Studio Vintage SV20 mods (and SC20 too)

[Freddy78]

yes for home use its way too loud , and i prefer not using attenuator as the tone changes

 

[_Steve]

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

You should be able to do a MV mod pretty easily without the need for a schematic. Just put a 1Meg audio pot across the grids of the power tubs. See here: https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Trainwreck_Type-3_Master_Volume

As far as I know theres no schematic publicly available for the SV20.

 

[Linster]

I bought a JCM SC 20 and the amp is just too brittle. I tried speakers, tubes but too much bad gain. I had a 78 2204 and it did not sound like that. The 78 had a ton more bass. I was going to shove this down the road then I decided to removed the bright cap. Total game changer for this amp. The amp is very usable now, but is darker/Less Treble in a very good way. You can now use the Treble an Presence controls other than being at 1 or 2 Gives it more of a Plexi sound, but a full tone, but high gain, that is controllable. Removing this does not change the 10 gain sound, only the lower levels. Best thing I ever did. Speakers and tubes now make a difference you can hear other than the nasal harshness it had. I only use about 3-5 on the pre amp gain, but it is still an beast. I have tried a couple other things using alligator clips, bypass the 820 on V2 #3 with a 20uf. Not a big gain boost but fuller sound, I like that seems to be a slightly boosted sound but fuller. Jumped another 10K on V2 #8 to equal a 4.7K, not sure I liked that but only gave it a few minutes because the bright cap was still in, will try that again. The amp is a keeper now just removing the bright cap. A little more testing to come but the amp now sounds the way I like. Not a fan of the low power mode, just sounds Blah, but I only gave it a few minutes there. I have not dug into that area at all on how that is achieved.

 

[_Steve]

I bought a JCM SC 20 and the amp is just too brittle. I tried speakers, tubes but too much bad gain. I had a 78 2204 and it did not sound like that. The 78 had a ton more bass. I was going to shove this down the road then I decided to removed the bright cap. Total game changer for this amp. The amp is very usable now, but is darker/Less Treble in a very good way. You can now use the Treble an Presence controls other than being at 1 or 2 Gives it more of a Plexi sound, but a full tone, but high gain, that is controllable. Removing this does not change the 10 gain sound, only the lower levels. Best thing I ever did. Speakers and tubes now make a difference you can hear other than the nasal harshness it had. I only use about 3-5 on the pre amp gain, but it is still an beast. I have tried a couple other things using alligator clips, bypass the 820 on V2 #3 with a 20uf. Not a big gain boost but fuller sound, I like that seems to be a slightly boosted sound but fuller. Jumped another 10K on V2 #8 to equal a 4.7K, not sure I liked that but only gave it a few minutes because the bright cap was still in, will try that again. The amp is a keeper now just removing the bright cap. A little more testing to come but the amp now sounds the way I like. Not a fan of the low power mode, just sounds Blah, but I only gave it a few minutes there. I have not dug into that area at all on how that is achieved.

Cool thanks for posting!

 

[shredgd]

I bought a JCM SC 20 and the amp is just too brittle. I tried speakers, tubes but too much bad gain. I had a 78 2204 and it did not sound like that. The 78 had a ton more bass. I was going to shove this down the road then I decided to removed the bright cap. Total game changer for this amp. The amp is very usable now, but is darker/Less Treble in a very good way. You can now use the Treble an Presence controls other than being at 1 or 2 Gives it more of a Plexi sound, but a full tone, but high gain, that is controllable. Removing this does not change the 10 gain sound, only the lower levels. Best thing I ever did. Speakers and tubes now make a difference you can hear other than the nasal harshness it had. I only use about 3-5 on the pre amp gain, but it is still an beast. I have tried a couple other things using alligator clips, bypass the 820 on V2 #3 with a 20uf. Not a big gain boost but fuller sound, I like that seems to be a slightly boosted sound but fuller. Jumped another 10K on V2 #8 to equal a 4.7K, not sure I liked that but only gave it a few minutes because the bright cap was still in, will try that again. The amp is a keeper now just removing the bright cap. A little more testing to come but the amp now sounds the way I like. Not a fan of the low power mode, just sounds Blah, but I only gave it a few minutes there. I have not dug into that area at all on how that is achieved.

The probable reason why you prefer the bright cap removed is the low gain settings (3-5 you said) you like to dial in the preamp gain control. Bright caps are often "too much" when the control they're attached to is set under about 2/3 of its run, so they give the best results above about 7 out of 10 in the dial.
Another cool mod is to raise the first 470pf treble peaker value (the one before the preamp gain pot), which increases mids. You might use the 1nf cap you removed to parallel it for a total of 1470pf, or go on to a total of 2200pf as seen in the 5150, Soldano SLO and AFD100/Slash mods.

Giulio

 

[_Steve]

The probable reason why you prefer the bright cap removed is the low gain settings (3-5 you said) you like to dial in the preamp gain control. Bright caps are often "too much" when the control they're attached to is set under about 2/3 of its run, so they give the best results above about 7 out of 10 in the dial.
Another cool mod is to raise the first 470pf treble peaker value (the one before the preamp gain pot), which increases mids. You might use the 1nf cap you removed to parallel it for a total of 1470pf, or go on to a total of 2200pf as seen in the 5150, Soldano SLO and AFD100/Slash mods.

Giulio

This is all spot on!

I experimented removing the bright cap, but for me it just lost its liveliness/mojo so I put it back. Horses for courses of course.

