Can I mod my stewmac plexi kit to be 100W?

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Mustang Dude28
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Can I mod my stewmac plexi kit to be 100W?

Post by Mustang Dude28 » Fri Mar 01, 2024 4:14 am

Recently I've been wanting to build my own plexi from a kit. The two best I've found are the Tube Amp Doctor Plexi kit and the Stewmac 45W plexi kit. Now, I really want those hundred watts and I heard some negative press about the tube amp doctor so I was wondering if there was anything i could do or parts i could change to build the stewmac one but for 100W?

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Re: Can I mod my stewmac plexi kit to be 100W?

Post by Dr Tony Balls » Fri Mar 01, 2024 7:07 am

Can you mod a JTM 45 kit to be 100W? Not easily. You need to replace both transformers which costs a lot, and likely add two power tubes and sockets. This is all presuming that everything fits in the chassis, which it likely will not.

Do you really want those 100 watts? 50 watts is already exceedingly loud. Many people arent used to amps without master volume controls and when faced with them are shocked at how loud 50 watts is at breakup level. Also a 100W amp, when used with the same cabinet, is only gonna be like ~10% more loud than the 50W version. If volume is really what you're after you'll get more mileage out of more/more efficient speakers than you will extra power.

If you *must* do 100W, Weber and Mojotone both sell 100W plexi kits.
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Re: Can I mod my stewmac plexi kit to be 100W?

Post by GilmourD » Fri Mar 01, 2024 7:15 am

Mustang Dude28 wrote:
Fri Mar 01, 2024 4:14 am
Recently I've been wanting to build my own plexi from a kit. The two best I've found are the Tube Amp Doctor Plexi kit and the Stewmac 45W plexi kit. Now, I really want those hundred watts and I heard some negative press about the tube amp doctor so I was wondering if there was anything i could do or parts i could change to build the stewmac one but for 100W?
The biggest difference between a 50W amp and a 100W amp are actually the expensive parts -- the power transformer and the output transformer. The rest are just component value changes and the difference in topology.

The JTM45 power wouldn't supply enough current for a quad of EL34s and the output transformer would cook itself trying to output that to speakers. If you tried to run two 6550s in the existing topology you'd never be able to bias it to 50W each in Class AB1 but, even if you did somehow accomplish that, you'd again cook the output transformer. That's why Hendrix's first 100W JTM45-100 amps actually had two output transformers in parallel.

Now, the question I do have to ask is... What "negative press" have you heard about the TAD kit? There's a lot of things out the on the greater Intarwebnetz as a whole that simply aren't true.

That said, there are other 100W Plexi-style kits out there, like the Weber, Mojo Tone, and others that aren't floating to the top of my brain at the moment.

Also... What do you need/want 100W for? Are you competing with other amps? Do you need to feel the air move from a 4x12 cabinet? Or do you think it's just a magical number? A 50W amp will compress a little bit more, but negative feedback and some other components could be tweaked to raise the B+ slightly (although even actual Marshall 100W B+ voltage varied depending on the year) and tighten up the bottom end compared to a 100 watter.

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Re: Can I mod my stewmac plexi kit to be 100W?

Post by mynameisjonas » Fri Mar 01, 2024 11:21 pm

As already mentioned, it’d be a giant hassle trying to mod a JTM45 kit into a Super Lead. There are plenty of Super Lead kits out there, where are you located?

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Re: Can I mod my stewmac plexi kit to be 100W?

Post by Mustang Dude28 » Tue Mar 05, 2024 8:53 am

Warsaw, Poland

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