Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bassden on September 12, 2023, 10:16:50 am
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Hello all, I have a problem, I have not seen before. The original in this amp was a AB165 And I updated it to a AA864 all new components. Transformers are original. This is for channel 1 only when the treble and bass pots are at 0. the volume pot in not working at all. When I increase either the treble, bass or both, the volume pot does then work. Channel 2 works great.
I've gone point to point a gazillion times but it is like they are connected wrong. Resoldered connections, all new pots, all new components on channel 1, new sockets, new tubes I do not have a scope. I've resolved a bunch of problems in all areas after updating but this one is stumping me. Any and all suggestions would be fantastic. Thank you.
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According to the schematic and from what I see, if you have treble and bass pots at zero, all signal is shunted to ground before it ever makes it to the volume pot. Path of least resistance. Seems like it is operating normally to me.
Dave
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Thank you Dave for your quick response. I understand what you are saying, I don't understand how the treble pot if at 0 would be grounding the signal since it is not grounded and has signal on both ends to one side of the volume pot. I would expect signal to pass to the center of the treble pot and then to one side of the volume pot. This would be giving signal to the volume pot without additional resistance of the treble and bass pots giving a volume adjustment without any treble or bass additions. I have built 2 AA864's with identical wiring and the volume pot does have volume without treble and bass added.
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When the tone pots are set to zero (max CCW) the wiper is connected to the left lug. That puts a ground directly on the volume pot and no signal can pass through. Turning up either tone pot removes the ground to the volume pot and allows signal to pass through.
I have built 2 AA864's with identical wiring and the volume pot does have volume without treble and bass added.
If that's the case, they are not wired according to the schematic.
Maybe this pic will help see the ground that exists when both pots are set to zero...
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Thank you Dave. I fully understand now. Sorry for not right away.
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Nothing to be sorry about. I've been doing this a long time and I still have trouble understanding what I see in a schematic very often.
Dave