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Post subject: Help Bandmaster blowing fuses
Posted: Mon Jul 09, 2012 7:22 pm
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Hi all, I hope I'm at the right place for help. I have a '73 Bandmaster that I have blackfaced. It sounded good and played well. I replaced the power tubes with tung-sol 5881's. I re-biased it and it sounded great. I was playing it and it made a crackle sound and the fuse blew. I could smell something that got hot. I replaced the fuse and turned on the power and it was ok, then I flipped the standby on and the fuse blew. I checked and two 100 ohm resistors going from the lamp to the bias circuit were bad. one burnt and the other cracked. I replaced them and the fuse popped with the standby switch. I removed all the tubes and turned it on and flipped the standby and it was ok. I put the rec. tube and was ok, put the preamp and rec tubes in and ok. Put one power tube in and was ok, pulled that tube out and put the other one in and it popped the fuse. Thinking I had a bad tube, I put the old power tubes in and it popped a fuse. I removed one of the power tubes in and it is ok but with two it pops it. Also when I had one tube in and it was on it had a low hum so I switched the ground switch and it popped. Any suggestions would be appreciated. Just wanting some direction as I'm fairly new at this. :roll:


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Mon Jul 09, 2012 8:11 pm
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Interesting, Musicman. So the power amp section has a short or over current draw, right? Initially, I'd guess a bad power tube. But the older power tubes(which were just tired)caused the same problem, right? I would start at the Phase Inverter tube socket and check voltages, all the way through the power amp, with a close eye on the bias supply. You mentioned the 6.3 filament artifical center tap 100ohm resistors were fried, so that supply off the power trans needs to be examined. The output trans could also use a static check, but this is an outside culprit. More than likely, you're just looking for resistors off the power tube sockets or bias supply that took a dump. Just a thought, does the amp have a three-prong AC power cable? Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Mon Jul 09, 2012 11:06 pm
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If the amp powered up fine and blew the fuse only when the standby switch was flipped to the "play" mode you can forget about a phase-inverter problem. The only thing the standby switch does is interrupt the B+ voltage to the power-tube plates. Look to the power supply and filter caps first and if they check out okay, examine everything between the 445VDC secondary tap of the power tranny to the power tube sockets -- wiring, sockets, screen resistors, even the standby switch itself.

HTH

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 2:07 am
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Arjay, we don't know about channel operation yet. And if I'm not mistaken, all B+ power follows the standby switch. IMHO, any power amp failure diagnosis would, yes, begin at the power trans and progress from there through the power supply and bias supply, right up to the power tube sockets. IMO Where the pre amp couples to the phase inverter, this begins the power amp section. Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 6:09 am
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" I replaced the power tubes with tung-sol 5881's. I re-biased it and it sounded great "

Are these the 1950-60 US made TungSols? Or the newly made Russian reissued ones?

Either way when you go to re-bias them... you must bias them @ lower idle current (so that max dissipation is not above 19 watts per tube--- better around 16-17 watts per tube). The 5881 started life as a mil spec tough version of the 6L6GB, not the 6L6GC.

For the Russian made 5881 (TS or EH)... the verdict is still out as to whether they can handle the plate volts of 425-475VDC for any length of time--- voltages that these vintage Fenders puts on their 6L6GC amps.

You maybe inducing instability & excess current draw, if the tubes are not biased correctly or are the type with not heavy enough grids and plate material to handle 425+ VDC. And that could be causing the fuse to blow.

Go back to some known, good 6L6GC and try reseting your values and see if this keeps the fuse happy.

BTW... you are using the proper amperage SLO-BLOW fuse?


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 7:41 am
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BMW2002Ti wrote:
BTW... you are using the proper amperage SLO-BLOW fuse?


As silly as that sounds, when I finally got my Weber 5F6A clone working, I had a problem with fuses popping when switching standby to on. Turned out that even though I had the correct amperage fuses, I had missread the box label and purchased fast blow instead of slow blow fuses. The sudden current in rush when the switch was flipped caused the fuse to blow. Installing the correct slow blow fuse solved all my problems. By all means, make sure the correct fuse is installed.

