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Forums :: Ask Amp Tech Expert Skip Simmons

5F2-A Circuit

8 replies [Last post]
Sun, 03/07/2021 - 09:03
JoelPM
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Joined: 07/10/2020

Hello Skip,

As a novice about to try and make an amp I would have thought it best not to mess about with a design classic. That said I just wanted to ask if there are any simple/minor tweaks to a Fender 5F2-A circuit that would be appropriate for harmonica amplification.

Thanks

Joel

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Mon, 03/08/2021 - 09:42
#1
Expert Skip Simmons
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Joined: 01/18/2010
Hello Joel- Thanks for

Hello Joel- Thanks for writing. The 5F6-A is a fine choice and won't really need any tweaking for harp. You might try a lower-gain preamp tube (12AY7) or a higher-value roll-off cap in the tone circuit. Instead of the stock value of .005, try a .01.

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Mon, 03/08/2021 - 09:54
#2
JoelPM
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Thank you

Thanks very much Skip.

Joel

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Tue, 03/30/2021 - 07:00
#3
JoelPM
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Hot Tube?

Hello Skip.

So, I've got a completed 5f2a circuit and chassis hooked up to a speaker in a cardboard box. It seems to work though I can't really test the sound with my makeshift cabinet. I though it would be sensible to test the voltages even though finding a definitive answer on the correct values seems to be difficult. My concern is that I've followed instructions for calculating the plate dissipation of the 6v6 GT tube and keep getting a figure of around 18 watts. Should I desolder the 325 output transformer leads and use the 275 volt ones instead? I've tried searching online for some answers but that is not proving fruitful... 

Thanks

Joel

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Tue, 03/30/2021 - 12:08
#4
JoelPM
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ps

Output transformer centre tap = 419.5 V

6V6 plate = 408 V

Cathode = 22 V

OT centre tap to plate resistance 328.4 Ohms

Cathode bypass resistor 464 Ohms

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Fri, 04/02/2021 - 16:28
#5
Expert Skip Simmons
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Hello Joel- Thanks for

Hello Joel- Thanks for writing. Yes, you should use the lower voltage (275V) taps on the power transformer. Remember, all the work has been done before you when you it comes to a Fender circuit. You don't need to worry about plate dissipation and other complicated stuff- just go by the DC voltages.

Mojotone.com sells beautiful reproductions of the Fender schematics and "layouts". Forget the internet, start there!

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Sat, 04/03/2021 - 04:34
#6
JoelPM
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Keep it simple

Thanks very much Skip, that Mojotone schematic is just what I was looking for. Everything looks fine now with the 275v transformer taps.

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Fri, 05/07/2021 - 14:28
#7
JoelPM
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Cabinet

https://www.instagram.com/p/COXVvUKpq-s/?igshid=143qh6498cav0

https://www.instagram.com/p/COXVmcap1P4/?igshid=h9fom8yvhuyb

Here are a couple of photos of the cabinet I've made for my amp prior to "tweeding".

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Sat, 05/08/2021 - 06:37
#8
David Barrett
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ExpertHead InstructorTeacher 10Level 10
Joined: 12/20/2009
Looks excellent Joel, well

Looks excellent Joel, well done

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