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Post subject: Blues DeVille has low power, distortion at all levels.
Posted: Fri Dec 02, 2011 2:34 pm
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I have been working on a '94 or '95 Blues DeVille 4x10 that suddenly lost power and has no clean sound. I've read about the common problems with this series of amps and have completed several steps.
1. new tubes. tried 2 different sets to be sure. preamp and power tubes.
2. new filter caps.
3. replaced/upgraded resistors R85 & R86.
4. re-flowed/re-soldered many connections including the tube sockets and jacks.
5. connecting another source directly to Power Amp In has no change leading me to believe the problem is NOT in the preamp section.

I feel like I'm chasing my tail. I've checked multiple test points, dozens of individual components and still haven't found the problem. Obviously there must be something I'm missing but at this point how do I back up and systematically go through the signal path to find where the problem is?

I have (successfully) fixed many amps but consider myself a technician, not an engineer. I know how to be safe around high voltage, how to solder, how to test components, etc. but I'm feeling inadequate right now. Any help would be appreciated.

Thanks.


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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Fri Dec 02, 2011 3:26 pm
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First make sure you have the schematic:
http://support.fender.com/schematics/gu ... ematic.pdf
Now let's twist !

I understand that your preamp is OK since you have connected preamp-out on another amp or connected another preamp in power in.

0. Make sure all valves are filament red
1. Measure on pin 3 of both 6L6 roughly 450V.
2. Measure on pin 5 of both 6L6 roughly -50V (bias)
3. Measure Y test point roughly 400V
4. Measure on plates (pin 1 an 6 ) of V3 roughly 200V

Let me know and take care of lethal voltages.

8) 8)

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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Mon Dec 05, 2011 1:40 pm
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Thanks for the swift reply. Here's what I found:

0. Make sure all valves are filament red
confirmed yes

1. Measure on pin 3 of both 6L6 roughly 450V.
both 6L6 at 467V

2. Measure on pin 5 of both 6L6 roughly -50V (bias)
-48V, -49V

3. Measure Y test point roughly 400V
test at 425V

4. Measure on plates (pin 1 an 6 ) of V3 roughly 200V
pin 1 @ 269V, pin 6 @275V


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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Mon Dec 05, 2011 2:06 pm
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Measure pin 4 of both 6L6, about 464 VDC. :idea:

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Mon Dec 05, 2011 2:15 pm
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Problem could be in Phase inverter circuit ; The 12AX7 just before power tubes. Check R51 , 91 K and R52, 100 K .

I understand that you work on this amp ( replacing components or... ? ) and sudenly amp lost its power and had no clean . Right ?

Check what you are doing just before this appened.


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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Wed Dec 07, 2011 5:38 am
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Seems correct, let us know about Shimmilou and Stratele measurements.

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*'52 Telecaster RI, '57 Stratocaster RI, Gibson ES175 RI, Washburn Hawk 1980, Epiphone Casino MIK 2000, 57' Deluxe Knotty Alder, BDRI ...*
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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Thu Dec 08, 2011 6:15 pm
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Measure pin 4 of both 6L6, about 464 VDC.
Both at 475V.

Problem could be in Phase inverter circuit ; The 12AX7 just before power tubes. Check R51 , 91 K and R52, 100 K .
R51 @ 91.3K
R52 @ 99.9K

...this amp ( replacing components or... ? ) and sudenly amp lost its power and had no clean . Right ?
Amp has been used only a few hours in the last 10 years and not pushed very hard. A friend bought this from me and the first time he used it the power dropped out. I'm not sure how long he played through it before the problem showed up but knowing him I'd say he used it no more than 2 hours and not very loud. Thanks again.


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Post subject: Re: Blues DeVille has low power, distortion at all levels.
Posted: Fri Dec 09, 2011 3:02 am
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raynerman, your resistors are good.

Can you mesure and tell us what is your cathodes milliamps on each 6L6. The bias ?

Do you have an audio signal generator and oscilloscope ? This is the electronic equipement to use in this case to follow the path.

___________________________________________________________________________________________
re : your post dec.02

For my personal information, in another country is often called things were different.
What is for you the difference between a technician and an engineer?
In my country, Canada, an engineer designs a circuit to obtain a certain result, sound, tone, power, cost.
A technician will know how to analyze and understand the circuit operation to repair it or modify it. He will use different measuring devices. It will have a solid knowledge of electronics.


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