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Post subject: 1974 Bassman Ten, broken Bass control Pot
Posted: Fri Feb 07, 2014 4:08 am
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Hello!,

I've just bought an original 1974 Fender Bassman Ten, and the Bass control pot of channel BASS/NORMAL is not working...

Anyone knows what potenciometer should I look for, in order to replace it??

Thanks a lot!!


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Fri Feb 07, 2014 4:22 am
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Location: Province de Québec, Canada
250 K audio pot . Schematic here ;

http://www.el34world.com/charts/Schemat ... 10_75w.pdf

If your amp have knob with screw, you need a solid shaft pot like this one ;

http://www.tubesandmore.com/products/R-VC-MA

Are you sure it is the pot ? Never see a pot not working if shaft it is not broken . Look inside the amp if it have some mode and if pots or a capacitor are disconnected .

More information about what the pot do ; dirty ? Or nothing ?


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Fri Feb 07, 2014 6:41 am
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Just a word of caution.

There are some very high DC voltages present in several parts of the circuit and will still be present for some time even if the amp is unplugged from the wall socket. Use caution while inspecting ANY amp internally - but especially tube type amps and in this case, pin 3 of the 6L6 sockets and terminals on any of the large electrolytic capacitors near the power transformer. Technicians and persons not familiar with tube circuits can be injured if proper high DC voltage safety procedures are not followed.

P-L

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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Fri Feb 07, 2014 7:02 am
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PaducahLuke wrote:
Just a word of caution.

There are some very high DC voltages present in several parts of the circuit and will still be present for some time even if the amp is unplugged from the wall socket. Use caution while inspecting ANY amp internally - but especially tube type amps and in this case, pin 3 of the 6L6 sockets and terminals on any of the large electrolytic capacitors near the power transformer. Technicians and persons not familiar with tube circuits can be injured if proper high DC voltage safety procedures are not followed.

P-L

==============



+100


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Sat Feb 08, 2014 11:02 am
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Thanks a lot!!

Yes, the pot does not make any noise, it just does nothing when you turn it around...


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Sat Feb 08, 2014 11:08 am
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PaducahLuke wrote:
Just a word of caution.

There are some very high DC voltages present in several parts of the circuit and will still be present for some time even if the amp is unplugged from the wall socket. Use caution while inspecting ANY amp internally - but especially tube type amps and in this case, pin 3 of the 6L6 sockets and terminals on any of the large electrolytic capacitors near the power transformer. Technicians and persons not familiar with tube circuits can be injured if proper high DC voltage safety procedures are not followed.

P-L

==============


I've heard that, if I you turn on the amp for a few seconds when its unplugged from the wall socket, that remain electricity goes off... is that true?? or it still remains there?? :|


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Sat Feb 08, 2014 11:42 am
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Paco Alonso wrote:

I've heard that, if I you turn on the amp for a few seconds when its unplugged from the wall socket, that remain electricity goes off... is that true?? or it still remains there?? :|



It may remain in the amp , you never know without voltage reading.


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Sat Feb 08, 2014 12:08 pm
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It depends upon the power supply design. I don't have a schematic handy at this time for the Ten series... but treat ALL tube type power supplies as RETAINING DC VOLTAGE for at least 2 to 4 hours after it is unplugged from the wall outlet. Some power supply filter capacitors (electrolytic) do NOT have a voltage equalizing bleed off resistor associated with them and some others do. Don't take a chance on working inside a tube type circuit unless you have specific training on those circuits. The voltage that is the most dangerous is referred to as "B+" and it has been known to stop a technician's heart. In the early electronic era when there were only tube circuits, technicians knew about these dangers. Modern technicians who are not that familiar with the high DC voltages present to bias and drive vacuum tubes may find out the hard way what a 'blast' they can really be, if not extremely careful.

Getting back to the main question... This sounds like a job for someone trained on Fender tube type amps. I say that because the control in question may not be the issue. It is simply a variable bias source for a circuit that may have issues other than the pot itself. A multi-meter, audio signal tracer and troubleshooting experience will probably be needed to get this one running correctly again.

Please keep us posted on your progress...


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Wed Feb 12, 2014 6:20 am
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Ok, thanks a lot!!


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Thu Mar 05, 2015 4:13 pm
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old thread but new post,
Fender screwed up the Baxandal tone circuit,
change the top cap on the bass pot to .002
cj


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Post subject: Re: 1974 Bassman Ten, broken Bass control Pot
Posted: Mon Feb 17, 2020 3:18 pm
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CJ wrote:
old thread but new post,
Fender screwed up the Baxandal tone circuit,
change the top cap on the bass pot to .002
cj


I agree with CJ on this one. I think Fender meant to have C2 = 10*C1 as would typically be expected in that type of Bandaxall tone circuit and they made a mistake on the schematic which then made it into the final product.

I could be wrong, but that is the only thing that really makes sense to me in regards to this mostly useless bass tone control.

By making the suggested modification you get a bass tone control that is more usable, albeit still more subtle than what you find in the typical fender tone stack.

I have also seen where people change the value of the pulldown resistor on the bass part of the tone circuit to achieve similar results.

Thanks, CJ for pointing this out.


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