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Post subject: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 10:48 am
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Hey Guys,
I just acquired a '68 Dual Showman that has been worked on a lot. The date code on the transformers read 606 810 and 606 816. I was told that it worked, so when I got it home I fired it up and it sounded fine but didn't feel like 85w. It does have a master volume where the trem jack was. I think the filter caps have been replaced but it looks like they replaced them with whatever they had laying around. The first two larger caps are Nichicon 82uf 450v and are pointing in the same direction, polarity wise. I don't think that's right. Is that bad for the amp to be run that way? A friend said it looks like they wired it like a Deluxe. I want to replace them all with Sprague Atoms and was wondering about the polarity of the first cap. I do have pics I can post if you want to see a mess.

Thanks for any help.


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 11:07 am
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The first 2 caps MUST be in the same direction. It's because the originals ones are 350 volts and the amps give more than 400 volts in working use and probably more than 450 in Stanby position ( no load ) . These caps , in serie can hold 350X 2 = 700 volts but their capacitance are the half; 70 / 2 = 35 Mfd.
Look a the schematic , you 'll see those 2 caps in series;

http://www.el34world.com/charts/Schemat ... _AA769.pdf
_________________________________________________________________________________________
Nichicon caps are not the best for a good amp like that , yes the use of Sprague give you a better sound. More bottom end and dynamic

You don't have enough power ? Maybe some news power tubes will help, if everything is good in the amp ; no mods , voltages alright. Change also the phase inverter tube V 6 , 12AT7, Check also if you have the right preamp tubes , some tube ( 12AT7, 12AU7 ) have less gain than originals 12AX7/ 7025
less gain = less power.


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 11:41 am
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Thanks for the prompt reply.
I am not an amp tech. I know how to bias tubes and safely discharge filter caps. I'm a guitar player that works on his own gear. I've modded effects petals and stuff but don't have much experience with amps. I don't fully understand schematics.

The reason I was concerned about the polarity of the caps is that pics I've seen of the filter caps show the first cap turned around on Showman's and Twins.
I don't know if it makes a difference or not but the tube chart has this amp being an AB763. It has no reverb and someone has put a master volume on it.

Thanks again


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 1:15 pm
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Here's a pic of a set of Sprague Atom's correctly installed in an AB763 Showman amp......

Image

Note the polarity of the caps. And while you're inside your amp's doghouse, don't forget to replace those resistors in the voltage-dividing network -- if they've drifted significantly, they might also be responsible for the symptoms you describe.

HTH

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: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 2:02 pm
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Cool, thank you very much.
This is what I have to work with. I hope this pic comes out. The blue resistor is not the same value as the one in your pic.
Image


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 2:07 pm
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I'll try this again.
Image


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 2:19 pm
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Your blue resistor as the same value than the one on the other picture, 1000 ohms . Color code ; brown-black-red = 1000 ohms.

it is simply not the same brand


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 2:50 pm
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Thank you, Retroverbial and stratele52 for your expertise. You guys are great.

I have a tube tester and all tubes test good.

I just got a Photobucket account and posted pics of this amp if you want to see what I have to deal with. I think it looks like a mess.

I guess this link will get you there.

http://s1214.photobucket.com/home/debeall/index

Thanks again.


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 3:44 pm
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It looks like all of the 470-ohm/1-watt resistors on your power tube sockets could stand to be replaced. I'd check those four 1500-ohm resistors as well. They all look pretty "toasty".

HTH

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: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 3:45 pm
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debeall,
I don't want to begin a new post but tube tester are not good for testing guitar amps tube.
I resume :
Tube tester can only tell you if tube filament is working or if tube is short , this last test is not always true.

A weak tube, a tube missing power , tube tester can't mesure that. Tubes on guitar amps work hard and at high voltage. Tubes tester work a low voltage without proper load ( for working hard ) on guitar tubes amp

Exception some "laboratory" or high end tube tester are good . But most of the people don't have them at home.

So don't trust your tube tester. The amp is your best and only tube tester.


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 3:50 pm
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+1 Retroverbial, You are right

This 470 ohms could be the cause of your power loss ,


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 4:28 pm
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Cool.
Now are resistors, resistors or is there better places to get them other than Radio Shack. I know the tone caps I put in my guitars never come from Radio Shack.

...and the tone caps in this amp. should I replace them with Orange Drops or something?

stratele52,
I agree about the tube tester. I have found tubes to be weak on occasion. That's all I check for.

I want to thank both of you very much for your advice.


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Mon Sep 26, 2011 5:45 pm
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I buy NOS (new old stock) Allen & Bradley carbon-comp resistors from various Ebay vendors. Those are the spec components that belong in your amp. If you wish to upgrade for reasons of longevity and/or durability, the metal-oxide types sold by AES, Mouser, and others are recommended. Radio Shack no longer sells anything I'd consider installing in a vintage Fender amp.

HTH

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: 1968 Dual Showman filter caps
Posted: Tue Sep 27, 2011 11:51 am
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I found the Sprague Atom caps and Allen & Bradley resistors I need and will install when they get here.

Thank you Retroverbial and stratele52 for your time and guidance.

You Rock!!
debeall


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Post subject: Re: 1968 Dual Showman filter caps
Posted: Tue Sep 27, 2011 2:00 pm
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Ok debeal, and don't forget to discharge your capacitors before .


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