SV20 over the SC20. I wanted to be able to play Jimi Hendrix, Zeppelin, AC/DC etc., and at the time the SV20 was a lot more affordable than a 1987X. I picked it up when it was $1,300 new.

I think that I was hesitant to make a mod to it because of how many people claim that the Plexi is supposed to sound like that and if it doesn’t sound good you’re not running it right. I spent a lot of time with this amp. I ran the amp volume high and rolled off the guitar volume etc, etc, and frankly these 20W models don’t sound like a 50W/100W model. The small transformers are likely part of the reason, as well as the low plate voltage running the tubes cold to keep the output at 20W.

Anyway… Today, I finally ended up changing the bright cap from the stock 4.7nF down to 100pF, and I must say I’m honestly impressed with how much better the amp feels. I first removed the cap and ran 2 wires from the pcb, and used alligator clips to try about 7 diffferent values ranging from no cap to 4.7nF (stock). I ended up going with 100pF because it gave back some of the clarity and air without feeling like the top end was falling apart.

Clipping the cap definitely felt too dark to me. It’s still brighter than the normal channel because of the 470pF cap on the channel mixing resistor, but with it completely removed it definitely felt like it lost a little magic.

I am running the amp head through an Origin 2x12 vertical cab which I replaced the stock Seventy-Eighty speakers (junk) with modern day G12-M Greenbacks. It now has the thick creamy bluesy overdrive that I was wanting. It does the AC/DC thing, Zeppelin riffs sound incredible as well, I run the High Treble volume on about 6-8, and it’s awesome. The 20W seems to move enough air to make me feel the magic without pissing off the neighbors.

If you’re an SV20 owner who feels the same way I did about your amp, I highly suggest you try the mod. You simply replace C101 on the control panel PCB. The whole PCB comes off by removing nuts from the pots, and pulling a couple of wire connectors.

I documented the whole process with photos if anyone is interested in a walkthrough. If you’re savvy, you’ve probably already done this, if you’re at all handy with a solder iron, it’ll be a breeze!
Glad you got it sounding the way you like!
And :welcome: to the forum!
 

purpleplexi

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Is the bright cap not removed from the circuit once the volume is halfway up?
 

ckaudio

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I would love to do that on mine, so please share the whole process. I know how to do it on vintage amps, but on this one I don't know where its located.
It will come in handy to many other users. Thanks!
I’ll post the walk through today, I will have all the photos in a google drive folder for reference (it seems like it’s hard to post good quality images without them being too large on the forum).
 

ckaudio

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Is the bright cap not removed from the circuit once the volume is halfway up?
The bright cap makes a capacitor-resistor high pass filter, the resistor being a variable potentiometer, as you turn up the volume you increase more lows. So at full volume the cap is “bypassed” but I’d argue that it’s effect still quite noticeable even at 75% volume. At least with the stock value.
 

ckaudio

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It's well worth it to experiment with bright cap values. They can totally change the character of the amp.

In the case of a Superlead, at least, the difference can be amazing. I changed the bright cap from 150 pF all the way up to 5600 pF, and it's a Jekyll and Hyde level of transformation. It's SO aggressive! Love it like that! For a more classic tonality I can just use my other Superlead which has the stock value bright cap.
Nice! Definitely something that’s easily tweakable to dial in the type of sound you’re after!
 

ckaudio

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I would love to do that on mine, so please share the whole process. I know how to do it on vintage amps, but on this one I don't know where its located.
It will come in handy to many other users. Thanks!

How to Mod Bright Cap on Marshall Studio Vintage (SV20)

This walkthrough is for the head unit not the combo, but the circuitry inside should be the same, you’ll just have to figure out how to remove your amp chassis if you have a combo.

As with all tube amps, be cautious when handling the inside of the amp as there are capacitors that can contain lethal voltages.

1. Remove the rear panel from the amp (4 screws)

2. Remove the amp chassis from the cabinet. (For the head I find it easiest to turn the amp on it’s side, and remove the 4 screws on the bottom panel while holding onto the power transformer to prevent the chassis from falling when the screws are all out.)

3. Remove the power tubes (allows you to lay the amp upside down so it rests on the transformers, not your precious EL34s.

4. Pull off all of the knobs from the front panel (I used a screw driver to pry up gently while pulling. They’re a little tight at first but they just pull off.)

5. Use an 11mm socket wrench to remove the nuts from all of the potentiometers.

6. Pull the control panel PCB out of the amp and remove the two wire connectors to free the board (pull from the base of the connectors, not from the wires.)

7. Remove C101 (4.7nF box film cap behind the bright channel volume pot) by removing the solder from the underside of the board and pulling the cap out from the top (desolder wick is the BEST way to do this without damaging the PCB, if you don’t own Chem-Wick, order some, it’s on Amazon and it’s great. It has resin in the wick that lifts and pulls the solder from the PCB.)

8. Replace C101 with the cap value of your choice, I ended up using a 100pF. To test multiple values I connected 2 wires from the PCB and used alligator clips. I tried 3.3nF, 2.2nF, 1nF, 820pF, 390pF, and 100pF and all but the 100pF seemed harsh and unpleasant to me. I would like to try 220pF as that’s another common value I’ve seen used, but I didn’t have any on hand. Note: the stock cap is rated for 400V, this cap only sees like 3-5 AC volts from the guitar signal after it’s boosted from the first preamp stage, so you **can** use a lower rated one if you wish, but be aware if you ever had a coupling cap blow/leak you could potentially blow the bright cap from the plate voltage, though it’s unlikely. Best practice is to have all caps rated for the highest voltage they could potentially come into contact with.

