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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 5:34 am
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Retroverbial wrote:
stratele52 wrote:
I never use those cheap caps in good amps.

I wouldn't use those caps in ANYTHING.

:roll:
Arjay

I couldnt find better than these caps.
The seller told me that the caps are built in japan , but now i think thats not true :(


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Fender Play April 2019
Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 5:52 am
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And your alibi for installing it backwards?

:roll:
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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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 5:54 am
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ali_k wrote:
[
If you now any other good toturials or videos please send me.



Forget to fix you amp this month.

Trouble shooting a amp can't be learn with Y-T.

You can only learn easy thing with Y-T like put strings on a guitar ; you start to A and go to B.
You can't learn electronics with Y-T.
It take me few years in electronic school

Gerald Weber Books are very good.

For a beginner ; The tubes amps Book, Aspen Pitman.

https://www.amazon.com/Tube-Amp-Book-De ... 0879307676

_________________
English is not my mother tongue , I do my best


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 6:40 am
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Posts: 480
ali_k wrote:
BMW2002Ti wrote:
Be sure to follow the layout, carefully. Like the polarity of those darn electrolytic caps. That's why the cap in your photo is popped. It was wired wrong.

Image

Thank you so much for telling the problem
I'll correct that.


Not only the polarity needs to be correct, it also needs to be the correct value. The schematic calls for 70uF, which will be hard to find, so you can use 100uF with at least a 100V rating.

22uF is woefully inadequate for the bias supply.


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 6:55 am
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vinyl wrote:
I'm surprised it works at all.

You need to take this amp to a pro before you burn it up.
Image
with this wrong cap position i measure the bias in it was about 45 volts.
After the correcting the cap direction
( wait for 15 minutes then check the bias) the bias is still 45 volts.
I'm so surprised....
I also put the filter cap photos, i think they have no problem.

https://www.mediafire.com/view/wnjt9ovp ... 7e4dcnhe7t


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 7:11 am
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ali_k wrote:


with this wrong cap position i measure the bias in it was about 45 volts.
After the correcting the cap direction
( wait for 15 minutes then check the bias) the bias is still 45 volts.
I'm so surprised....
I also put the filter cap photos, i think they have no problem.



Bias negative voltage, yes you must read negative voltage like -45 volts.

This voltage do no mean you have the right bias for the 6L6. Oh no !
It is the voltage you have to adjust to have the right bias.

1- You must read current at 6L6's cathodes
And multiply by anode voltage ( 400 to 425..... ? ) Lethal voltage
2- Current X voltage = power ( watts)
3- Adjust the negative voltage to have 60 % of tube power dissipation ; 6L6 ( wich one ?) go from 25 to 30 watts.

Without doing that, - 45 volts may give you a cold bias or a hot bias.
In the worse case a hot bias will blow your tubes in hour, in a day or in a week.

Why a "good" bias ?

A correctly biased amp will run efficiently and maximizes the life of your tubes.

Under - biased (hot ) amps will lack of punch and tubes will run hotter , shorter tube's life .

Overbiased amps ( cold ) will sound thin and brittle. with tubes running too cool for proper performance .
A correctly biased amps will sound clean and tight a moderate volumes, than at higher volumes breakup and distort musically .

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English is not my mother tongue , I do my best


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 7:46 am
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stratele52 wrote:
ali_k wrote:


with this wrong cap position i measure the bias in it was about 45 volts.
After the correcting the cap direction
( wait for 15 minutes then check the bias) the bias is still 45 volts.
I'm so surprised....
I also put the filter cap photos, i think they have no problem.



Bias negative voltage, yes you must read negative voltage like -45 volts.

This voltage do no mean you have the right bias for the 6L6. Oh no !
It is the voltage you have to adjust to have the right bias.

1- You must read current at 6L6's cathodes
And multiply by anode voltage ( 400 to 425..... ? ) Lethal voltage
2- Current X voltage = power ( watts)
3- Adjust the negative voltage to have 60 % of tube power dissipation ; 6L6 ( wich one ?) go from 25 to 30 watts.

Without doing that, - 45 volts may give you a cold bias or a hot bias.
In the worse case a hot bias will blow your tubes in hour, in a day or in a week.

Why a "good" bias ?

