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Post subject: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 2:03 pm
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Guys, I picked up a "loudly humming" VC today for $100. When looking at the schematic the PS caps are 20/20/20 at 450volts. In the amp now is 40/20/20 at 450volts.

Was there a change at some point, or has someone replaced this before? The transformer date codes put it at 1976.

Thanks.


Last edited by gear-head on Wed Jul 11, 2012 5:30 pm, edited 1 time in total.

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Post subject: Re: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 4:49 pm
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There should be an EIA date code stamped on the cap can.

HTH

Arjay

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Post subject: Re: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 5:30 pm
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Mallory, Type FP
235-74037 85 degC
01

I think that makes it later replacement doesn't it?


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Post subject: Re: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 6:29 pm
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My guess would be the 37th week of '74.

Arjay

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Post subject: Re: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 6:49 pm
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Ah, well then .. now I know what to replace it with ...


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Post subject: Re: Vibro Champ PS Caps
Posted: Wed Jul 11, 2012 7:58 pm
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Go this one:

http://www.tubesandmore.com/products/C-EC30-20X3-525

In lieu of driving ppl on board nutz, to reiterate.... the extra VDC rating = longer & quieter life. The can is also tougher at higher volume levels, IMO.

Thank you, for you patience.


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Post subject: Re: Vibro Champ PS Caps
Posted: Sat Jul 14, 2012 12:33 am
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gear-head wrote:
...When looking at the schematic the PS caps are 20/20/20 at 450volts. In the amp now is 40/20/20 at 450volts.

Was there a change at some point,...


AA764 has 20-20-20
AB764 has 40-20-20

Make sure that you have the correct schematic, I think that you have the AB764, so the 40-20-20 would be the correct value.

BTW, are you sure that the cap is bad? Could be something else, input jack, ground connection.

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Post subject: Re: Vibro Champ PS Caps
Posted: Sat Jul 14, 2012 1:35 am
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Shimmy, actually the 30/20/20/20 at 525 VDC can will work fine in either AA764 or AB764 circuit. You see the tolerance is -10%/+50%. Check the dimensions versus the 40/20/20/20 at 525VDC one. This one is much smaller.

More electrolytic fluid usually = longer life.


30/20/20/20 AT 525VDC:

http://www.tubesandmore.com/products/C-EC30-20X3-525


40/20/20/20 at 525VDC:

http://www.tubesandmore.com/products/C-EC40-20X3-525

40/20/20 AT 500VDC:

http://www.tubesandmore.com/products/C-EC40-20-20


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Post subject: Re: Vibro Champ PS Caps
Posted: Sat Jul 14, 2012 2:23 am
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BMW2002Ti wrote:
Shimmy, actually the 30/20/20/20 at 525 VDC can will work fine in either AA764 or AB764 circuit. You see the tolerance is -10%/+50%. Check the dimensions versus the 40/20/20/20 at 525VDC one. This one is much smaller.

More electrolytic fluid usually = longer life.


30/20/20/20 AT 525VDC:

http://www.tubesandmore.com/products/C-EC30-20X3-525

40/20/20/20 at 525VDC:

http://www.tubesandmore.com/products/C-EC40-20X3-525

40/20/20 AT 500VDC:

http://www.tubesandmore.com/products/C-EC40-20-20


+1

The larger first-stage value will help to tighten up the amp's bass response.

HTH

Arjay

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Post subject: Re: Vibro Champ PS Caps
Posted: Sat Jul 14, 2012 2:45 pm
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After closely comparing both the AA and AB schematics (never really compared them closely before), there really isn't much difference between the two circuits.

AB uses mostly J-taper pots, whereas the AA uses A-taper pots.
AB has an added 330 pF cap added to the output tube, between the Cathode and control grid. I presume this was to prevent unwanted oscillation.
AB uses the 40-20-20 cap can and the AA uses the 20-20-20.
The voltages listed on the schematics are higher for the AB. Not sure why, as both schematics list the same transformers and rectifier tube. Maybe the AB lists the correct voltages considering higher AC outlet voltage?

Other than these differences, the circuits are identical.

Thanks for the info guys. :)

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Post subject: Re: Vibro Champ PS Caps
Posted: Sat Jul 14, 2012 5:11 pm
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shimmilou wrote:
Not sure why, as both schematics list the same transformers and rectifier tube. Maybe the AB lists the correct voltages considering higher AC outlet voltage?


That may indeed have been the reason, shimmilou. The same holds true for many of the other tube-rectified amps where a 5U4GB began to replace the earlier GZ34, beginning in 1969.

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Post subject: Re: Vibro Champ PS Caps
Posted: Sun Jul 15, 2012 12:37 am
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" The voltages listed on the schematics are higher for the AB. "

That's prolly due to the higher cap section off the first section of the AB763 VC and Bronco. Bigger 1st cap usually allows a bigger voltage & current swing off the rectifier. This is what can lead to issues with too much capacitance off tube rectifiers (like 5X the amount recommended by original manufacturers--- values which tends to be conservative).

My understanding with the 40/20/20 versus 20/20/20 cap can, is that Fender got a bunch at good $$$ from Mallory. Really, the 5U4GB likes the 40mfd --- but a good 5Y3GT or 5AR4 can handle the extra capacitance ok.


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Post subject: Re: Vibro Champ PS Caps
Posted: Sun Jul 15, 2012 8:11 am
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I don't think so BMW2002Ti. Doubling the filter cap value would only result in a few volts difference, maybe about 3 or 4 volts, not 65 volts. The extra capacitance would affect ripple voltage, which is only maybe 7 volts at most to begin with, and doubling the capacitance might halve the ripple down to 3 volts, resulting in maybe an extra 4 volts on the supply. If the ripple is 3 volts to begin with, doubling the cap would only add about 1.5 volts to the supply. No way that the extra capacitance will give an additional 65 volts to the supply. :idea:

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Post subject: Re: Vibro Champ PS Caps
Posted: Mon Jul 16, 2012 4:29 am
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Shimmy, that's a really good question. They use the same PT, the same rectifier, only the first section of the filter cap can is different. But, really I don't think I've ever measured the first section of any 20/20/20 Champ amp at 355VDC. More like 400VDC.

Maybe, the AA764 circuit was based on 115-118VAC outlet voltages. And the AB764 on 120-125VAC? This change in primary side voltage can make a big difference in the secondary voltages. Just a WAG.

All the more reason to use a good 5Y3GT.


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

http://www.webphix.com/schematic%20heav ... _ab764.pdf


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Post subject: Re: Vibro Champ PS Caps
Posted: Mon Jul 16, 2012 6:59 am
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That must be the case. I believe that the outlet voltage used to be more like 110, and I remember that it was lower than that in some places. Just checked my outlets, and I read 124.7 VAC. The voltages varied much more back then from location to location than they do now. I believe that we also had several different frequencies and voltages used in different parts of the US for some time, and may not have been standardized until as late as the '70s. :?:

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