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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Fri Oct 30, 2015 2:14 am
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You have all good parts. I would not think about tweaking, right now. Think good layout and A-one soldering. Keep irons separated and at 90-degrees to each other. Pre-amp section physically away from output section. Separate grounding scheme for the PSU, pre-amp gain stage, and output stage.

Star-ground each section to different areas of the chassis. Ground for AC-cord closest to power cord's insertion point to the chassis.

Noise is your enemy. This is the time to construct a good grounding scheme and component layout to avoid EM interactions. You may want to make the grounding contacts temporary. Be able to move these around to get the lowest amount of hum from loops. Then, permanently soldered the grounding points.

Lot of ideas on layout schemes. Maybe, a drawing of your potential layout and post it, first. Kinda kick it around. And have enough room in wiring and spare space to experiment.

A heavy duty chassis is a must. For a long & happy transformer life.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Oct 31, 2015 11:22 am
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BMW2002Ti wrote:
......

Lot of ideas on layout schemes. Maybe, a drawing of your potential layout and post it, first. Kinda kick it around. And have enough room in wiring and spare space to experiment.


Thanks BMW.
I've already drilled the chassis but have a question about power tube placement and would appreciate your thoughts on these two options.

Image

My idea for option 2 is to keep power as far from the 12AX7 as possible. Is there any technical reason I shouldn't do this?

Image

The 3rd chicken head is for NFB, and please disregard the crappy grey primer. The aluminum chassis has some small scratches and I sanded and sprayed the top with a very light coat to see if I could hide them. I have more drilling and deburring to do and I'm in no way done with it.

Thanks


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Oct 31, 2015 2:27 pm
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I would go with option 1 but move the O/T a skosh closer to the P/T -- say, an inch and a half.

Arjay

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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Oct 31, 2015 6:14 pm
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Option #1 is prolly better, provided the tubes will be accessible, once the chassis is in the cab. Will you be having the controls at the top of the unit or on the side?

If they will be on the sides, move the tubes to the front of the chassis (nearest to controls). If the controls will be at the top --- then #1 is the best layout.

BTW... nice & heavy duty chassis! :)


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Oct 31, 2015 8:47 pm
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Good thing BMW suggested an upload of the layout. I was leaning hard toward option 2 because I wanted as much separation as practical between the power and input side of the amp. I went with option 1 after reading these replies. It looks fine. Also move the OT closer to the PT by an inch or two.

I thought about tube access but have no immediate desire for a cabinet so I went with the flat top. The plan is to mount the chassis to a piece of wood when done and then possibly fab some kind of mesh cover for it.

Making the chassis has taken way longer than expected and I now have a much greater appreciation for the $70 tag for the chassis kit, but I wanted to give this a personal look and and I got it. So far it's been a pain in the rear but a good learning experience. I've spent more hours than I care to admit measuring, marking grid\reference lines, moving and mocking up parts in various places, etc.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 01, 2015 12:15 am
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If the chassis is going to be upright, then put the tubes in front of the trannies. I'd put a cage on top to protect from prying little fingers. :lol:

Kinda like this:

http://www.valvepower.co.uk/cage%20mk2% ... deuced.jpg


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Nov 07, 2015 10:36 pm
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It's been drilled for option 1 so it's too late to turn back now, unless I fab another chassis which I might do depending on what happens tomorrow. I went with aluminum but painting has been problematic with the high humidity we've had the past couple of weeks. I'm trying again tonight but running out of patience so if it doesn't come out decent I'm going to pick up a piece of steel scrap Monday and fab another one. I think the layout will be okay but I've already seen a couple of design changes I would make if I had to start over.

I have another question. I intend to use the low input jack for an MP3 player for backing track practice but I don't want it to overpower the guitar. Can or should I change the resister (R2 below) to cut the input a little more on the low channel?

Image

Thanks.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 08, 2015 12:10 am
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Randy, the 68k-ohm resistors set the input impedance for the first gain stage. You can't vary the amount too much, as long as V1 is a 12AX7. Look at the schematic for the Fender amp. It is the same as yours, but maybe a bit easier to see the functions of the 68k-ohm and 1 meg-ohm resistors.

