Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big_Mike on December 23, 2025, 07:53:57 pm
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Hi guys,
I am thinking of building a Vox AC15 clone using the Hoffman AC30 board and layout. Having the normal and top boost channel will be great.
There are three small changes that I want to make. Obviously, I only need two EL84 outputs, so I assume I can just trim the layout down to two output tubes rather than 4? Any other component parts I need to change?
Second, I would like to make it solid state rectified instead of tube rectified. How would the layout look for that?
Finally, I am going to add a Lar-Mar master volume, but that will be fairly easy to do.
Any help on the first two points would be greatly appreciated!
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Ordered the chassis, transformers, and components for this build. I am using the Hoffman AC30 board and layout, just reducing the output to two tubes.
Really excited for this project. I figure I can build an AC15 clone that sounds much better and more like a JMI than the current Vox handwired series offers.
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I purchased the transformers for this build, and I am using a Hammond 290px. I have a couple of questions on the hookups with the Vox AC30 layout.
First, on the hookup for pins 1 and 2, I assume hot goes to pin 2 and neutral goes to pin 1?
Second, I am a little confused on the secondary 6.66 volt heater wiring. Pins 5 and 6. 5 is zero volts, 6 is 6.66 volts, and there is a black wire for 3.33 volts. How should I hook those up for the heaters?
Below is a link to the wiring diagram for the 290px:
https://www.hammfg.com/files/parts/pdf/290PX.pdf?v=1697661948 (https://www.hammfg.com/files/parts/pdf/290PX.pdf?v=1697661948)
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First, on the hookup for pins 1 and 2, I assume hot goes to pin 2 and neutral goes to pin 1?
That's correct. But you should also connect pin 3 to pin 1 and pin 4 to pin 2.
Second, I am a little confused on the secondary 6.66 volt heater wiring. Pins 5 and 6. 5 is zero volts, 6 is 6.66 volts, and there is a black wire for 3.33 volts. How should I hook those up for the heaters?
It's just a 6V winding with a center tap (black). Pins 5 and 6 are the filament string that you connect to the tube filaments. Connect the black wire to ground to reduce filament hum.
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First, on the hookup for pins 1 and 2, I assume hot goes to pin 2 and neutral goes to pin 1?
That's correct. But you should also connect pin 3 to pin 1 and pin 4 to pin 2.
Second, I am a little confused on the secondary 6.66 volt heater wiring. Pins 5 and 6. 5 is zero volts, 6 is 6.66 volts, and there is a black wire for 3.33 volts. How should I hook those up for the heaters?
It's just a 6V winding with a center tap (black). Pins 5 and 6 are the filament string that you connect to the tube filaments. Connect the black wire to ground to reduce filament hum.
Thanks for the info, I wonder why you have to wire up the 120v to both pins 1-2 and 3-4? Is that just to feed separate secondaries?