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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 6:37 pm
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"pi" is short for "phase-inverter", the 12AX7 tube that drives the output tubes, nothing to do with Greek pi. There is also a 33K ohm, 1/2 watt resistor between the Cathodes of the pi tube and ground, lose this resistor and you simply lose output, as no Cathode current flows in the pi tube. This is just as the OP described his problem.

If the 33K bias resistor had popped, it would have resulted in the output tubes idle current going very high, and probably would have done more damage, like maybe melted a tube or worse, as that "bias" resistor is in the bias supply for the output tubes, so there would have been sound output up until an output tube fried, or hopefully a fuse blew. :idea:

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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 8:01 pm
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Shimmy, Ok, I saw that 33k-ohm resistor in the voltage divider off the phase inverter circuit. So, it is possible. In any event, the amp is working.

Just what would be the cause if this resistor poppping, and no other feeding the PI? Bad resistor, perhaps?


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 8:11 pm
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Thanks for all of the responses and wealth of info. I really appreciate it.

Yes, the amp is working fine. In fact, it is sounding even better than before. There is virtually no hiss any more. I've had the amp for years and just wrote the hiss off as a "Pro Jr" thing. Now, you can barely tell the amp is on when the guitar's volume is turned all the way down. The amp is better than ever now am I am a happy man!

Thanks again!


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 8:26 pm
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Glad that it is fixed ES11. :)

BMW2002Ti wrote:
...Just what would be the cause if this resistor popping, and no other feeding the PI? Bad resistor, perhaps?


As you pointed out, this is a little unusual, I would say that you nailed it, maybe just a bad resistor. JAG

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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 8:55 pm
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Shimmy and ES11, my bet is that bad resistor that popped was a big part of the inherent hiss in this amp. You killed two birds with one stone.


Just for sake of complete-ness, if anything else... there is such a thing as a Pi-filter circuit:

http://en.wikipedia.org/wiki/Capacitor-input_filter


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 9:49 pm
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:lol: We'll have to start using "p.i." or even "pee-eye" to be clear from now on.

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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 9:51 pm
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Glad to hear it all worked out. Ditching the hiss is merely icing on the brownies. Now it'll let that ash Tele sing!

Rawk on!

8)

Arjay

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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Sep 11, 2011 4:45 am
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Thanks!

Here is the picture of the bad resistor:
Image

Here's a few more of the amp:

Image

Image

Image


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