SV20 - Bright Cap Mod is Worth it

[ckaudio]

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.

Good to know! That makes me feel a little better about being happier with the 100pF 😆 the higher values can definitely have their own sound, but that wasn’t the sound I was looking for with this amp!

 

[ckaudio]

For anyone interested -

I recorded this little jam with my SV20 post bright cap mod.

Running it straight into my Torpedo Captor X with a Pre Rola Greenback cab sim. It’s my favorite way to record because I can save/recall my settings without having to setup mics, plus I can crank the amp any time if the day 😄 and it sounds better than when I mic my cabs 😂

Settings are all listed in the description for those interested.

 

[blackeye-liner]

Just did this mod to my SV20, tried a couple of different values just for safety but yes, 100pF was sounding good - didn't try even lower values since I didn't have them on hand. Sounds very good right now, compared to stock cap it's so strange I could tolerate it for so long - that brightness was glaring.

Thank you for the instructions! That demo sounds awesome.

Did not expect that drop in gain though! With boost it's just fine, but without boost it's so much cleaner now!

 

[vivanchenko]

I just roll off the volume and/or tone pots a bit. Tames any excessive brightness like magic and sounds fantastic. Leaves your high end on tap which you will likely need when playing in a band.

 

[2203xman]

A really useful option is a bright cap selector switch where you can choose between two different bright caps or none with a 3 way mini toggle.Works great and one of my favorite mods on the Ceriatone plexi 51.

 

[ckaudio]

Just did this mod to my SV20, tried a couple of different values just for safety but yes, 100pF was sounding good - didn't try even lower values since I didn't have them on hand. Sounds very good right now, compared to stock cap it's so strange I could tolerate it for so long - that brightness was glaring.

Thank you for the instructions! That demo sounds awesome.

Did not expect that drop in gain though! With boost it's just fine, but without boost it's so much cleaner now!

Nice! That’s exactly how I felt after doing it on my SV20, how did I tolerate it for so long! Night and day difference to me.

There is a drop in gain, I usually run my presence knob dimed which helps but the amount of gain that was left is exactly the classic crunch Marshall tone I wanted with my Les Paul, and with single coils I can get that clean/edge if breakup Hendrix kind of tone.

 

[ckaudio]

I just roll off the volume and/or tone pots a bit. Tames any excessive brightness like magic and sounds fantastic. Leaves your high end on tap which you will likely need when playing in a band.

To each his own! That’s why it’s such a great value to be able to tweak. The stock bright cap was more than I could handle even adjusting the tone/volume pots I could never get the sound I wanted. The 100pF value leaves me plenty of highs to cut through in a mix with the way I’m running it.

 

[ckaudio]

A really useful option is a bright cap selector switch where you can choose between two different bright caps or none with a 3 way mini toggle.Works great and one of my favorite mods on the Ceriatone plexi 51.

Yes! I almost did this but was hesitant to drill any holes. I liked the 100pf so much that I decided I didn’t need a switch, but at some point I may add a 3-way toggle to have more flexibility!

 

[blackeye-liner]

A really useful option is a bright cap selector switch where you can choose between two different bright caps or none with a 3 way mini toggle.Works great and one of my favorite mods on the Ceriatone plexi 51.

That's what I was thinking, and I ordered a 3-way switch as well as multiple cap values. However, seems like the lowest value was the best one I've tried, and doing it with values higher than that made very little sense at the time. Now I'm thinking to try 2-3 even lower value options to see if that would work, and maybe that will go into the switch options.

 

[goldtop0]

Thanks ck, you've reminded me of what I've had done to one or two of my other M amps over the years.
I don't work on them like you guys I only swap tubes, speakers etc and tend to forget what I get my amp guru to do, so now it's the bright cap I'll get done on my SV20, voila, thanks
And all the best for 2023 !!

 

[ckaudio]

Thanks ck, you've reminded me of what I've had done to one or two of my other M amps over the years.
I don't work on them like you guys I only swap tubes, speakers etc and tend to forget what I get my amp guru to do, so now it's the bright cap I'll get done on my SV20, voila, thanks
And all the best for 2023 !!

Very cool, hope it helps you love your SV20 even more!

 

[tmingle]

I added a 330pf to my 40CR a while back & liked it better. I just finished installing a dual switch in place of the caps to make it switchable. I used 440pf & 100pf & really like the results with the 440pf. My ears cannot hear much change between 100pf & no cap.

 

[Figaro]

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!

I’m surprised you couldn’t get a good enough balance with just jumping the channels. Maybe going into the Normal channel first or using more of the Normal channel?

 

[paul-e-mann]

@ckaudio The higher the volume the darker the tone gets. I also like classic rock tones, I put a JHS volume box in the loop and have my bright channel volume dimed and get those tones with no mods and at any volume. Just pointing out theres other ways to get there.

 

[ckaudio]

@ckaudio The higher the volume the darker the tone gets. I also like classic rock tones, I put a JHS volume box in the loop and have my bright channel volume dimed and get those tones with no mods and at any volume. Just pointing out theres other ways to get there.

For sure, for me though, even with the bright channel dimed (or both channels) it had some nasty/harsh overtones that I wasn’t able to really tame. I’m sure I could have inserted an EQ or volume or something into the loop to try and dial it how I wanted but it was easier for me to just mod it to sound how I wanted without needing any additional gear going forward lol.

I’m sure not everyone feels comfortable modding an amp so it’s good to have other options.

 

[goldtop0]

On the High Treble inputs there's a good lot of hiss as you dial up the volume, will discuss this with my amp guru as I'm pretty sure that's to do with the resistors used whether they're carbon film or metal. Can't recall which one cuts down the noise factor.

 

[Central Scrutinizer]

Good information and write-up @ckaudio .

Cool video and editing, sounds good.
I think a lot of your tone issue is the fact that you are left handed !

 

[ckaudio]

On the High Treble inputs there's a good lot of hiss as you dial up the volume, will discuss this with my amp guru as I'm pretty sure that's to do with the resistors used whether they're carbon film or metal. Can't recall which one cuts down the noise factor.

Yes the high treble channel is very noisy. I wouldn’t be surprised if the issue is more related to the PCB layout/wiring rather than the type of resistors. A lot of things can cause noise, while carbon resistors are noisier, metal film won’t help much if the noise is coming from a bad grounding path design on the pcb etc.

 

[ckaudio]

Good information and write-up @ckaudio .

Cool video and editing, sounds good.
I think a lot of your tone issue is the fact that you are left handed !

I don’t doubt that 😂 left handedness caused me all sorts of trouble

 

[goldtop0]

Yes the high treble channel is very noisy. I wouldn’t be surprised if the issue is more related to the PCB layout/wiring rather than the type of resistors. A lot of things can cause noise, while carbon resistors are noisier, metal film won’t help much if the noise is coming from a bad grounding path design on the pcb etc.

Cheers ck, I'll discuss with it my chap. He'll source the schematic and then we'll take it from there. Undoubtedly it'll be more problematic working with a PCB than handwired. Used to have all sorts of fun and games many years ago dealing with the '80s M solid state amps, dry joints etc were rife