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Fender bandmaster rectifire problem.
Fender bandmaster rectifire problem.
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Old 10th May 2011, 04:26 PM   #1
mario13 is offline mario13
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Join Date: Jul 2010
Default Fender bandmaster rectifire problem.

While changing the tubes in my Fender the other day I made a stupid mistake and put a preamp tube in the wrong way( i thought it was in but the legs bend and shorted but the heater worked) The short was from one anode of the 12at7 reverb tube to the heater. Needless to say it went up in smoke! After replacing a vapourized bypass cap and the anode load resistors I turned on the amp only to find that the rectifire tube's( a 5u4 ) heater was not glowing. Not completly off just a VERY faint pre red kinda glow. I had a gig so rather that fart around anymore I stuck a coulpe of 1N4007 diodes in its place and got on with it. Is it posible that I blew the 5v rectifire heater winding? Unloaded it measured 4.9v. I was in a rush and didnt measure the voltage with the tube in place. I find this od because the short was with the 6.3v winding for the rest of the tubes. Can anybody please diagnose the problem or point me in the right direction. I can live without the tube rectifire just want to know possible causes.
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Old 10th May 2011, 08:36 PM   #2
jazzguitar is offline jazzguitar  Germany
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Join Date: May 2011
I don't think you blew the rectifier secondary, there is no connection to the 12at7 preamp tubes. But maybe your transformer was damaged in that shorting event ... then it won't be only the 5V tap.
If your amp worked afterwards though I think the power transformer is good and you should repair the other damages asap.

(can't for the life of me think how to insert a 9-pin type the wrong way ...) Good luck!
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Old 10th May 2011, 11:10 PM   #3
mario13 is offline mario13
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Thanx for the input mate! I didnt think you could insert a 9 pin wrong either thats why I thougt it was in place!!!
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Old 10th May 2011, 11:40 PM   #4
tomchr is offline tomchr  Canada
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Have you tried a different rectifier tube? I wonder if the heater got cranky by getting B+ on it. I'm kinda doubtful, as the current would be limited by the plate resistor for the 12AT7, but still...

It would also be interesting to know the heater voltage under load.

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Old 11th May 2011, 09:42 PM   #5
dcgillespie is offline dcgillespie  United States
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My bet is the rectifier tube is physically damaged, such that a small leak is now present. Over the years, I've ever seen a few tubes with a very small physical breach, and one of the characteristics I've seen happen before the inside finally turns chalky white, is that the heater starts to glow dimly for a period, before it finally burns out.

What I don't understand is how there was any damage to the reverb circuit in the first place. If a plate pin bends over and touches a bent over heater pin, so what? Assuming the heater pin was either pin #4 or #5 for the section that had the bent plate terminal, one section would still glow -- and drive the reverb springs -- while the other simply would not. Either that, or I'm not sure which circuit you are using, as the only position that Fender used a 12AT7 in in the reverb portion of a Bandmaster Reverb was the driver tube, right? Just trying to figure this out myself!

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Old 11th May 2011, 10:30 PM   #6
mario13 is offline mario13
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Thank you all for your usefull input on the matter. I finnaly found that the problem is the rectifire tube itself. I didnt have a spare so i connected the 5v fillament secondary to another tube a GZ33 to check and it lights up just fine. The secondary reads 4.8v under load. Does anybody know if I can use this tube in the bandmaster? The old bassman used a GZ34 so it seems to be an ok option...I would re wire the socket ofcourse since it has a diferent pinout.
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