9. Replace the wire connectors and place the control panel PCB back in the front panel, and fasten all of the pot nuts snuggly (just till it feels tight with your wrist, don’t crank on them too much.) Replace the knobs (they just slide back on).

10. Replace the power tubes in their sockets. (They only go in one way, line up the notch on the power tube base and the socket.)

11. Place the amp chassis back in the cabinet and fasten the 4 screws holding it in place. (Do this by hand not with a power tool, you don’t want to cross thread them and if you overtighten them the tolex will start to twist around the screw heads/washer.)

12. Replace the rear panel with the 4 screws (again by hand for the same reason above.)

13. Turn on the amp, plug into the bright channel and enjoy!

Pics for reference can be found here:

SV20 Bright Mod Pics

Hope this helps someone!
 

wakjob

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Whelp.... I'm not gonna tell ya that you're wrong for doing it.
Makes sense for what you are gunning for tonally.

Thanks for the post.
Threads like this will help when I get mine. :cool:
 

marshallmellowed

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A bit off-topic, but not totally... One of the things I love about the Axe Fx, is the ability to change key components of an amp model, with the click of the mouse. You can change the bright cap value, as well as preamp and power amp tube types, of any amp model in the Axe Fx. Changing the bright cap value of an amp model gives you a pretty close approximation of how it would sound in the "real" amp, if you happen to have the real amp, and are thinking about changing the bright cap.
 

ckaudio

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A bit off-topic, but not totally... One of the things I love about the Axe Fx, is the ability to change key components of an amp model, with the click of the mouse. You can change the bright cap value, as well as preamp and power amp tube types, of any amp model in the Axe Fx. Changing the bright cap value of an amp model gives you a pretty close approximation of how it would sound in the "real" amp, if you happen to have the real amp, and are thinking about changing the bright cap.

That’s pretty awesome! Good to know! Kinda like how I use my Torpedo Captor X to try different speaker IRs to figure out what speakers I like for my amps 😂 technology has come along way.
 

Ramhead

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How to Mod Bright Cap on Marshall Studio Vintage (SV20)

This walkthrough is for the head unit not the combo, but the circuitry inside should be the same, you’ll just have to figure out how to remove your amp chassis if you have a combo.

As with all tube amps, be cautious when handling the inside of the amp as there are capacitors that can contain lethal voltages.

1. Remove the rear panel from the amp (4 screws)

2. Remove the amp chassis from the cabinet. (For the head I find it easiest to turn the amp on it’s side, and remove the 4 screws on the bottom panel while holding onto the power transformer to prevent the chassis from falling when the screws are all out.)

3. Remove the power tubes (allows you to lay the amp upside down so it rests on the transformers, not your precious EL34s.

4. Pull off all of the knobs from the front panel (I used a screw driver to pry up gently while pulling. They’re a little tight at first but they just pull off.)

5. Use an 11mm socket wrench to remove the nuts from all of the potentiometers.

6. Pull the control panel PCB out of the amp and remove the two wire connectors to free the board (pull from the base of the connectors, not from the wires.)

7. Remove C101 (4.7nF box film cap behind the bright channel volume pot) by removing the solder from the underside of the board and pulling the cap out from the top (desolder wick is the BEST way to do this without damaging the PCB, if you don’t own Chem-Wick, order some, it’s on Amazon and it’s great. It has resin in the wick that lifts and pulls the solder from the PCB.)

8. Replace C101 with the cap value of your choice, I ended up using a 100pF. To test multiple values I connected 2 wires from the PCB and used alligator clips. I tried 3.3nF, 2.2nF, 1nF, 820pF, 390pF, and 100pF and all but the 100pF seemed harsh and unpleasant to me. I would like to try 220pF as that’s another common value I’ve seen used, but I didn’t have any on hand. Note: the stock cap is rated for 400V, this cap only sees like 3-5 AC volts from the guitar signal after it’s boosted from the first preamp stage, so you **can** use a lower rated one if you wish, but be aware if you ever had a coupling cap blow/leak you could potentially blow the bright cap from the plate voltage, though it’s unlikely. Best practice is to have all caps rated for the highest voltage they could potentially come into contact with.

9. Replace the wire connectors and place the control panel PCB back in the front panel, and fasten all of the pot nuts snuggly (just till it feels tight with your wrist, don’t crank on them too much.) Replace the knobs (they just slide back on).

10. Replace the power tubes in their sockets. (They only go in one way, line up the notch on the power tube base and the socket.)

11. Place the amp chassis back in the cabinet and fasten the 4 screws holding it in place. (Do this by hand not with a power tool, you don’t want to cross thread them and if you overtighten them the tolex will start to twist around the screw heads/washer.)

12. Replace the rear panel with the 4 screws (again by hand for the same reason above.)

13. Turn on the amp, plug into the bright channel and enjoy!

Pics for reference can be found here:

SV20 Bright Mod Pics

Hope this helps someone!

Many thanks! :)
 

Kinkless Tetrode

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I ended up with a 100pf in my JTM45, even with the shared cathode. I think the most common value for JTM45s was 500pf, but someone posted their all original JTM45 guts here a few years ago and it had 100pf. I think certain values were more common on certain models and during certain periods, but there was some variation, so there's not really correct values and incorrect values. The schematics I have examined didn't start specifying real high values until the 70's.
 

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