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Post subject: How to Blackface an AA768 Bandmaster Reverb
Posted: Mon Nov 06, 2017 12:08 pm
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Hello from a noob to the forum! I have a teeny amp/guitar repair shop and a customer brought in a "68 Silverface Bandmaster Reverb" to be restored. He wants it Blackfaced, along with a cap job and new output tubes etc. The date code on the PT is 606847, which would make it a very late 68 or a 69 right? It has the 150 ohm 7 watt cathode resistors on the output tubes, which would make it "The Dreaded" AA768 I believe. Firstly, why do folks refer to this circuit in this way? Is it from a tone standpoint, or a tech's nightmare standpoint? I've never done a blackface mod before and have found info on how to do one on an AA1069, but can only find the one almost illegible AA768 schematic out there to compare to. My question is, after installing a trim pot to make the bias adjustable, what is to be done as far as converting the output tubes to fixed bias and getting rid of the cathode resistors? Is trying to blackface this particular circuit a nightmare or maybe pointless job? Thanks for any help from a guy who has to learn everything the hard way!


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Post subject: Re: How to Blackface an AA768 Bandmaster Reverb
Posted: Tue Nov 07, 2017 2:16 am
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Joined: Fri Feb 05, 2010 4:31 am
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With cathodes resistors it look a AC 568 or AA768
It could be a 1978 amp too

You write ;
"My question is, after installing a trim pot to make the bias adjustable, what is to be done as far as converting the output tubes to fixed bias and getting rid of the cathode resistors? "

My answer ; just remove cathodes 's resistors and send cathodes direct to ground

Some good schematic here ; http://ampwares.com/line/bandmaster/

or
http://www.amprepairparts.com/schematics.htm

You may know these link if you fix tubes amps


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Post subject: Re: How to Blackface an AA768 Bandmaster Reverb
Posted: Tue Nov 07, 2017 12:35 pm
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Thanks very much! I appreciate the help. I was thinking that it would be too simple to just remove the cathode resistors. I will do as you suggest and rebias the amp after.


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Post subject: Re: How to Blackface an AA768 Bandmaster Reverb
Posted: Tue Nov 07, 2017 1:47 pm
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Joined: Fri Feb 05, 2010 4:31 am
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scottbrown wrote:
Thanks very much! I appreciate the help. I was thinking that it would be too simple to just remove the cathode resistors. I will do as you suggest and rebias the amp after.


Yes very easy.


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