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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 8:24 am
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I think that the iso tranny fuse blew for few reasons. The inrush of the iso tranny plus the inrush of the amp, combined with the lower fuse value. No need for the iso in the first place, and if a variac, it should be brought up slowly anyway, not turned on at full voltage, otherwise what's the point of using it?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 8:25 am
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As for the high Plate voltages on some preamp/reverb/trem tubes, it will be high if the tube is not conducting, no voltage drop on Plate resistor. But, the voltage seems high from the supply to begin with.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 8:26 am
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This whole "black-facing" thing is so convoluted it is hard to tell what is happening. You need a step by step procedure, starting with the voltage and current from the rectifier.

What rectifier is used? What is the voltage and current from the rectifier? What is the Plate current and Plate voltage on output tubes? :? What is this; "removed the resistors to ground from tubes..." What resistors from which tubes?

Yes, this stupid effing forum won't let me post my whole response in one post! :?

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Last edited by shimmilou on Sun Aug 05, 2012 8:46 am, edited 1 time in total.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 8:37 am
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aclempoppi wrote:
Shimmy, please tell us about the re-temper concept...


Well, sort of another red-herring argument on my part, hinting at the "Cathode stripping" nonsense, just throwing out a complicated term to confuse people. But, heating and cooling of metal quickly can change the temper in the metal, or early fatigue, leading to shorter tube life. Metal expands when heated, and contracts when cooled. Which you probably already know about.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 9:37 am
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BMW2002Ti wrote:
Bluesky,

OK... Looked back and actually read the thread this time.

Very serpentine tale, for sure.. for sure. :D

MusicMan, prolly best for you to update us on the current status.

1.) Is the amp making good vibes?

2.) Are you still blowing fuses? Which ones? In amp or in isolation tranny or both. And what is the VA rating of that isolation tranny?

3.) Are the voltages still wacky on which specific reverb tube {driver: 12AT7 or recovery tube: 12AX7 (7025)}. Does the reverb work? How does it sound?

Thanks. Thread re-boot. :mrgreen:


According to the schematic, the 12AT7 has 400 VDC on both plates and the 7025 has 230 VDC on the plate. But need to confirm.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 9:42 am
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shimmilou wrote:
I think that the iso tranny fuse blew for few reasons. The inrush of the iso tranny plus the inrush of the amp, combined with the lower fuse value. No need for the iso in the first place, and if a variac, it should be brought up slowly anyway, not turned on at full voltage, otherwise what's the point of using it?


All that would correspond to my thought that the iso tranny was just the victum of bad mojo. :lol:

I'm still not sure from MusicMan's post whether the 3 amp or 2 amp fuse is what blew in the iso tranny.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 9:44 am
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shimmilou wrote:
As for the high Plate voltages on some preamp/reverb/trem tubes, it will be high if the tube is not conducting, no voltage drop on Plate resistor. But, the voltage seems high from the supply to begin with.


So possibly bad tube, bad socket, or bad resistor.

Without looking back in the thread, were all PS voltages measured and posted?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 9:46 am
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shimmilou wrote:
This whole "black-facing" thing is so convoluted it is hard to tell what is happening. You need a step by step procedure, starting with the voltage and current from the rectifier.

What rectifier is used? What is the voltage and current from the rectifier? What is the Plate current and Plate voltage on output tubes? :? What is this; "removed the resistors to ground from tubes..." What resistors from which tubes?

Yes, this stupid effing forum won't let me post my whole response in one post! :?


Definitely needs clarification.

You need to be more concise in your posts. :lol: :lol: :lol:

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 9:59 am
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bluesky636 wrote:
shimmilou wrote:
As for the high Plate voltages on some preamp/reverb/trem tubes, it will be high if the tube is not conducting, no voltage drop on Plate resistor. But, the voltage seems high from the supply to begin with.


So possibly bad tube, bad socket, or bad resistor...


No, I meant that it is normal to see a higher voltage if the tube isn't actually "on", or conducting, as when the tremolo is off.

My understanding is that the iso tranny fuse is what blew, and again why use a transformer to feed the amp, because in this case it's useless at best.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 2:19 pm
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shimmilou wrote:
bluesky636 wrote:
shimmilou wrote:
As for the high Plate voltages on some preamp/reverb/trem tubes, it will be high if the tube is not conducting, no voltage drop on Plate resistor. But, the voltage seems high from the supply to begin with.


So possibly bad tube, bad socket, or bad resistor...


No, I meant that it is normal to see a higher voltage if the tube isn't actually "on", or conducting, as when the tremolo is off.

My understanding is that the iso tranny fuse is what blew, and again why use a transformer to feed the amp, because in this case it's useless at best.


True, due to the fact there is no load on the tube to pull it down, but not 200 VDC high! (MM says he is measuring 460 VDC when it should be 230 to 280 VDC). :shock:

I really think the iso ransformer fuse blowing is another red herring. :wink:

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 4:38 pm
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Agreed, we need an update(reboot). I for one got this thread mixed up with a Vibrolux thread. So my comment about the first filter reservoir being post standby switch is irrelevant! My Bad! But flippin" that switch 5 times in one minute isn't a good idea IMO. The Iso trans, yeah what is the VA? Shimmy, I think Musicman meant the 2000pf caps off the power tubes were clipped. Bill, no PT voltages were posted. Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 5:53 pm
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This is the comment about the resistors below. There are a couple of Bandmasters that have Cathode resistors on the output tubes (AC568, AA768) but if you remove them I would expect the output tubes current would be excessive. I'm not sure what other resistors might have been removed, or why, or even exactly what circuit it was to begin with. The OP mentioned looking at both the ab763 and the aa1069 schematics, the former has a bias pot and no reverb, and the latter has a balance pot and reverb, and both have a SS rectifier. :?

musicman3858 wrote:
...and removed the resistors on the tubes going to ground...

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 6:07 pm
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Right, it's confusing! But I think we've got an AA1069 Bandmaster Reverb. That has been changed to a generic AB763 bias control setup. And that amp would have a 5U4GB rectifier. HTH Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 6:22 pm
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Yeah, time to take a step back until MusicMan can respond to our questions.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 7:11 pm
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:D Ok... cautiously going where NO MAN has gone before... :lol:

You know, I read that thread about the 400VDC on the 12AT7 anodes about three times before it hit me that it's supposed to be around 400VDC. Both halves are tied together to get the most mojo (ok, current flow) out of that 12AT7. And the 12AX7 (7025) recovery half should be around 250VDC.

Awaiting re-boot...

http://www.webphix.com/schematic%20heav ... _schem.pdf


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