Princeton kit build

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yello
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Princeton kit build

Post by yello »

A friend sent me a Mojotone 5f2a Tweed Princeton kit a while back, and I think that'll be my next amp build.

This is the schematic and layout:

https://www.mojotone.com/Amp_Kits/Tweed ... 2A_SCH.pdf

https://www.mojotone.com/Amp_Kits/Tweed ... F2A_WD.pdf

Here are my introductory questions:

1) Does anyone have some fun mods they like on a 5f2a? Modify or remove the NFB? Lower the 12ax7 cathode cap? Different value for the 6V6 cathode cap?

2) I'm using a small chassis I have on hand, and using a cap can would save me some space. I happen to have a 20/20/10/10 uf cap can on hand, which should work fine for the 16/16/8/8 uf in the schematic. My question is, the cap can shares a ground for all caps, but that makes a common ground point for caps in different amp stages - is that a problem, and what would the problem be?
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Re: Princeton kit build

Post by Bieworm »

yello wrote:
Sun 05/22/22 1:20 pm
A friend sent me a Mojotone 5f2a Tweed Princeton kit a while back, and I think that'll be my next amp build.

This is the schematic and layout:

https://www.mojotone.com/Amp_Kits/Tweed ... 2A_SCH.pdf

https://www.mojotone.com/Amp_Kits/Tweed ... F2A_WD.pdf

Here are my introductory questions:

1) Does anyone have some fun mods they like on a 5f2a? Modify or remove the NFB? Lower the 12ax7 cathode cap? Different value for the 6V6 cathode cap?

2) I'm using a small chassis I have on hand, and using a cap can would save me some space. I happen to have a 20/20/10/10 uf cap can on hand, which should work fine for the 16/16/8/8 uf in the schematic. My question is, the cap can shares a ground for all caps, but that makes a common ground point for caps in different amp stages - is that a problem, and what would the problem be?
1. Build it stock first, do the mods later.. you might wanna anticipate on possible modding, building those areas with additional turrets

2. I think you know that you've already answered your own questions. If not, read the docs on grounding schemes again 😉
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yello
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Re: Princeton kit build

Post by yello »

Bieworm wrote:
Sun 05/22/22 1:38 pm

1. Build it stock first, do the mods later.. you might wanna anticipate on possible modding, building those areas with additional turrets

2. I think you know that you've already answered your own questions. If not, read the docs on grounding schemes again 😉
On #2, yeah my instinct is that it is bad, but why? I assume because it is a potential noise/hum issue?

Related question on the cap can - can I just use 3 of the 4 cap terminals in the cap can and leave one unused? Then use a separate same value cap for the preamp section? I don't know if it is standard or advisable to not use all sections of a multi-section cap can (or why/why not).
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Re: Princeton kit build

Post by Bieworm »

You can use half the can cap without issues.
If you tie preamp and power amp grounds together you will likely get hum. In fact, it's a safe bet for getting hum for sure.
You will harvest the most success when you put the filter cap for a stage as close as possible to that stage's circuit.
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yello
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Re: Princeton kit build

Post by yello »

I did a little bit of work on my Princeton project. I am using an old film projector chassis, thus the 16mm lens for the pilot light. It needed a bit of metalwork to comply with my layout. The chassis has a couple little turret boards inside which I will use for the input section and a couple mods. And I will utilize a cap can to save some space as well. I do have a faceplate for it.

Pondering using a SS diode rectifier instead of the 5y3 - or could maybe make it switchable between them? Or could use a 5r4wgb, as I like that tube. My PT is a Hammond 290ax, and has 275v and 325v wiring options. That could be handy in sorting out my rectifier decision, or I've seen people make a voltage selector switch with that PT.

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Re: Princeton kit build

Post by Unit_1 »

Hey and you can put an LED behind that lens and use it as the indicator light!!! Awesome!
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yello
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Re: Princeton kit build

Post by yello »

Unit_1 wrote:
Wed 06/22/22 7:58 pm
Hey and you can put an LED behind that lens and use it as the indicator light!!! Awesome!
Yep, that’s the plan!
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Re: Princeton kit build

Post by yello »

Unit_1 wrote:
Wed 06/22/22 7:58 pm
Hey and you can put an LED behind that lens and use it as the indicator light!!! Awesome!
Here is a photo from another similar amp I've been slowly working on, showing how I implemented the pilot light behind the lens:

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