A correctly biased amp will run efficiently and maximizes the life of your tubes.

Under - biased (hot ) amps will lack of punch and tubes will run hotter , shorter tube's life .

Overbiased amps ( cold ) will sound thin and brittle. with tubes running too cool for proper performance .
A correctly biased amps will sound clean and tight a moderate volumes, than at higher volumes breakup and distort musically .

Thank you so much for your awesome answer...
I check the voltage of both 6l6 and it was 430 vdc. Both tubes
Have same voltage.
Would you please tell me to how to adjust negative voltage?


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 8:32 am
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1581
Location: Coastal Bend, Tx.
Ali, are your multi-meter leads connected correctly? The black lead should be connected to COM
http://www.voltextra.com/product-detail/meter/SD9205A.html
Measure voltage on both sides of the diode, one reading should be positive(resistor&diode eyelet) and the other negative(diode and jump wire to capacitor). If the latter point is also positive = replace the diode, ensure proper polarity.
Do you understand the the three lugs on the bias potentiometer?
left lug=input, from diode/capacitor- negative supply voltage
middle lug=output/wiper - negative voltage adjusted to supply the bias for the output tubes
right lug=ground w/ resistor to back of potentiometer
Quote:
Would you please tell me to how to adjust negative voltage?

With no output tubes installed, find the center point of your potentiometer. With your multimeter attached(black to ground-red to center lug) insert a small flat screwdriver in the pots slot. Turn all the way counter clockwise(meter reading) turn all the way clockwise(meter reading), then adjust the pot so it reads half way(meter reading). Check the schematic for the correct bias supply voltage and adjust accordingly.

Based on the plate voltage and the current/quality of your tubes, the correct bias adjustments should be very small at the potentiometer.


Last edited by sfceric64 on Wed Oct 09, 2019 9:19 am, edited 5 times in total.

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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 8:47 am
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ali_k wrote:


Would you please tell me to how to adjust negative voltage?


I don't want to read all the post;
Adjusting negative voltage ; By a bias pot ( not balance pot ) or change resistor value in the bias circuit.

You must know what is the bias before.

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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 10:30 am
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1581
Location: Coastal Bend, Tx.
Well, I took the time to look at all your posted photos.
There are some mods to your amplifier, IMO they should be removed and the circuit returned to the stock-modified AB763 circuit.
For instance:
most of the caps are .2 instead of .02
150 ohm 7 watt resistors on output sockets
capacitor ran parallel between both output socket
The photos didn't yield all the info I was looking for, but enough inconsistency leads me to believe that all components should be cross-checked vs the schematic!


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Wed Oct 09, 2019 4:33 pm
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Joined: Thu Dec 12, 2013 2:54 pm
Posts: 480
vinyl wrote:
I'm surprised it works at all.

You need to take this amp to a pro before you burn it up.
Image


So I'll try to explain it very specifically.

What is wrong with this picture?
1. The capacitance value is incorrect. It should be a at least a 70uF 70 volt capacitor according to the schematic. Not 22uF. That value and voltage rating is very difficult to find, but you can substitute a 100uF 100 volt capacitor.

2. The capacitor is shown in the wrong polarity, the positive end is not going to chassis ground.

3. Because of #2, operating the amplifier in this condition causes the capacitor to overheat causing the capacitor to vent some of its electrolytic fluid out of the end of the capacitor. (that brown stuff oozing out the end of the capacitor) That means the capacitor is "blown", and is no longer able to provide the necessary filtering (smoothing of the ripple current necessary to provide the power tubes with proper bias current) to operate the power tubes in a correct manner. Once that happens, you can't just simply reverse the "blown" capacitor in the circuit.

I can not tell exactly from from your later pictures, but it would appear that you just unsoldered the existing capacitor, and soldered it back in the correct polarity. If that is the case, then you need to replace it with the correct value.

I have the following questions:

1. You said there is "no pro around here" in a previous post. Where is "around here"? It might help us tell you how to get the correct capacitors that you need.

2. Your pictures indicate that your amp is a domestic (made for the U.S.) model. Are you operating it in the U.S., or are you plugging it into another transformer to allow you to operate it in a foreign country with a different voltage?

