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Post subject: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sat Aug 06, 2011 7:26 am
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when the amp is turned on. The sounds comes on within the first 10 seconds. I've tried changing out the 6v6 and 12ax7 but nothing changed. As far as I can tell the amp is all original aside from a 3 prong cord (that is missing the third prong ha).

Wouldn't mind teaching myself a thing or two with this amp. My skill level is at a 0 but I can read! Unfortunately I have more time than money at the moment. Any help is appreciated!


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sat Aug 06, 2011 9:10 am
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Likely the amp is ready for a new cap can.

You should fix the the ground on that plug.

Or ensure that your will is updated.

Arjay

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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sat Aug 06, 2011 10:02 am
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would you by chance have a link to a place that sells cap cans?

I found this...is this what you are referring to?
http://www.amplifiedparts.com/products/ ... EC40-20-20

thank you!


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sat Aug 06, 2011 10:14 am
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That one will probably work although Champ and Vibro Champ cap cans are typically 20/20/20/450 in value. Check Ebay -- I've seen several listed for reasonable $$$.

HTH

Arjay

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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sat Aug 06, 2011 6:17 pm
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I agree with Arjay, that a new cap can will prolly lower the noise level and keep the components happy. Might want to try this first, though. Is the guitar plugged in? If you disconnect the guitar, does the hum go away? If so... it maybe the guitar or patch cord.

I like 20/20/20 cap cans. They have a browner tone, for these amps than the 40/20/20, IMHO. AES carries both.

Good luck. Love these amps. Keep us informed.


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sun Aug 07, 2011 12:51 pm
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leftcoast wrote:
would you by chance have a link to a place that sells cap cans?

I found this...is this what you are referring to?
http://www.amplifiedparts.com/products/ ... EC40-20-20

thank you!
That's the right can cap.I put one of them in in my 79 champ,mine came stock with a 40/20/20 from the factory. The 40 is the first part of the DC filtering,it does not hurt if you increase it here.Just better filtering you might end up with a bit more low end versus the 20 but it does not hurt.

Just make sure you got a big enough soldering iron tip that can get hot enough for the grounding on the cap.You are grounding it to the chassis and most normal soldering irons tip like the pencil are too small.


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Sun Aug 07, 2011 3:59 pm
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That's the main difference between the AA764 and the AB764 (which runs 6V6GTA). For a 5Y3GT rectifier, I like no more than 20mfd on the first cap section. More sag. A little less in-rush issues. But, if you are using mil spec or other good NOS 5Y3GT... should be not too much of a problem.

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Tue Aug 09, 2011 5:17 pm
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Hey Beemer, can you elaborate on the "in-rush issues"? I don't see it on this amp. If I'm not mistaken the 5Y3GT will take almost half a second to develope full potential voltage, and the larger first reservoir section would take longer to charge up(although a miniscule amount). It is class A, so the greatest current draw is at idle. Just not sure what you were getting at. Thanks, Art

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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Tue Aug 09, 2011 5:54 pm
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Simpler than that. Tube manufacturers put ideal upper mfd limits on the first electrolyic filter stage, because there is large flow of electrons and current when the amp is first turned on---- to fill this stage's cap. And the rectifier is not at operating temp (lower internal resistance---as R is a direct function of heat. Colder plates = lower resistance, by a lot).

This influx causes cathode stripping and premature failure (arcing) many times. It's why it is not a good idea to put 1000mfd cap in first filter stage of GZ34 rectified amps. Though several hi-end Hi-Fi amps are noted for doing this.

Then, there the ripple current effect. Recharging this cap on each cycle is much harder with over-capacitance filtering. Check out The Valve Wizard Site:

http://www.freewebs.com/valvewizard1/smoothing.html

" if you are using a valve rectifier you must check the data sheet for the maximum allowable reservoir capacitor. THIS VALUE MUST NOT BE EXCEEDED or too much ripple current will flow and the valve rectifier will almost certainly be destroyed.

For the GZ34 the limit is 60uF, and it would be a good idea to always use a value smaller than this, rather than push the valve to its maximum limits, especially since there is rarely any need for a capacitor larger than 60uF. "


I like 8-20 mfd range caps, in all of the filter pi-stages of a 5Y3GT rectified amp. It "breathes" better. And sags better. Bluesier and more responsive to used input (IMHO).

Fender may have used the 40/20/20 can because of the 6V6GTA. Which can run higher idle bias. A tiny bit more power and headroom, than the standard 6V6GT of the time. Yes, my experience is that the 40/20/20 with a healthy power tranny will putout a bit more juice, esp that first stage. At the expense of being a bit harder on the 5Y3GT.

Link to original Sylvania 5Y3GT data (which recommends 10mfd in first filter stage).

http://tdsl.duncanamps.com/dcigna/tubes ... yl-49h.gif


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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Tue Aug 09, 2011 6:21 pm
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Very good ! I should have looked up the spec data. Thanks, Art

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Post subject: Re: picked up a '76 Silverface Champ but there is a loud hum
Posted: Tue Aug 09, 2011 6:56 pm
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BMW2002Ti wrote:
Though several hi-end Hi-Fi amps are noted for doing this.


+1

Many of these amps use timed relays to facilitate the initial start-up procedure thus protecting critical components from the initial surge current.

Arjay

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