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 Post subject: Fender Bassman 50 Head advice
PostPosted: Sun Jul 17, 2011 5:22 pm 
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Hi,

I have just purchased what I believe to be a 1971 Fender Bassman Head.

I would like to ask the oirum some advice regarding some niggles that the amp has.

Firstly on the Bass channel, wheneverthe deep switch is puched UP a POP sound is made.

Secondly, again mainly on the BAss channel, but also slightly on the other, there seems to be a bit of background noise (it sounds like a swish/swirl noise not really a crackle)

and finally on the other channel, when the bright switch is in the UP position, and you touch the switch, a static noise can be heard through the cab.

So my question to you is what 'jobs' does the amp need doing? It has already been sugested that the Caps should be replaced (do people agree?)

Also before anyone asks (thank you for your consideration for my safety... I have friends who are able to work on amps in a competant and safe manner, so they will be assisting me in all work that needs to be carried out)

Some pics-
Image
Image
Image
Image
Image
Image
Image
(Valve was removed to ease taking amp out of head box)

Thanks for reading, Ian.

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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Sun Jul 17, 2011 5:41 pm 
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The "swishing/swirling" noise you've noted is likely a coupling cap that is leaking DC voltage to ground or a carbon-comp resistor that's reached the end of its usable service life (both are common phenomena with most vintage amps).

The "popping" deep switch is not unusual, even when the amp was new.

This is an export-model amp, is it not?

HTH

Arjay

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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Sun Jul 17, 2011 5:55 pm 
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Thanks, how can I identify if it is a problem as you mentioned?

I live in th UK, so I would presume that it is an Export model.

Ian.

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Fender Mustang 1966
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Rickenbacker 4003 Bass 2003

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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Sun Jul 17, 2011 6:03 pm 
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You unsolder one end of a suspect component and perform the usual battery of static tests.

Arjay

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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Tue Jul 19, 2011 6:28 pm 
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Hi there Ian,Just a couple of ideas to chew on. the Deep switch is located directly in the signal path, after the first gain stage. So it's mechanical/electrical switching is very susceptible to "pop" noises, unless it is switched with the volume at '0' or the amp on standby. Most of these switches can be cleaned, using a quality contact cleaner. Otherwise, a replacement can sometimes help. A guieter or lower gain valve in the V1 socket can also help. A change of cap type for the 0.1 may also help. But after seeing the effort and components in your amp, I would not recommend that.
Secondly, the"swirling" or "ghost note" sound would make me suspect the phase inverter feed coupling cap. In the picture(top photo-near center-Wima cap with green insulated leads), it seems to be a .01uf/400vdc. I would use a cap rated for 600vdc. At this point in the circuit, it could be operating at 400-480vdc. Also if the amp is to be used for guitar only, a smaller value cap could be used(.005 to .001). But too low can thin out the signal.
And lastly, the bright switch is in the same conditions as the deep switch. So cleaning and watching the volume at switching should apply. Good Luck ! Art

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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Wed Jul 20, 2011 7:55 am 
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Someone went to quite a bit of work to change out all those coupling caps. In my experience, sometimes changing to newer technology higher bandwidth caps can cause some instabilities and osciallations in these older amps.

Do you have access to a signal generator and scope to go section by section? If not or you don't know of anyone that has these and knows how to use them--- start pulling tubes, section by-scetion, until the noise goes away (or changes for the better).

Hopefully, you can find that section. Then concentrate on the coupling caps and voltage settings of that section.

HTH. Or at least a good head start.


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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Wed Jul 20, 2011 12:14 pm 
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Ok, some of you guys know so much about amps and I am so freaking limited, but I want to learn. Is there a book you guys can recommend? I'm afraid to dig in and get buzzed to death.


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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Wed Jul 20, 2011 7:16 pm 
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Try this site first. His books are good, too.

http://www.freewebs.com/valvewizard/

I learned mostly, from an old amp tech. Just watching over his shoulders and asking stupid questions. And yes, he had nice permanently toasted fingers. Sorry, it comes with the territory--- no matter how careful you are.


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 Post subject: Re: Fender Bassman 50 Head advice
PostPosted: Wed Jul 20, 2011 10:51 pm 
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Hey, at least he had all his fingers.

I've known a few HV pole jockeys who had their entire hand blown off!

:shock:

Arjay

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 Post subject: Re: Fender Bassman 50 Head advice-books
PostPosted: Tue Jul 26, 2011 7:53 pm 
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Atoll, one of the first books I started with was the Navy Basic Electronics ca.1960 or so. That book is a good intro and reference, but that was quite a few years ago. Here's something more up-to-date: http://www.phy.davidson.edu/instrumentation/NEE.htm. This should help with understanding the theory and language(terms). Then a beginning amp repair book would get you off on the right foot. Art

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 Post subject: Re: Fender Bassman 50 Head advice-books
PostPosted: Tue Jul 26, 2011 10:39 pm 
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aclempoppi wrote:
Atoll, one of the first books I started with was the Navy Basic Electronics ca.1960 or so. That book is a good intro and reference, but that was quite a few years ago. Here's something more up-to-date: http://www.phy.davidson.edu/instrumentation/NEE.htm. This should help with understanding the theory and language(terms). Then a beginning amp repair book would get you off on the right foot. Art


+1

Likewise, there is a substantial amount of vacuum-tube theory contained in the RCA Receiving Tube handbook. Westinghouse also offered a similar publication.

Arjay

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