Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MacGwyn on December 08, 2018, 03:18:21 pm
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Just getting started on a Super Reverb clone, AB763. I hooked up the power transformer first and connected the red/blue wire only to test the bias board... nothing else connected... no tubes yet. I thought I could apply the AC and check the bias supply before proceeding farther. Right off the bat, my light bulb current limiter lit brightly. Is that normal with nothing connected to the b+... and only the bias supply connected? I also disconnected the bias output from the trans, but still got a bright limiter.
The AC wiring is green to chassis ground, black through fuse to trans and white wire from trans through on/off switch to the AC line. Secondary center tap red/yellow is also grounded to chassis. Any ideas on what to check?
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Disconnect every PT wire except the two primary wires. Tape the ends of all secondary wires so they cannot touch each other. Is the light bulb current limiter still lit brightly?
The preferred wiring for the AC cord is green to chassis. Black to end of fuse. Side of fuse to switch. Other side of switch to one PT primary. The other PT primary lead connects directly to the power cord white wire.
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The primary wires appear to be:
black, white, black/red, black/yellow, black/green and black/blue.
I asked the supplier and they said black and white were correct for 120 AC.
The others mentioned were cut to different lengths (so they could not touch) and sealed.
I double-checked that they are not touching, and still got the bright limiter light.
I could easily change to the preferred wiring you mentioned, but it is
different than the plans supplied.
Thanks for your reply... What else can I check?
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Whoops, I misread what you wrote! I was concentrating on the primary side...
I'll try disconnecting the secondaries... the red wires are going to an empty
tube socket, but the greens are connected to heater wiring. I'll try disconnecting
those and report back.
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Give us more info on your PT. A wiring diagram would be great.
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Here's a link to the diagram, it's a Mojotone 762 ex.
https://www.mojotone.com/amp-parts/Power-Transformers/BF-Bassman-Super-Rev-Vibroverb-Mojo-Pro-Super-Power-EX#
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Sluckey, I re-wired the AC supply according to your directions, and the limiter light is now out! I'm getting 52 volts AC out of the bias supply, but the output from that board is not negative voltage. Think I must have the diode in backwards or something. On the Super Reverb, doesn't the band on the diode face toward the 470 ohm resistor at the board input?
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Yes and the positive end of the bias cap goes to ground.
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All working OK now, thanks for the help!