3. What is the fuse rating that is in the amplifier?

From your previous posts, my guess is English is not your first language. There is no shame in that, it still is pretty good. Maybe even better that Stratele52's :P (Sorry Louis, we still like your posts around here)

I admire your effort to solve how to get your amplifier to run in the way it was meant to run when it left Fender.

If you could answer those questions, it would be helpful. Be warned, you might not get the answers you want, but will get answers that will help you.


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Thu Oct 10, 2019 6:38 am
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vinyl wrote:


From your previous posts, my guess is English is not your first language. There is no shame in that, it still is pretty good. Maybe even better that Stratele52's :P (Sorry Louis, we still like your posts around here)
.


You are right, no offense. :lol:

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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Fri Oct 11, 2019 8:13 am
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Joined: Fri Nov 07, 2014 7:33 am
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Location: Iran - Tehran
Retroverbial wrote:
And your alibi for installing it backwards?

:roll:
Arjay

its just a mistake.


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Fri Oct 11, 2019 8:14 am
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Joined: Fri Nov 07, 2014 7:33 am
Posts: 77
Location: Iran - Tehran
stratele52 wrote:
ali_k wrote:
[
If you now any other good toturials or videos please send me.



Forget to fix you amp this month.

Trouble shooting a amp can't be learn with Y-T.

You can only learn easy thing with Y-T like put strings on a guitar ; you start to A and go to B.
You can't learn electronics with Y-T.
It take me few years in electronic school

Gerald Weber Books are very good.

For a beginner ; The tubes amps Book, Aspen Pitman.

https://www.amazon.com/Tube-Amp-Book-De ... 0879307676

ok, thanks.


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Post subject: Re: Fender super reverb SF ab763 modification
Posted: Fri Oct 11, 2019 8:36 am
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Joined: Fri Nov 07, 2014 7:33 am
Posts: 77
Location: Iran - Tehran
vinyl wrote:
vinyl wrote:
I'm surprised it works at all.

You need to take this amp to a pro before you burn it up.
Image


So I'll try to explain it very specifically.

What is wrong with this picture?
1. The capacitance value is incorrect. It should be a at least a 70uF 70 volt capacitor according to the schematic. Not 22uF. That value and voltage rating is very difficult to find, but you can substitute a 100uF 100 volt capacitor.

2. The capacitor is shown in the wrong polarity, the positive end is not going to chassis ground.

3. Because of #2, operating the amplifier in this condition causes the capacitor to overheat causing the capacitor to vent some of its electrolytic fluid out of the end of the capacitor. (that brown stuff oozing out the end of the capacitor) That means the capacitor is "blown", and is no longer able to provide the necessary filtering (smoothing of the ripple current necessary to provide the power tubes with proper bias current) to operate the power tubes in a correct manner. Once that happens, you can't just simply reverse the "blown" capacitor in the circuit.

I can not tell exactly from from your later pictures, but it would appear that you just unsoldered the existing capacitor, and soldered it back in the correct polarity. If that is the case, then you need to replace it with the correct value.

I have the following questions:

1. You said there is "no pro around here" in a previous post. Where is "around here"? It might help us tell you how to get the correct capacitors that you need.

2. Your pictures indicate that your amp is a domestic (made for the U.S.) model. Are you operating it in the U.S., or are you plugging it into another transformer to allow you to operate it in a foreign country with a different voltage?

3. What is the fuse rating that is in the amplifier?

From your previous posts, my guess is English is not your first language. There is no shame in that, it still is pretty good. Maybe even better that Stratele52's :P (Sorry Louis, we still like your posts around here)

I admire your effort to solve how to get your amplifier to run in the way it was meant to run when it left Fender.

If you could answer those questions, it would be helpful. Be warned, you might not get the answers you want, but will get answers that will help you.

I'm in Iran / tehran... the voltage is 220AC here in iran and i use extra transformer inside the amp cab for changing 220 v to 110 v.
my amp has "2.5 A" fuse.

i decide to order new tubes...for both power and preamp section.
i think 99% problem will be solved after using new tubes.
because i check precisely the wiring and circuit and found nothing wrong.
i have lot of experience in IT and networks, Medical equipment engineering , and also guitar pedals and effect building and cloning, but my english is weak.

I'm really Sorry for my weak english. I'll improve it in near future.


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