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Post subject: Re: Replacing filter caps
Posted: Sat Dec 27, 2014 7:20 pm
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Aspiring Musician
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Thank you! The amp sounds fantastic!

I do not have a bias probe and was considering ordering one. Eurotubes sells one for $25. I do have a multimeter. I have never measured plate current. If I am correct you can do so by attaching one lead to the OT center tap and another to pin three. Then set you meter to milliamps as in the video below.

https://www.youtube.com/watch?v=rvxYaX5zHZg

I'm open to suggestions and tutorials on how to measure plate voltage.


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Post subject: Re: Replacing filter caps
Posted: Sat Dec 27, 2014 8:20 pm
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Plate voltage = VDC. Pin 3 of each 6V6GT to red DVM lead. Black on chassis (ground point). Should be around 425-450VDC, with 120VAC outlet voltage.

:D


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Post subject: Re: Replacing filter caps
Posted: Sat Dec 27, 2014 9:51 pm
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Aspiring Musician
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DJ,

The method that Gerald is describing in the video is called the transformer shunt method. The idea is the DC resistance of your multimeter in the current measuring mode is much less than the DC resistance of the output transformer, so most of the current is going through the meter, and only a very small amount is going through the output transformer.

So, here's where I disagree with the video to some degree. Plate dissipation in watts is equal to plate current times plate voltage, so the current you want to adjust for won't necessarily be 25mA, but a value that equals about 8-11 Watts of plate dissipation. The "hotter" you bias the tubes, the shorter their life span will be. Remember that a small amount of plate current is still going through the transformer winding, so your meter will not be indicating the total plate current, but most of it.


Lastly, to measure plate voltage, you set your meter to the highest DC voltage setting and clip the red lead to pin 3 on the tube, and clip the black lead to ground. (chassis) If you forget to change the switch on your meter to measure voltage and still have it set to measure current, you will blow the internal fuse in your meter, and possibly damage it. If you don't have alligator clips for you meter, get some alligator clip leads from Radio Shack or some other electronic supplier.

Remember to have at least one hand in your pocket, and no hands touching the amp when powered on. One hand holding an insulated handle of a screwdriver to adjust bias current is OK.

It is important to remind you that you are dealing with possibly lethal voltages here, so you shouldn't be trying to hold either of the meter leads in your hands, but have them clipped on the measuring points with alligator clips.

Remember there is still a small amount of current going through the transformer winding, so the total dissipation is slightly more than what you're measuring.

Sorry for the lengthy reply, but I thought you should have some idea of what you are doing, and why.

Be careful!
vinyl


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 2:10 am
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BMW2002Tii wrote:
Good job, indeed! Amp should now have a nice strong , clean tone.

:mrgreen: :D :mrgreen:



IMO one cap in dog house may have wrong polarity

djwhitjr, are you sure there is no mistake ?

Don't power amp to ON , until you are sure


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 2:15 am
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vinyl wrote:
DJ,

The method that Gerald is describing in the video is called the transformer shunt method. The idea is the DC resistance of your multimeter in the current measuring mode is much less than the DC resistance of the output transformer, so most of the current is going through the meter, and only a very small amount is going through the output transformer.


vinyl


This is a very dangerous method , both voltmeter's probes (and your hands ) are connected on lethal voltage , as G. Weber write in one of his book . I never see the video.


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 3:58 am
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Stratele, that is the correct polarity for all of the filter caps, under the hood. For Deluxe Reverbs and Vibrolux Reverbs, the first two caps are paralleled and not in series ---- as the larger 6L6GC amps.

http://www.thevintagesound.com/ffg/sche ... _schem.gif

http://www.thevintagesound.com/ffg/sche ... _schem.gif


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 4:06 am
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BMW2002Tii wrote:
Stratele, that is the correct polarity for all of the filter caps, under the hood. For Deluxe Reverbs and Vibrolux Reverbs, the first two caps are paralleled and not in series ---- as the larger 6L6GC amps.

http://www.thevintagesound.com/ffg/sche ... _schem.gif

http://www.thevintagesound.com/ffg/sche ... _schem.gif


+1

Thank's


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 3:06 pm
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Aspiring Musician
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stratele52 wrote:
vinyl wrote:
DJ,

The method that Gerald is describing in the video is called the transformer shunt method. The idea is the DC resistance of your multimeter in the current measuring mode is much less than the DC resistance of the output transformer, so most of the current is going through the meter, and only a very small amount is going through the output transformer.


vinyl


This is a very dangerous method , both voltmeter's probes (and your hands ) are connected on lethal voltage , as G. Weber write in one of his book . I never see the video.


