Mojotone GA-5 Kit

Cjsinla

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It looks like Mojotone has a kit version of the GA-5. It’s actually PTP construction. I bet that would make for a fun afternoon of building. Any thoughts on this kit? It is close to the Champ build circuit-wise but there are a few differences. I think I might buy one.

 

D'tar

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IMHO. If you have a 5f1 you are covered. I have a 1960 GA-5 and they are sweet but not different enough to have both.

Nothing wrong with having two however :naughty:
 

D'tar

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just looked at their schematic. That is a mojotone hybrid 5f1/ga5

Mojotone schem shows multi tap OT using 4ohm tap /4ohm speaker and a 47k nfb resistor

Ga5 uses 8ohm tap/ 8ohm speaker and 47k nfb resistor

5f1 uses 4ohm tap/ 4ohm speaker and 22k nfb

So just fyi if you build to that schematic, for original sonic value, use the 8 ohm tap with the 47k nfb or use a 22k resistor with the 4 ohm tap. Better yet, use the 4ohm tap and a 10k nfb resistor in series with a 50kl pot for a nice variable nfb.
 

Cjsinla

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just looked at their schematic. That is a mojotone hybrid 5f1/ga5

Mojotone schem shows multi tap OT using 4ohm tap /4ohm speaker and a 47k nfb resistor

Ga5 uses 8ohm tap/ 8ohm speaker and 47k nfb resistor

5f1 uses 4ohm tap/ 4ohm speaker and 22k nfb

So just fyi if you build to that schematic, for original sonic value, use the 8 ohm tap with the 47k nfb or use a 22k resistor with the 4 ohm tap. Better yet, use the 4ohm tap and a 10k nfb resistor in series with a 50kl pot for a nice variable nfb.
Not a bad idea to hook the 8ohm tap to the nfb. I did a similar thing with another amp.
 

Cjsinla

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@D’tar so if I’m reading the diagram and schematic correctly, the green 8ohm tap is just hanging out on terminal 6. That would mean that I connect the 47k resistor between terminals 6 and 7, right and not from terminal 7 to the speaker jack.
 

D'tar

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@D’tar so if I’m reading the diagram and schematic correctly, the green 8ohm tap is just hanging out on terminal 6. That would mean that I connect the 47k resistor between terminals 6 and 7, right and not from terminal 7 to the speaker jack.

Had to look at the layout...

That is correct.
 

sonar

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The cab looks cool.

Unless it's a super anal, perfect 90 degree angle leads on everything I'm not a fan of true PTP. Visually it's really ineloquent (unless perfectly assembled) and I've yet to read or hear credible evidence that it makes for a better sounding amp.

That said, I own an old 50's Supro and Conn Strobe that are PTP. As long as I don't have to work on them it's all good.
 

Cjsinla

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I’m just intrigued by older technology. I decided to order one because I have been interested in a GA-5. I know it’s not exactly the same as the 50’s models and I don’t think that PTP is any better. It just amazes me that such a hodgepodge of wires and resistors works at all. On top of that, the price was reasonable and I was looking for a project. It arrives by FedEx this Friday.
 

sonar

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I’m just intrigued by older technology. I decided to order one because I have been interested in a GA-5. I know it’s not exactly the same as the 50’s models and I don’t think that PTP is any better. It just amazes me that such a hodgepodge of wires and resistors works at all. On top of that, the price was reasonable and I was looking for a project. It arrives by FedEx this Friday.

Very cool and I applaud your interest. Most of the early amps were electronically very simple, so it should be a straightforward build. I look forward to your progress.
 

D'tar

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Cool!!! Waiting for fedex today eh! I've been wanting to copy this circuit for my three boys and one for myself. If they turn out well enough I may let go of the original as it does not get used like it should. Cj, if you are interested in a more traditional ptp layout let me know. I've measured the chassis layout but have been putting off the circuit layout. You are right, it is very simple and amazing how it is so quiet seemingly disregarding all the practices of lead dress that are pounded into our learning process. Good luck with your build.

Subscribed:)
 

Cjsinla

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Cool!!! Waiting for fedex today eh! I've been wanting to copy this circuit for my three boys and one for myself. If they turn out well enough I may let go of the original as it does not get used like it should. Cj, if you are interested in a more traditional ptp layout let me know. I've measured the chassis layout but have been putting off the circuit layout. You are right, it is very simple and amazing how it is so quiet seemingly disregarding all the practices of lead dress that are pounded into our learning process. Good luck with your build.

Subscribed:)
I’m going to go with their layout for now as the tubes are different than the original and I don’t know enough about circuits to tailor an amps. If you ever make a copy of the layout of your original GA-5 I would like to see it.
 

Cjsinla

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The box arrived today, the cat approves of the contents.

FB8277D0-D7A0-4A70-BDA5-48F8FB5F9CB8.jpeg

DF22FE38-9EAE-40D8-B135-BFB09DA0A547.jpeg
 

Cjsinla

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The cabinet is very nice and covered in a blond tolex. It looks like it’s finger-jointed ply construction. All of the small parts were packed into the cabinet.

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Cjsinla

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A bunch of parts and descent documentation.

DAD190E6-63F9-402A-9B7A-BE799E84F1C2.jpeg

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CB91710

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Sweet.
Would be interesting to hear the difference between the 5F1 and GA5
I've been debating an amp build, and leaning toward the tweed Princeton, but the Studio One also has my attention.
 

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