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Post subject: Re: Help Bandmaster blowing fuses
Posted: Tue Jul 10, 2012 9:44 pm
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Yeah Musicman, you can check PT voltages with the standby switch in the"standby" position. Readings will be higher than normal(no load). IMO better to pull all the tubes, switch to play mode and check all of the B+ power supply and bias supply. Then I would begin replacing the pre amp tubes one at a time(starting with V1) and recheck all voltages. BTW I'd get a 3 prong plug in the amp first(sounds like the ground lug was removed). Two questions: How long did the amp work correctly after the mod and power tube R&R? And just for a schemo, is this a reverb or non-reverb unit? OOPPS, one more, which OT are you using? Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Jul 20, 2012 8:26 am
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Hi, just wanted to update you on the progress. Since I can only really concentrate on this project on weekends it takes me a little while. I have been looking at the schematics and layouts of both the ab763 and the aa1069, and I have replaced the power cord and been checking voltages. I did notice something, that the power and standby will stay on until the amp is on for @ 10 min., then the fuse goes. After checking power I found that my power at my phase inverter should be around 260 at both resistors and I'm getting 257 at one and 215 at the other. I have replaced the original 47k resistors with 100k and 82k ones respectively. While I would expect the voltage to be lower at the 100k, the schematics are showing the voltages should be +260. Any ideas? Bad resistor? Also going to replace the cap in the bias circuit from 50/70 to 25/50 as in the ab763 and the resistor to 470- 1W. Could the diode be bad in this circuit? and how could I check.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Fri Jul 20, 2012 11:42 am
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Ok... this is what I would do. Keep the values of the OEM resistors the same for now. That is, if they drifted more than 25% out of nominal value, replace with similar value (5% tolerance) carbon comp resistors. Keep the circuit as OEM as possilbe, until you can get rid of the blowing fuse issue. Then, mod it to AB763 (or whatever) once the amp is stable.

Now, measure voltages--- but with no tubes (or spare tubes which are known good ones & expendable). NO tube will give you about 20-25% more voltage values (in general)-- because of reduced load.

Measure the NEGATIVE DC voltage off the Bias Power Supply at the grid-to-ground resistor branch point, to the output tubes.

If your amp is an AD1269 circuit this would be (-48 VDC) at the BALANCE pot. It should be ROCK STEADY -48VDC. If not you could have a weak lytic cap in this circuit. Or a bad solder point. Or a hinky reisistor. Or bad pot. Wooden Chopsticks test should isolate the problem(s).

http://www.webphix.com/schematic%20heav ... ad1269.pdf

Like I said before, the scenario of all is seemingly ok for 5-10 mins, then the fuse pops could indicate an instability in the amp. Like parasitic oscillations. Which take awhile in a cold amp, to get to the point of red-plating or fuse blowing.

Go through this section to isolate the issue:

http://www.geofex.com/ampdbug/fuseblow.htm

Mr Harnish (on Telecaster Board), has some nice tips on how to help eliminate instabiliblities and parasitic oscillations:


" For a good description see Kevin O'Conner's Tonnes of Tone Vol. 3. The book is pretty pricey but well worth the $$$ if you're serious about amp building; the other 2 volumes are also very good. He covers all the basics which can't be easily described in a post. However, in a nutshell, good lead dress practices include:

1) Making sure wires cross at right angles
2) Making sure there is a proper distance between various signal carrying wires
3) Proper soldering techniques
4) Avoiding excess lengths of wire
5) Creating twisted pairs when dealing with AC signals (where phase cancellation helps lower noise radiation)
6) Proper grounding techniques
7) Use of shielded cable in certain cases

Regarding your observations about insulation on resistors; this makes little to no difference in terms of noise and stability. It does improve reliability marginally b/c it can help avoid short circuits but this is a very rare case; where it's probably most critical is in true PTP wired amps (e.g., Matchless) where you have a lot of leads running all over the place. With turret or eyelet boards, it really isn't needed.

