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Posted: Thu Dec 02, 2010 11:20 am
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I think you are right Arjay :-)

When I first powered it up last night there was arcing so I suspect that was the last gasp of life ....

Thanks! So question - the schematic for the AB763 shows a GZ34 but this amp has a5U4GB - what should I choose as a replacement - stick with 5U4 or get something else?

Chris


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Posted: Thu Dec 02, 2010 12:05 pm
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1- Remove your power tubes and preamp tubes

2- Fix your voltages problem before putting tubes in their sockets

Because if you don't have bias voltage at 6L6's you can scrap these tubes

3- Remove your 5U4 and mesure if you have your AC voktage comming fron power transformer
A) 5U4 pin 4 to ground around 350 volts AC
B) 5U4 pin 6 to ground, same
C) Between pin 2 and 8, 5 volts AC
AC volts not DC

If all these voltages are good, plug in 5U4 in its socket an standby switch IN standby position, mesure DC volts at pin 8 to ground : around 480 volts DC.

If no voltage or vey low :5U4 is bad or 2 caps on "dog box" are defective or something wrong in this box


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Posted: Thu Dec 02, 2010 12:12 pm
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I would use a 5AR4 rectifier.

And Stratele's advice is wise. Check your voltages as per his suggestions.

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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Posted: Thu Dec 02, 2010 12:17 pm
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5U4 is more powerfull tube, less voltage drop than GZ34 ( 5AR4)


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Posted: Thu Dec 02, 2010 12:29 pm
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Look over there :http://www.el34world.com/charts/fenderservice7.htm

And on he right picture, replace the 2 X 100 Mfd by your 2 X 70 Mfd this must be your "dog box"


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Posted: Thu Dec 02, 2010 12:33 pm
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stratele52 wrote:
1- Remove your power tubes and preamp tubes

2- Fix your voltages problem before putting tubes in their sockets

Because if you don't have bias voltage at 6L6's you can scrap these tubes

3- Remove your 5U4 and mesure if you have your AC voktage comming fron power transformer
A) 5U4 pin 4 to ground around 350 volts AC
B) 5U4 pin 6 to ground, same
C) Between pin 2 and 8, 5 volts AC
AC volts not DC

If all these voltages are good, plug in 5U4 in its socket an standby switch IN standby position, mesure DC volts at pin 8 to ground : around 480 volts DC.

If no voltage or vey low :5U4 is bad or 2 caps on "dog box" are defective or something wrong in this box


Thanks Louis,

A) 5U4 pin 4 to ground around 350 volts AC - OK 373
B) 5U4 pin 6 to ground, same - OK 373
C) Between pin 2 and 8, 5 volts AC - Pin 2 WRT ground 4.5v, pin 8 wrt ground 0v - between pins 2 and 8 - zero v.

So now I will check dog box :-)


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Posted: Thu Dec 02, 2010 12:35 pm
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stratele52 wrote:
5U4 is more powerfull tube, less voltage drop than GZ34 ( 5AR4)


So what does this mean? Is 5U4 preferred? Some of the discussion say 5U4 draws higher current and it is easier on PT to have lower current draw. But then again, discussion says Leo Fender over-spec'd tx and any GZ34 amp can use the 5U4 :?

Chris


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Posted: Thu Dec 02, 2010 1:16 pm
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You can use both in your amp for now, sag is différent and more voltage with 5U4 .This is not the problem now unless your tube are bad.


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Posted: Thu Dec 02, 2010 1:19 pm
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stratele52 wrote:
Look over there :http://www.el34world.com/charts/fenderservice7.htm

And on he right picture, replace the 2 X 100 Mfd by your 2 X 70 Mfd this must be your "dog box"


OK. looking at this picture, I think I have a cap the wrong way around - which confirms what I thought from schematic .... Back to work :-)

Chris


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Posted: Thu Dec 02, 2010 1:32 pm
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You can show us picture of your cap if can help


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Posted: Thu Dec 02, 2010 1:32 pm
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The AB763 power supply was designed to use a GZ34/5AR4 rectifier. Were this my amp, I would not substitute for a 5U4GB. The later AC568 circuit uses this tube in the power supply but there are other differences in addition to a tube change. YMMV

Arjay

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Posted: Thu Dec 02, 2010 1:42 pm
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I think is some kind of "matter of taste"

I look at AB763, it use GZ34 and I look at AB568 ( not AC) it use 5U4. It's the same circuit, two different rectifier : mather of taste.

taskerc could choose later if he want to try another one, now this is not the problem


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Posted: Thu Dec 02, 2010 2:04 pm
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Retroverbial wrote:
The AB763 power supply was designed to use a GZ34/5AR4 rectifier. Were this my amp, I would not substitute for a 5U4GB. The later AC568 circuit uses this tube in the power supply but there are other differences in addition to a tube change. YMMV

Arjay


Hi Arjay,

This has been part of my confusion all along. Everything points to AB763 circuit but it came with a 5U4GB rectifier. It is a SF with BF innards, except for the 5U4GB :-) Or more or less .....

Back to the dog house ....

Chris


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Posted: Thu Dec 02, 2010 2:21 pm
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Reoriented cap and no explosion, sparks or flames ....

A) 5U4 pin 4 to ground around 350 volts AC - OK 367
B) 5U4 pin 6 to ground, same - OK 367
C) Between pin 2 and 8, 5 volts AC - Pin 2 WRT ground 4.5v, pin 8 wrt ground 0v - between pins 2 and 8 - 4.9 v. So looks good.

So I put 5U4GB back in and from pin 8 to GND is ZERO.

Since caps are new and no-tube measurements are as expected is rectifier the probable cause? How can I tell if cap(s) are gone without spares?

Chris


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Posted: Thu Dec 02, 2010 5:06 pm
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If you mesure pin 8 to ground WITH your standby switch to stanby the tube is look bad,
You have the proper fuse in ?


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