Studio Vintage SV20 mods (and SC20 too)

[Pete Farrington]

J taper master volume

Where did you buy that?

 

[C-Grin]

Here's my second go at a hand wired mod, this time with board mounted tube sockets and using the existing standoffs to make install easier. Used a 5H choke on this build. Push pull high gain mode and push pull fat boost for stage 3.

Modified Zero Loss FX Loop
Modified DI
Modified Power Supply for 12VDC circuit, uses a chassis heatsinked voltage regulator instead of voltage dropping resistors that heat up.
Switchable Fixed Depth Circuit for the negative feedback loop to add a bit extra low end fullness when needed.
Post phase inverter J taper master volume.

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Cool, what about taking an origin and doing that?

 

[Xenrelic]

Where did you buy that?

They don't exist, so I had to custom make it. I took a dual gang 24mm CTS pot, opened it up and since most of their 24mm pots use the same size track boards and wiper system, I just replaced the carbon track boards with matched CTS 250k J tapers as close as I could get to 220k from the bag of 20 that I bought. I settled on some that measured super close to 220k and then I used resistors from the wiper to ground to get them to read 220k when the control is full clockwise.

 

[Pete Farrington]

CTS 250k J tapers as close as I could get to 220k from the bag of 20 that I bought

Where from?

 

[Xenrelic]

Cool, what about taking an origin and doing that?

I've done hot-rodded JCM800 style mods to the Origin 20 and Origin 50 heads and it looks like the SV20HW board should fit in the Origin series with slight adjustments to the layout.

The boards in the SV and Origin don't seem all that different dimensionally.
The only thing is cosmetically, the Origin is better suited for a single input design. Although more inputs could be added to the front panel if you wanted a second or third one to do like the top inputs on the SV or do a modified version where you get a '68 Super Lead Bright Channel Input on the Hi input that switches into a high gain modded tone and a have a cleaner 2204 Low gain input with some slight '68 Normal channel hybrid tone going on that takes pedals really well.

 

[Xenrelic]

Where from?

I get my single gang 250k J taper pots from CE Distribution, which is either Amplified Parts or Antique Electronic Supply on the retail end if you don't have a resale license to sign up with CE. Purchase link

 

[Xenrelic]

I cant imagine doing mods to my SV20H it sounds great as is!

Wait until you play a modded one though. There are tonal possibilities there to unlock. One of the best ones is polyester film and foil capacitors like Sozo mustard caps. Then you can cascade the channels and get some really tasty higher gain overdrive on a switch.

 

[AbstractSnow]

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Am I seeing it right that the NFB is on the 4 ohm tap? Compared to the 1959 HW, which has the same NFB resistor value and has it on the 8 ohm tap. Curious. Should make a somewhat big difference, shouldn't it?

 

[Native34]

Question for you amp modders and builders. I have a stock SV20H, and would like it to be more in the realm of the Master Volume JMP of the mid 70's, how can I achieve this? I believe that I just have to cascade V1A, and V1B, then add MV. If this is correct does anyone have pics of the items that need to be changed on this board. I'm new to modding amps and understanding schematics, so I'm a little unsure how to proceed.

 

[shredgd]

Question for you amp modders and builders. I have a stock SV20H, and would like it to be more in the realm of the Master Volume JMP of the mid 70's, how can I achieve this? I believe that I just have to cascade V1A, and V1B, then add MV. If this is correct does anyone have pics of the items that need to be changed on this board. I'm new to modding amps and understanding schematics, so I'm a little unsure how to proceed.

If you don’t have any experience in amp modding, you shouldn’t proceed for a number of reasons.
1. There’s a very high risk to do something wrong and ruin your amp
2. Modding a PCB based amp is not as straightforward as modding a point to point turret board one. PCB traces are often fragile and you really have to know what you’re doing.
3. The 70s tone is a cranked volume stock Plexi tone, not a cascaded one. At the same time, adding a MV isn’t enough to achieve your goal, unless we’re talking a PPIMV, which is quite an involved and complex mod
4. You’re probably asking for a step by step guide, which is something one should create for you for this particular amp, with pictures and text. A time consuming task, which still doesn’t guarantee success or to avoid damages to your amp.

I suggest you to get an attenuator and crank your stock amp!

Giulio

 

[Native34]

Thank you for your input, although I don't necessarily have experience modding an amp, I'm no stranger to a soldering iron and a PCB. I'm also not afraid of damage to my amp as everything could be fixed with additional work if needed. I'm just not certain how to read the schematic for the 1959, and JCM 800 and translating that to the layout and relate it to what needs to be changed to make one closer to the other. I also am currently running this amp through an Ironman Mini Attenuator, and using a RYRA 808 OD pedal in the front end. I would just like to get more gain from the amp and use the pedal more like a solo boost or something like that. I also was under the impression that the JMP was based off of a 1959 with connected V1A, and V1B together to achieve a higher gain structure as well as a PPIVM, which then was renamed to be a JCM 800.

 

[_Steve]

Thank you for your input, although I don't necessarily have experience modding an amp, I'm no stranger to a soldering iron and a PCB. I'm also not afraid of damage to my amp as everything could be fixed with additional work if needed. I'm just not certain how to read the schematic for the 1959, and JCM 800 and translating that to the layout and relate it to what needs to be changed to make one closer to the other. I also am currently running this amp through an Ironman Mini Attenuator, and using a RYRA 808 OD pedal in the front end. I would just like to get more gain from the amp and use the pedal more like a solo boost or something like that. I also was under the impression that the JMP was based off of a 1959 with connected V1A, and V1B together to achieve a higher gain structure as well as a PPIVM, which then was renamed to be a JCM 800.

The JMP/JCM800 has quite a few more changes in the preamp than that. Not something I would want to do on the flimsy PCB in my SV20. The "one wire" mod you may of heard about for 1959s doesn't sound great in my opinion - it's certainly not a JCM800.

Moreover, and please take this in the spirit intended, tube amps (and microwave ovens) are far more dangerous to work on than regular electronics due to their lethal voltages and ability to still kill you after they're switched off and unplugged. You really need to be at least confident understanding schematics and HV safety precautions before you dive in. It's not like the warnings you get on regular electronics that are mostly to keep legal departments happy - tube amps are proper dangerous.

Have you considered putting a tube pedal in the FX loop? It'd be cascading another tube stage AND pre-phase inverter MV (like the JCM800) into the signal chain, pretty much just like what you are trying to accomplish.

 

[Xenrelic]

Be advised, it seems that Marshall has switched up their color coding on the SV20H Power Transformers with the (MAV 19 09) and newer date codes.

The PT's before that with (MAV 19 08) and (MAV 19 07) have 141VAC on the Red and Brown secondary leads and 108VAC on the Blue and White secondary leads.

These newer ones have it reversed, so there's 141V on the Blue and White secondary leads and 108V on the Red and Brown secondary leads.

Here's the files and photos for the SV20H for reference. Notice the latest schematic that Marshall has released reflects these changes to the power transformer. SV20H Files