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Post subject: Re: Help Bandmaster blowing fuses
Posted: Wed Aug 01, 2012 12:38 am
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Know what you mean about discerning the old resistor color codes, Musicman. Maybe that 470 was a Saturday hung-over installation. Neat bunch of equipment you procured. Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Wed Aug 01, 2012 9:07 am
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" I've found a 460 ohm where there should be a 2200 ohm resistor. "

MusicMan, The one 2200ohm resistor I find in the AA1069 BMR (TFL 5005D) is the cathode-to-ground resistor on the tied halves of the 12AT7 (reverb driver). Be sure the 470 ohm resistor you are seeing on the board is this resistor.

It's easy to mixed up the placement of the components with-respects-to the layout versus the schematic.

See the layout, on the bottom of link? Is it the 2200-ohm resistor sitting above the 12AT7 socket on the tagboard?

FWIW -- this resistor helps set the idle bias of that 12AT7. Wrong value here would throw that tube out of the linear range. Causing excessive distortion (even "farting") if the idle is too hot (too little resistance). Or poor reverb, if set too cold (too much resistance).

Knowing which 2200-ohm resistor has been replaced by a 470-ohm one, would tell you a lot about the response (or lack of) in the amp.

Anyhow, good luck.

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


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Wed Aug 01, 2012 11:23 am
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Yes it was that resistor. I changed it out last night back to the 2200 shown. My voltage on the vibrato 12ax7 in the vibrato stage is 411 on pin 6 and 407 on pin 1. should I be looking at the .02 and .01 caps? I think that Art suggested I look at these if that is correct. Since I've changed my circuit to look more like an AB763, there is a 5-25 cap going to the 12ax7 in the vibrato circuit. Should I change this to the 25-25 as in the SR? Thank you :)


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Wed Aug 01, 2012 2:36 pm
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I use 8 mfd/150VDC in the 5mfd/25VDC bypass position.

http://www.tubesandmore.com/products/C-SA8-150

BTW... I've been using 25mfd/50VDC rated caps for OEM 25/25 ones. Higher outlet voltages and the smaller size of modern caps make this a good choice for longer life IMHO.

http://www.tubesandmore.com/products/C-SA25-50


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 7:58 am
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Well, I got the amp working!! Hooray! I replaced the .01's and the .02 caps and rechecked my voltages. It stayed on so I tried to switch the standby off and on which I did about 5 times in about 1 min. It blew the fuse in my iso tranny. I let it sit for about 10 min in the on position then I turned it back on. It held... I plugged my guitar in and played... WOW! Still a little worried about my voltage on the pin on the reverb tube. Should be 280 but I'm still getting 460. Maybe the sockets which I'm sure could be replaced. Gonna give it a few more test runs before I put it back into the cabinet. Now to find a new project. I would just like to say a BIG THANK YOU to ART, ARJAY and all who so graciously gave me info and the help to get to this point.. I'm sure I'll be needing more soon! :D


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 8:57 am
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Congrats.

However, that blown fuse in your isolation transformer gives me pause for concern. What's it's current rating and what type of fuse was installed?

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 9:07 am
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Yeah. I don't have a warm feeling about it either.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 9:11 am
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It was a 3 amp, I replaced it with a 2 amp which I had as replacements for my amp. Do you think I still have a problem in there? I thought it could have been switching the standby off and on. I didn't have my speaker load on at the time. :?


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 8:23 pm
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MusicMan, I don't think that not having your speakers connected would cause the fuse to blow. HOWEVER, shouldn't do this, as it's not good for the OPT.

Standby switch popping fuses is prolly another issue.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Aug 03, 2012 11:19 pm
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Can't say I'm confident with the situation. Switching that standby, slams the first filter cap pretty hard. But since the first reservoir is post standby, it kind of takes the brunt. That's good news for the choke and the rest of the guys following on the B+ supply. They get a slight break, while those caps charge up. To be honest, I use the standby function as little as possible. Prefer to turn volumes down, if I'm taking a 10 minute break. As far as using that function as a cool-down, I'm willing to debate the heater running on a hot tube vs ambient temperature cool-down. Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 2:01 am
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Art, there's a bit of controversy over the efficacy of standby swtiches in tube amps. Whether guitar or hi-fi. If you haven't already already read it, here's a nice little ditty on them (pro & con):

http://psg.com/~dlamkins/lamkins-guitar ... dby-switch


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 7:32 am
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It is sad that people want to make things more complicated than they need be. No one claims Cathode stripping as a reason to use a standby switch on a tube amp, except for those that bring it up when decrying it as a reason. The simple fact is, that slowly warming up the tubes is beneficial for longer tube life, and a standby switch helps in this regard. Same for slowly cooling down the tubes. How about "tempering" for a term to consider. :idea:

I can agree with conservative use of the standby, maybe not necessary when taking breaks, maybe not necessary when switching power level or impedance setting on the amp, but let's not throw the baby out with the bath water.

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 7:46 am
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There are a few things that I do religiously......

Like always wearing my seatbelt, documenting every line on my 1040 form, checking my altimeter setting prior to taxiing to the active, remembering my mother-in-law's birthday.

Using an amp's standby switch falls into this category.

:mrgreen:

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 8:25 am
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I've put snubber caps across switches in amp, to reduce pitting. And early failure. Another option. FWIW.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 9:40 am
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BMW2002Ti wrote:
I've put snubber caps across switches in amp, to reduce pitting. And early failure. Another option. FWIW.


+1

Especially since contemporary Carlson switches are but a pale substitute for the US-made OEM versions of yore.

Arjay

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