Studio Vintage SV20 mods (and SC20 too)

[Anson]

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.

Agreed. I’ll most likely have an amp tech trace it down for me. I also have two SV20s, modded one with a shared cathode which puts it a bit closer to super bass territory. Just need to change the pi couplers to enhance the lower frequency.

 

[Anson]

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.

Decided to study the SV20 circuit myself and convert the components to Super Bass specs. Keep in mind that this amp is only 20 watts and will not put out as much lows as it’s 100 watt counterpart. Nevertheless, the results came out pretty well and my SV20 now has accurate characteristics of a Super Bass!!

Here’s a demo link:

 

[Ramhead]

Decided to study the SV20 circuit myself and convert the components to Super Bass specs.

Or you could use an EQ pedal, getting the same results and keep the amp stock.

 

[Pete Farrington]

Or you could use an EQ pedal, getting the same results

I suppose that with the amp set clean, it should be possible to get a broadly similar frequency response that way.
But overdriven, that will change, as the different RC values in the grid circuits of the 2nd and output stages of the circuits will tend to affect their overdrive characteristics.

 

[Anson]

I suppose that with the amp set clean, it should be possible to get a broadly similar frequency response that way.
But overdriven, that will change, as the different RC values in the grid circuits of the 2nd and output stages of the circuits will tend to affect their overdrive characteristics.

I agree. I’m starting to understand why people lean towards the super bass than the lead. Something about the lesser gain that presents a certain clarity when OD/Fuzz pedals are used (At least for my preference). Changing the components in the circuit gives a completely different feel to this amp.

 

[Anson]

Or you could use an EQ pedal, getting the same results and keep the amp stock.

I have a stock SV20 that I don’t tamper with and did try doing that. Compared the two and can honestly say that switching values in the circuit changes the overall feel of the amp, especially when pushed with a OD/Fuzz. I tend to lean towards the modded SV20 when playing, mainly due to the lesser gain that has a certain clarity when used with pedals.

 

[Xenrelic]

 

[Xenrelic]

You guys want to see something retarded? Here's an SV20H hand-wired prototype with a Zero Loss FX Loop and some hot-rodded push pull switching. Oh and a post phase inverter master volume, so you can crank the shit out of the front end at any volume level.

 

[Filipe Soares]

@Xenrelic , not retarded at all.... it is wonderful, beautiful, awesome and the list goes on.

 

[Xenrelic]

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 can do on the rectifier tube. It's set up for dual B+ and full wave bridge rectification, so the voltages aren't there for the typical full wave tube rectification. Not to mention, no 5V tap, so you'd need an entirely different power transformer. You're better off just building a hand wired version of the amp if you're going to go tube rectifier.

I tried a Metro LND150 Zero Loss FX Loop with it and with a few modifications to the power supply it will work just fine with the Studio 20 series amps.

 

[PelliX]

@Xenrelic Dare I trouble you by asking whether you might have and share the schematic for the SC20?

 

[Xenrelic]

If someone wants to pull their SC20 Main PCB and ship it out to me for a couple of days or take some hi-res photos for me, I will trace it out and post a schematic of it and then ship it back. Or if I get one in to modify I’ll post it, whatever happens first I suppose.

 

[L0oniX]

View attachment 111689

 

[L0oniX]

Thank you ...thank you ...thank you ...you rock

 

[L0oniX]

I

If someone wants to pull their SC20 Main PCB and ship it out to me for a couple of days or take some hi-res photos for me, I will trace it out and post a schematic of it and then ship it back. Or if I get one in to modify I’ll post it, whatever hap

 

[L0oniX]

I may be able to do that when I get the 110VAC power transformer to replace the 220VAC. Unfortunately the stock ones do not have a multi tap input so I had to order a TXMA-91085-01 from Antique Electronic Supply. BTW do you happen to know the difference between the SV20 and the SC20 other than cosmetics?

 

[Xenrelic]

I may be able to do that when I get the 110VAC power transformer to replace the 220VAC. Unfortunately the stock ones do not have a multi tap input so I had to order a TXMA-91085-01 from Antique Electronic Supply. BTW do you happen to know the difference between the SV20 and the SC20 other than cosmetics?

The SC20 is based on the JCM800 and the SV20 is based on a late 60's plexi.

 

[L0oniX]

I got the actual Marshall factory service manual from a store in Austria where I bought my SC20CNB.

 

[Xenrelic]

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.

 

[paul-e-mann]

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