Studio Vintage SV20 mods (and SC20 too)

[Pete Farrington]

I wrote them down if you need them.

I’m interested to see them, if you don’t mind looking them out, thanks

 

[_Steve]

I’m interested to see them, if you don’t mind looking them out, thanks

No problem. Im on holiday right now, will upload in when im back this weekend.

 

[_Steve]

I’m interested to see them, if you don’t mind looking them out, thanks

Here you go. Bear in mind this was the first ever one I traced back when I was just beginning learning about tube amps. One is for 20W mode, the other is for 5W mode.

 

[Franksamps]

I replaced the 4n7 bright cap with a 100pf. Any 100-470pf will work well. Choose for your required brightness, the higher the brighter..

 

[shredgd]

I replaced the 4n7 bright cap with a 100pf. Any 100-470pf will work well. Choose for your required brightness, the higher the brighter.. View attachment 95310

Actually the lower value a bright cap, the higher the frequencies it lets go through. As you increase the value of the bright cap, you let progressively more high mids and then mids (for big values like the stock one) go past the potentiometer. Of course, when you start passing high mids, you have more volume, that’s why a lot of people go wrong with saying it’s brighter, but if you normalize volumes, it’s definitely not.
The stock value is so big, the volume pot actually really acts as a bass roll off control more than anything.

Giulio

 

[_Steve]

Actually the lower value a bright cap, the higher the frequencies it lets go through. As you increase the value of the bright cap, you let progressively more high mids and then mids (for big values like the stock one) go past the potentiometer. Of course, when you start passing high mids, you have more volume, that’s why a lot of people go wrong with saying it’s brighter, but if you normalize volumes, it’s definitely not.
The stock value is so big, the volume pot actually really acts as a bass roll off control more than anything.

Giulio

I think you may have misread the post you replied to.

 

[shredgd]

I think you may have misread the post you replied to.

No, I didn’t. He wrote “the higher [value], the brighter”, instead it’s the lower, the brighter and I explained why.

Giulio

 

[_Steve]

No, I didn’t. He wrote “the higher [value], the brighter”, instead it’s the lower, the brighter and I explained why.

Giulio

Perhaps re-read it again

 

[shredgd]

Perhaps re-read it again

I’m sure it’s you, which should read again

 

[Ed A]

ANY MODS FOR MORE BASS?

Hey guys. I just picked up a new SV20 head. I’m playing it through a 1x12 with an old JBL D120. Amp sounds great, better than I expected. I play it with High Treble volume on 8 to 9. And mixing in Normal volume at around 4 to 5. Convincing plexi tones, I just wish I can get more bottom out of the amp! At the volume I’m playing the bright cap is pretty much already pulled out. My issue is if I try to mix in more of the Normal volume the bottom end gets muddy. I’m guessing due to the small transformers. Same happens if I try to mix in more bass. Overall the amp sounds great but a bit hard and brittle. So I am going to try a set of Mullard XF2s I have on the shelf and some other preamp tubes. I’m sure I can soften it and sweeten it a bit. But I don’t think those things will give me a bigger and tighter bottom end.

Is it possible with the high treble volume on eight the bright Is still somewhat engaged? Should I try snipping it?

I am new to this amp and sorry I have not gone through the couple hundred pages here but wondering if there are any mods I can do to the circuit to fatten that bottom end?! Thanks!

 

[_Steve]

Hi - the method @Franksamps shows in post #44 above is the easiest as the bright cap is on the daughter board and easily accessible. IMO ~470pF is a sweet spot, but experiment with some alligator clips to find what you like.

Also you could increase the V1B coupling cap from .0022uF to .02uF. This would move the amp a little more towards a bass-spec plexi.

Good luck!

 

[shredgd]

ANY MODS FOR MORE BASS?

Hey guys. I just picked up a new SV20 head. I’m playing it through a 1x12 with an old JBL D120. Amp sounds great, better than I expected. I play it with High Treble volume on 8 to 9. And mixing in Normal volume at around 4 to 5. Convincing plexi tones, I just wish I can get more bottom out of the amp! At the volume I’m playing the bright cap is pretty much already pulled out. My issue is if I try to mix in more of the Normal volume the bottom end gets muddy. I’m guessing due to the small transformers. Same happens if I try to mix in more bass. Overall the amp sounds great but a bit hard and brittle. So I am going to try a set of Mullard XF2s I have on the shelf and some other preamp tubes. I’m sure I can soften it and sweeten it a bit. But I don’t think those things will give me a bigger and tighter bottom end.

Is it possible with the high treble volume on eight the bright Is still somewhat engaged? Should I try snipping it?

I am new to this amp and sorry I have not gone through the couple hundred pages here but wondering if there are any mods I can do to the circuit to fatten that bottom end?! Thanks!

You might also try working on the power amp section: you can increase bass by adding a fixed depth/resonance circuit in the negative feedback. Try to add a 220k (or 100k) and a 4.7n (or 2.2n) cap in parallel to it before the feedback resistor (sorry I can’t locate it, I don’t own an SV20): this is a very common mod for hot rodded Marshalls, but might work for you as well.

Giulio

 

[Ed A]

Hi - the method @Franksamps shows in post #44 above is the easiest as the bright cap is on the daughter board and easily accessible. IMO ~470pF is a sweet spot, but experiment with some alligator clips to find what you like.

Also you could increase the V1B coupling cap from .0022uF to .02uF. This would move the amp a little more towards a bass-spec plexi.

Good luck!

Thanks for the reply and sorry to bug ya all on Christmas day!.... so I will experiment with the bright cap for sure, I generally removed them completely from my vintage plexis... also for the most part I always replaced the .0022 with .02 on my vintage plexis as well...

BUT, Im not much of a tech guy and never heard of this amp until a few days ago, not familiar with it... would anyone have a pix of where I can find the V1B coupling cap?

Thanks!

 

[_Steve]

Thanks for the reply and sorry to bug ya all on Christmas day!.... so I will experiment with the bright cap for sure, I generally removed them completely from my vintage plexis... also for the most part I always replaced the .0022 with .02 on my vintage plexis as well...

BUT, Im not much of a tech guy and never heard of this amp until a few days ago, not familiar with it... would anyone have a pix of where I can find the V1B coupling cap?

Thanks!

Sorry I dont know off the top of my head but should be easy to find with a DMM. Happy to walk you through it if you are unsure

 

[Ricardo Simas]

Hi, a bit of a random question from a new member: does anyone know if the power voltage on the SV20s can be converted somewhat easily?

 

[_Steve]

Hi, a bit of a random question from a new member: does anyone know if the power voltage on the SV20s can be converted somewhat easily?

I dont believe so sorry. I think it requires the power transformer to be changed and I dont think Marshall sell replacements.

 

[Ricardo Simas]

I dont believe so sorry. I think it requires the power transformer to be changed and I dont think Marshall sell replacements.

I was hoping it used a multi-tap transformer. Oh well, that’s what step up/down transformers are for.
Thanks!

 

[Anson]

Does anyone know where the phase inverter coupling caps are located on the SV20? I think I have an idea, but just want to be sure. Converting it into a mini Superbass.

 

[junk notes]

Does anyone know where the phase inverter coupling caps are located on the SV20? I think I have an idea, but just want to be sure. Converting it into a mini Superbass.

whats up!
I have a second SV waiting to be converted to a 1992. The amp has been out since 2019 and will be waiting longer for the schematic. ridiculous.

 

[Jordan Prysmiki]

From my experience the boards are very durable and not all that delicate, anywhere. It's a high grade pcb. The fact that parts may not be easy to get to with human hands holding a soldering iron does not make it less durable or made of spider silk.