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Post subject: NEED HELP w '75 Bassman 10
Posted: Fri Feb 01, 2013 1:47 pm
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The "Bass Normal" channel isn't bassy enough. The "Bass Studio" channel has plenty of bass and the control pot works as it should. The Normal channel basically sounds like it's on 1 and only gets bassier when tuned all the way up, but sounds like it's on 2. The pot itself measures out fine with a meter. I have changed the 2 caps across the pot and the 2 at the deep switch, which makes it only slightly bassier. I have also changed the 25uf 25V cap coming from pin 3 of V1 12ax7. I have also swapped out the tube to no avail. All the resistors in the area measure out close enough. The treble control works fine. This is driving me nuts. PLEASE HELP!
I prefer the Normal channel for bass, when it worked. I have modded the Studio channel per Jeff Bober's recommendations in Premier Guitar mag and am happy with it for guitar.
Still want the Bass Normal channel to work right.

P.


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Post subject: Re: NEED HELP w '75 Bassman 10
Posted: Fri Feb 01, 2013 10:34 pm
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Have you checked the plate voltages at pins 1 and 6 of V1? You should be seeing 240VDC at each location. If they measure low, check the 100k resistor tied to pin 1 and the 220K resistor tied to pin 6 for any evidence of geriatric damage or value drift.

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: NEED HELP w '75 Bassman 10
Posted: Sat Feb 02, 2013 6:19 pm
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After I posted, I did check the voltages. Instead of being low, they're high: Pin 1 is 258V, Pin 6 is 267V of V1. V2, the Studio Channel, has 273V at Pin 1 and 259V at Pin 6, just for comparison's sake. The 100k resistor from pin 1 measures 106k, the 220k resistor measures 147k. But so does the corresponding 220k resistor on pin 6 on the other channel. Huh.....?
Thanks for responding.
P.


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Post subject: Re: NEED HELP w '75 Bassman 10
Posted: Sat Feb 02, 2013 6:21 pm
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That's some pretty significant component drift for those 220K resistors -- I'd be inclined to replace them.

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: NEED HELP w '75 Bassman 10
Posted: Sat Feb 02, 2013 6:28 pm
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Yes, I belive I will.
I'll post later when I get it done.
Thanks,

P.


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Post subject: Re: NEED HELP w '75 Bassman 10
Posted: Sat Feb 02, 2013 8:09 pm
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pellcio, you need to pull one leg of each resistor, before measuring the static DC resistance. This is because many resistors sit in a complex network of caps, resistors, and even chokes --- that effect onboard readings.

Good luck!


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Post subject: Re: NEED HELP w '75 Bassman 10
Posted: Sun Feb 03, 2013 12:07 pm
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Great. You are correct. The 220k resistor measures 224k; reaonable for a 10% tollerance.


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Post subject: Re: NEED HELP w '75 Bassman 10
Posted: Wed Aug 14, 2013 6:13 am
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I know this thread is a little old-ish but I
just had the same problem with a Bassman 10. What I did was rewire the normal channel to the same configuration as an AB763 Super normal channel. It required an hour, five capacitors, two resistors and some wire. It's very easy to do if you have the layout diagram.

Here's a little illustrated piece I posted about this.

http://judyboxamp.blogspot.com/2013/08/ ... rsion.html


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