Mercury magnetics FBfd-om in matchless spitfire - diyAudio
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Old 22nd June 2012, 11:10 PM   #1
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Default Mercury magnetics FBfd-om in matchless spitfire

I have just completer a spitfire build

http://www.diyguitarist.com/Schemati...eSchematic.bmp

I used an old output transformer from a teisco AMP, but it is Way to small, and overall the AMP sunds thin and Weak...

Just ten kilometers from my house, a guy is selling a mercury magnetics OT.

It is made for a delux reverb, so the primary is 8k, and I believe the spitfire uses a OT with a much lower primary impedance. 6k or sommerens like that

I am sure it Will work, but curious to hear if there are anyone out there, Who believe that the original/untraditional OT in a spitfire is important in a build like this?
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Old 22nd June 2012, 11:11 PM   #2
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Sommerens= something
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Old 22nd June 2012, 11:13 PM   #3
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Oh, obviously this transformer is interesting because I Can get it cheap, and save the expensive shipping from abroad
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Old 23rd June 2012, 01:21 AM   #4
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8k is perfect for EL84's.
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Old 29th June 2012, 03:48 AM   #5
shanx is offline shanx  Canada
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The spitfire output is designed around a 4 K primary. But having said that, the Mercury Mag transformer is going to be quite/very good. If it has multiple secondaries, you can ''mismatch'' the speaker impedance to reflect back the load to get about 4K primary.Hope this makes sense: Connect an 8 ohm speaker into the 4 ohm secondary tap. You reflect back to primary at 4K instead of 8 K. That should get you to an optimal impedance. If you have a 4 ohm speaker that may drive it a tad hard. 16 ohm cab can plug into 8 ohm tap.
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Old 29th June 2012, 04:02 AM   #6
shanx is offline shanx  Canada
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PS: Printer2 is right that EL 84s like the 8K. Even if you match the speaker taps, the Mercury OT will sound very good and you will only get a slightly lower volume. It will not blow anything up. The deluxe reverb OT I believe is rated much higher voltage etc than the spitfire.
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Old 29th June 2012, 06:15 AM   #7
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Thanks for the replies...
it only has an 8 ohm tap on the secondary....But IŽve got both 8 ohm and 16 ohm speakers that I want to try out.
IŽll install it over the weekend, and let you know what sounds the best....
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Old 2nd July 2012, 08:02 PM   #8
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Hooked up the output trandsormer tonight, with an 8ohm speaker.

The first impression is a much better sound and way more bass! But the volume is not impressive? And there is no gain/distortion what so ever. Volume almost fades out when toneknob is turned towards the bass side?

The voltages seem fine:

Powertubes:
pins 7: 351 vdc
pins 9: 352 vdc

V1
pins 1,6: 150 vdc
pins 3,8: 1,25 vdc

V2
pins 1,6: 253,258 vdc (should they be exactly the same?)
pins 3,8 : 66 vdc


B+ is 356 vdc

1st 33uf cap 356 vdc
2nd 33uf cap 354 vdc
1st 22 uf cap 335 vdc
2nd 22 uf cap 323 vdc

heater voltage (read between pins 4,5 - 9) is 6,8 vac ( a tad high?)

Cathode resistor is 150 ohms, and the voltage across the resistor is 13,2 vdc
Plate voltage pins 7-3 reads 338 vdc

Using the weber bias calculator gives 14w/41ma per tube
i know this is hot, but shouldnŽt it be biased hot?
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Old 2nd July 2012, 08:06 PM   #9
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Sorry
Forgot to post the schematic:

http://www.diyguitarist.com/Schemati...eSchematic.bmp
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Old 2nd July 2012, 09:12 PM   #10
tictac is offline tictac  United States
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The Deluxe Reverb had an OT with 6.6K impedance... the Princeton Reverb is the one with the 8K OT.... If it truely is a Deluxe Reverb OT you've likely got an OT with a 6.6K secondary...

just sayin....

TT
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