Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnwitt on July 20, 2017, 07:59:29 am
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So here's an odd one. Hooked up a new Fender style 2 button footswitch to the Hoffman AB763 I'm building and no Tremolo. I opened up the case and found no shield wire only a red and white insulated conductor connected as shown in the photo below. I ohmed out the RCA plugs and shields of the two male connectors and found that the red conductor is connected to the red plug's outer shield and the white conductor is connected to yellow plugs pin. There is no conductor coming from the red plug's pin. Strange?
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Well that ain't gonna work! The reverb switch must use a shielded cable. The shield also serves as a ground for the tremolo switch. Doug sells the proper cable. Or return the FS and get a proper one.
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Steve,
Yep. Poor work indeed. It turns out there is a conductor from the red pin it just wasn't connected properly and the red wire is shielded so it's on the reverb switch. The shield and conductor were shorted together. Probably why the amp volume was dropping when the reverb was engaged as well. I'm spending more time with product manufacturing issues on this amp build then I should be.
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That's the proper cable. The red should be for the reverb and the white should be for the tremolo. The red wire and it's shield should be connected to the reverb switch and the white wire should connect to the trem switch. Then put a jumper from the shield to the trem switch.
At the RCA plug end... Red cable center conductor should connect to the center pin and shield should connect to the shell. White wire connects to the center pin of the other plug and nothing connects to the shell.
And the footswitch jacks on the amp chassis cannot be isolated. IOW, the shell must connect to chassis ground.
You can work with that cable but you may have to replace the RCA plugs if they ain't wired as stated above.
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Yep, got that all figured out and rewired. Just a case of sloppy wire hookup. Both the red conductor and shield were shorted together on the reverb switch. I'm showing good conductivity between all rca jack shields and chassis ground but I'm going to wire the grounding tabs to a chassis ground to prevent future problems anyway.