Personally im a low-gain guy too and I prefer to run strapped channels with the Normal at around 4 and then I use a tiny bit of the Bright volume to mix in some highs (which some people call 'sizzle'). Rock and roll!

 

[Linster]

Some thoughts after trying different things. Removing the bright cap has suggested above does make the amp sound dull under 3. With it off about 2 on the Volume and 8 on the master you can get into a plexi cranked sound, but it can get dark depending on the speaker. With 20w you can only get a certain amount of bass out of the amp, lows take more power to reproduce thank highs. In my opinion, the studios are much brighter then the 50w or 100w just due to power to produce the low frequencies. The circuit is vintage correct from what I traced out. I don't do metal so I really like a moderate amount of gain. To keep things consistent, all tone knobs stay at 5. I use a R9 for all tests. I also use a 2/12 cab. I started with a mix of a G12H and a G12M-25w. OK, but lacked some clarity I want. Tried 2 G12H went got the clarity but not right for me. I had a new ET65 I never tried, paired that with a G12H and that seemed just right. I got another ET65 coming to try a pair. He are the tests, I used alligator cable to not hack the amp. I put a 50uf across the 820 in V2, the BE's do that. Sounds good, not a big gain change. Removed that the jumped a 10K across the 10K on V1-8. That is a gain changer, sounds good but adds more gain than I want. I would do that with a switch. Back to the bright cap. Volume on 2 Master on 8. I jumped a 100pf, better but not enough. Tried a 120pf, that may be the right value for me. Tried a 250pf very good. A 330pf getting to be too much, 470pf too much. The 250pf seems to give the amp the correct mix to sound like the 50w or 100w. All the sizzle is there, but with good lows. A tad brighter than I like but you can cut the presence and treble down a notch to my liking. I did find a 150pf but it was too late to make noise. I great mod could be off/no bright cap, on 120pf or on 250pf. I need to try my strat next. On the mod above about removing the 470cap, let me look where that is on the board. Another suggestion in that same area was to add a 470K across the other 470K which would reduce the bright effect. I dont want to change too much or I need another amp. I think with speakers and a bright cap adjust I can get it where I like it.

 

[Jordan Prysmiki]

Hey guys. Just starting to read on this amp. Would anyone be able to speak on installing a GZ34?

Also I require a decent loop for most of my playing. I hear that this has a passive loop which to me means tone suck. So can i swap in one of those discrete series high voltage loops? The spacing on the existing loop may not line up with one of those modules but I can look for one without jacks built on or I can make something work.

Limited amp knowledge but can follow diagrams and instructions well.

Would need to know if the tube rec would fit (yes, I'd drill the chassis and get a chassis mount socket) and would need some other tips if someone is out there.

Also the loop.

What do you think.

 

[junk notes]

schematics.




*ad nauseam*

 

[_Steve]

Hey guys. Just starting to read on this amp. Would anyone be able to speak on installing a GZ34?

Also I require a decent loop for most of my playing. I hear that this has a passive loop which to me means tone suck. So can i swap in one of those discrete series high voltage loops? The spacing on the existing loop may not line up with one of those modules but I can look for one without jacks built on or I can make something work.

Limited amp knowledge but can follow diagrams and instructions well.

Would need to know if the tube rec would fit (yes, I'd drill the chassis and get a chassis mount socket) and would need some other tips if someone is out there.

Also the loop.

What do you think.

No it's not possible sorry for several reasons like the transformer not having a 5V filament supply for the tube rectifier.

Your best bet would be to emulate a tube rectifier with a "sag resistor"

 

[Jordan Prysmiki]

What's stopping me from grabbing power from what's feeding the other tubes? You don't need a dedicated tap for that do you

 

[_Steve]

What's stopping me from grabbing power from what's feeding the other tubes? You don't need a dedicated tap for that do you

Yes you'd need a dedicated tap. It needs a different voltage and much higher current than the other tubes.

Sorry to bring bad news.

 

[Jordan Prysmiki]

It's all good. What about an active loop

 

[_Steve]

It's all good. What about an active loop

Sorry I dont play with effects or put loops in my builds so I don't have any experience with them to advise.

 

[javimusic]

hi , for me 500pf and 1000 pf is perfect for rock-hard and 2200pf added more mids good for heavy-hard rock , i have a master volumen .
one question , where i can buy de 12 pins connector on CMI02 ? thanks

 

[OdgeUK]

Does anyone know how the power section works in these yet? Sounds like the Preamp circuit is the same as a 50 watter, but the power section layout must be interesting. Would love to see someone do a handwired turret layout of this amp so it could be DIY'd and modded at will.

 

[junk notes]

Studio Series schematics are an anomaly.

 

[_Steve]

Does anyone know how the power section works in these yet? Sounds like the Preamp circuit is the same as a 50 watter, but the power section layout must be interesting. Would love to see someone do a handwired turret layout of this amp so it could be DIY'd and modded at will.

I havent traced it all out but i did investigate it and it looked to me like it was just running low plate voltages. I wrote them down if you need them.

p.s. I lived briefly in Worthing around 20 years ago. I worked at Alliance & Leicester there.

 

[Jordan Prysmiki]

Studio Series schematics are an anomaly.

?

 

[burger]

I've got a bit of info up on my channel about the SC20, along with mods and diy guide if you want to explore.

https://www.youtube.com/c/HeadfirstAmplification