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 Post subject: Tweed Princeton
PostPosted: Tue May 29, 2007 11:36 pm 
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Hello, I'm thinking about building a tweed fender practice amp. I'm looking for something that will clean up nice and quiet for TV levels and something I can open right up and not get evicted. I think I've settled on the 5F2-A. It's dead simple, classic, and not as loud as a 5E3.

Anyway, I'm looking at some schematics and layouts here and I'd like to know if everything looks right before I go and build it. BTW. the Fender amp field guidehas the stock shcem/layout to look at.

It looks like he's got some different component values. (I'm guessing higher gain and increased low frequency response.) These arent' necessarily bad. I'd just like to know.

Any input would be appreciated. Thanks!


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PostPosted: Wed May 30, 2007 6:34 am 
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That looks ok but i cant understand why he's taken the centre point of the artifical centre tap on the heaters and ran it to the cathode side of the 6V6's cathode resistor instead of ground? If you want something more classically Fender, try this layout here:

http://www.diycustomamps.com/images/pri ... layout.gif

I'd probably build the original version myself then mod it later on.

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 Post subject:
PostPosted: Wed May 30, 2007 9:38 am 
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I didn't totally understand what was going on there either. I was looking for something hopefully with a gounded cord and improved heater wiring but it doesn't look like it's quite right.

What do you folks think? Is it worth improving those things or should I just leave well enough alone?


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PostPosted: Wed May 30, 2007 10:56 am 
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I'd recommend you mod the stock layout to wire the amp up the way you did your Deluxe.

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 Post subject:
PostPosted: Wed May 30, 2007 1:26 pm 
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I think I'll do that. I might need some help from some knowlegable friends though :wink:


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 Post subject:
PostPosted: Wed May 30, 2007 4:07 pm 
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Send me your regular email address and I will send you a copy of a champ beast designed by Plexi/mark Huss. It is in PDF format and I can not send it through the forum email. It is a se amp that use a 6V6 through a EL34 power tube . You can use the master volume setup like the last princton build pictures on the DIY site.


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 Post subject:
PostPosted: Wed May 30, 2007 4:19 pm 
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I think I actually pulled a copy of that when he posted it.

But send it to me anyway! I'd like to take a second look. adam AT bradovka DOT com

Thanks.


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 Post subject:
PostPosted: Wed May 30, 2007 8:45 pm 
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Cool, thanks for the files. I must have an older one. The one I'm looking at has a GZ34 rectifier, and some different component values.

I seem to remember hearing some clips and thinking it wasn't for me... If 18watt ever gets back up I might be able to double check. Does anyone have the link?


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 Post subject:
PostPosted: Wed May 30, 2007 9:42 pm 
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what layout are you shooting around
jerryrigstudio@gmail.com
just in case :D
Thanks
Jac


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 Post subject:
PostPosted: Wed May 30, 2007 11:14 pm 
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It is a hot rodded champ . A Rock and Roll/Blues Champ. There is a cut switch in the circuit to take some of the bite out of it. You could also change some caps and resistors to tune it down to your taste. Improved filtering,SS rectifier,and layout are some of the pluses of doing this one.

JAC you got mail 8)


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 Post subject:
PostPosted: Wed May 30, 2007 11:52 pm 
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Thanks all for the layout's
Looks like a way fun build :D :D


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 Post subject:
PostPosted: Thu May 31, 2007 2:01 pm 
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For what it's worth, I took the vintage Fender schematic & layout and updated them for modernized heater wiring, less random grounding, and a three prong plug (plus removing the "death cap"):
modifed 5F2-A schematic

modified 5F2-A layout

PLEASE NOTE: I am not convinced that my layout shows an ideal grounding scheme! The speaker jack is isolated, but the input jacks are not - among other things.

I originally wired the heaters as shown with the virtual center tap going to ground. Bad hum problems! Moved the center tap to pin 8 of the 6V6 and it reduced the hum a lot. This provides an elevated DC ground of about 20 volts for the heaters. You can tap off the power rail and create an elevated DC ground refence for the heaters, but this way is dead simple. (That's what Steve did.) If you do it separately, people generally take it off the screen supply node and shoot for a somewhat higher voltage somewhere between 30 & 60VDC.

Personally, I'd stick with an NOS 5Y3GT rectifier tube but others insist it makes no difference in a single-ended amp. Note: the Sovtek "5Y3" does not match vintage specs - my voltages were 20+ with the new rectifier tube vs. NOS.

Also, IMHO you don't need for a standby switch in this amp, but there are arguments on both sides of that opinion.

I have the parts on hand to modify the power supply as follows:
PT -> 22uF -> choke -> 22uF -> OT/plate supply. This should reduce the noise level appreciably.

Also, I may need to move the speaker jack ground to the ground end of the last filter cap before the preamp. This amp is still a bit noisey, but not much worse than my stock Blues Jr.

Notes on Steve's other mods:

Reducing cathode bypass cap on 1st preamp stage to 10uF should reduce bass. Not necessarily a bad idea if you have a 10" or 12" speaker.

I'm not sure why he added the 1k5 grid stopper resistor on the 6V6 and pulled the 100k going to ground before the coupling cap, but he must have a reason.

The 1k5 screen grid resistor may be a good idea. Don't know what the effects will be, but I plan on trying out a 470k resistor there (more typical Fender value).

Please let me know if you want some reference voltages. The circuit for the Harvard 6G10 is virtually identical, but those voltages don't look Tweedie to me! I shot for voltages close to the 5F2 instead, although mine are slightly higher. Note that the choke here is probably a good idea, but the 8uF caps are WAY too small and tying the plate & screen supplies together is far from ideal.