Unfortunately there is a lot of voodoo when it comes to lead dress; you literally can nudge a wire a few millimeters and go from an unstable amp that has major problems with parasitic oscillation to a perfectly functioning circuit. This is the case with many SF Fenders which have the dreaded parasitic oscillation caps on the output sockets. Sometimes you can remove these and have no problems at all; other times the amp has major problems. I had a early (~69) SF Deluxe Reverb which had <u>major</u> oscillation problems when I removed the suppression caps; after I spent a few hours redoing the lead dress (shortening and re-routing the various leads and wires) I got the amp working correctly again. However, you can spend a long time working on these problems before you find the correct solution. In most cases the diagrams commonly floating around for the vintage Fender amps are pretty good guides to use. "
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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Jul 21, 2012 12:56 am
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With all due respect, MusicMan, I will ask again if this amp is the reverb or non-reverb model? Reason being, the B+ power supply. Are we looking at a solid state or tube rectifier? Also, how long did the amp work correctly after the mod? Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Jul 21, 2012 3:34 am
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MusicMan, Art is spot-on. You need to know the very basics (like the circuit specifics) before measuring voltages or even trying to figure out what is wrong.

Believe me --- the " 5 minutes on, then fuse blows " can be a difficult puppy to hunt down. May require a scope and signal generator (read: good amp tech).


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Jul 21, 2012 3:34 am
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BMW2002Ti wrote:
" I replaced the power tubes with tung-sol 5881's. I re-biased it and it sounded great "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.


Interesting, BMW.
Question. When Biasing any "amp head", should there be a speaker load
at idle or no speaker connected to the speaker out jack?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Jul 21, 2012 3:38 am
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nedorama wrote:
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?



I have some amps with the three prong power cord
but the chassis output plug is still two prongs. Do you or
does anybody change the amp's output plug to three prong as well???

Toppscore

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Jul 21, 2012 3:59 am
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" When Biasing any "amp head", should there be a speaker load at idle or no speaker connected to the speaker out jack? "


Topp, ALWAYS have a load on the OPT. NO input (no guitar plugged in). And volume(s) at zero --- when checking the idle bias.


Hierarchy of 6L6 tubes:

6L6G, GA = max 15-18 watts per tube.

6L6GB, 6L6WGB, 5881 = 19 watts per tube

6L6GC = 30 watts per tube

All in push-pull, Class AB1 configuration.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 11:05 am
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Hey guys, this is a reverb model tfl 5005 D. It played for aprrox. 3 hrs before it spazzed. Once for about 2 hrs. before I replaced the power tubes and about an hour after. I did hook a foot switch into the vibrato/ reverb to see if the vibrato worked. It was weak. Then I unplugged the switch and played approx. 20 min. and it popped. I just pulled all the preamp tubes and let it idle with the power and standby on. It stayed on about 40 min. Then I put pre amp tubes in after it cooled off and same thing,, on for about 20 min then off. I just want to say thanks to everyone who has taken the time and patience with my inexperience. I won't give up....lol


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 1:28 pm
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Just did some more voltage checking. I found that on the vibrato tube where I should have 390 on one side and 280 on the other I have 415 and 460. Then I traced it back to a 220K resistor and I have 460 on one side and 453 on the other. Keeping fingers crossed! Let me know what y9u all think... :idea:


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 1:51 pm
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Could be that 220KΩ resistor is toast -- actually a pretty common problem in older amps.

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 4:18 pm
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Check your home's electrical power output.
Simple as that :lol:

Toppscore 8)

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 5:23 pm
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Toppscore wrote:
Check your home's electrical power output.
Simple as that :lol:

Toppscore 8)


What a rocket scientist!

:roll:

We'll nominate you for the Nobel Prize.

:lol:

Arjay

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 5:29 pm
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Retroverbial wrote:
Toppscore wrote:
Check your home's electrical power output.
Simple as that :lol:

Toppscore 8)


What a rocket scientist!

:roll:

We'll nominate you for the Nobel Prize.

:lol:

Arjay


Thanks. Sometimes the simplist things are overlooked.
I have a house that at one time had different sections blow-fuses
and found out it needed an improved electrical system installed
as certain rooms were getting higher watts/amps/voltage, etc.
Never know. Hope my suggestion solved the problem :wink:

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Jul 22, 2012 6:08 pm
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Best to just lift one leg of the 220K-ohm resistor and measure its resistance. BUT, before you do that... check ALL voltages in the gain stages. Your problem(s) maybe up- or down-stream from that resistor.

Usually, pre-amp tubes (12AX7 or 12AT7), even if they short out won't cause the fuse to blow, as they can only draw a few hundred mA max. Blown fuses usually lie within power supply (PT, power supply caps, and/or rectifier section) and/or output section (6L6GC).

Good luck with all that.

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

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


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