Image

Notice the jumper line between INPUT 1 and 2? That ties the 1 meg-ohm resistor to ground, on both inputs. This resistor is in a grid-to-ground (G2G) position. Currently, one 1 meg-ohm is shared between the two inputs. You could split the two inputs and have a 1 meg-ohm on INPUT 1 (high) and say, a 470k-ohm on INPUT 2 (low). This would allow you to have an normal & attenuated inputs, yet maintain the correct impedance.

The two inputs would need ground-out type of input jacks. So as to isolate one G2G resistor from the other. Break the jumper seen on the Fender amp diagram and insert a 470k-ohm resistor from the jack's hot terminal to ground.

Hope that made some kinda sense... :)


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 08, 2015 2:15 pm
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I think I got it.

I see the Input 1 and 2 relationship with the 1M resistor, so adding a 470k between them like this would pad the #2 input but not alter the impedance of the gain stage.

Image


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 08, 2015 3:15 pm
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Randy, not quite. More like this:

Image

INPUT 2 will have lower input strength, as more of the signal is fed to ground. You may need to experiment with resistance --- eg: try 220K-ohm or even 100k-ohm --- if the input is still overloading the front end of the input stage.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 08, 2015 3:56 pm
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BMW2002Ti wrote:
Randy, not quite. More like this:

Image

INPUT 2 will have lower input strength, as more of the signal is fed to ground. You may need to experiment with resistance --- eg: try 220K-ohm or even 100k-ohm --- if the input is still overloading the front end of the input stage.


WOW..... embarrassed that I missed that. Makes way more sense now. Thanks!


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Fri Nov 27, 2015 6:36 pm
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Making some progress and have a simple question.

Where it says "PIN 3" in the diagram below, is that referring to pin 3 on the rectifier tube? I've checked the schematic but I don't see it.
Image

This is what I have so far. I've done some wiring of the internals and have more to do. I've already found a few screw ups in my layout that probably wouldn't be an issue for a pro, but for a first timer with a home built chassis I find that not everything flows as well in the application as it does on paper. Many thanks!

Image


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Fri Nov 27, 2015 6:40 pm
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The "pin 3" mentioned refers to your power tube (ie: the plate).

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Nov 28, 2015 11:14 am
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Hello,
I've been keeping up with this build as I've been contemplating a 5f2a build as well. It's looking good, I patiently wait in anticipation of the completion of your build and hopefully some sound samples as well.

Anyhow, I know this has been answered but I saw this video on YouTube with a pretty good description of the functionality and purpose of the screen grid resistors at the input. It provided me with some insight I had not been previously aware of. It definately enlightened me as to why it the values are what they are.

Hope it helps.

http://youtu.be/QFHcmST7AZ0

The 5 minute mark explains its purpose and the 17 minute mark shows the example based on the fender input values.

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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Nov 28, 2015 11:49 am
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socal323 wrote:
Hello,
I've been keeping up with this build as I've been contemplating a 5f2a build as well. It's looking good, I patiently wait in anticipation of the completion of your build and hopefully some sound samples as well.

Anyhow, I know this has been answered but I saw this video on YouTube with a pretty good description of the functionality and purpose of the screen grid resistors at the input. It provided me with some insight I had not been previously aware of. It definately enlightened me as to why it the values are what they are.

Hope it helps.

http://youtu.be/QFHcmST7AZ0

The 5 minute mark explains its purpose and the 17 minute mark shows the example based on the fender input values.


Thank you for your interest. I have also been following Uncle Doug's videos for a while and can assure you that nothing I have done looks anywhere near as good as his. Check out his Supro Tremoverb scratch build if you haven't already.

My intent is to post a clip of some of the building stages, just not in any great detail. I tried that and setting up a camera, checking the lighting, remembering what I already covered , what I left out, and I can tell you it's a royal pain to produce a decent vlog . Now I'm more interested in getting it completed and then possibly assemble some stills and clips to post.


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