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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sat Jan 30, 2016 7:25 pm
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Arjay, real good point. This link may help (if you have access to OEM part number):

https://www.tubesandmore.com/tech_corne ... ifications

Edit: What I can find on the Net. Typical reverb tank used in Fender combos during SF and BF era: 4AB3C1B


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 5:23 am
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Yeah this is definitely the original tank. I've checked it out and it's the correct model # and it's pretty old and beat up too. The amp manual says it came with a 3 spring tank while the 4 series only has 2. Mine is the 9AB3C1B. The only thing is that spring is missing which could probably change the impedance and grounding properties, right? It seemed to be a big difference while testing the other tank which was the 2 spring version.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 5:42 am
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Harris,

Well, at least the input and output impedance seem ok. 8-ohm input (letter A of 9AB3C1B) and 2,250-ohm output (letter B).

:)


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 5:48 am
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Yup and correction, mine is a 9AB2C1B, not 3C1B. So the only difference is the decay time. And on the 9 series the B is 2575 ohms according to that page. So do you guys agree that this could be causing a change in the properties and a new tank could help? Or should I also look into changing some of the amp components too?


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 11:15 am
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The missing spring (with the concurrent unloading of the transducers which feed it) *may* cause a change to the overall impedance of the tank but the grounding scheme should not be affected. A new replacement tank might cure the issue. In any case, I would not change any of the amp's circuit componentry unless there were a compelling reason such as a damaged or measurably bad part.

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: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 1:40 pm
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Agreed. All the parts seem to measure up ok so I don't think they need to be replaced. All grounding measurments look good too. Ill keep checking the components and if o e is really bad then I'll change it out. But I think the next step is to get a new tank and see how that works, I probably need one anyway considering the shape mine is in. Sucks I can't get another American made accutronics but I'll deal with it. Thanks again for all your help fellas. I'll let you know what happens and hopefully this takes care of it to my liking.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sun Jan 31, 2016 5:51 pm
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Keep an eye peeled on Ebay -- vintage tanks show up periodically (I've bought several over the years).

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: Fender 75 Reverb Problem??
Posted: Tue Feb 16, 2016 10:30 am
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Well I think I finally got it. I did replace the tank, which didn't help but that extra spring really adds a lot to the reverb, so I am fine with that. Someone suggested to me to take the lead from the reverb pot wiper and ground it to the recovery tube grid, V2A. so I removed the lead going to the board where the wiper lead goes, the other end is to the ring terminal on the plain pedal jack, and attached that to the center lug on the reverb output jack which then goes to the tube grid. This grounds the signal right out of the recovery tube by the footswitch. And finally, the reverb signal is gone unless I switch it on. Makes sense since coming from the wiper of the pot the signal could go either to ground but could also go through the other resistors thus carrying some reverb signal with it. BWM was right, a definite design flaw. Only quirk is that as I turn the reverb up it's picking up noise and buzzing louder, but I can live with that since I don't turn it up that much. any suggestions to fix that are appreciated but I can deal. Thanks again for all of your help with this.


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