Agreed. Which is why I specifically warned the OP further down in my reply that you have already quoted part of:

vinyl wrote:
Remember to have at least one hand in your pocket, and no hands touching the amp when powered on. One hand holding an insulated handle of a screwdriver to adjust bias current is OK.

It is important to remind you that you are dealing with possibly lethal voltages here, so you shouldn't be trying to hold either of the meter leads in your hands, but have them clipped on the measuring points with alligator clips.


N'est–ce pas?

I still have nothing but respect for you, sir.

Regards,
vinyl


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 4:59 pm
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vinyl wrote:
stratele52 wrote:
vinyl wrote:
DJ,

The method that Gerald is describing in the video is called the transformer shunt method. The idea is the DC resistance of your multimeter in the current measuring mode is much less than the DC resistance of the output transformer, so most of the current is going through the meter, and only a very small amount is going through the output transformer.


vinyl


This is a very dangerous method , both voltmeter's probes (and your hands ) are connected on lethal voltage , as G. Weber write in one of his book . I never see the video.


Agreed. Which is why I specifically warned the OP further down in my reply that you have already quoted part of:

vinyl wrote:
Remember to have at least one hand in your pocket, and no hands touching the amp when powered on. One hand holding an insulated handle of a screwdriver to adjust bias current is OK.

It is important to remind you that you are dealing with possibly lethal voltages here, so you shouldn't be trying to hold either of the meter leads in your hands, but have them clipped on the measuring points with alligator clips.


N'est–ce pas?

I still have nothing but respect for you, sir.

Regards,
vinyl


Très juste .
+1000 :D


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 6:07 pm
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Aspiring Musician
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Test results:

Plate Voltage
V7= 436v
V8 = 433v
The results were achieved by placing red probe on pin 3 and black probe to ground.

Bias Current
V7 = 23mA
V8 = 25mA
I tried the same method I used to measure plate current but couldn't get a reading and noticed a fair amount of hum coming from the speaker. Therefore I used the transformer shunt method with no problems.

I was a little shaky through the entire test as it was my first time and the thought of death did cross my mind a few times. :shock: :lol:


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 6:39 pm
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Aspiring Musician
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DJ,
Awesome work, and thanks for replying with the results.

My hat's off to you, sir. I hope my reply was not to confusing. But safety is more important than anything else.

Enjoy your vintage amp!

regards,
vinyl


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Post subject: Re: Replacing filter caps
Posted: Sun Dec 28, 2014 6:58 pm
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Vinyl

Your detailed response was exactly what I was looking for. I can't thank you enough. For that matter I appreciate everyone's input. My main goal when visiting this forum is education and you guys are doing a great job!

Keep on rockin!


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Post subject: Re: Replacing filter caps
Posted: Mon Dec 29, 2014 2:25 am
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djwhitjr wrote:
Test results:

Plate Voltage
V7= 436v
V8 = 433v
The results were achieved by placing red probe on pin 3 and black probe to ground.

Bias Current
V7 = 23mA
V8 = 25mA
I tried the same method I used to measure plate current but couldn't get a reading and noticed a fair amount of hum coming from the speaker. Therefore I used the transformer shunt method with no problems.

I was a little shaky through the entire test as it was my first time and the thought of death did cross my mind a few times. :shock: :lol:


IMO bias could be colder than your reading . Fender amp work well with colder ( 60 % or less ) bias and save tubes . It do not sound better with hot bias .

70% is 19 ma , you reading is too high.

http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Replacing filter caps
Posted: Mon Dec 29, 2014 8:57 am
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Knowing one tube runs hotter than the other, which would you adjust to 19ma? The hotter or colder tube?


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Post subject: Re: Replacing filter caps
Posted: Mon Dec 29, 2014 10:01 am
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djwhitjr wrote:
Knowing one tube runs hotter than the other, which would you adjust to 19ma? The hotter or colder tube?


The hotter tubes as Fender amp play very well with cold bias


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