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Post subject: 66' AB165
Posted: Wed Apr 01, 2015 3:52 pm
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I have a 1966 Fender Bassman circuit AB165.... I faithfully replaced the electrolytic caps and some resisitors..and re-tubed... Just turned it on and I have a massive HUM... tried lifting the ground.. but, didn't help.. Any suggestions?


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 4:08 pm
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Are the electrolytic caps in the filter array and bias supply installed correctly with respect to polarity?

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: 66' AB165
Posted: Wed Apr 01, 2015 4:36 pm
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Did you change the global feedback network off the secondary of the output tranny? Or the polarity of the output tranny's wires to the 6L6GCs?

Check the continuity of each replaced cap, under the doghouse --- negative side to ground. Amp off, power and standby switch to ON.

The polarity of the string of caps should look like this (speaker cable jack on the bottom of the photo). First stage has one cap (+) opposite of all other caps.



Image


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 5:08 pm
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As I said, I was very careful about polarity when installing the new caps... Curious how 2 guys were thinking about this section... I may need to do an inspection.. Thank you.. BN


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 5:11 pm
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I haven't performed the bias adjustment yet... Would that cause the HUM?


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 5:25 pm
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Yes. If the tubes are biased too hot --- hum can be a result. For plate voltages around 475VDC --- the idle bias in this amp is around 38-42mA. Around 18-20 watts per tube.

The stock AB165 does NOT have a bias pot adjustment. Just a hum pot. To change the bias point of both 6L6GCs, you need to play with fixed resistors, in the bias supply.

Check for red-plating.

Good luck!


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 5:42 pm
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Okay... thank you for the information... You were very helpful... I'll have to check this out.


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Post subject: Re: 66' AB165
Posted: Wed Apr 01, 2015 10:29 pm
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If you plan on keeping this amp, may want to revert the bias supply to bias pot. Fairly easy process.


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