What circuit do you have? Do you have a good schematic? I can;t find one that is legible.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 8:21 am
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Thank you all for the help, I have a three wire cord and a two prong plug. I've replaced the fuse with a 2 amp slo-blo which is the correct size. The Tung-sol's are the re-issue ones, and I put the old ones back in and the fuse popped with them now too. It will stay on with one power tube in. It had a small hum and I flipped the ground switch and it popped again with 1 power tube. Like I said I am new to this and want to learn so any information or instruction would be great. It is a 73 bandmaster that I blackfaced according to G. Weber. Could it be a pre-amp tube? I don't see any visible signs of any more damage than the 2 resistors. If the power tube shorted could it have taken something else out? Can I check the voltages with out the standby on?


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 9:43 am
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musicman3858 wrote:
Thank you all for the help, I have a three wire cord and a two prong plug.


Not that this is likely the problem, but why?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 10:17 am
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musicman3858 wrote:
Thank you all for the help, I have a three wire cord and a two prong plug. I've replaced the fuse with a 2 amp slo-blo which is the correct size. The Tung-sol's are the re-issue ones, and I put the old ones back in and the fuse popped with them now too. It will stay on with one power tube in. It had a small hum and I flipped the ground switch and it popped again with 1 power tube. Like I said I am new to this and want to learn so any information or instruction would be great. It is a 73 bandmaster that I blackfaced according to G. Weber. Could it be a pre-amp tube? I don't see any visible signs of any more damage than the 2 resistors. If the power tube shorted could it have taken something else out? Can I check the voltages with out the standby on?


If the fuse is OK with one tube plugged in, it seems clear to me that the problem is in the power amp circuitry. With two tubes, excess current is obviously the problem. It is my understanding that depending on the circuit you have, "Blackfacing" an amp primarily consits of modifying the bias circuit. I would take a VERY close look at the power supply and bias circuit from the power transformer to the tube as has already been suggested. If you cannot find anything wrong, then it is time to take the amp to a pro amp tech before any further damage is done.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 10:25 am
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Thanks I will definitely be checking voltages. But the ground switch thing is puzzling me. The cord is the one that came with the amp and is going to part of the changeover.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 11:48 am
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bluesky636 wrote:
It is my understanding that depending on the circuit you have, "Blackfacing" an amp primarily consits of modifying the bias circuit.


It's a little more involved than that -- there's also the matter of the center-tapped secondary on the power tranny plus those tone-thieving snubber caps and concrete resistors tying the screen grids of the power tubes to ground. And problematic is the fact that there never was a blackface version of the Bandmaster reverb to replicate. The best bet is to follow the AB763 schematic for a Super Reverb (the BMR's closest "living" relative).

HTH

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 12:51 pm
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Retroverbial wrote:
bluesky636 wrote:
It is my understanding that depending on the circuit you have, "Blackfacing" an amp primarily consits of modifying the bias circuit.


It's a little more involved than that -- there's also the matter of the center-tapped secondary on the power tranny plus those tone-thieving snubber caps and concrete resistors tying the screen grids of the power tubes to ground. And problematic is the fact that there never was a blackface version of the Bandmaster reverb to replicate. The best bet is to follow the AB763 schematic for a Super Reverb (the BMR's closest "living" relative).

HTH

Arjay


Understood. Could any of those changes cause the problems musicman3858 is experiencing? We also don't know what was done to "Blackface" the amp in question.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 1:03 pm
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Hey guys, I rewired the bias circuit as on the blackface ab763, changed a few resistors and caps and removed the resistors on the tubes going to ground. I also changed out the ot for an 8 ohm speaker. Also it doesn't matter which socket the tube is plugged in to. Thanks for responding because I'm really excited about this project and would really like to do and learn myself if I can.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 1:49 pm
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musicman3858 wrote:
Hey guys, I rewired the bias circuit as on the blackface ab763, changed a few resistors and caps and removed the resistors on the tubes going to ground. I also changed out the ot for an 8 ohm speaker. Also it doesn't matter which socket the tube is plugged in to. Thanks for responding because I'm really excited about this project and would really like to do and learn myself if I can.


Then I suggest you go back and review everything you did, comparing your resulting circuit to the AB763 schematic again, because there is obviously a problem in your wiring.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 2:48 pm
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bluesky636 wrote:
Then I suggest you go back and review everything you did, comparing your resulting circuit to the AB763 schematic again, because there is obviously a problem in your wiring.


+1

Or an aging component has finally given up the ghost. Start from "T1" and work forward from there.

HTH and let us know how it turns out.

Arjay

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