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Posted: Thu Dec 02, 2010 5:11 pm
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stratele52 wrote:
If you mesure pin 8 to ground WITH your standby switch to stanby the tube is look bad,
You have the proper fuse in ?

yes fuse is correct value and i measured both on standby anot not and same zero v.

Chris


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Posted: Thu Dec 02, 2010 9:43 pm
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Just checking in, Chris. The cap board as viewed with the tubes pointing up, and the back of the chassis towards you: the first 70/350(to the right) has the plus side towards the back; the other four caps have the plus side towards the front(controls) side of the chasssis. The cap that you describe is the supply cap for the phase inverter's plates. How does the B+ voltage look on the amp starting at the standby switch and checking all the way through the tube plates ? I mean with no tubes. Art

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Posted: Thu Dec 02, 2010 10:32 pm
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Ah, just noticed in a previous post. Pin 8 off the 5U4 is a rectified DC voltage. Probably +490vdc or so without tubes. BTW what negative DC voltage are you getting at the 220K apex of those resistors and at pin 5 of the power tubes ? I apologize for not keeping up to date on your progress. Art

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Posted: Thu Dec 02, 2010 11:03 pm
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aclempoppi wrote:
Pin 8 off the 5U4 is a rectified DC voltage. Probably +490vdc or so without tubes.


Correct, Art.

And Chris said his measurement showed 0.0 volts there.

I think the tube is toast.

Arjay

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Posted: Fri Dec 03, 2010 12:56 am
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Yeah, I'm just wondering if Chris had the meter on DC ? I've made plenty of mistakes by setting the meter on another measure. And there goes the meter's fuse or if I'm lucky just confusion. Art

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Posted: Fri Dec 03, 2010 2:49 am
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" Yeah, I'm just wondering if Chris had the meter on DC ? I've made plenty of mistakes by setting the meter on another measure. And there goes the meter's fuse or if I'm lucky just confusion. " Art

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Posted: Fri Dec 03, 2010 4:37 am
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aclempoppi wrote:
Just checking in, Chris. The cap board as viewed with the tubes pointing up, and the back of the chassis towards you: the first 70/350(to the right) has the plus side towards the back; the other four caps have the plus side towards the front(controls) side of the chasssis. The cap that you describe is the supply cap for the phase inverter's plates. How does the B+ voltage look on the amp starting at the standby switch and checking all the way through the tube plates ? I mean with no tubes. Art


Hi Art,

I have aligned the caps to the picture shown here:

http://www.el34world.com/charts/fenderservice7.htm

Doing this change brought the 5v feeds to expected:

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.

Regards, Chris


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Posted: Fri Dec 03, 2010 4:42 am
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aclempoppi wrote:
Yeah, I'm just wondering if Chris had the meter on DC ? I've made plenty of mistakes by setting the meter on another measure. And there goes the meter's fuse or if I'm lucky just confusion. Art


Hi Art, and thanks for your comments.

I think I checked both AC and DC at pin 8 but will recheck measurements you suggest. Just quick question, my layout diagram shows pin 5 of power tubes going to pin 1 through 1.5K Resistor then going to each end of apex of 220K resistors you mention. Is this the point I need to measure for DC - where the 220K resistors meet?

Regards, Chris


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Posted: Fri Dec 03, 2010 5:15 am
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Yes you should have -30 to -40 volts DC anywhere, 220 k jonction or after or at the 1.5 K. But the most important you must mesure this voltage at pin 5 of EACH of your 6L6.

That's the purpose of the bias, this negative voltage; going to pin 5


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Posted: Fri Dec 03, 2010 5:47 am
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stratele52 wrote:
Yes you should have -30 to -40 volts DC anywhere, 220 k jonction or after or at the 1.5 K. But the most important you must mesure this voltage at pin 5 of EACH of your 6L6.

That's the purpose of the bias, this negative voltage; going to pin 5


Thanks Louis, I will do this today.

I really appreciate the support you all have given to me. I could take this amp into the shop, but I feel better knowing it can be achieved personally with the help of others. Many years ago I was a young guitar player with a passion for electronics. And as a young guy I never had the opportunity to have such interesting equipment or projects to work on. Life gets in the way, but now I have time again to devote to the long lost passion and your help has been very good for this. Thanks.

Chris


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Posted: Fri Dec 03, 2010 6:07 am
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Yes, good to know how to repair amp, but never forget lethal voltage inside.

If you want to know more about tubes amps, there's some books from Gerald Weber .

Titles ;

1- A desktop reference of hip vintage amps

2- Tube amp talk for the guitarist and tech

and two others books

These book are easy to read and I think if you like what you are doing now you will like these book


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Posted: Fri Dec 03, 2010 7:12 am
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OK - more measurements made:

With rectifier in: Pin 8 of 5U4, 0vdc

With rectifier out:
pin 5 of 6L6GC (nearest rectifier) = -40.2vdc
pin 1 of 6L6GC (nearest rectifier) = -40.2vdc
pin 5 of 6L6GC (farthest from rectifier) = -39.8vdc
pin 5 of 6L6GC (farthest from rectifier) = -39.8vdc
At apex of 220K resistors, -50.4vdc

So from schematic, it looks like voltage going into bias circuit should be -52vdc so it looks like power supply and bias feed are ok.

So am I correct in thinking PS filter caps are ok?

Chris


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Posted: Fri Dec 03, 2010 7:17 am
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stratele52 wrote:
Yes, good to know how to repair amp, but never forget lethal voltage inside.

If you want to know more about tubes amps, there's some books from Gerald Weber .

Titles ;

1- A desktop reference of hip vintage amps

2- Tube amp talk for the guitarist and tech

and two others books

These book are easy to read and I think if you like what you are doing now you will like these book


Thanks Louis - I have had experience with tube amp voltages before. One was when we did one of our first shows and the mike was a bit "hot" - ouch. And last year I completed a tube phono stage. So my rule of one hand in pocket is followed with diligence.

Thanks for the Weber references - will look into them.

Chris


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Posted: Fri Dec 03, 2010 7:38 am
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Good to know you had a tube phono stage, you know enough for starting in guitar tube amp


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Posted: Fri Dec 03, 2010 8:52 am
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While I am waiting for more parts, I thought I would ask another question about my project. Are the stories true about how loud this amp really is? If so how do I tame it down a bit for use in civilized environments? I know it is possible to reduce the speaker load by taking one or more out of the output impedance loop (since all are in parallel - right?). But do I need to add a soaker as well to take some of the volume out of it? Is there any danger to the amp in doing so?

Chris


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