Hope this helps. Good luck and have fun!

Chip

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 Post subject:
PostPosted: Thu May 31, 2007 2:48 pm 
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IMHO I think the standby switch should always be added. It's not that hard and the safety factor is well.... obvious.
As you said though, it's up to argument.
Later


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 Post subject:
PostPosted: Thu May 31, 2007 3:37 pm 
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Wow. You rock fresh_start. This is exactly what I was looking for. I would deffinately appreciate some voltages and/or the transformers make/model numbers.

It's funny, I was entertaining the idea of building a Harvard a little while ago, but never thought to compare the 2. The voltage difference must be the PT. I'll probably be shooting for something along the lines of the 5F2. I want that gritty, dusty sound. And a quiet clean sound that you can talk over.

I think it's 'Bout time I started making some decisions.... :D getting excited.


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 Post subject:
PostPosted: Fri Jun 01, 2007 12:21 am 
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Here are my first voltage readings, along with vintage benchmarks:


6V6 5F2-A 5F2 6G10
3-Plate 337 305 350
4-Screen 302 305 320
8-Cathode 19 19 20

12AX7
1-Plate A 170 150 190
3-Cath A 1.3 1.5 1.4
6-Plate B 175 150 190
8-Cath B 1.2 1.5 1.4

Can't find my latest voltages, but I think the 6V6 plate was about 325 with the screen grid about 295. Close enough for gov't work. The preamp voltages didn't change much and aren't as sensitive anyway.

You don't want the power transformer I used if you want Tweed-type voltages (my "kit" came with a PT rated at 350-0-350 and I had to use a 50 volt zener to get down an effective secondary voltage of 300-0-300). The Hammond 272BX looks about right with 300-0-300 on the secondary rated at 100ma and 3 amps for the heaters. IIRC you could go as low as 75ma for the high voltage secondary and 2.5 amps for the heaters, but Hammonds are good PTs at good prices (for me anyway).

For the OT, I'd go with Allen Amplification's TO8C if I were starting over. May get one anyway! The TO11C is a bit beefier and would allow an 8 ohm or 16 ohm speaker load, while the TO8C is for a 4 ohm load with a 6V6. (Call David Allen about the OT if you want to go that way. He's got a $50 order minimum, but you can get there fast enough with other various parts - really good Carling switches to start with.)

Please have someone else verify these transformer specs before you order anything!

BTW with a 1.5K cathode resistor on a 12AX7 with a 100K plate resistor, the approximate cutoff point for a 22uF bypass cap is roughly 9.3 Hz according to a table I just dug up. A 10uF bypass cap raises the cutoff point to 20.5Hz. According to Wikipedia (yeah, I know, check your sources ;) ) the normal range of human hearing is 20Hz to 14kHz. Why do we want to amplify frequencies lower than we can hear? Don't know. Sometimes I'm OK with "because that's how Leo did it!" Other times I want to experiment...

HTH

Chip

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 Post subject:
PostPosted: Fri Jun 01, 2007 12:36 am 
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While we're at it, Mojo will make a Tweed cab for a narrow panel chassis that fits a 10" speaker. Not the cheapest cab in the world, but it's pine with box joints and really well made in my opinion.

I just couldn't see using an 8" speaker, especially with the wider range of choices in 10". Mine's a BBQ 10A100 for what it's worth.

Chip

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 Post subject:
PostPosted: Fri Jun 01, 2007 1:34 am 
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And I'm sure Coco can make one also, using a 10" or a 12"... ummm 12" nice tone. All ya gotta do is ask :D


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 Post subject:
PostPosted: Fri Jun 01, 2007 1:51 am 
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Awesome. Thanks for all the input. Hammonds *SHOULD* also cheap for me as I'm about 150 km from where they're made. I just need to find a local distributor (Toronto, anyone?).

Quote:
BTW with a 1.5K cathode resistor on a 12AX7 with a 100K plate resistor, the approximate cutoff point for a 22uF bypass cap is roughly 9.3 Hz according to a table I just dug up. A 10uF bypass cap raises the cutoff point to 20.5Hz. According to Wikipedia (yeah, I know, check your sources Wink ) the normal range of human hearing is 20Hz to 14kHz. Why do we want to amplify frequencies lower than we can hear? Don't know. Sometimes I'm OK with "because that's how Leo did it!" Other times I want to experiment...


Sometimes you can just *feel* it... I dunno. I'll just take Leo's word for it too.

Quote:
While we're at it,

...
Quote:
I'm sure Coco can make one also


I'm all for suppporting the local guy, but my main reason for this project is to hack appart a larger home-made cabinet and make an even better amp; saving precious space, and materials! I think Stephen has some speakers that I might be interested in though...


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 Post subject:
PostPosted: Fri Jun 01, 2007 2:24 am 
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I am all for building and saving old stuff. My one upcoming project I'll be transforming this way cool radio cabinet. It's all flamed and burled maple. Stands about 4 feet high and is just ultra cool looking. There is no way I could ever trash this. So I'm going to put an amp in it.
Right there with you.My girlfriend is sanding it as we speak. It's amazing looking I don't even know how to build it yet, but I will. :D
I'll also try to build it as inexpensive as possible. But, if I get the cash, I'll still splurge and put Trinity Iron in it.That's 1 area I prefer to not cheap on.
Later


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 Post subject:
PostPosted: Fri Jun 01, 2007 6:23 am 
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In the Toronto area, you can get/order Hammond Transformers at:

http://www.partsconnexion.com/

but I've never used them.

I heard a Hammond on an 18 watt once and compared it immediately to one of ours. We were not amused!

Not sure how they sound on other builds.

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