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Post subject: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Thu Oct 30, 2014 10:11 pm
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Hi Guys.....opened up this Bassman (its owned by Eddie Cochran's original Bassist) Conrad, for a retube and I noticed this "White Wire" and wondering what the H&LL is this...a mod?


Thanks

[img][IMG]http://i7.photobucket.com/albums/y286/69targa912/IMG_6746_zps64075876.jpg[/img][/img]

[img][IMG]http://i7.photobucket.com/albums/y286/69targa912/th_IMG_6746_zps64075876.jpg[/img][/img]


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 1:55 am
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Photo is poor but look to me a B+ from power supply feed both 5881 tubes . The wire are connected to pins 4 screen grids .

It look that somebody remove the two 100 ohms resistors feeding these grids and replace them by a wire .


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 1:59 am
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It's hard to be sure from the picture, but it looks like the screen grids of the two output tubes are tied together.

The wires from the two screen grid resistors look like they are still connected. :? If the screen grid resistors were disconnected, I would expect no sound from the amp.

I would check to see if a screen grid resistor is blown. Maybe some doofus put in a jumper instead of replacing a blown resistor?

BTW, a blown screen grid resistors is usually the result of a shorted output tube.

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Last edited by shimmilou on Fri Oct 31, 2014 2:03 am, edited 1 time in total.

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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 2:03 am
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Or somebody want to push the tube more ........?


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 2:05 am
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It's hard to say, but likely an attempted repair by a hack.

Hack: Someone that knows just enough to be dangerous. :shock:

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 2:08 am
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+1 Shimmilou :lol:


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 7:16 am
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Like Stratele and Shimmy stated, the white line seems to be tying the screen grids (pin 4) of each 6L6GC (or 5881) together. I am not sure what is the idea of this mod? :?:

The two screens should have separate 100-ohm resistors tied to the power supply rail, after the choke.

If the screens are tied directly to the PSU, after the choke, the difference in voltage between the anode and the screen would only be 2VDC (432 versus 430VDC). In essence, taking out much of the screen's effect. I suppose making the tube a pseudo-triode.

If the white line ties the two screens together, after the 100-ohm resistors, I suppose this may balance the voltage between the two screens. I am not sure what this gains, in terms of tone or tube life.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 7:54 am
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I'll post better pics.

http://i7.photobucket.com/albums/y286/6 ... 075876.jpg
http://i7.photobucket.com/albums/y286/6 ... ce64e9.jpg


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Post subject: Re: 1956 Tweed Fender Bassman 4x10 (Dual Rectifier)
Posted: Fri Oct 31, 2014 1:48 pm
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Some old Bassman don't have any resistors , just the wire ;

http://el34world.com/charts/Schematics/ ... _schem.pdf

That is the layout for BMW schematic . You can see the two 100 ohms resistors missing where you have a white wire . Look at your amp ;

http://el34world.com/charts/Schematics/ ... AN_5F6.pdf


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