Studio Vintage SV20 mods (and SC20 too)

Trapland

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Well I started the first stage of modding my SV20. I’ve been playing bass, pa and organ plexis rather than 1987 model lead amps for awhile now. I just don’t care for the tone of a split cathode treble peaked lead amp.

Today I started by putting both V1 cathodes on the normal side. The next step will be changing the tonestack. Then if needed changing coupling caps, NFB, and adding a choke.

There’s no schematics out that I am aware of, and I haven’t found any sources of mod pioneers, or suggestion. So I just started. I’m only learning the design as I go upstream and need to know.

So far it’s essentially the same preamp as a 1987 Lead amp. Only honkier.

A word of caution. Wreck your own amp your own way. These things are made of spider silk and delicate wisps of foam. They are delicate. If you touch it expect to convert everything you touch to point to point as the solder pads and traced just disappear into space as soon as any human comes near.

Take out some screws and unplug some stuff.
do it some more.

Also the tube retainer clips must be removed as the screws are part of the board mounting.
This is the cathode bypass cap from the bright channel and the cathode resistor from the normal.
and the 330mfd cathode bypass cap for the normal channel and under the socket is the bright channel cathode resistor. Arggg!

So I lifted the socket leg from the bright channel cathode because to disconnect resistor and cap any other way would have required Insanity. I then connected the 2 cathodes to the normal channel cathode bias circuit like a bass model.
I STUPIDLY lifted the bright channel grid socket leg. Something about not counting upside down. No problem, it came out easy. Nope, as soon as I pushed the clean leg back into the hole the trace was gone. So I started my conversion to point to point today to keep it running.

Anyway, it took a lot of the nasal quality out and added some clarity. Next is change the tone stack and I hope I’m done.
 
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Trapland

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Maybe someone could help me with something. Since I’m sharing the cathode circuit I’ve noticed an occilation when jumping channels. I can make it go away by turning down presence and treble A LOT, and there is no issu if I don’t jump channels. I though lead dress and maybe it still is, but I have tried quite a few adjustments.

Ideas? I still haven’t done the tone stack or played with NFB.
 

Mach70

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Did you notice the bright cap while you were in there? I've clipped/reduced values on a 1987x before and am curious if this board is similar? Location or cap# would be very helpful, thank you!!
Doug
 

tompilompa

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Did you notice the bright cap while you were in there? I've clipped/reduced values on a 1987x before and am curious if this board is similar? Location or cap# would be very helpful, thank you!!
Doug

Im interested about the bright cap as well!
 

nickstomp

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Hi i Just got a Marshall sv20 nice amp for now
The schematic is basically a super lead with the exception of 820 ohm instead of 2.7k on the bright channel cathode and effects loop/speaker simulation.
The effects loop is bypassed with two relays so is totally out of the circuit when disabled.
For now I only swapped the resistor with a2.7k to tame the high a bit and I was thinking of replacing the 4.7nf as well with a mica/less harsh cap. What do you think?
 

nickstomp

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Ok my mod so far:

1) I changed the input cathode bypass to superbass style -> removed the 820 ohm from v1b and connect the cathode of v1b to v1a
2) I changed the 100k cathode resistor of v2b to carbon film instead of metal film

The amp seems to be slightly better that way(but is personal)

I think the capacitor and the pots are good in this amp and they did a good job also on the resistors using metal film and carbon film where it matters.
Now I was thinking to mod it with a master volume and maybe adjustable bias
 

shredgd

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Hi, I noticed there are two .68uF caps on the board but the 1987 only calls for one...
Did anyone discover what the second cap does?

Thank you,
Giulio
 

shredgd

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I found out some 1987 schematics report another .68uF cap at V2A cathode, which might be a more common version which people at Marshall decided to reproduce.
 

Trapland

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Hi i Just got a Marshall sv20 nice amp for now
The schematic is basically a super lead with the exception of 820 ohm instead of 2.7k on the bright channel cathode and effects loop/speaker simulation.
The effects loop is bypassed with two relays so is totally out of the circuit when disabled.
For now I only swapped the resistor with a2.7k to tame the high a bit and I was thinking of replacing the 4.7nf as well with a mica/less harsh cap. What do you think?

Where did you find the schematic? Or have you traced it yourself? Either way, please share.
 

Trapland

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Regarding the SV20H bright cap. It’s been a year since I modded mine and I can’t understand why anyone would want to remove it but....

The bright cap is one the same board the volume pot is on. It’s pretty easy to see the cap across the pot legs. Crush it or cut the trace on the board.
 

Trapland

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Ok my mod so far:

1) I changed the input cathode bypass to superbass style -> removed the 820 ohm from v1b and connect the cathode of v1b to v1a
2) I changed the 100k cathode resistor of v2b to carbon film instead of metal film

The amp seems to be slightly better that way(but is personal)

I think the capacitor and the pots are good in this amp and they did a good job also on the resistors using metal film and carbon film where it matters.
Now I was thinking to mod it with a master volume and maybe adjustable bias


I am VERY much interested in your plan to convert to adjustable bias.

The master is easy, but honestly a MV on a plexi is kind of meh.
 

_Steve

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Hey! So glad I found this thread!

I've literally just started tracing my SV20H to try to learn how the power section works. I'd also be EXTREMELY keen to know more about any plans to add fixed/adjustable bias!

Also wondering if anyone knows what the second 5W 150 Ohm resistor in the power section does??? As far as I can tell it is NOT a cathode resistor as both the tubes are sharing the other nearby 150 Ohm one as their shared cathode resistor.

Attaching some (admittedly poor) underside shots of the mainboard i took last night:

20200428_141639_sized.jpg

20200428_141647_sized.jpg

20200428_141652_sized.jpg
 

Trapland

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I
Hi i Just got a Marshall sv20 nice amp for now
The schematic is basically a super lead with the exception of 820 ohm instead of 2.7k on the bright channel cathode and effects loop/speaker simulation.
The effects loop is bypassed with two relays so is totally out of the circuit when disabled.
For now I only swapped the resistor with a2.7k to tame the high a bit and I was thinking of replacing the 4.7nf as well with a mica/less harsh cap. What do you think?

know it’s been some months but where did you get a schematic? I have more mods to do and a schematic would be very helpful.
 

Freddy78

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Great info , there is any progression with the MV mod as i would like to have one installed on my SV20 ?
 


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