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Post subject: Problem with a Vibrolux reverb reissue
Posted: Fri Aug 27, 2010 3:55 pm
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Hi

Ivgot a weak ouput / distortion sound with a fender vibrolux reverb reissue 1992.

I test biais and tube. I check all voltage with the reference on this schematic

http://www.schematicheaven.com/fenderamps/custom_vibrolux_manual.pdf

the only difference on my reading is the cathod voltage on the v6 the PI
I read 70 vdc. On the shcematic Its 17 vdc. All of my others reading are ready close to what are the schematic values.
is this a clue to my problem?

I interchanged the PI tube, same reading. I measure the 4 resistors that make the cross form with the PI, all is good. I check dc leakage with coupling caps.


someone have an idea or a Hint?

R.


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Post subject: Amp
Posted: Fri Aug 27, 2010 6:15 pm
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Hi nitrosonique,

Make sure to have the knobs and switches set to where the schematic shows before you take your readings. Nothing plugged into input jack.

Reverb and Vibrato off
Reverb and Vibrato controls Full CCW
All Volume and Tone controls at 50%

Try that and see if you still have the high reading. Just to make sure.

Maybe you lost a preamp tube?

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Post subject:
Posted: Sat Aug 28, 2010 6:06 pm
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Hi Remi, did you check the voltage between the grid and cathode on the PI ? You'd want to see 0 to -5 volts, but not positive. If that reading is OK, then I wouldn't worry about the 70vdc. Instead I would continue to look elsewhere for the problem. Art

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Post subject:
Posted: Sat Aug 28, 2010 6:50 pm
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Hi Art

I read -1.5 Vdc between the grid and the cathode of each side of my PI.

So this confirm me that the PI work properly?

Iv got the same problem(low output/fat distortion) if I plug the guitar in channel one or two. So that look like a problem after the first two gain stage on each channel. I'm gonna the check further around the V4 tube.

thanks


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Post subject:
Posted: Sat Aug 28, 2010 8:02 pm
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Yeah, Remi, the PI tube is doing it's job. But like you say the signal from V4b needs to be examined, and of course the feed cap to V6. After that, it's the power amp section. Is there any reason to suspect the power supply caps or power tubes of not holding up their share of the work-load ? BTW I'm looking at the Custom Vibrolux Reverb('94-95) schem. Is that the correct schem ? Art

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Posted: Sun Aug 29, 2010 3:23 am
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Just a remark: did you measure directly the Kathode voltage on a valve socket pin 3 and 8 ? Did you measure plate resistors R43/44 ? ( You could have a PCB issue). What is the voltage on plates (directly on pin 1 and 6 260V?)

By the way, as you and aclempoppi proposed, V4b is not DC connected to V6 you can disconnect R36 and/or C18 to make sure leakage. It should be at normal DC condition 17V on the Kathodes.

(That strange, when I calculate with DC current on this circuit I don't find the value on the schematic ! I found 75V !!! on my BDRI it is written 34V with 15K that 's make sense with 39K on Vibro. Or I lost my latin!!! If one fellow member can measure on it own and performs as expected it will be a good indication. we can also check with power of resistors)

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