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Post subject: Amp Issue Diagnosis
Posted: Sat Aug 27, 2011 9:01 pm
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I had to drop my amp off at the local music shop today. I was playing this morning and all of the sudden it just went quiet. I was wondering if I could get some opinions of what you more experienced tube amp aficionados think might be wrong with it. Here are the details:

Like I said, it just went quite when I was playing it. Before that (over the past 4 to 5 times I recently played it) it started to make some hissing noises. The amp is a Pro Jr, which are known for their hiss, so I didn't think much of it....figuring there might be some electrical interference in the outlet or something like that. So, when it died today I took it to the shop. We tried new tubes in it which didn't resolve the issue. To hear anything out of it, you need to turn the amp up all the way....this only results in a faint sound coming out of the speaker.

Any idea of what might be wrong with it? Here is a picture of the amp as your reward for reading this long post and (hopefully) giving me some info. Thanks!

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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Aug 27, 2011 10:52 pm
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Hi ES11,

Did all of the tubes light up, glowing orange? Did you change all of the tubes? Do you have the amp, or is it at the shop?

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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 4:27 am
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Hi Shim,
Thanks for the response. Yes, all of the tubes light up and glow orange like they they always did. Yes, all of the tubes were changed. No, I do not have the amp now...I left it at the shop.
Does this info help?
Thanks!


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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 6:17 am
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If you don't have the amp in front of you, any more guessing would be pointless. I presume that you took the amp to a shop that knows how to fix it?

Nice looking amp and Tele! :D

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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 12:32 pm
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Yes, I left it at a reputable shop. We'll see what they say. I was hoping to just hear some different ideas from the folks here. My hope is that it is a cheap fix. I'm going to have a tough time spending a lot of money fixing it when I'm gassing for a Vox AC15 right now... :D


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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 12:41 pm
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That sure is a nice-looking ash Tele......

8)

Arjay

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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 12:51 pm
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Thanks Arjay!

Image

Image


With black PG:

Image

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Post subject: Re: Amp Issue Diagnosis
Posted: Sun Aug 28, 2011 9:45 pm
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Looks good with either pickguard 8)

Although I am a fan of the white ones...

T2

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Post subject: Re: Amp Issue Diagnosis
Posted: Mon Aug 29, 2011 11:26 am
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T2,
That is a mighty fine Esquire! What year?


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 10:10 am
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Thanks everyone for the responses. I got my amp back today. The issue was an open resistor in the PI circuit. Could have been worse so in all, I'm just glad to have the amp back.
Thanks again.


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 10:22 am
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" The issue was an open resistor in the PI circuit. "

Ok, this is prolly not an issue... but, if "open" meant popped---it could be. Pi-circuit is part of the power supply filter circuit. And if the resistor popped due to excess current flow, you could (operative work is "could") have an inherent problem.

Do you plug the amp into one of those filtered, multi-outlet adapters? Or directly into the wall? Sometimes, the cheaper multi-outlet adapters can cause problems with tube amps. If this is the case, plug the puppy into the wall outlet. The amp has filter circuits that handles line noise and surges better than cheap MOV type multi-outlets.

Good luck! Just a heads-up.


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 12:06 pm
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BMW,
I plug the amp directly into the wall. The resistor was 'popped'. The tech gave me the broken one when I picked-up the amp.
I'm really hoping that the issue is fixed once and for all. Thank you for your insight!


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 5:10 pm
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Most likely, that resistor acted like a fuse. For whatever reason. A really good reason NOT to arbitrarily replace too high wattage-rated resistors, esp in certain critical sections of the amp.

Good luck with the amp. It could easily have been a defective resistor (knowing the quality of many new resistors on the OEM market these days...)


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 5:36 pm
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BMW,
The old resistor was replaced with a 33K 1/2 watt new one. I don't know much about electronics and amps, so my hope is that this is the proper resistor that should be in the amp.
Thanks!


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Post subject: Re: Amp Issue Diagnosis
Posted: Sat Sep 10, 2011 6:17 pm
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ES11, the 33k-ohm, 1/2 watt resistor (R30) I see in the schemo & part of a Pi-circuit is in the Bias Power Supply. This is part of circuitry that sets the idle bias of the power tubes. Very important part.

BTW... it's called "Pi-circuit" because if you look at the schematic, the sections are repeated cap-to-ground --- then resistor (or choke) in series --- then another cap to ground. So it kinda looks like the Greek letter Pi = π

Good luck with amp! Sorry for the small image. Maybe it can be enlarged upon D/L.


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