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Post subject: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 6:47 am
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Roadie
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Just a general question.
In an other wise healthy amp experiecing low volume output can an OT be weak?
Particular amp in question is 1973 Band Master Reverb. As stated all other aspects of the amp and cab are good to go.
thanks


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 6:52 am
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Rock Star
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I've rarely seen a bad OPT in a Fender amp. I've seen other amps with fried OPT (usually not the OPT's fault). There are no moving parts in a tranny & Fender used some very well-made ones, so it should not "wear-out" from use.

You need to do a voltage read on the amp. PS supply, bias supply (including bias current), plate voltages, the whole nine-yards. This sounds like a weak tube somewhere. Or poor jack-plug contact. Or bad pot. Or something that does wear out.

BTW--have you tried a different speaker cab?

Keep us informed!


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 9:30 am
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BMW2002Ti wrote:
I've rarely seen a bad OPT in a Fender amp. I've seen other amps with fried OPT (usually not the OPT's fault). There are no moving parts in a tranny & Fender used some very well-made ones, so it should not "wear-out" from use.

You need to do a voltage read on the amp. PS supply, bias supply (including bias current), plate voltages, the whole nine-yards. This sounds like a weak tube somewhere. Or poor jack-plug contact. Or bad pot. Or something that does wear out.

BTW--have you tried a different speaker cab?

Keep us informed!



Thanks B...
The amp was like this when I got it a few mothns ago. Its a BMR I am cloning to a VR type of thing. The stock Utahs suck so I assumed that was the low volume out put issue. I have redone the PI and power supply like the BFVR. All tubes are good and
all the voltages are good. PV at 460, biased at 35mA. Bias voltage is at -55.
Pots douched out and have used three different cabs with dif types speakers.
This amp was not played alot at all. I am thinking maybe the OT was just not right from day one. I guess its possible. I realize an OT as no moving parts to wear out but can they be weak?? I gues they can...right?


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 9:36 am
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I've seen Fender OT's go bad before, usually the result of shorted windings caused by thermal overload. A quick method to check its health is to isolate the primary and measure the DCR -- for a 2 x 6L6 configuration you should see 7.6KΩ to 8.0KΩ.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 9:53 am
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Frick it!!! I hate it when the damn site dumps your post. Here goes attempt number 2.

You need to pull your output tranny. And measure the static DC resistance of the secondary windings. From end to end of the secondary.

And the primary. Measure the DC resistance from each end of the primary to the center tap. Now, their resistance will not be the same. But, should be within 25% of each other.

To test more accurately, you need a signal generator and a scope (or good DVM). this requires a tech that knows HOW to do these tests. I've found that most guitar amp (even hi-fi amp) tech do not know the proper procedure. So, I find a good Ham Radio or Vintage Radio tech to do the work. They do this a lot, since replacement trannies are hard to find.

The resistance should read in the DC 20-100 ohm range. If it reads in the 1000 plus-ohm range, you prolly have a damaged winding.

You can have the tranny re-wound. There are several good services. Or get a proper Hammond or Merc Mag replacement.

Good luck, keep us informed!

Note: Arjay, I believe those measurements you posted are AC impedance values. Not static DC values.


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 10:01 am
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Roadie
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Retroverbial wrote:
I've seen Fender OT's go bad before, usually the result of shorted windings caused by thermal overload. A quick method to check its health is to isolate the primary and measure the DCR -- for a 2 x 6L6 configuration you should see 7.6KΩ to 8.0KΩ.

HTH

Arjay


Thanks RJ...
can this be accomplished with a DVM? Or is special equipment needed. I apologize for my lack of tech savy here. I have replaced OTs/PTs before but never really troubleshot them other then narrowing it down them being the faulty part.


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 11:37 am
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mxvin, sorry to butt-in, but read the STATIC DC resistance of both primary and secondary windings, as I posted. This will give you an idea of continutity issues.

Yes, you need a signal generator (to make 1KHz sine wave) and a scope or good DVM to read the dynamic AC IMPEDANCE values of the secondary and primary windings.

Good luck! Tell us your results.


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 11:42 am
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Roadie
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BMW2002Ti wrote:
mxvin, sorry to butt-in, but read the STATIC DC resistance of both primary and secondary windings, as I posted. This will give you an idea of continutity issues.

Yes, you need a signal generator (to make 1KHz sine wave) and a scope or good DVM to read the dynamic AC IMPEDANCE values of the secondary and primary windings.

Good luck! Tell us your results.

Static reading it is then..
I actually have a scope and sig gen. Never learned how to use them though. Got them free from a closed down avionics shop years ago.


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 2:19 pm
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Ok, if the static DC resistance measurements doesn't give you the answer to the problem --- post this. And we can go over how to use a signal generator and DVM to measure the dynamic impedance of the OPT.

Good luck!


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Thu Jan 17, 2013 2:34 pm
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Roadie
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will do BMW....thnx


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Sat Jan 19, 2013 4:02 am
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Easy test here but only the first test , There is a better test with audio generator with AC sine wave at 0.5 volts and a voltmeter with reading the ration of primary and secondary. Like BMW write ?

The first easy test;

books.google.ca/books?id=fvUVBP3wW1gC&pg=PA127&lpg=PA127&dq=gerald+weber+transformer+test&source=bl&ots=0xmzkay1_F&sig=uTadyznmggIW3nh18rfGDLOqVuc&hl=fr&sa=X&ei=tHr6UIuODtGF0QHt-YGgCQ&ved=0CC0Q6AEwADgK#v=onepage&q=gerald%20weber%20transformer%20test&f=false


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Tue Jan 29, 2013 11:36 am
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Roadie
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well after doing all the troubleshooting that i "want" to I made an eductaed guess and figure its a weak OT.
Purchased a Classic Tone from Magentic Components. 40 watter, 4K primary, multi tap OT. When I get it and install it I will follow up..

http://www.classictone.net/40-18006.html


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Tue Jan 29, 2013 11:46 am
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Rock Star
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Good luck with the new OPT. Be interested in eeeing the results. If it is the OPT, I'd be curious to open up the bad one, to find out what happened.

:mrgreen:


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Tue Jan 29, 2013 1:34 pm
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Roadie
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BMW2002Ti wrote:
Good luck with the new OPT. Be interested in eeeing the results. If it is the OPT, I'd be curious to open up the bad one, to find out what happened.

:mrgreen:


you may be curious but I am not...:)

:lol:


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Post subject: Re: Can Output Tranfromers get "weak"
Posted: Tue Jan 29, 2013 5:26 pm
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Reason I say that is for every bad Fender OEM OPT --- I see, maybe, 50 fried output stages (prolly closer to 100). Which may toast the OPT. Usually due to shorted output tubes, bias stage gone wild, current sucking instabilities in previous stages of the amp, shot main filter caps, and arcing rectifiers.

Anyhow, good luck with the new iron!

:D


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